Important words and concepts from Chapter 24, Campbell & Reece, 2002 (4/6/2004):
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(1) Chapter title: The Origin of Species
(a) "It is not enough to explain how adaptations evolve in populations... Evolutionary theory must also explain the multiplication of species, the radiation of an existing species that gives rise to two or more new species."
(b) "Status as a peripheral isolate merely gives a lottery ticket to a small population. A population can't win (speciate) without a ticket, but there are very few winning tickets." Stephen Jay Gould, p. 443, Campbell, 1996
PATTERNS OF SPECIATION (MACROEVOLUTIONARY)
(2) Speciation (see also speciation)
(a) Speciation is the formation of a new species from an older, immediately ancestral species
(3) Anagenesis (see also anagenesis)
(b) Anagenesis involves the extinction of the older, ancestral species
(c) Anagenesis is converse to "branching" evolution; that is, it is "non-branching" evolution
(d) Contrast anagenesis with cladogenesis
(e) See Figure, Two patterns of speciation
(b) Cladogenesis does not (or, at least, does not necessarily) involve the extinction of the parental species
(c) Cladogenesis is branching evolution
(d) Only via branching evolution can species increase in number
(e) The "evolution of many diversely adapted species from a common ancestor is called adaptive radiation." (p. 471, Campbell & Reece, 2002)
(f) Cladogenesis is probably more common than anagenesis
(g) For whatever it is worth, anagenesis is probably just a special case of cladogenesis where the parental population either
(i) goes extinct coincident to the formation of the progeny species, or
(ii) the parental species is driven to extinction by the progeny species soon after the latter's genesis
(h) (there two scenarios are effectively the same thing so far as the fossil record is concerned)
(j) See Figure, Two patterns of speciation
(5) Species (see also species)
(a) Just what the heck is a species?
(c) Instead, the genotypes and phenotypes of populations of organisms appear to be constrained to discrete types
(d) Populations of types which satisfy certain criteria are termed species
(a) The idea of reproductive isolation is a squishy one
(b) Absolute reproductive isolation means that genes (alleles) do not pass from one population to a second population, one with which the first population is reproductively isolated
(c) Note that reproductive isolation does not mean that individuals within two populations are not mating nor producing offspring within populations; instead, if there are offspring, those offspring are not contributing their alleles to either of the parental populations (e.g., because these hybrid offspring are sterile and/or do not survive to reproduce)
(d) Also note that reproductive isolation need not be 100%; it is possible for two populations to maintain a large degree of reproductive isolation with some small amount of gene exchange still occurring (a.k.a., introgression)
(e) Thus, the phrase "reproductive isolation" describes some point along a spectrum ranging from something greater than a total lack of reproductive isolation (free gene exchange between populations) to complete reproductive isolation (no gene exchange between populations)
(a) There is more than one way to define just what a species is
(b) That is, there are various species concepts
(c) Species concepts include:
(iii) Recognition species concept (mating recognition)
(iv) Cohesion species concept (phenotype space, e.g., as applied to bacteria)
(vi) Evolutionary species concept
(d) We will emphasize in particular the first two of these species concepts
(8) Biological species concept (see also biological species concept)
(b) A biological species is a "...population or group of populations whose members have the potential to interbreed with one another in nature to produce viable, fertile offspring, but who cannot successfully interbreed with members of other species. In other words, a biological species is the largest unit of population in which genetic exchange is possible, and that is genetically isolated from other such populations." (emphasis mine)
(c) "Put still another way, each species is circumscribed by reproductive barriers that preserve its integrity as a species by blocking genetic mixing with other species."
(d) "Remember that biological species are defined by their reproductive isolation from other species in natural environments. In the laboratory or in zoos, hybrids can often be produced between two species that do not interbreed in nature."
(9) Conspecifics (see also conspecific)
(a) Conspecifics are two (or more) individuals who are members of the same species
(a) Two problems with the biological species concept are
(i) that it requires sex and
(ii) it requires sex
(b) (that is, both the concept and the act)
(c) Thus, the biological species concept is difficult to apply to organisms that reproduce asexually (though not impossible to apply if gene exchange still occurs such as between bacteria via transduction, transformation, and conjugation), and there are examples of populations that we otherwise might want to call separate species but which nevertheless at some low level share a gene pool (i.e., exchange genes, a.k.a., introgression)
(d) The biological species concept is also difficult to apply to organisms that are dead (e.g., extinct organisms)
(e) The biological species concept also typically must be inferred; confirming reproductive isolation is not a simple task
(a) A widely employed alternative to the biological species concept is the morphological species concept
(b) That is, two very similar organisms are more likely conspecifics than two less-similar organisms
(c) This is the same, familiar species concept that all of us have been employing most of our lives
(d) The morphological species concept is useful particularly since it is as applicable to fossils as it is to extant, sexually reproducing species
(e) However, the morphological species concept is not a terribly useful in terms of understanding processes of speciation since ultimately such processes are intimately tied to matters of reproductive isolation
(a) The ecological species concept is based on ecological competition:
(i) "A species is a number of related populations the members of which compete more with their own kind than with members of other species." (p. 152, Colinvaux, P. 1986, Ecology. John Wiley & Sons. New York. p. 152)
(ii) The more similar two organisms are, the more likely their needs will overlap, the more likely they will compete, and therefore the more likely that they are of the same species
(b) Caveat: intraspecific life history divergence:
(i) Even the ecological species concept has problems since it requires that members of individual species not have divergent life histories (which, in practice, is not always the case)
(ii) It also runs into a problem also seen with the morphological species concept: At what point does one stop the process of splitting divergent forms into new species?
(iii) It also is not necessarily trivial to determine the degree to which two or more individuals are competing ecologically
(a) While a given species concept may be preferred in a given circumstance, that species concept probably will not have universal application
(b) To understand all species, living at all times, should require a broader concept of what it means to be a species than any one species concept indicated above
(c) The need to mix and match species concepts as applicable gives rise to the idea of a pluralistic species concept which recognizes, essentially, that "the factors that are most important for the cohesion of individuals as a species vary." (p. 468, Campbell & Reece, 2002)
(14) Subspecies (see also subspecies)
(a) A subspecies is a morphologically distinct population that nevertheless enjoy incomplete reproductive isolation from another such population
(b) Typically two members of different subspecies are more reproductively isolated than two members of the same subspecies
(c) "Population biologists are discovering more and more cases where the distinction between subspecies with limited genetic exchange and full biological species with segregated gene pools blurs. It is as though we are catching populations at different stages in their evolutionary descent from common ancestors."
