Chapter 23: The Evolution of Populations
The Smallest Unit of Evolution
 Individual organisms do not evolve. Nature selection acts on individuals. Each organism’s
traits affect its survival & reproductive success compared with other individuals, but the
evolutionary impact of natural selection is only apparent in the changes of population of
organisms over time
 Ex: G. fortis medium ground finch on the Galapagos Islands. In 1977, there was a drought,
decimating a lot of the population. During the drought, small, soft seeds were scarce, while
hard seeds were more plentiful, so the finches mostly fed on those. Birds with larger, deeper
beaks were better able to crack and eat these seeds, so they survived at a higher rate than
finches with smaller beaks. Beak depth is inherited; the average depth in the next generation
was greater. The finch population evolved by natural selection; the individual finches didn’t
evolve. Their beak sizes didn’t grow during the drought; the proportion of large beaks in the
population increased from generation to generation
 Microevolution- change in allele frequencies in a population over generations; smallest scale
of evolution
 Genetic drift & gene flow also cause allele frequency change, but only natural selection
consistently improves the match between organisms & their environment
23.1 Genetic Variation makes Evolution Possible
 Darwin proposed natural selection as the primary mechanism for evolution. Individuals
differed in their inherited traits & that selection acted on such differences, leading to
evolution. Variation in heritable traits was required for evolution. Darwin didn’t know how
organisms passed heritable traits to their offspring
 Soon after, Mendel proposed a particulate model of inheritance in which organisms transmit
discrete heritable units (genes) to their offspring
Genetic Variation
 Genetic variation- different among individuals in the composition of their genes or other DNA
segments
 Some phenotypic variation isn’t heritable. Phenotype is affected by inherited genotype &
environmental influences. Only the genetically determined part of phenotypic variation can
lead to evolution
 Without genetic variation, evolution can’t occur
Variation within a Population
 Many discrete characters are determined by single gene locus with different alleles that
produce different phenotypes
 Most heritable variation involves quantitative characters (results from influences of 2(+) genes
on a single phenotypic character), which vary within a population
 2 ways to measure genetic variation in a population
1. Gene variability: genetic variation at whole-gene level
2. Nucleotide variability: genetic variation at molecular level of DNA
 Gene variability can be quantified as average heterozygosity- average percentage of loci that
are heterozygous
 Average heterozygosity provides raw material for natural selection to operate, leading to
evolution
 How scientists identify heterozygous loci to determine gene variability:
1. Use gel electrophoresis to survey the protein products of genes
 Disadvantages: can’t detect silent mutations that alter DNA sequence of gene
but not amino acid sequence of protein
2. To measure nucleotide variability, DNA sequences of 2 individuals in a population are
compared, & then the data is averaged

Gene variability tends to exceed nucleotide variability because a gene consists of thousands of
nucleotides. A difference at only one of these nucleotides can be sufficient to make 2 alleles of
that gene different
Variation between Populations
 Geographic variation- differences in the genetic composition of separate populations
 Ex: geographic variation in Mus muculus (house mice) population: they were separated by
mountains on the Atlantic Island. Several populations of mice have evolved in isolation from
one another. There are differences in the karyotypes (chromosome sets) of these isolate
populations. Some populations’ chromosomes have been fused, but each population’s fusions
differed. The chromosome-level changes leave genes intact, so their phenotypic effects are
neutral. The variation between these populations results from chance events (drift) rather than
natural selection
 Cline- graded change in a character along a geographic axis.
o Some produced by gradation in an environmental variation (ex: temp on frequency of
a cold-adaptive allele in mummichog fish)
o Some result from natural selection
 Ex: in mummichog fish, the frequency of the Ldh-Bb allele for the enzyme
lactate dehydrogenase-B decreases in warmer environments. The allele codes
for a form of that enzyme that’s a better catalyst in cold water. Individuals
with the allele can swim faster in cold water
 Selection can only operate if multiple alleles exist for a given locus
Sources of Genetic Variation
 Mutation, gene duplication, & other processes that produce new alleles & new genes lead to
genetic variation
 Many new genetic variants can be produced in short periods of time in organisms that
reproduce rapidly. Sexual reproduction can result in genetic variation as existing genes are
arranged in new ways
Formation of New Alleles
 Can arise by mutation (change in nucleotide sequence of an organism’s DNA); can’t be
predicted 100% accurately beforehand which segments will be altered or how
 In muticellular organisms, only mutations in reproductive cells can be passed on to offspring.
