Important words and concepts from Chapter 23,
Campbell & Reece, 2002 (3/25/2005):
by Stephen T. Abedon (abedon.1@osu.edu)
for Biology 113 at the Ohio State University
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(1) Chapter title: The Evolution of
Populations
(a)
"An organism exposes its phenotype—its
physical traits, metabolism,
physiology, and behavior—not its genotype, to the environment. Acting
on phenotypes, selection indirectly adapts a population to its environment by
increasing or maintaining favorable genotypes in the gene pool."
(p. 430)
(b)
"One obstacle to understanding evolution is the common
misconception that individual organisms evolve, in the Darwinian sense, during
their lifetimes. In fact, natural selection
does act on individuals; their characteristics affect their chances of survival
and reproductive success. But the evolutionary impact of this natural selection
is only apparent in tracking how a population of organisms changes over time…
Thus, it is the population, not its individuals,
that evolves, as some heritable variation becomes more common at the expense of
others." (p. 416)
(c)
["To the people gathered there, most of whom had no more than a
first- or second-grade education, some genetic principles seemed to make
intuitive sense, whereas others did not. No one had trouble, for instance,
understanding that traits can be inherited. But the fact that the probability
of inheriting a trait is unrelated to the previous births was more difficult to
grasp. If one parent has the Alzheimer's mutation, there is a 50 percent risk
that each child will have it too. But, just as parents who have had three girls
in a row may expect their chance of having a boy to increase, the villagers
endorsed a logical fallacy. One man announced to the assembled group: ‘We the
families in which there are only a few affecteds must be grateful to those
families with many affecteds.’ Local ideas of guilt and collective burden were
deeply ingrained, and clashed with the principles of population genetics."
p. 15 of Kenneth S. Kosik, 1999, The fortune teller, The Sciences 39:13-17]
(d)
[the evolution of populations
(Google Search)] [population evolution and
speciation (BSC Courseware)] [index]
MICROEVOLUTION TOOLS AND
OVERVIEW
(a)
Population genetics is essentially the study of allele and genotype frequencies within populations of
organisms
(b)
[population genetics
(Google Search)] [DNA technology in forensic
science (welcome to applied population genetics) (Committee on DNA
Technology in Forensic Science)] [index]
(a)
The ultimate triumph of Darwinism required its integration with Mendelian genetics
(b)
That is, evolution is a genetic phenomenon so cannot be fully (or even
well) understood without an understanding of Mendelian genetics
(c)
This synthesis between Darwinism and Mendelian genetics did not occur
until the 1930s (recall that Darwinism and Mendelian genetics both came into
being during the mid to late 1800s)
(d)
The combination of Darwinism and Mendelian genetics is called the
modern synthesis (of Darwinism and Mendelian genetics as well as paleontology, taxonomy, biogeography, and population genetics)
(e)
"The modern synthesis emphasizes the importance of populations as
the units of evolution, the central role of natural selection as the most important
mechanism of evolution, and the idea of gradualism to explain how large changes
can evolve as an accumulation of small changes occurring over long periods of
time."
(f)
["…the Modern Synthesis is
a theory about how evolution works at the level of genes, phenotypes, and
populations whereas Darwinism was concerned mainly with organisms, speciation
and individuals. This is a major paradigm shift and those who fail to
appreciate it find themselves out of step with the thinking of evolutionary
biologists. Many instances of such confusion can be seen here in the
newsgroups, in the popular press, and in the writings of anti-evolutionists."
(Talk.Origins)]
(g)
[modern synthesis, the modern synthesis of genetics
and evolution (Google Search)] [index]
(a)
The term population is more complex than you may realize
(b)
However we will delay our discussion of the complexity inherent in the term
population until our considerations of speciation
(c)
For now, consider a population to be a localized group of interbreeding
individuals (with lots of emphasis on interbreeding)
(d)
[interbreeding population
(Google Search)] [index]
(a)
The term species is also fraught with complexity
which will be considered more fully when we discuss speciation
(b)
For now, consider a species to be a group of populations whose
members are capable of interbreeding
(c)
Species and populations possess certain properties (or
characteristics) such as their location that we will discuss in more detail
when considering population ecology
(d)
One property is that the population's underlying a species may undergo
different degrees of gene exchange ranging from very little (an
isolated population) to quite a bit
(e)
[species (Google Search)] [index]
(a)
In a population
genetics
sense, a population or species consists of a gene pool
(b)
A gene pool "consists of all alleles at all gene
loci in all individuals in a population."
(c)
Recall that diploid individuals possess two alleles at
each locus
(d)
[gene pool (Google Search)] [index]
(7)
Fixed locus (fixed
allele)
(a)
A locus for which only a single allele exists for an entire gene pool is
considered to be fixed, i.e., a fixed locus
(b)
We would describe the frequency of a fixed allele within a gene pool as
1.0
(c)
We would describe the frequency of all other alleles as 0.0 (i.e., they
are not present)
(d)
An allele with a frequency of 0.0 is said to be extinct
(e)
["fixed locus" and
genetics, "fixed locus" and
evolution, fixed allele, fixed alleles (Google Search)] [index]
(8)
Gene frequency (allele
frequency)
(a)
All alleles at not-fixed loci possess a frequency that is somewhere between 0.0 and 1.0
(b)
We describe this frequency as allele
frequency or, less correctly but more commonly, as gene frequency
(c)
Remember that gene (or allele) frequency refers to the frequency of
alleles in an entire gene pool, not in single individuals
(d)
Remember that each diploid individual
has two alleles at each locus
(e)
[gene frequency, gene frequencies, allele frequency, allele frequencies
(Google Search)] [index]
(a)
Remember that any one individual may be homozygous for only one allele of the one
or more present in the population (at a given locus) or a given individual may be heterozygous at that locus
(b)
Thus, three alleles (or many more) can exist in a population (with
associated allele frequencies) but only up to two alleles at a time can exist
within a given individual
(c)
The frequency of genotypes within a
population is dependent on the frequency of alleles (and visa versa, actually)
(d)
It is only within genotypes that evolution acts on alleles
(e)
Consequently, to understand evolution and evolutionary change, it is
usually important to keep track of allele frequencies and to keep track of genotype frequencies
(f)
That is, make sure the following makes sense to you:
(i)
natural selection acts on phenotypes
(ii)
genotypes underlie phenotypes
(iii)
alleles underlie genotypes
(g)
[genotype frequency
(Google Search)] [index]
(10)
Genetic structure (supplemental concept)
(a)
Genetic structure is a population genetics term used to refer to a population’s
allele and genotype frequencies
(b)
Evolution may be defined as change over time of a population's genetic
structure
(c)
"Evolution is a generation-to-generation change in a population's
frequencies of alleles and genotypes—a change in a population's genetic
structure."