(d) It is important to keep in mind that the concept of a subspecies is a somewhat fuzzy one that can differ from scientist to scientist and perhaps even from mood to mood since subspecies represents one of those catchall categories where one throws populations that are divergent, but not too divergent, from other populations; clearly, however, subspecies legitimately exist as morphological distinctive populations that, however, are not enormously reproductively isolated from other such populations
What is a Subspecies? (supplemental discussion)
Subspecies are morphologically distinct from other subspecies of the same species
Members of subspecies are more likely to breed within their own subspecies than with other members of their species
Subspecies are geographically localized
Some researchers argue that the subspecies concept is sufficiently flawed as to be irrelevant
It doesn't really matter because apparently the rallying cry of humanity goes something like: "Prosperity before subspecies!"
If you really want a good cry, try doing a "subspecies and extinct" search on the web; you will find things like, "Three tiger subspecies are now extinct (all of them are dead): Caspian tiger (P.t. virgata), Javan tiger (P.t. sondaica), Bali tiger ( P.t. balica)."
Below are the mountain zebra, the grevy zebra, the plains zebra, and the quagga (extinct), all of which are subspecies of a single zebra species:
(b) Reproductive barriers may be classified into two general categories
(ii) Postzygotic barriers
(c) See Figure, A summary of reproductive barriers between closely related species
(d) The term "zygotic" refers to the product of conception
(e) Thus prezygotic barriers prevent conception while postzygotic barriers interfere with the Darwinian fitness of the hybrid progeny
(f) Note that key to understanding the speciation process is the cost to potential parents as increasing levels of prezygotic barriers are breached, as well as increasing levels of postzygotic barriers are breached
(g) The ultimate Darwinian disaster (besides dying before reproducing) is to invest in the raising of an offspring that never succeeds in contributing to the gene pool
(h) The earlier such an offspring may be aborted or prevented, the greater the Darwinian fitness of the potential parents
(i) Keep these ideas in mind as we walk through various reproductive barriers
(j) Below is a tabular summary of reproductive isolating mechanisms/barriers:
Increasing Fitness Cost
to Would-Be Hybridizers
(going from bottom to top)
Has Genetic Component
(right & above)
(16) Prezygotic barriers (see also prezygotic barrier)
(a) Prezygotic barriers include
(ii) Habitat isolation
(iii) Behavioral isolation
(vi) Gametic isolation
(b) See Figure, A summary of reproductive barriers between closely related species
(17) Geographical isolation (see also geographical isolation)
(a) Geography supplies distance and other obstacles to gene flow between populations
(b) Two organisms that are not able to meet cannot mate, period (well, at least, two individuals whose gametes are not able to meet cannot fertilize)
(c) The most profoundly important mechanism to speciation (particularly allopatric speciation) is some kind of mechanism whereby individuals from different populations fail to meet and thereby fail to mate
(d) We'll return to this concept
(e) All of the barriers that we will subsequently discuss are ones that function within geographically overlapping populations
(f) See Figure, Two modes of speciation
(g) See Figure, Allopatric speciation of squirrels in the Grand Canyon
(18) Habitat isolation (see also habitat isolation)
(a) Two individuals living within overlapping ranges may nonetheless never meet if both avoid the same spots within their ranges
(b) For example, high up in a tree is a long way from the ground though both share geographical ranges; an individual that sticks to the canopy will only rarely contact an individual who sticks to the ground
(c) Other examples of habitat isolation include preferences for habitat on/within different species
(d) Habitat isolation probably explains, at least in part, why the world has so darn many beetles (i.e., a given range has a lot of distinct habitats for such a small and versatile creature; those are Beatles to the right -->)
(19) Behavioral isolation (see also behavioral isolation)
(a) Two individuals who meet, who are very similar, will nevertheless fail to progress to mating if behaviors (especially mating behaviors) are incompatible
(b) Basically, the problem is that individuals belonging to two populations may not "speak the same language" (though note that "language" deserves ironic quotes since we are referring particularly to nonverbals)
(c) This is typically observed in courtship rituals
(d) An individual in a courtship ritual simultaneously is justifying his or her worthiness to mate as well as indicating status as a conspecific
(e) See Figure, Courtship rituals as a behavioral barrier between species
(f) See Figure, Mate choice in two species of Lake Victoria cichlids
(20) Temporal isolation (see also temporal isolation)
(a) Two individuals who breed only at certain times and not at overlapping times are effectively reproductively isolated
(21) Mechanical isolation (see also mechanical isolation)
(a) Mechanical isolation refers to an inability to mate even given a willingness to mate by the two participants, due to morphological incompatibility
(b) Note that with mechanical isolation we start down a road toward an increased costliness of lack of reproductive isolation
(c) Particularly, attempting to mate is not without cost, e.g., susceptibility to predation for animals during the mating process
(d) An important example of mechanical barriers is found in flowering plants that may be adapted to pollination by different insects
(22) Gametic isolation (see also gametic isolation)
(a) Given successful mating, both male and females bear the costs of mating, but so far only the male has managed to waste gametes
(b) The female does not lose gametes to hybridization until conception has occurred
(c) Keep in mind that a female's eggs are typically a lot more expensive than a male's sperm, especially when the female (but not the male) is charged with the brunt of the cost of raising the offspring, and when the female is much more limited in reproductive opportunity
(d) Thus, a female typically has more incentive to avoid conception than does a male
(e) Mechanisms whereby conception is avoided following mating are termed gametic isolation
(f) This can involve either a destruction of sperm prior to their reaching the egg, or an incompatibility between sperm and egg such that the sperm is unable to penetrate and thereby fertilize the egg
(g) Gametic isolation additionally occurs when pollen is excluded by flowers
(23) Postzygotic isolation (see also postzygotic barrier)
(a) Postzygotic isolation is very costly, to be avoided if possible
(b) This is because postzygotic isolation basically represents the formation of an offspring with reduced Darwinian fitness
(c) Postzygotic reproductive isolating mechanisms include
(iii) Hybrid breakdown
(d) Viability and fertility, of course, are what define Darwinian fitness
(f) See Figure, A summary of reproductive barriers between closely related species