In most animals, the majority of mutations occur in somatic cells & are lost when the
individual dies. But in plants & fungi, many different cell lines can produce gametes.
 Point mutation (change in 1 nucleotide base) can impact phenotype (ex: sickle-cell disease),
have no effect on the protein’s function if the amino acid composition isn’t changed
(redundancy in genetic code), and some changes may not affect the protein’s structure &
function, & point mutations in introns are harmless
 Organisms reflect thousands of generations of past selection, so their phenotypes usually
provide a close match to their environment, so it is unlikely that a new mutation that alters a
phenotype will improve it. But on rare occasions, a mutant allele may make an organism better
suited to the environment, enhancing reproductive success
Altering Gene Number or Position
 Chromosomal changes that delete, disrupt, or rearrange many loci at once are usually harmful;
when such large-scale changes leave genes intact, their effects are neutralized.
 Chromosomal rearrangements may be beneficial
o Ex: translocation of 1 chromosome to a different chromosome could link DNA
segments in a beneficial way
 Important sources of variation: genes are duplicated due to errors in meiosis, slippage during
DNA replication, or activities of transposable elements.
 Duplications of large chromosome segments are more harmful than smaller

Expanded genome with new genes with new functions may result from mutations accumulated
from gene duplications that don’t have harmful effects over generations
 These beneficial increases in gene number play a major role in evolution
o Ex: remote ancestors of mammals had a single gene for detecting odors. That gene has
been duplicated many times, resulting in 1000 olfactory receptor genes for humans.
Dramatic proliferation of olfactory genes helped early mammals & enabled them to
detect & distinguish among odors.
Rapid Reproduction
 Mutations are low in plants & animals & lower in prokaryotes
 But prokaryotes & viruses have short generation spans, so mutations can generate genetic
variation in population quickly
o Ex: HIV has life span of 2 days & has RNA genome (has higher mutation rate than
DNA because lack of RNA repair mechanisms in host cell). Unlikely that a singledrug treatment would be effective against HIV since mutant forms of the virus that are
resistant to a particular drug would proliferate in short time. The most effective AIDS
treatments have been drug cocktails that combine several medications, lessening the
likeliness that multiple mutations conferring resistance to all those drugs will occur in
a short time period
Sexual Reproduction
 Most of genetic variation in population results from unique combos of alleles that each
individual receives from parents
 At nucleotide level, all differences among alleles came from past mutations & other processes
that produce new alleles, but sexual reproduction shuffles existing alleles & deals them at
random to produce individual genotypes.
 Mechanisms that contribute to shuffling of existing alleles:
1. Crossing over: during meiosis, homologous chromosomes (1 inherited from each
parent) trade their alleles by crossing over
2. Independent assortment of chromosomes: distributed at random into gametes
3. Fertilization: brings together gametes that have different genetic backgrounds
23.2 The Hardy-Weinberg Equation can be Used to Test Whether a Population is Evolving
 Presence of genetic variation doesn’t guarantee that population will evolve.