(d)
[genetic structure (Google Search)] [index]
(11) Calculating
allele frequencies
(a)
Remember that a diploid organism has two (not necessarily
different) alleles at each locus
(b)
The frequency of an allele within a population is equal
to the number of alleles of a given type within the population divided by the
total number of alleles found at a given locus
(c)
Thus, if 200 A alleles and
400 a alleles are found within a
given population, then the frequency of A
alleles is 200 / (200 + 400) = 1/3 = 0.33.
(d)
If this is a diploid population, how many individuals are in this
population? (answer: 300… make sure that these ideas and calculations make
sense to you)
(e)
[calculating allele frequencies,
determining allele frequencies
(Google Search)] [index]
(12) Calculating
allele frequencies from genotype frequencies
(a)
Note that very often one knows (or can infer) genotype frequency
(b)
If so, then genotype frequency information can be used to calculate allele
frequency. How?
(c)
If a population has 100 Aa individuals, 200 aa
individuals, and 300 AA individuals
then the number of A alleles is 100*1 + 300*2 = 700; the number of a alleles is 100*1 + 200*2 = 500; the
frequency of A therefore is 700 /
(500 + 700) = 7/12 = 0.58
(d)
Remember, diploid individuals contribute two alleles
from each locus to the gene pool (hence
the *2 in the above calculations); How many diploid individuals are present in
the above example?
(e)
[Calculating allele frequencies
from genotype frequencies (Google Search)] [index]
CALCULATING GENOTYPE
FREQUENCIES FROM ALLELE FREQUENCIES
(13) Calculating
genotype frequencies from allele frequencies
(a)
Calculating genotype frequencies from allele
frequencies is also possible, but requires quite a bit of fudging
(b)
In fact, much of this chapter deals with this fudging
(c)
However, the calculations are simple: One assumes simply that alleles are picked at random from the gene pool to
assemble genotypes
(d)
Assume that the frequency of allele A
is 0.4 and that the frequency of allele a
is 0.6; What is the frequency of genotypes AA,
Aa, and aa?
(i)
Frequency AA = 0.4 * 0.4 =
0.16
(ii)
Frequency aa = 0.6 * 0.6 =
0.36
(iii)
Frequency Aa = 0.4 * 0.6 +
0.6 * 0.4 = 0.48
(e)
Remember that there are two paths by which the heterozygote may be constructed, A from mom and a from
dad, or a from mom and A from dad (make sure that this idea
makes sense to you)
(f)
Now, substitute the letter p
for the frequency of A (i.e., in this
example p = 0.4) and the letter q for the frequency of a (i.e., in this example q = 0.6); what
is the frequency of the genotypes AA,
Aa, and aa?
(i)
Frequency AA = p * p
= p2
(ii)
Frequency aa = q * q
= q2
(iii)
Frequency Aa = p * q
+ q * p = 2pq
(g)
Note that for a locus for which only two alleles are
present in a population:
(i)
p2 + 2pq + q2 = 1 = (p
+ q)2
(h)
In addition, of course, keep in mind that
(i)
p = 1 – q
(ii)
q = 1 – p
(iii)
1 = p + q
(i)
Again, make sure that these ideas and generalizations make sense to
you, particularly to the point where you are able to apply these ideas
(j)
[(Google Search)] [index]
(a)
Genotype frequencies do not necessarily
coincide with phenotype frequencies (e.g., as a
consequence of complete dominance)
so calculating genotype frequencies from phenotype frequencies is not
necessarily straightforward
(b)
However, if the frequency of the recessive allele is q then
(i)
the frequency of the recessive homozygote is q2
(ii)
the frequency of the dominant homozygote is (1 - q)2
(iii)
the frequency of the heterozygote is 2 * q * (1 - q)
(iv)
(these ideas and concepts should eventually make intuitive sense to you
and you should be working towards that point, so make sure that these ideas
makes sense to you to this point, before you move on, such that you are at
least able to recapitulate and then utilize the underlying logic, e.g., why is
the frequency of the dominant homozygote equal to (1 - q)2?)
(c)
Thus, if the recessive allele, a,
has a frequency of 0.01, then
(i)
Frequency AA = 0.99 * 0.99 =
0.98
(ii)
Frequency Aa = 2 * 0.99 *
0.01 = 0.02
(iii)
Frequency aa = 0.01 * 0.01 =
0.0001
(d)
(do you know where the above numbers come from? if you don’t, then you
don’t understand the concept so go back and try again)
(e)
In other words, in this example there are 200 times more heterozygotes carrying the recessive allele than there are
recessive homozygotes carrying the recessive allele—rare recessive alleles are hidden in populations
within heterozygotes (where does “200 times” come from? you should be able to
understand this… if you don’t then go back and try again)
(f)
"The rarer the recessive allele, the greater the degree of
protection afforded by heterozygosity."
(g)
That is, as recessive alleles become more and more rare, many, many
more carriers of this allele will be heterozygotes (who are assymptomatic in
the case of complete dominance, i.e., are hidden recessives) rather than
homozygotes
(h)
[hidden recessives, hidden recessive (Google Search)] [index]
(15) The
Hardy-Weinberg theorem
(a)
The above calculations are stated more formally as the Hardy-Weinberg
theorem
(b)
“The theorem states that the frequencies of alleles and genotypes in a
population’s gene pool remain constant over the generations unless acted upon
by agents other than Mendelian segregation and recombination of alleles.” (p.