(24) Reduced hybrid viability (see also reduced hybrid viability)
(a) Reduced hybrid viability means that the hybrid basically dies, either before successfully reproducing or before reproducing at a rate that is as high as that experienced by the non-hybrid progeny of the parent species
(b) The earlier during the period of parental care (if any) that the offspring becomes inviable, in general, the better for the caring parent
(c) Thus, ideally inviability, if it is going to occur, occurs soon after conception, especially if this frees up the female for subsequent mating and reproduction
(d) For the parents, the most costly time for inviability to occur is after parental care is over but prior to the occurrence of successful reproduction by the hybrid
(25) Reduced hybrid fertility (see also reduced hybrid fertility)
(b) Hybrids can display reduced fertility, that is, relative to the fertility displayed by either parent
(c) Often this reduced fertility occurs as a consequence of problems during meiosis
(e) See Figure, Hybrid sterility, a postzygotic barrier
(26) Hybrid breakdown (see also hybrid breakdown)
(a) Hybrids may display neither reduced viability nor reduced fertility, but the offspring of hybrids may still go on to display reduced viability or fertility; this is hybrid breakdown
(27) Selection for reproductive isolation (see also selection for reproductive isolation)
(a) Two populations that fail to achieve prezygotic reproductive isolation display a reduced average fitness as a consequence of reduced hybrid viability and reduced hybrid fertility (or hybrid breakdown)
(b) Individuals who can avoid mating such that hybrids are therefore not produced can enjoy a Darwinian fitness advantage over individuals who fail to avoid such matings
(c) Thus, selection often will favor the evolution of prezygotic isolating mechanisms among populations whose ranges overlap and that produce hybrids sporting reduced Darwinian fitness
(28) Breakdown of barriers (see also breakdown of reproductive barriers)
(b) While this movement toward prezygotic reproductive isolation is occurring, gene exchange may be occurring between the two populations such that genetic differences are lost
(c) Thus, from the point in time at which two distinct populations overlap in range there exists a race between the evolution of the mechanisms of prezygotic reproductive isolation and the reformation of a single, not internally reproductively isolated population
(d) Additionally, one population may out-compete the other, driving the latter to extinction
(e) See Figure, Has speciation occurred during geographic isolation?
(29) Introgression (see also introgression)
(b) Note that the existence of introgression causes the biological species concept to be a very fuzzy concept
(c) Indeed, separate species can remain intact despite a low level of sharing of alleles
(30) Allopatric speciation (see also allopatric speciation)
(a) Perhaps the best way to understand the process of speciation is to follow a hypothetical speciation scenario
(c) Allopatric speciation is speciation that is initiated via the geographical isolation of populations (allopatric = "originating in or occupying in different geographical areas" The Random House Dictionary of the English Language)
(d) Key to the occurrence of allopatric speciation is the occurrence of geographical barriers--big things in the landscape that get in the way of the movement of organisms from place to place, i.e., from one population to another
(e) See Figure, Two modes of speciation
(f) See Figure, Allopatric speciation of squirrels in the Grand Canyon
(g) See Figure, Has speciation occurred during geographic isolation?
(31) Geographical barriers (see also geographical barrier)
(a) Geographical barriers can include such things as mountains, oceans, valleys, rivers, land between water bodies, glaciers, etc.
(b) Key, again, is simply that the geographic barriers arise (typically via geographical or environmental processes) thus splitting a formerly contiguous population into a discontiguous one (note: "discontiguous" really is a word: click here for Google search with over 8,000 hits in April of 2002)
(c) Note that not all geographical barriers are of the same significance to all organisms
(32) Peripheral isolates (see also peripheral isolates)
(a) "Whenever populations become allopatric, it is possible for speciation to occur as the isolated gene pools accumulate genetic differences by microevolution. But an isolated population that is small is more likely than a large population to change substantially enough to become a new species."
(b) "The geographical isolation of a small population usually occurs at the fringe of the parent population's range. The splinter population, or peripheral isolate, is a good candidate for speciation for (these) reasons:"
(i) The peripheral population probably occurs at the extreme of the population's range consequently potentially reflecting extremes of variation within the parental population due to both incomplete mixing within the whole population and environmental extremes at the periphery of a species' range (= selection before formation of geographical barrier)
(ii) Founding of a small peripheral population typically involves a founder effect which differentiates the founding population from the parent population further (= founder effect)
(iii) A new population cut off from its parent population typically will not immediately become large (if there were space to do this in, chances are it already would be large; exceptions are when populations are founded in new locales by accident, such as on islands) so consequently genetic bottlenecking further differentiates the peripheral population from the parental population (note once again that genetic bottlenecking and the founders effect are not identical phenomena) (= genetic drift/bottleneck)
(iv) Environmental differences between ranges will result in natural selection following different paths in the peripheral population relative to the natural population (= selection after founder effect/geographical isolation)
(33) Hybrid zone (see also hybrid zone)
(b) The area over which the population's ranges overlap, and within which hybridization occurs, is called a hybrid zone
(c) Note that it is within the hybrid zone that the reproductive isolation of two populations is tested and evolves
(d) Two populations that come into contact at a hybrid zone will either evolve more-robust reproductive isolating mechanisms, e.g., behavioral isolation, or will fail to, thus setting the stage for a melding of the two populations back into one
(e) Two populations may be able to stably retain something resembling species status as a consequence of ecological considerations and only limited gene exchange at the hybrid zone (i.e., some form of hybrid inviability or hybrid infertility)
(f) "Stabilizing selection would restrict phenotypic variation to a range narrow enough to define the species as separate from other species... The genetic basis for this cohesion of phenotype may involve specific combinations of alleles and specific linkages between gene loci on chromosomes."
(34) Scenario for speciation (see also peripatric speciation)
(a) How, then, is speciation typically thought to occur?