Gene Pools & Allele Frequencies
 Population- group of individuals of same species that live in same area & interbreed,
producing fertile offspring
o Different populations of a single species may be geographically isolated from one
another, only exchanging genetic material rarely
 Gene pool- consists of all copies of every type of allele at every locus in all members of the
population; described to characterize a population’s genetic makeup
o Fixed- only 1 allele exists for a particular locus
o If there are 2(+) alleles for a particular locus, individuals may be either homozygous
or heterozygous
The Hardy-Weinberg Principle
 One way to see whether natural selection or other factors are causing evolution at a particular
locus is to determine how the genetic makeup of a population would be if it wasn’t evolving at
that locus, and then we can compare that scenario with data from real population. If no
difference, not evolving, but if there are significant differences, might be evolving
Hardy-Weinberg Equilibrium
 Hardy-Weinberg principle- the frequencies of alleles & genotypes in a population will
remain constant from generate to generate, given that only Mendelian segregation & recombo
of alleles are at work; describes gene pool of a population that’s not evolving


Consider combo of alleles in all the crosses in a population
Selecting alleles at random from a gene pool analogy:
o Imagine that all the alleles for a given locus from all the individuals in a population
are placed in a large bin (holds population’s gene pool).
o Reproduction occurs by selecting alleles at random from the bin (mating occurs at
random; all male-female matings are equally likely)
Conditions for Hardy-Weinberg Equilibrium
 Hardy-Weinberg describes hypothetical population that’s not evolving.
 All 5 conditions have to be met in order for a population to be in equilibrium
1. No mutations: gene pool’s modified if mutations alter alleles or if entire genes are
deleted or duplicated
2. Random mating: if individuals mate with preference, random mixing of gametes
doesn’t occur, & genotype frequencies change
3. No natural selection: differences in the survival & reproductive success of individuals
carrying different genotypes can alter allele frequencies
4. Extremely large population size: smaller population increases likeliness that allele
frequencies will fluctuate by change from one generation to the next (genetic drift)
5. No gene flow: inflow/ outflow of alleles into population can alter allele frequencies
 If at least one of these conditions is not met, the population is not in equilibrium
 It is common for natural populations to be in Hardy-Weinberg equilibrium for specific genes,
because a population can be evolving at some loci, yet simultaneously be in equilibrium at
another loci
 Also, some populations evolve so slowly that changes in their allele & genotype frequencies
are hard to distinguish from a hypothetical nonevolving one
Applying the Hardy-Weinberg Principle
 The Hardy-Weinberg equation is used as an initial test of whether evolution is occurring in a
population
 The equation also has medical applications, like estimating the percentage of a population
carrying the allele for an inherited disease
o Ex: phenlyketonuria (PKU), a metabolic disorder that results from homozygous
recessive
1. No PKU mutations are being introduced into the population
2. People neither choose their mates on the basis of whether or not they carry
this gene
3. Effects of differential survival & reproductive success among PKU genotypes
are ignored
4. No effects of genetic drift
5. No gene flow from other populations into US
 Allele frequencies : p + q = 1
o Legend: p= dominant allele, q= recessive allele
 Genotype frequencies : p2 + 2pq + q2 = 1
o Legend: p2 = homozygous dominant, 2pq = heterozygous q2 = recessive
 Tips: solve for q2 (homozygous recessive) first, take the square root to get q, and subtract
from 1.
23.3 Natural Selection, Genetic Drift, & Gene Flow can Alter Allele Frequencies in a Population
 Mutations can have a large effect on allele frequencies when it produces new alleles that
strongly influence fitness. Nonrandom mutations can affect the frequencies of homozygous
and heterozygous genotypes. Natural selection, genetic drift, & gene flow can alter allele
frequencies directly & cause evolutionary change
Natural Selection
 Selection results in alleles being passed to the next generate in proportions that differ from
those in present generate
 Natural selection can cause adaptive evolution (better match between organisms & their
environment) by consistently favoring some alleles over others
Genetic Drift
1. The smaller a population, the more likely it is that chance alone will cause a deviation from
the predicted result
2. Genetic drift- chance events that cause allele frequencies to fluctuate unpredictably from one
generation to the next, especially in small populations
3. Allele frequencies can be affected by chance events that occur during fertilization, since every
egg & sperm pair that generated offspring could have happened to carry either allele.
4. Circumstances that can result in genetic drift having significant impact on population
1. Founder Effect- when a few individuals become isolated from larger population, this
smaller group may establish a new population whose gene pool differs from source
population
 ex: occurs when a few members of a population are blown by a storm to a
new island
 accounts for relatively high frequency of certain inherited disorders among isolated
human populations
2. Bottleneck effect- by chance alone, certain alleles may be overrepresented,
underrepresented, or absent after a population has been severely reduced due to
sudden change in environment
 Ongoing genetic drift has substantial effects on gene pool until population becomes
larger so that chance events have less impact.