447, Campbell & Reece, 2002)
(c)
“The system operates somewhat like shuffling a deck of cards: No matter
how many times the deck is reshuffled to deal out new hands, the deck itself
remains the same. Aces do not grow more numerous than jacks. And the repeated
shuffling of a population’s gene pool over the generations cannot, in itself,
increase the frequency of one allele relative to another.” (p. 448, Campbell
& Reece, 2002)
(d)
In the Hardy-Weinberg theorem it is assumed that matings between
individuals within a population occur randomly and that no evolution
is occurring within the population
(e)
Under such conditions genotype frequency
may be calculated from allele frequency information as described above (i.e., p2 + 2pq + q2)
(f)
The existence of this calculation given these assumptions is termed the
Hardy-Weinberg theorem
(g)
See Figure 23.3, The
Hardy-Weinberg theorem
(h)
(i)
“The Hardy-Weinberg theorem is important conceptually and historically
because it shows how Mendel’s theory of inheritance plugs a hole in
(j)
[Hardy-Weinberg theorem
(Google Search)] [Hardy-Weinberg Equilibrium (a guide to teaching H-W at the pre-college
level) (see also…) (Judith Stanhope)] [population genetics, Hardy-Weinberg equilibrium, and
the modes of evolution (a lecture) (Rebecca Irwin)] [model construction and the
Hardy-Weinberg equilibrium (model construction and hypothesis
testing using Hardy-Weinberg as example) (Biomathematics -- Sally Otto)] [index]
(16) Hardy-Weinberg
equilibrium
(a)
Note that so long as no evolution is occurring within a population
then allele frequencies are not changing within that population
(b)
So long as allele frequencies are not changing, then genotype frequencies may be calculated using the Hardy-Weinberg theorem
(c)
Furthermore, so long as these conditions stay the same, then genotype
frequency will remain the same, as calculated above (this is true because
genotype frequencies under these conditions are not a function of genotype
frequencies so much as allele frequencies, and allele frequencies we are
assuming are not changing)
(d)
The constant genotype frequencies in the absence of evolution and given
Hardy-Weinberg conditions is called Hardy-Weinberg equilibrium
(e)
That this is an equilibrium is implied by the absence of change over
time (particularly change in genotype frequency since a lack of change in
allele frequencies is, in fact, one of our assumptions)
(f)
Remember that, given the appropriate conditions (above), it takes only
a single generation to generate a Hardy-Weinberg equilibrium
(g)
["The discrete genes
Mendel discovered would exist at some frequency in natural populations.
Biologists wondered how and if these frequencies would change. Many thought
that the more common versions of genes would increase in frequency simply
because they were already at high frequency. Hardy and Weinberg independently
showed that the frequency of an allele would not change over time simply due to
its being rare or common." (Talk.Origins)]
(h)
[Hardy-Weinberg equilibrium,
the Hardy-Weinberg equilibrium
simulator, Hardy-Weinberg problems
(Google Search)] [population genetics, Hardy-Weinberg equilibrium, and
the modes of evolution (Biology 391: Organic Evolution)] [index]
(17) “No evolution”
(= absense of microevolution)
(a)
Hardy-Weinberg equilibrium represents the
no-evolution ground state for population genetics
studies; it is what you "expect if a population is not evolving"; it is the null
hypothesis
(b)
"If the frequencies of alleles or genotypes deviate from values expected from Hardy-Weinberg equilibrium, then the population is evolving… If we
track allele and genotype frequencies in a population over
a succession of generations, some loci may be at
equilibrium, while frequencies of alleles at other loci may be changing."
(c)
More specifically, Hardy-Weinberg conditions assume an absence of microevolutionary forces
(d)
To accomplish this, Hardy-Weinberg conditions include
(i)
Very large (infinite) population sizes
(ii)
Isolation from other populations
(iii)
No net mutations
(iv)
Random mating
(v)
No natural selection
(e)
These conditions correspond to the five mechanisms of microevolution
(i)
Genetic drift
(ii)
Gene flow between populations (migration)
(iii)
Mutation
(iv)
Nonrandom mating
(f)
“…we do not really expect a natural population to be in Hardy-Weinberg
equilibrium. And a deviation from the stability of a gene pool—and from
Hardy-Weinberg equlibrium—usually results in evolution.” (p. 449, Campbell
& Reece, 2002)
(g)
[microevolution, microevolutionary forces
(Google Search)] [index]
NON-DARWINIAN EVOLUTION
(18) Non-Darwinian
evolution (Darwinian evolution)
(a)
"Of all the causes of microevolution, only natural selection generally adapts a population
to its environment. The other agents of microevolution are sometimes called
non-Darwinian because of their usually non-adaptive nature."
(b)
These are genetic drift (i.e., sampling
error), migration (i.e., gene
flow), and mutation
(c)
That is, only natural selection is always an agent of positive change
(at least over the short term and in the sense of adapting a population to its
local environment); all other mechanisms can be agents of positive change, but
are not necessarily so
(d)
Only natural selection represents Darwinian evolution, all other mechanism
of microevolution may be termed non-Darwinian evolution
(e)
[Darwinian evolution,
non-Darwinian evolution
(Google Search)] [index]
(a)
Genetic drift is sampling error
(b)
Whenever a population size is
less than infinite, then the Hardy-Weinberg equation will
fail to predict genotype frequencies exactly
(c)
This problem increases in magnitude as population sizes become smaller
(d)
Sampling error affects the transfer of alleles from one generation to
the next, resulting in a random increase or decrease in the frequency of a
given allele
(e)
This latter effect is known as genetic drift
(f)
Note that the alleles lost are not necessarily the same
alleles as may have been lost due to natural selection
(g)
Again, this problem only increases with smaller population sizes
(h)
See Figure 23.4, Genetic
drift
(i)
Two situations in which the effects of genetic drift are particularly
dramatic include
(ii)
Founder effect
(j)
["Sharp drops in
population size can change allele frequencies substantially. When a population
crashes, the alleles in the surviving sample may not be representative of the
precrash gene pool. This change in the gene pool is called the founder effect,
because small populations of organisms that invade a new territory (founders)
are subject to this. Many biologists feel the genetic changes brought about by
founder effects may contribute to isolated populations developing reproductive
isolation from their parent populations. In sufficiently small populations,
genetic drift can counteract selection. [genetic drift: a random change in
allele frequencies] Mildly deleterious alleles may drift to fixation."