(i) Start with a single population
(iii) Note that the parental population may
· remain more or less intact while one or more peripheral populations may form, or
· the parental population may be broken up entirely into a number of remnant populations
(iv) The peripheral populations
· may be different from the parental population before becoming separated
· may be founded by only a small number of individuals
· may not have an opportunity to increase in size over the medium term
· may find themselves in environments that differ from that of the parental population
(v) Key is that the geographical barrier prevents gene flow between the peripheral population and the parental population
(vi) Thus, the peripheral population is in the position to diverge genetically from the parental population
(b) Note that the fate of the majority of peripheral populations is extinction
(c) Note that the fate of the "parental" population, if it has been essentially broken up into a number of remnant populations, likely is extinction
(i) Testing of reproductive barriers occurs only should the geographical barrier fail thus allowing the peripheral population's range to come to overlap the range of the parental population
(e) When the ranges of two isolated populations come to overlap, one of three things can result:
(i) The two populations evolve effective reproductive barriers thus preventing significant allele exchange between populations--speciation occurs
(ii) The two populations exchange genes to a sufficient extent that speciation fails to occur and the two populations turn into one population
(iii) One population can drive the other population to extinction
(f) Recall that costly postzygotic isolating mechanisms will drive the evolution of less-costly prezygotic isolating mechanisms
(g) Note that should the formerly peripheral population succeed in driving the parental population to extinction, then that would appear (in the fossil record) as anagenesis
(h) Note that should speciation occur such that the formerly peripheral population and the parental population coexist, that would be an example of cladogenesis
(i) Note that should the parental population be reduced to remnant populations, two of which succeed in forming new species, i.e., ones that differ morphologically from the parental population, then this would appear in the fossil record as one species "suddenly" diverging into two (or more) different species
(35) Parapatric speciation (supplemental discussion) (see also parapatric speciation)
(a) "In parapatric speciation there is no specific extrinsic barrier to gene flow. The population is continuous, but nonetheless, the population does not mate randomly. Individuals are more likely to mate with their geographic neighbors than with individuals in a different part of the population's range. In this mode, divergence may happen because of reduced gene flow within the population and varying selection pressures across the population's range." Evolution 101
(36) Sympatric speciation (see also sympatric speciation)
(b) This may occur as a consequence of isolation between microenvironments (different trees in the same forest, for example)
(c) Alternatively it may involve the founding of new populations that are reproductively isolated from the parent population from day one
(d) One way this latter, sympatric mechanism of speciation occurs is via a transition from sexual to asexual reproduction (asexual reproduction immediately isolates populations since it is sex that ties populations together genetically)
(e) Another way, common among plants, occurs as a consequence of changes in ploidy (i.e., number of sets of haploid genomes possessed by individual cells)
(f) See Figure, Two modes of speciation
(37) Autopolyploidy (see also autopolyploidy)
(c) A polyploid individual may not be able to mate with non-polyploid individuals with and resulting viable progeny
(d) Autopolyploid individuals may still be able to produce gametes but can mate successfully only within other autopolyploids
(e) In the case of plants (where all of these goings on typically are going on), the other autopolyploid could be the same plant
(f) See Figure, Botanist Hugo de Vries and his new primrose species
(38) Allopolyploid (see also allopolyploidy)
(b) "Interspecific hybrids are usually sterile because the haploid set of chromosomes from one species cannot pair during meiosis with the haploid set from the other species. Though infertile, a hybrid may actually be more vigorous than its parents and propagate itself asexually."
(c) See Figure, One mechanism for allopolyploidy speciation in plants
ISSUES RELATED TO SPECIATION
(a) This idea that populations do most of their evolving in small, isolated populations forms the basis of a concept known as punctuated equilibrium
(b) The idea is that because most morphological change occurs in small populations and this change occurs over "only" a few 10s, 100s, or 1000s of generations, there is a reduced likelihood that fossilization will document these morphological changes step by step as they occur
(c) Instead, one would expect the fossil record to be represented by dominant morphotypes (i.e., those represented by the parental population) and then for the fossil record to suddenly record a change in morphotype should the parental population be replaced by a peripheral population (i.e., a population that has a different morphotype)
(d) The "equilibrium" of the concept of punctuated equilibrium refers to the persistence of stable morphotypes in the fossil record over long periods (millions of years) while the "punctuated" part of the concept refers to the "sudden" appearance of morphological change over a period of "only" a few tens of thousands of years
(e) In other words, the likelihood of fossilization is directly proportional to population size and population duration:
(i) small, short-lived populations should result in proportionately fewer fossils
(ii) population biological theory predicts that the majority of morphological change/speciation events may occur as a consequence of evolution occurring in small/short-lived populations (e.g., allopatric speciation)
(f) "Suppose that a particular species survives for 5 million years, but most of its morphological changes occurred during the first 50,000 years of its existence. In this case, the evolution of the species-defining characteristics was compressed into just 1% of the lifetime of the species [and probably less-than 1% of the cumulative population of the species]. On the time scale that can generally be determined in fossil strata, the species will appear suddenly in rocks of a certain age and then linger with little or no change before becoming extinct. During its formative millennia, the species may have accumulated its modifications gradually, but relative to the overall history of the species, its inception was abrupt (short time scales/small populations)... If the species is adapted to an environment that stays the same, then natural selection would counter changes in the gene pool. In this view, the tendency for stabilizing selection to hold a population at one adaptive peak results in long periods of stasis."
(g) (Note that the idea of punctuated equilibrium may be to some extent artifactual. This is because the grouping of fossils into a coherent "species" is a very human and imperfect process. How much variation does one allow before declaring that one fossil is a different species from another group of fossils? The very concept of grouping fossils together as morphotypes of a certain similarity and abundance may artificially result in an idea that species tend to persist "unchanged" over relatively long periods. Clearly paleontology and population biology continue to be very dynamic sciences.)
(h) See Figure, Two models for the tempo of speciation
(i) Birth of an individual » speciation (birth of a species)
(ii) Death of an individual » extinction of a species
(iii) Genetic drift » species extinction due to random (not foreseeable) events
(iv) Natural selection » differential species birth and extinction (i.e., species selection)
(b) All else held constant, those species that survive the longest will give birth to more new species, so any characteristic of a species that tends to prevent extinction (large populations, broad range, generalist rather than specialist) will tend to increase the representation of that species' descendant species among the total number of species present on Earth (especially given changing environments)
(c) Other characteristics of species tend to increase the likelihood of speciation events. These include an ability to disperse to new locations and an ability to adapt to new environments
(d) Note that these characteristics are not necessarily the same that will assure a maximum short-term reproductive success by individuals in specific environments (i.e., why do generalists persist if specialists are so much better at doing what they do? Answer: because specialist are more susceptible to extinction than are generalists)
(e) Certain characteristics of a species might also make that species less susceptible to random changes in the environment (e.g., asteroid impact); such characteristics might include small size, wide range, a lack of specific dietary needs, etc.
(f) "The species that endure the longest and generate the greatest number of new species determine the direction of major evolutionary trends."