 Even if a population recovers in size from bottleneck, it may have low levels of
genetic variation for a long time due to the genetic drift that occurred when population
was small
 Human actions sometimes create severe bottlenecks for other species
Case Study: Impact of Genetic Drift on the Greater Prairie Chicken
 Millions of greater prairie chickens once lived on the prairies of Illinois. As these prairies
were converted to farmland & other human uses, only 2 populations remained, less than 50
birds. The few surviving ones had low levels of genetic variation & less than half their eggs
hatched
 Genetic drift during the bottleneck may have led to a loss of genetic variation & an increase in
the frequency of harmful alleles
 Juan Bouzat extracted DNA from the birds before the bottleneck and during. By studying the
DNA of these specimens, the researchers were able to obtain a minimum, baseline estimate of
how much genetic variation was present in the Illinois population before the bottleneck.
 The researchers looked at 6 loci, and found that the birds after the bottleneck had lost 9 alleles
& had fewer alleles per locus. Drift had reduced the genetic variation of the small reduced
population & increased frequency of harmful alleles, leading to a lower egg-hatching rate
 271 birds from neighboring states (that did not go under bottleneck) were added, allowing new
alleles to enter the population & the egg-hatching rate improved to >90%
Effects of Genetic Drift: A Summary
1. Genetic drift is significant in small population: chance events can cause an allele to be
disproportioned in the next generation. Chance events tend to alter allele frequency seriously
in only small populations
2. Genetic drift can cause allele frequencies to change at random: the change in allele frequency
is random over time
3. Genetic drift can lead to a loss of genetic variation within population: random fluctuations in
allele frequencies over time can eliminate alleles from a population. Such losses can influence
how effectively a population can adapt to a change in the environment
4. Genetic drift can cause harmful alleles to become fixed: alleles can be lost or become fixed
(only 1 allele exists for a particular locus) entirely by change through genetic drift. In very
small populations, genetic drift can cause alleles that are harmful to become fixed, threatening
population’s survival
Gene Flow
 Gene flow- transfer of alleles into or out of a population due to the movement of fertile
individuals or their gametes; tends to reduce genetic differences between populations; if
extensive enough, can result in 2 populations combining into a single population with
common gene pool
 Alleles transferred by gene flow can affect how well populations are adapted to local
environmental conditions
 Ex: great tit songbird on small Dutch island, Vlieland, had survival differences between two
populations on the island.
o Females in eastern population survive twice as well as females born in central
population
o Females born in eastern population are better adapted to life on the island than
females born in the central population
o But studies have shown that the 2 populations are connected by high levels of gene
flow (mating), which should reduce genetic differences between them
 Eastern population better adapted to life on Vlieland than central because of
the unequal amounts of gene flow from the mainland
 43% of first-time breeders in central population are immigrants from
mainland while only 13% from eastern population
 Birds with mainland genotypes survive & reproduce poorly; in the
eastern population, selection reduces the frequency of those
genotypes
 In the central population, gene flow from the mainland is so high that
it overwhelms the effects of selection. So, females born in central
population have many immigrant genes, reducing the degree to which
members of that population are adapted to life on the island

Gene flow can also transfer alleles that improve the ability of populations to adapt to local
conditions
o Ex: gene flow has resulted in worldwide spread of several insecticide resistance
alleles in the mosquito Culex pipins
o
In their population of origin, these alleles increased because they provided insecticide
resistance. Then they were transferred to new population, where their frequencies
increased due to natural selection
 Gene flow has become increasingly important in evolutionary change in human populations.
o Lots of immigration & emigration, allowing mating to be more common with
members of populations that previously had very little contact, leading to exchange of
alleles & fewer genetic differences between those populations
23.4 Natural Selection is the Only Mechanism that Consistently Causes Adaptive Evolution
 Natural selection is a blend of chance (creation of new genetic variations) & sorting (favors
some alleles over others).