(Talk.Origins)]
(k)
[genetic drift (Google Search)] [genetic drift and gene flow
(EEOB 208 -- Processes of
Evolution)] [random genetic drift
(The Talk.Origins)] [gentic drift and gene flow (Biology 391: Organic Evolution)] [index]
(20)
Bottleneck effect (genetic bottleneck)
(a)
When a population is reduced in size by some non-microevolutionary
means (e.g., via a natural disaster), sampling error results in the allele
frequencies of the new population not likely matching what were the
allele frequencies in the old population
(b)
See Figure 23.5, The
bottleneck effect: an analogy
(a)
(b)
Furthermore, the longer a population remains at a reduced size, the
greater the effect of genetic drift on allele frequency
(c)
Ultimately, the result of genetic bottlenecks
is the loss of allelic variation (i.e., the fixing of alleles)
(d)
Genetic bottlenecks can lead to the fixing of maladaptive alleles
(i.e., genetic defects)
(e)
["bottleneck effect"
and "genetic drift" (Google Search)] [index]
(a)
Genetic bottlenecks are characterized by declines in population size due to individual mortality
(b)
An alternative means by which population size may be decreased is via
the transplanting of part of a population to a new locale
(c)
This new population is smaller than its parent population and therefore
is subject to the effects of sampling error
(d)
(Consequently, new populations tend to possess a different genetic structure than their parent populations)
(e)
Consequently, new populations tend to possess different allele
frequencies than their parent populations
(f)
This effect is enhanced with smaller founder populations
(g)
This effect is also enhanced by (but are not identical to) bottleneck effects that result from the new population
remaining small over many generations
(h)
The net effect is for new populations to not resemble parent populations in terms of allele frequencies (and
genetic structure)
(i)
Founder effects probably play relevant roles in speciation since they set it up so that two populations are
genetically dissimilar essentially from the start
(j)
(k)
["founder effect" and
"genetic drift", "founder's effect"
and "genetic drift" (Google Search)] [index]
(22)
Migration (gene
flow)
(a)
The degree to which two populations resemble each other depends on
the degree of gene flow between those two populations
(b)
Gene flow is essentially the movement of alleles into and
out of populations
(c)
Of course, these alleles are carried within individuals who are doing
the actual moving (migration) between populations
(d)
Depending on the degree of gene flow between populations, allele
frequencies of populations will be dissimilar to different degrees: the more
gene flow, the more similar, the less gene flow, the less similar
(e)
Note that migration is a microevolutionary
event since the movement of an allele into or out of a population automatically
changes allele frequency (either increasing or
decreasing allele frequency)
(f)
[migration and evolution,
gene flow (Google Search)] [index]
(a)
Mutation (or, at least, net mutation) also
automatically changes allele frequency
(b)
For example, a mutation involves the conversion of one allele into another allele
(c)
Typically mutation does not play a big, direct role in changing allele
frequency because mutation rates per locus tend to be
low
(d)
However, indirectly mutation is absolutely essential to microevolutionary
processes because all allelic variation ultimately has a mutational
origin
(e)
That is, mutations make the new alleles that then either increase or
decrease in frequency via the other microevolutionary forces
(f)
However, keep in mind that mutations are not only rare but also often
represent random changes in highly evolved (i.e., information laden) nucleotide
sequences
(g)
As a consequence, mutation typically results in a loss in gene
(product) function—that is, adaptations are
lost (though this loss is limited to those organisms carrying the mutation)
(h)
"Organisms are the refined products of thousands of generations of
past selection, and a random change is not likely to improve the genome any more than firing a gunshot blindly through the hood
of a car is likely to improve engine performance."
(i)
Consequently, most mutations are recessive
(j)
Every now and then, though, a mutational change is adaptive (and even
less often, both adaptive and dominant or codominant), i.e., novel functions or novel expression of old
functions
(k)
"On rare occasions, however, a mutant allele may actually fit its
bearer to the environment better and enhance the
reproductive success of the individual. This is not especially likely in a
stable environment, but becomes more probable when the environment is changing
and mutations that were once selected against are now favorable under the new
conditions."
(l)
[mutation and evolution,
mutation and microevolution
(Google Search)] [are mutations harmful? (The Talk.Origins)] [index]
NATURAL SELECTION
(a)
“An organisms exposes its phenotype—its physical traits, metabolism,
physiology, and behavior—not its genotype, to the environment. Acting on
phenotypes, selection indirectly adapts a population to its environment by
increasing or maintaining favorable genotypes in the gene pool.” (p. 458,
Campbell & Reece, 2002)
(b)
Natural selection can act during either the haploid or diploid stage
(c)
If it acts during the diploid stage, then natural selection affects the
contribution of genotypes to the gene pool
(d)
That is, natural selection serves to reduce certain genotypes relative
to others in terms of their contribution of alleles to the gene
pool
(e)
Natural selection acting at the haploid stage serves to reduce allelic
frequency directly
(f)
Note that in either case the effect of natural selection is to reduce
(not to increase) the absolute number of genotypes or alleles
(g)
That is, mutation places alleles into a gene pool,
other microevolutionary forces can serve to increase the frequency
of the allele, but selection acts to selectively
remove maladaptive alleles (mutation in, selection out)
(h)
Natural selection is differential reproductive success: in the absence
of natural selection an organism contributes x gametes to the next generation; in the
presence of natural selection an organism contributes <x gametes to the next generation
(i)
"Of all agents of microevolution that change a gene pool, only
selection is likely to be adaptive. Natural selection accumulates and maintains
favorable genotypes in a population. If the environment
should change, selection responds by favoring genotypes adapted to the new
conditions. But the degree of adaptation can be extended only within the realm
of the genetic variability present in the population."
(j)
Natural selection serves to increase the information content contained
within the genomes of the organisms of a population; more
specifically, it increases the prevalence of information which has been
time-tested to allow the increased survival and reproduction of genotypes
within the environment in which a population resides
[natural selection (Google Search)] [a population genetics model of natural selection
(a lecture) (Rebecca Irwin)] [index]
(a)
“Darwinian fitness is the contribution an individual makes to the gene
pool of the next generation relative to the contributions of other
individuals.” (p. 457, Campbell & Reece, 2002)
(b)
Darwinian fitness is the allelic contribution an individual makes to
the next generation
(c)
Thus, the more likely an individual is to survive and reproduce (i.e., to contributes its alleles to the next
generation), the higher that individual's Darwinian fitness
(d)
Note that the Darwinian fitness is a quantity equal to the average reproductive
output associated with a given genotype
(e)
Thus, Darwinian fitness is an environment-specific quantity (i.e., it
may change depending on environment)
(f)
Darwinian fitness is often simply called fitness
(g)
People typically consider Darwinian fitness on a locus-by-locus basis since things (e.g.,
experimentation as well as theorizing) get very complicated the more loci there are involved
(h)
[Darwinian fitness (Google Search)] [index]
(a)
“In a more quantitative approach to natural selection, population
geneticists define relative fitness as the contribution of a genotype to the
next generation compared to the contributions of alternative genotypes for the
same locus… The relative fitness of the most reproductively successful variants
is set at 1 as a basis for comparison.” (pp. 458-459, Campbell & Reece,
2002)
(b)
For a given locus, the Darwinian fitness associated with different genotypes may be determined
(c)
Typically these fitnesses are presented as relative quantities rather
than as absolutes
(d)
Typically the genotype with the highest Darwinian
fitness is given a relative fitness value of 1.0
(e)
All other genotypes, i.e., those with lower than the highest Darwinian
fitness, then have relative fitness values of less than 1.0
(f)
If one genotype produces on average 4 progeny per generation and
another produces on average 1 progeny per generation, then what is the relative
fitness of the latter genotype? The former? (answer: 0.25 and 1.0,
respectively)
(g)
Relative fitness and the “selection coefficients” employed when
mathematically following the impact of selection on a population are
essentially identical concepts
(h)
Relative fitness especially comes into play
when considering competition between conspecifics—do you
understand why?