(g) Thus, to impact greatly on the evolution of the diversity of life, an organism must possess qualities that go beyond simply being highly adapted to life within a specific environment
(41) Adaptive landscapes (supplemental discussion) (see also adaptive landscape)
(b) Because Darwinian fitness is environmentally dependent, an adaptive landscape is valid only for one given environment
(c) Because allele combinations are approachingly infinite in their permutations (and certainly vary over more than two dimensions), the graphical representation of an adaptive landscape is itself merely a metaphorical representation
(d) That is, look at Figure not as a lumpy plane which shows the Darwinian fitness of all of the genotypes possible in a population (higher peaks represent greater Darwinian fitness); instead think of the plane as a two-dimensional compression of the idea of representing all of these genotypes within a single plane
(f) Nevertheless, the idea here is that certain genotypes display greater fitness in a given environment than do others
(g) A variety of genotypes may display similar Darwinian fitnesses, e.g., there exists more than one peak (combination of alleles) on this adaptive landscape
(h) Natural selection will drive a population up an adaptive peak; that is, the effect of selection is to eliminate those genotypes which are found at lower elevations in this metaphorical landscape
(i) Note that a population will tend to congregate around a single adaptive peak, and the peak chosen will be chosen not because that peak is the highest but because that peak is the easiest to attain given the alleles which are present in the population
(j) Once natural selection has driven a population up an adaptive peak, stabilizing selection will tend to keep that population on that peak (this is especially true as alleles become more and more co-adapted, adapted especially to the presence of certain alleles found at other loci)
(k) Only non-adaptive evolution (drift, migration, mutation) can move a population off of an adaptive peak thus allowing that population to explore another, perhaps taller adaptive peak (i.e., become even better adapted to its environment)
(l) Alternatively, environmental change will serve to completely change the adaptive landscape thus allowing for adaptive evolution up a different peak (or more likely, the population will fail to climb up a new peak and as a consequence go extinct)
(m) Thus, small, isolated populations in novel environments essentially find themselves in new adaptive landscapes which give them the opportunity to find and explore new adaptive peaks; and most such populations go extinct but those which survive display rapid adaptation (and, potentially, correlated morphological change than can be followed in the fossil record)
(o) Hybrid breakdown
(p) Hybrid zone
(hh) Species concepts
(ii) Species selection
(kk) Sympatric speciation
(ll) Temporal isolation
(43) Practice questions [index]
(a) Describe allopatric speciation in terms of (i) geographical barriers, (ii) peripheral isolates, (iii) hybrid zones, (iv) genetic drift, (v) diversifying selection, (vi) founders effects, (vii) prezygotic reproductive isolating mechanisms, (viii) speciation, and (ix) postzygotic reproductive isolating mechanisms. (order of use and order of presentation above are not necessarily synonymous--basically I want you to describe allopatric speciation using all of the above terms in a manner which convinces me that you know what the terms mean and how they impact on allopatric speciation)
(b) Name two processes capable of moving a population from the top of one adaptive peak to the area around another adaptive peak.
(c) What is the most basic difference distinguishing sympatric speciation from allopatric speciation?
(i) branching evolution ________
(ii) sympatric speciation ________
(iii) same species ________
(iv) replacement of parent species by progeny species ________
(v) overlapping range ________
(e) Physiologically, what is the difference between prezygotic mechanisms of reproductive isolation and postzygotic mechanisms of reproductive isolation?
(f) Why does natural selection favor the evolution of prezygotic reproductive barriers if postzygotic reproductive barriers are already present?
(g) Give an example of a population to which the biological species concept is difficult to apply to.
(h) Which isolating mechanism is likely least costly to the participants (i.e., the potential parents):
(i) reduced hybrid viability
(ii) behavioral isolation
(iii) gametic isolation
(iv) reduced hybrid fertility
(i) Name three general ways (reasons) by which peripheral isolates can vary genetically from their parental population.
(j) Contrast anagenesis and cladogenesis.
(k) The idea of reproductive isolation is crucial in defining the _________ species concept.
(l) Why is the morphologically defined species concept not particularly useful for understanding processes of speciation?
(m) Postzygotic reproductive barriers serve, in general, to interfere with (or otherwise impact negatively on) the __________ of hybrid progeny.
(n) Which of the following prezygotic barriers is possible only when the individuals making up two populations cannot possibly meet?
(i) Mechanical isolation
(ii) Temporal isolation
(iii) Behavioral isolation
(iv) Gametic isolation
(v) Geographical isolation
(o) Which of the following prezygotic barriers is likely the most costly to the participating individuals?
(i) Mechanical isolation
(ii) Temporal isolation
(iii) Behavioral isolation
(iv) Habitat isolation
(v) Geographical isolation
(p) Given reduced hybrid viability as a mechanism of reproductive isolation, describe a situation in which reduced hybrid viability would be maximally costly, in a Darwinian fitness sense, to the participating individuals (i.e., the parents).
(q) Define introgression.
(r) Give three mechanisms by which peripheral populations will either differ or may come to differ from the parental population during the process of allopatric speciation.
(s) What is the fate of the majority of peripheral populations that have become reproductively isolated from parental populations by geographical barriers?
(t) Describe a scenario of allopatric speciation that could be interpreted as anagenesis in a fossil record.
(u) What is sympatric speciation?
(v) Define autopolyploidy.
(w) What important role does non-adaptive (i.e., non-Darwinian) evolution play within adaptive landscapes?
(x) Another name for speciation without branching is __________.
(i) Allopatric speciation
(v) Sympatric speciation
(y) The idea that small populations evolve rapidly (e.g., as in allopatric speciation) while large populations evolve slowly, especially morphologically, gives rise to the concept of __________, which serves as an explanation for the sudden appearance of new species in the fossil record even though those same species may survive in the fossil record for comparatively long periods.
(z) What non-biological entity is required for the occurrence of allopatric speciation but is conspicuously not required for sympatric speciation.
(aa) Given that two speciation events have occurred via allopatric rather than sympatric processes, if the two formerly peripheral populations and the parental population continue to coexist (temporally as well as spatially) then this is an example of __________, a kind of speciation.
(bb) What is the name of the species concept that is defined by reproductive isolation from other species in natural environments?
(cc) Prezygotic reproductive barriers interfere with conception while postzygotic barriers interfere with the __________ of the hybrid progeny.