 Favoring makes natural selection not random, since natural selection consistently increases the
frequencies of the alleles that provide reproductive advantage & leads to adaptive evolution
A Closer Look at Natural Selection
A. Relative Fitness- contribution an individual makes to the gene pool of the next generation
relative to the contributions of other individuals
a. Selection acts more directly on the phenotype than on the genotype. It acts on the
genotype indirectly, via how the genotype affects the phenotype
b. Natural selection is not direct competitive contests among individuals. Reproductive
success is more subtle than just outright battle.
c. In a given environment, certain traits can lead to greater relative fitness
i. Ex: a barnacle that’s more efficient at collecting food than its neighbors have
greater stores of energy & will be able to produce more offspring
ii. Ex: a better camouflaged moth will give more offspring, since it can conceal
itself from offspring better
B. Directional, Disruptive, & Stabilizing Selection
 Natural selection can alter the frequency distribution of heritable traits in these 3
ways, depending on which phenotypes in a population are favored
a. Directional selection- conditions favor individuals exhibiting one extreme of a
phenotypic range, thus shifting a population’s frequency curve for the phenotypic
character in one direction
i. Common when a population’s environment changes or when members of a
population migrate to a new & different habitat
ii. Ex: an increase in the relative abundance of large seeds over small seeds led
to an increase in the beak depth in a population of Galapagos finches
b. Disruptive selection- conditions favor individuals at both extremes of a phenotypic
range over individuals with intermediate phenotypes
i. Ex: a population of black-bellied seedcracker finches in which small-billed
birds feed mainly on soft seeds, while large-billed birds specialize in cracking
hard seeds. Intermediate-sized bills are inefficient at cracking both types of
seeds & have less relative fitness
c. Stabilizing selection- acts against both extreme phenotypes and favors intermediate
variants; reduces variation & tends to maintain the status quo for a particular
phenotypic character
i. Ex: birth weights of most human babies are 3 – 4 kg. Babies who aren’t in the
range suffer higher rates of mortality
 For all 3, selection favors individuals whose heritable phenotypic traits provide higher
reproductive success
The Key Role of Natural Selection in Adaptive Evolution
 Adaptations that better survival arise gradually over time as natural selection increases the
frequencies of alleles that enhance survival & reproduction

As the proportion of individuals that have favorable traits increases, the match between a
species & its environment improves, allowing adaptive evolution to occur
 Genetic drift & gene flow can increase the frequencies of alleles that improve the match
between organisms & their environment, but don’t do so consistently.
o Genetic drift can cause the frequency of a slightly beneficial allele to increase, but it
can also cause the frequency of that allele to decrease
o Gene flow may introduce alleles that are advantageous or ones that are
disadvantageous
 Natural selection is the only evolutionary mechanism that can consistently lead to adaptive
evolution
Sexual Selection
 Sexual selection- individuals with certain inherited characteristics are more likely than other
individuals to obtain mates
 Can result in sexual dimorphism- difference between the 2 sexes in secondary sexual
characteristics (ex: size, color, ornamentation, behavior)
 Ways in which sexual selection operate:
1. Intrasexual selection- individuals of one sex compete directly for mates of opposite sex
o For most species, occurs in males.
 Ex: a single male may patrol a group of females and prevent other males
from mating with them
2. Intersexual selection (mate choice)- individuals of one sex (usually females) are choosy in
selecting their mates from the other sex
o Female’s choice often depends on showiness of male’s appearance or behavior
 Male showiness may pose risk of being more easily spotted to predators,
but if it helps a male gain a mate, & if this benefit outweighs the risk from
predation, then both the brightness & female preference for it will be
reinforced because they enhance overall reproductive success
o A hypothesis of why female preferences for certain male characteristics evolve
because females prefer males with “good genes” so that her offspring will have
them too
 Ex: female gray tree frogs prefer to mate with males that give long mating
calls. To test if genetic makeup of the long-calling males is superior to
that of short-calling (SC) males, researchers fertilized half the eggs of
each female with sperm from an LC male & fertilized the remaining eggs
with sperm from an SC male.