(i)
“Survival alone does not guarantee reproductive success. Relative
fitness is zero for a sterile plant or animal, even if it is robust and
outlives other members of the population. But , of course, survival is a
prerequisite for reproducing, and longevity increases fitness if it results in certain
individuals leaving more descendants than other individuals leave.” (p. 458,
Campbell & Reece, 2002)
(j)
[relative fitness (Google Search)] [index]
(27) Action of
natural selection
(a)
"Survival alone does not guarantee reproductive success. Relative
fitness is zero for a sterile plant or animal even if it is robust
and outlives other members of the population. But, of course, survival is a
prerequisite for reproducing, and longevity increases fitness if it results in certain individuals
leaving disproportionately high numbers of descendants. Then again, an
individual that matures quickly and becomes fertile at an early age may have a
greater reproductive potential than individuals that live longer but mature
late. Thus, the components of selection are the many factors that affect both
survival and fertility."
(b)
In following a population through generations, one that is undergoing natural selection but is otherwise adhering to Hardy-Weinberg conditions, A population will go through the
following contortions (note typical animal sexual cycle):
(i)
Diploid 4 Meiosis 4 Haploid 4 Fertilization 4 Diploid 4 Mitosis
(ii)
Natural selection can reduce the frequency of certain genotypes relative to others; note that this happens formally
by multiplying genotype frequency by relative
fitness and then recalculating genotype
frequencies (do you know how to do this/understand what I mean?)
(iii)
Natural selection can also act at the level of gamete
generation; obviously an individual who can't make gametes has a Darwinian
fitness (and relative
fitness)
of zero
(iv)
An individual fails to fertilize if an individual fails to mate (or if
an individual dies before reaching sexual maturity…)
(c)
"A genotype at a particular locus may
have multiple effects, especially if it influences the development or growth of
the organism. This ability of genes to influence many phenotypic characters is
called pleiotropy. The overall fitness of a genotype
depends on whether its positive effects outweigh any harmful effects it may
have on the survival and reproductive success of the organism… The finished
organism subjected to natural selection is an integrated composite of its many
phenotypic features, not a collage of individual parts. The relative fitness of
a genotype at any one locus depends on the entire genetic context in which it
works." —in other words, things get very complicate, very fast, and
organisms are always at best compromises between competing selective forces
(d)
[action of natural selection
(Google Search)] [index]
MANY MODES/TYPES OF NATURAL
SELECTION
(28) Modes of
natural selection
(a)
Natural selection edits out mutational variation
(b)
However, natural selection is considered to display different modes
depending on what is being edited out
(c)
Consider a character that is controlled by many loci and whose associated traits span a
spectrum such as from short to tall (for the character height) or light to dark
(for the character hair color), etc.
(d)
In such a case, depending on where on a given spectrum natural
selection acts most strongly, selection may be classified as
(iii)
Diversifying selection
(e)
See Figure 23.12, Modes of
selection
(f)
(g)
[modes of natural selection
(Google Search)] [index]
(a)
Stabilizing selection eliminates phenotypic extremes within a
population thus increasing the frequency of genotypes
underlying intermediate phenotypes
(b)
Thus, stabilized populations tend to be reasonably well
adapted to their environments
(c)
(what does that statement mean? It means that we can infer, minimally,
that the extreme phenotypes are less well adapted to the environment than the
intermediate phenotypes. Why? Because natural selection is selectively removing
the extreme phenotypes rather than the now by-definition better-adapted
intermediate phenotypes. If those intermediate phenotypes were not reasonably
well adapted to their environment, then the population would be in fairly rapid
decline and therefore, in all likelihood, either not be observed or be only
briefly observed)
(d)
[stabilizing selection
(Google Search)] [index]
(a)
Directional selection is natural selection
against only one phenotypic extreme
Directional Selection (in
Macroevolution)
The
fossil lineage of the horse provides a remarkable demonstration of
directional succession. The full lineage is quite complicated and is not just
a simple line from the tiny dawn horse Hyracotherium of the early
Eocene, to today's familiar Equus. Overall, though, the horse has
evolved from a small-bodied ancestor built for moving through woodlands and
thickets to its long- legged descendent built for speed on the open
grassland. This evolution has involved well- documented changes in teeth,
leg length, and toe structure. |
(b)
The net effect of directional selection is to increase or decrease the
character along the spectrum of its possible expression (i.e., taller, shorter,
etc.)
(c)
[directional selection
(Google Search)] [index]
(31) Diversifying
selection (a.k.a., disruptive selection)
(a)
In diversifying selection it is the intermediate form that is selected against
(b)
Diversifying selection can result in balanced polymorphisms
(c)
[diversifying selection,
disruptive selection
(Google Search)] [index]
(32)
Sexual selection (sexual dimorphism,
secondary sexual characteristics)
(a)
Males and females often differ phenotypically
other than in their possessing different sexual organs
(b)
Such differences are referred to as sexual dimorphisms or secondary
sexual characteristics
(c)
For example, human males generally are taller, heavier, and hairier
than human females
(d)
Sexual dimorphisms often are involved in mate procurement
(e)
We can distinguish sexual selection into an intrasexual and intersexual
selection
(f)
[sexual selection, sexual dimorphism (Google Search)] [index]
(a)
“Intrasexual selection is a direct competition among individuals of one
sex (usually the males in vertebrates) for mates of the opposite sex. Males may
use secondary sexual equipment such as antlers to battle competitors. This is
especially common in species where a single male garners a harem of females.