(dd) Which species concept is most reasonably applied to dead organisms, e.g., their fossil remains?
(ee) Two populations of organisms are morphologically distinct and between them there exist two or more prezygotic reproductive barriers. These two organisms likely are examples of two different __________.
(ff) What is temporal isolation?
(gg) Viability and fertility, of course, define Darwinian fitness, but also are what is reduced or lacking given the existence of a __________ isolating mechanism.
(hh) Geographical isolation initiates what kind of speciation?
(ii) Two individuals that live within the same geographical area at the same time but otherwise never meet may be experiencing __________, a kind of prezygotic isolating mechanism.
(i) Geographical isolation
(ii) Habitat isolation
(iii) Hybrid breakdown
(v) Reduced hybrid fertility
(jj) Because of introgression, the biological species concept is not quite as robust as it might have been. Why not?
(kk) Why does reduced hybrid viability drive two sympatric populations further down the road towards the erection of more robust reproductive isolating mechanisms?
(ll) A transition from sexual to asexual reproduction is one means by which sympatric speciation may occur. Name or describe another that does not involve a sexual to asexual reproductive transition.
(nn) All else held constant, and not considering mechanisms of new speciation or speciation rates, what is the single most important factor governing success in species selection?
(oo) Which has contributed least to increasing the number of existing species?
(iii) Punctuated equilibrium
(iv) Reproductive isolation
(pp) The process that results in the transformation of one ancestral species into more than one descendant species is called "__________genesis."
(qq) Which species concept is most closely tied to the idea of reproductive isolation?
(rr) Name two situations in which the biological species concept would be expected to be difficult to apply.
(ss) Why is the morphological species concept less applicable to the actual process of speciation than the biological species concept?
(tt) Arrange in terms of increasing cost or loss to the hybridizing parents (or would-be parents):
(i) Behavioral isolation
(ii) Gametic isolation
(iii) Hybrid breakdown
(iv) Mechanical isolation
(v) Reduced hybrid fertility
(uu) Which is not a prezygotic reproductive barrier?
(i) Behavioral isolation
(ii) Geographical isolation
(iii) Habitat isolation
(iv) Hybrid breakdown
(v) Temporal isolation
(vv) Which prezygotic isolation mechanism has the least (e.g., no) genetic component?
(ww) Which type of prezygotic isolation involves courtship rituals?
(xx) Which type of prezygotic isolation involves the wasting of sperm but not the wasting of eggs?
(yy) What is hybrid breakdown?
(zz) What does introgression mean?
(aaa) Contrast sympatric speciation with allopatric speciation.
(bbb) Describe why it is, even if they are subjected to otherwise identical selective forces, that most peripheral isolates are reasonably likely to experience a founder effect following the formation of a geographical barrier between them and their parental population.
(ccc) When two formerly isolated populations come back into contact, the geographic range over which this contact occurs (not necessarily the entire geographic range of either population) is called a(n) __________, within which the reproductive isolation of the two populations is tested.
(ddd) Speciation requires that two populations diverge genetically from each other which requires that there exists strong prezygotic reproductive barriers between the two populations. What, by far, is the most common prezygotic reproductive barrier whose presence can initiate the occurrence of allopatric speciation?
(eee) Other than "nothing", what happens to two genetically divergent but now sympatric populations if reproductive isolation mechanisms fail to develop?
(fff) How, fundamentally, does autopolyploidy differ from allopolyploidy? Be sure to indicate which definitions are associated with which term.
(ggg) What is species selection?
(hhh) A mechanism of sympatric speciation that is common among plants involves a process that is easily detected by karyotype. This process particularly involves __________.
(i) Gametic isolation
(ii) Geographical isolation
(iii) Increase in ploidy
(v) Mechanical isolation
(iii) What is sympatric speciation?
(jjj) What is the meaning of the term "equilibrium" in the phrase "punctuated equilibrium"?
(kkk) At what point in allopatric speciation are reproductive barriers tested?
(lll) Under what circumstances following allopatric speciation might the fossil record be interpreted as an example of anagenesis?
(mmm) What do we call the populations that are on the fringe of the parental population's range, i.e., that often are subject to diversifying selection, founders effects, and genetic bottlenecking over the course of allopatric speciation.
(i) Border collection
(ii) Fringe group
(iii) Marginal assemblage
(iv) Outlying population
(v) Peripheral isolate
(nnn) What is the meaning of "Introgression"?
(i) Existence of a hybrid zone
(ii) Lack of pre-zygotic isolation mechanisms
(iii) Movement of alleles between species
(iv) Overcoming pre-zygotic isolation mechanisms
(v) Reduced hybrid vitality
(vi) In general, what circumstances, in addition to sympatry, will tend to for prezygotic reproductive isolation between two populations?
(ooo) Which of the following is a postzygotic isolating mechanisms?
(i) Behavioral isolation
(ii) Gametic isolation
(iii) Geographical isolation
(iv) Hybrid inviability
(v) Mechanical isolation
(ppp) What is the pluralistic species concept?
(qqq) What is temporal isolation?
(rrr) Explain why, though allopatric speciation is defined in terms of the geographical barriers, in fact geographical barriers must eventually be lost if prezygotic isolation is to evolve as a selected adaptation.
(sss) Give three reasons why you might expect peripheral isolates to differ from the parent populations both prior to and following the establishment of geographic isolation.
(ttt) What are the terms (prefixes, actually) that we associate with these two patterns of speciation (illustration below): _______genesis and _________genesis. Please indicate which is which in the illustration:
(uuu) What is introgression?
(vvv) Which of the following species concepts is most applicable to the classification of species that are known only from the fossil record?
(i) Biological species concept
(ii) Ecological species concept
(iii) Morphological species concept
(iv) Recognition species concept
(www) Which of the following species concepts is most applicable to our understanding of speciation among sexually reproducing organisms such as animals and plants?
(i) Biological species concept
(ii) Ecological species concept
(iii) Morphological species concept
(iv) Recognition species concept
(xxx) These phylogenies represent two hypotheses as to the tempo and mode of evolution, particularly with regard to the relationship between speciation events and morphological change. Which one of these better fits the concept of punctuated equilibrium?
(yyy) Which are the postzygotic reproductive isolating mechanisms?