 The resulting half-sibling offspring were raised in a common
environment & their performance was measured for 2 years.
 Because offspring fathered by an LC male outperformed their
half-siblings fathered by an SC male, the duration of a male’s
mating call indicates the male’s overall genetic quality.
 Female mate choice can be based on a trait that indicates whether
the male has good genes
The Preservation of Genetic Variation
 Some genetic variation in populations represent Neutral variation- differences in DNA
sequence that don’t confer a selective advantage or disadvantage
 The tendency for directional & stabilizing selection to reduce variation is countered by
mechanisms that preserve or restore it
A. Diploidy: in diploid eukaryotes, genetic variation is hidden from selection in the form of
recessive alleles. This latent variation is exposed to natural selection only when both parents
carry the same recessive allele & 2 copies end up in the same zygote.
 Heterozygote protection maintains a huge pool of alleles that might not be favored
under present conditions, but which could bring new benefits if the environment
changed
B. Balancing Selection- natural selection maintains 2(+) forms in a population. Two types:
1. Heterozygote advantage- individuals who are heterozygous at a particular locus have
greater fitness than homozygotes. Natural selection maintains 2(+) alleles at that locus
 Genotype, not phenotype. Thus, whether heterozygote advantage represents
stabilizing or directional selection depends on the relationship between the
genotype & the phenotype
o Ex: if the phenotype of a heterozygote is intermediate to the phenotypes
of both homozygotes, heterozygote advantage is stabilizing selection
 Ex: heterozygote advance at locus in human gene that codes for B polypeptide
subunit of hemoglobin
o In homozygous recessive individuals, a certain recessive allele at that
locus causes sickle-cell disease. The RBCs of people with sickle cell
disease become sickled under low-oxygen conditions
o Heterozygotes for sickle-cell allele are protected against the most severe
effects of malaria, especially in Africa. This partial protection occurs
because the body destroys sickled RBCs rapidly, killing the parasites
they harbor.
a. In these tropical regions, selection favors heterozygotes over
homozygous dominant individuals, who are more vulnerable to
the effects of malaria, & over homozygous recessive individuals,
who develop sickle-cell disease
2. Frequency-dependent selection- fitness of a phenotype depends on how common it is in
the population
 Ex: scale-eating fish in an African Lake
o These fish attack others from behind, darting to remove a few scales
from their prey’s side
a. Some are left mouthed (recessive), while some are right
mouthed (dominant).
i. Because left-mouthed fish’s mouths twist to the left,
they always attack their prey’s right flank, & same with
right-mouthed fish.
b. Prey species guard against attack from whatever phenotype of
scale-eating fish is most common, so selection favors whichever
phenotype is least common
c. Thus, the frequency of the phenotype of the fish oscillates over
time
d. Balancing selection keeps the frequency of each phenotype
~50%
Why Natural Selection Cannot Fashion Perfect Organisms
1. Selection can act only on existing variations: Natural selection favors only the fittest phenotypes
among those currently in the population. New advantageous alleles don’t arise on demand
2. Evolution is limited by historical constraints: each species has a legacy of descent with
modification from ancestral forms. Evolution doesn’t scrap the ancestral anatomy & build each
new complex structure from scratch. Evolution chooses existing structures & adapts them to new
situations. Evolution operates on the traits an organism already has
3. Adaptations are often compromises: e.g. the colorfulness of male peacocks’ feathers help it attract
mate, but also makes it more visible to predators
4. Chance, natural selection, & the environment interact:
a. Ex: when a storm blows birds from an ocean to an island, the wind doesn’t necessarily
transport those individuals that are best suited to the new environment; not all alleles
present in the founding population’s gene pool are better suited to the new environment
than the alleles that are left behind
b. Environment at a particular location may change unpredictably, limiting the extent to
which adaptive evolution results in a close match between the organism & current
environmental conditions