These males may gain their status by defeating smaller, weaker, or less fierce
males in combat; more often, they are the victors in ritualized displays that
discourage would-be competitors.” (p. 460, Campbell & Reece, 2002)
(b)
The better fighter typically is the fighter that gains preferred access
to the female and, of course, genes can underlie fighting ability
(c)
[intrasexual selection (Google Search)] [index]
(34)
Intersexual selection (mate choice)
(a)
Any trait that increases the attractiveness of an
individual to sexually mature members of the opposite gender will confer a
selective advantage to the bearer because this attractiveness will increase the
bearer's likelihood of either
(i)
depositing its gametes in the gene pool or
(ii)
having its gametes fertilize the gametes of an individual possessing a
preferred genotype
(b)
This can all get very complicated since just what a preferred genotype
is can depend on just what a preferred genotype is (i.e., positive feedback)
(c)
Regardless, the alleles that lead to an increase an individual’s
ability to either secure a mate or to secure the mate of one's choice are
selected, at least in part, by a form of natural selection known as sexual selection
(d)
“In intersexual selection, also called mate choice, individuals of one
sex (usually females) are choosy in selecting their mates from individuals of
the other sex. Apparently, males with the most impressive masculine features
are the most attractive to females. A peacock strutting in front of hens with
his tail feathers spread into a showy fan is an example of this ‘choose me’
statement. What intrigued Darwin about such behavior is that some of the
features that appear to help attract mates do not seem to be adaptive in any
other way and may in fact pose some risk in natural environments [i.e., negatively
impact on survival]. For example, showy plumage may make male birds more
visible to predators. But if such secondary sexual characteristics help a male
gain a mate, then they will be reinforced over the generations for he most
Darwinian of reasons—because they enhance reproductive success. Every time a
female chooses a mate based on a certain appearance or behavior, she
perpetuates the alleles that caused her to make that choice and allows a male
with a particular phenotype to perpetuate his alleles.” (p. 461, Campbell &
Reece, 2002)
(a)
[intersexual selection (Google Search)] [index]
NATURAL SELECTION’S RAW
MATERIAL
(a)
"Heritable variation is at the heart of
(b)
A polymorphism describes the situation where more than one allele is present within a gene pool at a
given locus
(c)
Note that in practice polymorphism tends to mean that one allele does
not overwhelmingly dominates a given locus (such that all other alleles are
very rare)
(d)
Finally, the term polymorphism is derived from the idea that a population
consists of individuals having two or more distinct morphologies; this concept
has since been extended to the broader idea of more than one phenotype (or even more than one genotype); at the level of
molecules, phenotypes vary dependent on variations in alleles; consequently, a
locus that possesses more than a single allele across a single gene pool is
said to be polymorphic, regardless of the effects of the individual alleles on
morphology
(e)
In general, there is a lot more polymorphism in wild populations than
one might otherwise expect
(f)
Heritable variation within a population is synonymous with polymorphism
(if only a single allele exists at a given locus, then by definition there is
no heritable variation at that locus)
(i)
Therefore, the raw material of natural selection are polymorphisms
(ii)
Therefore, the fewer polymorphisms, the less ably a population can
respond to environmental change (i.e., the population is less able to adapt)
(iii)
Therefore, the less polymorphism, the more likely a population will go
extinct given environmental change
(iv)
The effect of reduced population size is genetic
drift which tends to reduce genetic variability (i.e., reduce the
number of polymorphisms)
(v)
Lack of adaptation to environmental change further reduces population
size which further reduces polymorphism
(vi)
The effect of man on the environment generally is to reduce population
sizes while simultaneously effecting environmental change (guess what comes
next)
(g)
Note that “Polymorphism applies only to discrete characters, not to
characters such as human height, which varies among people in a continuum.” (p.
453, Campbell & Reece, 2003)
(h)
[polymorphism and allele
(Google Search)] [index]
(36) Hiding deleterious
recessives
(a)
“The diploid nature of most eukaryotes hides a considerable amount of
genetic variation from selection in the form of recessive alleles in
heterozygotes. Recessive alleles that are less favorable than their dominant
counterparts, or even harmful in the present environment, can persist in a
population through their propagation by heterozygous individuals. This latent
variation is exposed to selection only when both parents carry the same
recessive allele and combine two copies in one zygote. This happens only rarely
if the frequency of the recessive allele is very low. The rarer the recessive
allele, the greater the degree of protection from natural selection [using the
Hardy-Weinberg theorem concepts, do you understand why?]. Heterozygote
protection maintains a huge pool of alleles that may not be suitable for
present conditions but that could bring new benefits when the environment
changes.” (p. 456, Campbell & Reece, 2002)
(b)
Note that this hiding of alleles from natural selection is not the same
as a balanced polymorphism; based on your
understanding of balanced polymorphism (defined below) do you understand why?
(a)
Polymorphisms tend to be eliminated by either natural selection (should one of the alleles be selectively
disadvantageous) or by genetic drift (especially if population sizes
are small)
(b)
Recessive alleles are more difficult to eliminate
than are dominant alleles because the former tend to be tied up in
phenotypically dominant heterozygotes
(c)
A number of additional mechanisms tend to preserve polymorphisms
(ii)
Hybrid vigor
(iii)
Frequency-dependent selection
(iv)
Neutral variation
(d)
A stably persisting polymorphism
is known as a balanced polymorphism
(e)
The existence of a balanced polymorphism implies a lack of allele
fixation and, furthermore, the existence of a mechanism or
mechanisms that inhibits the fixation of a given allele
(f)
[balanced polymorphism
(Google Search)] [index]
MECHANISMS FOR MAINTAINING
POLYMORPHISMS
(38) Heterozygous advantage (balancing selection)
(a)
In certain circumstances the heterozygote is
better adapted than either homozygote
(b)
In such circumstances, the greater adaptedness of the heterozygote
prevents the extinction of one or the other allele
(c)
An example is sickle-cell disease where the heterozygote is more
resistant to malaria than either homozygote while the affected homozygote dies
young from sickle-cell disease, but the allele is nevertheless maintained in
areas experiencing high incidences of malaria
(d)
["Balancing selection is
rare in natural populations. [balancing selection: selection favoring
heterozygotes] Only a handful of other cases beside the sickle-cell example
have been found. At one time population geneticists thought balancing selection
could be a general explanation for the levels of genetic variation found in
natural populations. That is no longer the case. Balancing selection is only
rarely found in natural populations. And, there are theoretical reasons why
natural selection cannot maintain polymorphisms at several loci via balancing
selection." (Talk.Origins)]
(e)
[heterozygous advantage
(Google Search)] [index]
(a)
Hybrid vigor is related to heterozygous advantage
(b)
The idea here is that when two inbred
individuals are mated, the hybrid offspring typically displays greater vigor
(is better adapted) than either parent
(c)
This occurs in part due to heterozygous advantage and also due to a
simple masking of deleterious recessives (do you understand the difference?