(i) Behavioral Isolation
(ii) Gametic Isolation
(iii) Geographical Isolation
(iv) Habitat Isolation
(v) Hybrid Breakdown
(vi) Mechanical Isolation
(vii) Reduced Hybrid Fertility
(viii) Reduced Hybrid Viability
(ix) Temporal Isolation
Which pattern least ambiguously illustrates sympatric speciation. Circle the whole column:
(aaaa) List three pre-zygotic reproductive isolation mechanisms.
(bbbb) "Status as a __________ merely gives a lottery ticket to a small population. A population can't win (speciate) without a ticket, but there are very few winning tickets." Stephen Jay Gould
(iii) Peripheral isolate
(iv) Punctuated equilibrium
(cccc) What is anagenesis?
(dddd) The genotypes and phenotypes of populations of organisms appear to be constrained to discrete types. Populations of types which satisfy certain criteria are termed __________.
(eeee) Which of the following is most important in defining the biological species concept?
(ii) Ecological niche occupied by a population
(iii) Morphological distinctiveness
(iv) Reproductive isolation
(v) Species are products of evolution
(ffff) Reproductive barriers may be distinguished into at least two general types (which together encompass all existing types of reproductive isolating mechanisms). Name and distinguish those two types.
(gggg) Which is less costly to potential parents, behavioral isolating mechanisms or gametic isolating mechanisms? Why?
(hhhh) What specifically does "Hybrid breakdown" mean?
(iiii) Describe a hypothetical scenario for allopatric speciation.
(jjjj) Name three consequences of peripheral isolation that could abet (aid) peripheral population divergence from a main population from which it has become isolated.
(kkkk) Which of the following characteristics of a species is least likely to result in a high likelihood of leaving numerous descendant species?
(i) Ability to adapt to new environments
(ii) Less susceptible to random changes in environments
(iii) Small body size
(iv) Specific dietary needs
(v) Wide distribution
(llll) True or False, the concept of peripheral isolates is closely linked to hypotheses on mechanisms of sympatric speciation. A: False
(mmmm) What does the term Anagenesis mean? A: Anagenesis is non-branching evolution, i.e., speciation that does not result in an increase in the total number of species in the world
(nnnn) True or False, reproductive isolation between populations by definition is 100%, i.e., implying zero gene flow between two populations. A: False
(oooo) What is a conspecific? A: A conspecific is a second member of the same species
(pppp) Give an example of a type of organism for which it is difficult to apply the biological species concept. A: extinct (dead) organisms, asexual organisms
(qqqq) The morphological species concept is quite useful. Why, then, is it not emphasized in a chapter (24) dealing particularly with the concepts of speciation? A: because speciation is all about reproductive isolation while morphological differences between species does not directly address considerations of reproductive isolation
(rrrr) What is the term we use to describe two populations that are divergent but not too divergent from each other, that are reproductively isolated from one another, but too reproductively isolated, and which are morphological dissimilar but not too morphologically dissimilar from each other? A: subspecies
(ssss) Complete this sentence: "The ultimate Darwinian disaster (besides dying before reproducing) is to..." A: raising offspring that never succeed in contributing to the gene pool
(tttt) What do the increasing fitness costs of postzygotic barriers to reproduction tend to select for? Please be specific. A: prezygotic barriers to reproduction (or more than one specific example)
(uuuu) What does it mean for two individuals to be temporally isolated? A: It means that they are receptive to/able to/motivated to mating at different times, thereby reducing their likelihood of mating
(vvvv) In gametic isolation, typically which gamete is lost and which is preserved? A: the sperm is lost while the egg is retained
(wwww) What does hybrid breakdown mean? A: It means that it is the grandchildren of the hybridizing individuals, i.e., the offspring of the hybrids, that display a reduced fitness
(xxxx) True or False, prezygotic isolating mechanisms select for postzygotic isolating mechanisms.
(yyyy) What does introgression mean? A: Introgression represents low-levels of gene flow between two populations such as from one species to another
(zzzz) Describe a scenario for allopatric speciation including explanations for how the two species-to-be come to genetically diverge. A: One species, one population --> peripheral isolates, living under different environmental conditions so selection for different allelic frequencies --> geographical barrier cutting off peripheral isolate resulting in founder effect --> perhaps multigenerational small population size resulting in genetic bottleneck (changing allelic frequencies further from those of parent population) --> different environmental conditions resulting in further selection for evolutionary change --> loss of geographical barrier resulting in potential for hybridization between the two populations --> if hybrids are reasonably fit then the two populations may blend into a single population or if hybrids are not terribly fit then there will be selection for the evolution of mechanisms assuring pre-zygotic reproduction isolation; if the latter than speciation effectively has occurred.
(aaaaa) What type of speciation do we associate with autopolyploidy? A: Sympatric speciation
(44) Practice question answers [index]
(a) A single population is split into two by the formation of a geographical barrier. The smaller of the two populations is termed a peripheral isolate. Founders effects and genetic drift effect undirected change. Diversifying selection causes the two populations to evolve in different directions, adapting differently to different environments. Loss of the geographical barriers results in the formation of a hybrid zone where the two populations display an overlap in ranges. The cost of postzygotic reproductive isolating mechanisms within this hybrid zone selects for the evolution of prezygotic isolating mechanisms. Thus speciation occurs.
(b) Only non-adaptive evolution can move a population off of an adaptive peak thus allowing that population to explore another, perhaps taller adaptive peak (i.e., become better adapted to its environment): mutation, genetic drift, migration, or environmental change; assortative mating probably can also do the trick
(c) sympatric speciation does not involve geographical isolation, while allopatric speciation does
(d) (i) c, (ii) a, (iii) d, (iv) b, (v) e
(e) prezygotic mechanisms prevent fertilization while postzygotic mechanisms do not
(f) because prezygotic isolating mechanisms are generally cheaper to implement than are postzygotic reproductive isolating mechanisms
(g) asexual populations, extinct organisms
(h) (ii) behavioral isolation.
(i) founders effect, genetic drift, extremes of range correlating with genotypic extremes (prior to founding of peripheral population), natural selection to divergent environments following cut off of gene flow
(j) both anagenesis and cladogenesis are mechanisms of speciation; they differ in that anagenesis does not give rise to more than one new species from the ancestral species, does not result in branching evolution (no increase in biodiversity), and involves the extinction of the ancestral species upon the genesis of the progeny species; cladogenesis can give rise to more than one species from a single ancestral species, results in branching evolution (increase in biodiversity), and does not involve the extinction of the ancestral species
(l) the morphologically defined species concept fails to directly address reproductive isolation and the occurrence of reproductive isolation is key to understanding the processes of speciation
(m) Darwinian fitness
(n) (v) geographical isolation
(o) (i) mechanical isolation, because this is more than just a byproduct of other behaviors and because individuals must invest in mating before this barrier comes into play
(p) inviability that occurs following the investment by the parents in the raising of the offspring, but prior to reproductive success by the offspring
(q) introgression is where alleles manage to make it past reproductive barriers between species and thereby move from one species to another (as a consequence of successful hybridization)
(r) (i) possessing adaptations to extreme of range prior to geographical isolation, (ii) founders effect simultaneous with occurrence of geographical isolation, (iii) bottleneck effect before isolated population recovers to reasonable size, (iv) continued environmental differences between ranges of parental and geographically isolated population
(t) peripheral isolate is geographically isolated and diverges genetically from parental population. Breakdown of geographical barrier results in testing of reproductive barriers between former peripheral isolate and parental population. Parental population driven to extinction given competition with former peripheral isolate. Morphological differences between two populations are interpreted in fossil record as one species replacing its parental species: anagenesis.
(u) sympatric speciation is speciation that occurs without (i.e., independent of) geographical barriers
(v) polyploidy in which all of the involved chromosomes are derived from a single species
(w) non-adaptive evolution is pretty much the only way that a population can move down from an adaptive peak thus allowing them to explore a different, perhaps taller adaptive peak
(x) (i) anagenesis
(y) punctuated equlibrium
(z) geographical barriers
(bb) biological species concept
(cc) fertility or viability
(dd) morphological species concept
(ff) temporal isolation means that the receptivity two mating of two populations occurs at different times, thereby eliminating the ability of those populations to successful attempt mating
(hh) allopatric speciation
(ii) (ii) Habitat isolation
(jj) the biological species concept defines species in terms of reproductive isolation, but introgression represents a violation of the reproductive isolation between species, i.e., a low-level gene exchange between what might otherwise have been robustly defined biological species
(kk) hybrid inviability is costly because it represents wasted reproduction which directly and negatively impacts on Darwinian fitness by reducing the reproduction component
(mm) extinction of a species
(nn) Duration of survival of a parent species
(oo) (ii) Anagenesis
(qq) Biological species concept
(rr) Fossils, asexual species (etc.)
(ss) A fundamental aspect of the occurrence of speciation is the development of reproductive isolation, a concept which the morphological species concept does not adequately address
(tt) (i) Behavioral isolation (avoidance of mating), (iv) Mechanical isolation (mating but no or less wastage of gametes), (ii) Gametic isolation (mating and wastage of sperm but no or less wastage of eggs), and then either (iii) Hybrid breakdown or (v) Reduced hybrid fertility
(uu) (iv) Hybrid breakdown
(vv) Geographical isolation
(ww) Behavioral isolation
(xx) Gametic isolation
(yy) Hybrid breakdown is a reduced Darwinian fitness among the progeny of hybrids, i.e., the grandchildren of the original cross
(zz) Introgression is a low-level passage of alleles across an otherwise fairly robust reproductive barrier
(aaa) Allopatric speciation involves, at least initially, the existence of geographical barriers while sympatric speciation specifically is that which occurs in the absence of geographical barriers
(bbb) Peripheral populations tend to have small population sizes
(ccc) Hybrid zone
(ddd) Geographical barriers
(eee) Speciation does not occur
(fff) Allopolyploidy involves hybridization while autopolyploidy does not
(ggg) Species select refers to the mechanisms that result in differential rates of speciation among different taxa; those taxa that, for whatever reason, tend to undergo more speciation events and whose speciation products (new species) are longer lived tend, over time, to accumulate more species than do those taxa that do not produce reasonably long surviving new species at the same rate
(hhh) (iii) Increase in ploidy
(iii) Sympatric speciation occurs without geographical barriers to gene exchange
(jjj) Stasis, i.e., lack of morphological change
(kkk) Following the removal of geographical barriers; reestablishment of sympatry
(lll) Anagenesis = non-branching evolution, i.e., the descendent species completely replaces the parental species such that in the fossil record there is little or no overlap
(mmm) (v) Peripheral isolate" (or "Splinter population")
(nnn) (iii) Movement of alleles between species
(ooo) (iv) Hybrid inviability
(ppp) The pluralistic species concept represents an amalgam between the various traditional species concept, essentially drawing on a given species concept as appropriate to achieve a higher level of understanding of how one species is distinct from others
(qqq) Temporal isolation refers to the reproductive isolation of two populations due to inter-population differences in the timing that members are receptive to mating
(rrr) For natural selection to favor the formation of prezyogitc reproductive barriers, members of two populations must be able to form reduced Darwinian fitness hybrids, which they are able to do only so long as geographic barriers are not sufficiently robust as to keep the too populations separated
(sss) Founders effect, genetic bottleneck and extreme of range resulting in natural selection potentially favor different alleles from those most prevalent in the main population
(ttt) Anagenesis (on the left) and Cladogenesis (on the right)
(uuu) Introgression is a low level of gene exchange between two otherwise reproductively isolated populations
(vvv) (iii) Morphological species concept
(www) (i) Biological species concept
(xxx) The one on the left (c)
(yyy) (v), (vii), and (viii)
(zzz) Sympatric speciation is the middle column
(aaaa) Geographic, Habitat, Behavioral, Temporal, Mechanical, or Gametic isolation
(bbbb) (iii) Peripheral isolate
(cccc) Anagenesis is non-branching evolution
(eeee) (iv) Reproductive isolation
(ffff) Distinguishable types include pre-zygotic vs. post-zygotic (the answer I expect to most-often see) but also could be genetically based vs. not
(gggg) Gametic isolating mechanisms because, at the very minimum, mating has occurred with gametic isolating mechanisms and behavioral isolating mechanisms
(hhhh) Hybrid breakdown means that the grandchildren of a between-population cross display lower fitness than either of the two parents
(iiii) Recall that allopatric means that specification is initiated as a consequence of the formation of geographical barriers... at the very minimum that concept must be in your answer
(jjjj) Extreme of range (adapted to different environment relative to rest of population), small population size resulting in genetic drift (founders effect and then genetic bottleneck), ongoing natural selection following geographical isolation from main population
(kkkk) (iv) Specific dietary needs