Essentially heterozygous advantage is a product of a codominant relationship
between alleles while the masking of deleterious recessives is a product
instead of a dominant-recessive relationship)
(d)
Hybrid vigor is best exemplified in the production of hybrid
horticultural varieties, e.g., hybrid corn
(e)
Note that the concept of hybrid vigor makes the most sense particularly
when considering the formation of a heterozygote across more than one locus
(f)
[hybrid vigor (Google Search)] [index]
(40) Frequency-dependent selection
(a)
Frequency-dependent selection occurs in situations where the advantage
of possessing an allele increases with a decreasing frequency
of that allele (i.e., increasing rarity); that is, there is more
survival and increases in reproductive success as a genotype gets rarer,
thereby interfering with the elimination of the underlying alleles from the
population
(b)
Rarity tends to be a factor when an allele is involved in combating an
enemy
(c)
For example, frequency-dependent selection is found among butterflies
who mimic other, foul-tasting butterflies—the more
prevalent the mimic, the less likely the predator (a bird) will worry about
accidentally consuming a foul-tasting species; this
places a natural cap on the mimic's population size,
thus resulting in an advantage associated with mimicking a different-looking
foul-tasting species (i.e., employing a different, rarer morphology)
(d)
Major histocompatibility proteins (the killer T-cell receptor) also are the product of frequency-dependent
selection; different alleles result in different abilities to recognize
different pathogens (particularly viruses); individuals
with common alleles are susceptible to viruses which have evolved to evade
alleles common to the rest of the population
(e)
Parasites, similarly, may be subject to frequency dependent selection
as hosts develop resistance to more common parasite phenotypes
(f)
See Figure 23.11,
Frequency-dependent selection in a host-parasite relationship
(a)
[frequency-dependent selection
(Google Search)] [index]
(a)
Neutral mutations minimally disrupt the
adaptedness of an organism, e.g., impacts extremely minimally on phenotype, in such a way that natural selection
cannot distinguish alleles
(b)
Such neutral mutations tend, by definition, not to be eliminated from populations
by natural selection
(c)
Thus, in populations that are large enough to display minimal genetic
drift, neutral variation can be very abundant
(d)
Note that neutral variation by definition need be neutral only within
the context of the environment in which an organism resides
(e)
Transfer to a new environment may increase or decrease the selective
advantage associated with an allele; thus, neutral alleles serve as a
reservoir of allelic variation
(f)
“There is no consensus among evolutionary biologists on how much
genetic variation is neutral or even if any variation can be considered truly
neutral. Variations that appear to be neutral may in fact influence survival
and reproductive success in ways that are difficulty to measure. It is possible
to show that a particular allele is detrimental, but it is impossible to
demonstrate that an allele brings no benefits at all to an organism.
Furthermore, a variation may be neutral in one environment but not in another.
We can never know the degree that even if only a fraction of the extensive
variation in a gene pool significantly affects the organisms, that is still an
enormous reservoir of raw material for natural selection and the adaptive
evolution it causes.” (p. 457, Campbell & Reece, 2002)
(g)
[neutral variation (Google Search)] [index]
NATURAL SELECTION’S LIMITATIONS
(42) Does evolution
fashion perfect organisms?
(a)
No! Why not?
(b)
"An organism's phenotype is
constrained by its evolutionary history"
(i)
Some snakes may be able to glide, but it's unlikely one will ever flap
a wing
(c)
"Adaptations are often compromises"
(i)
Any engineer knows the truth that optimizing one aspect of a design
inevitably compromises another
(ii)
That is why you might be able to take out all of your friends in your
mom's minivan, but you'll never beat a motorcycle in a drag race with it
(d)
"Not all evolution is adaptive"
(i)
It takes too much energy to optimize everything so much of
most organisms is simply good enough to get the job done (a.k.a., the principle of allocation)
(ii)
If it ain't broke, don't fix it
(e)
"Selection can only edit variations that exist"
(i)
In part this is continuation of the first point
(ii)
Also, it is an indication that it is tough to get all of the right
components of an adaptation into a single individual, then keep them there in
descendents
(iii)
Finally, even if an optimal allele can in
theory be reached mutationally, expect within large populations the
needed mutation likely will not be achieved
(f)
Environments change
(i)
Even if a perfect organism existed, it would only remain perfect so
long as its environment remained unchanged (i.e., the one to which it is
perfectly adapted)
(ii)
Environments change all the time, not necessarily catastrophically, but
change nonetheless
(g)
Environments vary over even individual life spans
(i)
To make matters worse, environments even change over single
individual's life spans
(ii)
Thus, the environment to which the hypothetical perfectly adapted
organism is adapted tends to always be a moving target
(h)
[perfect organisms (Google Search)] [index]
(a)
"Natural selection is usually thought of as
an agent of change, but it can also act to maintain the status quo. Stabilizing selection probably prevails most of the time,
resisting change that may be maladaptive. Evolutionary spurts occur when a population
is stressed by a change in the environment, to a new place, or a change in the genome. When challenged with a new set of problems, a
population either adjusts through natural selection
or becomes extinct. The fossil record
indicates that extinction is the more common outcome."
NONRANDOM MATING (AND
CONSEQUENCES)
(44)
Nonrandom mating (random
mating)
(a)
The gene pool within a population in Hardy-Weinberg equilibrium completely lacks structure (i.e., there is complete mixing)
(b)
I typically like to think of the pool very literally as a well mixed
tub of randomly colliding sperm and eggs
(c)
Anything that interferes with this nonrandom fertilization constitutes
structure within the gene pool
(d)
Since many organisms make babies employing a more controlled, less promiscuous mixing of gametes, structure
within a gene pool is typically manifest as nonrandom mating
(e)
Anything that interferes with the random mating between individuals
(now think of a well mixed bowl of males and females randomly . . . oh, never
mind) is nonrandom mating
(f)
Nonrandom mating results in deviations from a Hardy-Weinberg generation
of genotypes from a given frequency
of alleles
(g)
Two aspects of nonrandom mating are
(i)
Inbreeding
(ii)
Assortative mating
(h)
[nonrandom mating (Google Search)] [index]
(45)
Inbreeding
(supplemental discussion)
(a)
Inbreeding typically results from individuals nonrandomly
mating over geographical distances
(b)
Basically, in a population which is not well mixed (i.e.,
not highly mobile) individuals more likely mate with neighbors than with non-neighbors
(the guy next door versus the guy 8,000 miles down the road)
(c)
This nonrandom mating and minimal movement means that, with time,
individuals are more likely to mate with relatives than with non-relatives
(more precisely: with closer relatives than with more distant relatives)
(d)
Inbreeding requires effectively small population sizes (i.e., mating within an
artificially constrained gene pools) so is a mechanism associated
with genetic drift (e.g., inbreeding results from bottleneck and founder effects)
though is not a mechanism of genetic drift (associated here meaning found
occurring under the same circumstances)
(e)
Thus, inbreeding tends to be associated with the expression of rare
recessive alleles (in a homozygous state) and even a local fixing
of alleles of alleles that are otherwise rare in larger populations
(f)
[inbreeding, inbreeding and evolution
(Google Search)] [index]
(46)
Assortative mating
(supplemental discussion)
(a)
Assortative mating is where an individual chooses a mate based on how
that mate resembles the individual
(b)
For example (and I'm not exactly sure how I pulled this off), I married
a girl who looks like my mother
(c)
Note that assortative mating serves to constrain the breadth of one's gene
pool potentially resulting in inbreeding and all
of the consequences of inbreeding discussed above
(d)
That is, one major consequence of inbreeding is a decline in heterozyogosity
(e)
Note, however, that in an infinite population with no
selection, no migration, and no mutation, assortative mating will not have an
impact on allele frequency because decreasing heterozygosity only results in
changes in allele frequency given the existence of natural selection
(f)
Assortative mating will certainly impact on genotype frequency, however (Why? If you understand
assortative mating, genotype frequency, and the impact of the various Hardy-Weinberg assumptions, then the answer should be obvious)
(g)
[assortative mating
(Google Search)] [index]
STRATEGIES FOR SOLVING
HARDY-WEINBERG PROBLEMS
(47) Strategies for solving Hardy-Weinberg problems
(a)
General strategies to employ
when attempting Hardy-Weinberg problems
(i)
Think of these problems as puzzles and then keep telling yourself that
it's only a puzzle or a game; that is, just go with the flow; stressing
yourself out at any juncture in life is counterproductive
(ii)
For introductory problems one can usually assume one locus, two allele,
diploid genetics; if this is not the case, often one will be told so with the
exception of questions that ask for speculation as to the number of loci,
alleles, or the ploidy involved
(iii)
Always assume Hardy-Weinberg equilibrium unless information contradicts
that assumption; you can always reject this hypothesis down the road
(b)
Work with decimals
(i)
Decimals typically are easier to work with than percentages, fractions,
or absolute numbers. Consequently:
·
Always convert percentage information into decimal information (i.e.,
frequencies; 97% à 0.97)
·
Always convert fractions into decimals
·
Always convert absolute information (e.g., numbers of individuals) into
decimal information (e.g., frequencies of phenotypes)
(c)
Convert phenotypes to
genotypes
(i)
Always look for how you might convert phenotype information
into genotype information, and then do so; remember, solving Hardy-Weinberg
problems is a game; whenever you see phenotype frequencies, you should start
looking forward to deducing genotype frequencies
(ii)
Remember, frequencies must add up to one
(iii)
When phenotypes map onto genotypes one-to-one (e.g., codominance), then genotype frequencies
are equal to phenotype frequencies
(iv)
When phenotypes do not map onto genotypes one-to-one, then try to
figure out the phenotype frequency of one homozygote, and then assume Hardy-Weinberg equilibrium (unless you have
reason not to)
(v)
Remember, again, frequencies must add up to one
(d)
Convert genotypes to alleles
(i)
When you have genotype frequencies, use that information
to calculate allelic frequency; remember again that solving
Hardy-Weinberg problems is only a game; whenever you see phenotype frequencies, you should start looking forward to deducing genotype frequencies
(ii)
If you have Hardy-Weinberg equilibrium
and have the frequency of one homozygote, then you can calculate at least one
allele's frequency as the square root of the homozygote's frequency.
(iii)
If you don't know allelic frequencies but do know genotype frequencies
then you can calculate allelic frequencies by multiplying the frequency of each
genotype by a coefficient equal to the number of alleles of one type in each
genotype, then divide the quantity by 2 (e.g., f(A) = (f(AA)*2 + f(Aa)*1 + f(aa)*0)/2)
(iv)
If you have not yet calculated genotype frequencies, but do know the
absolute number of each genotype, then it is possible to skip a step and
calculate allelic frequencies directly from absolute genotype numbers; just
multiplying absolute numbers by the coefficient, as above, and then divide by
twice the sum of the population size (i.e., the sum of the absolute numbers of
genotypes)
(v)
Remember, frequencies must add up to one
(e)
Convert alleles to genotypes
(i)
When you have allele frequencies, you can then calculate
genotype frequencies using the Hardy-Weinberg equation, i.e., f(AA) = p2, f(Aa) = 2pq, and f(aa) = q2
(ii)
Remember, frequencies must add up to one
(iii)
Remember that you can generate genotype frequencies using the
Hardy-Weinberg equation only if Hardy-Weinberg conditions apply.
(f)
Incorporating selection
(i)
Incorporating selection into Hardy-Weinberg problems
complicates things somewhat
(ii)
Selection is doable, however, so long as you keep in mind that the
effect of selection is to reduce absolute genotype/phenotype/allele numbers
(iii)
Operationally this may be accomplished by multiplying frequencies
(typically genotype frequencies) by a selection coefficient
(iv)
Remember that by following such a procedure the resulting
"frequencies" will no longer add to one, but must be re-calculated
such that genotype frequencies following selection are presented as decimals
which do add up to one
(g)
Practice
(i)
As in much of life, the key to success is practice and dedication
(ii)
Anything that you can do to make a difficult task an enjoyable one will
always serve to make that task (and your life) easier; so lighten up and learn
to enjoy doing population genetics
(iii)
Remember, it's only a game
(h)
[Hardy-Weinberg calculation
(Karen Bjorndal)] [population genetics and
evolution (exercises on solving Hardy-Weinberg problems) (Advanced Placement Labs)] [population genetics
(exercises on solving Hardy-Weinberg problems) (Autotutorial Genetics)]
[index]
VOCABULARY
(48)
Vocabulary [index]
(a)
Action of natural
selection
(b)
Allele frequency
(f)
Calculating allele frequencies
(g)
Calculating allele frequencies from genotype frequencies
(h)
Calculating genotype
frequencies from allele frequencies
(n)
Does evolution fashion perfect organisms
(o)
Fixed alelle
(p)
Fixed locus
(q)
Founder effect
(r)
Frequency-dependent
selection
(s)
Gene frequency
(t)
Gene flow
(u)
Gene pool
(w)
Genetic drift
(z)
Hardy-Weinberg
equilibrium
(aa)
The Hardy-Weinberg
theorem
(cc)
Hidden recessives
(dd)
Hiding deleterious recessives
(ee)
Hybrid vigor
(ff)
Inbreeding
(ii)
Mate choice
(jj)
Microevolution
(kk)
Migration
(ll)
Modern synthesis
(mm)
Modes of natural
selection
(nn)
Mutation
(oo)
Natural
selection
(pp)
Neutral
variation
(qq)
No evolution
(ss)
Nonrandom
mating
(tt)
Polymorphism
(uu)
Population
(vv)
Population
genetics
(ww)
Relative
fitness
(xx)
Secondary sexual characteristics
(yy)
Sexual dimorphism
(zz)
Sexual
selection
(aaa)
Species
(bbb)
Stabilizing
selection
(ccc)
Summary