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22The Mechanisms of Evolution
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22 The Mechanisms of Evolution
22.1 What Facts Form the Base of OurUnderstanding of Evolution?
22.2 What Are the Mechanisms of EvolutionaryChange?
22.3 What Evolutionary Mechanisms Result inAdaptation?
22.4 How Is Genetic Variation Maintained withinPopulations?
22.5 What Are the Constraints on Evolution?
22.6 How Have Humans Influenced Evolution?
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22.1 What Facts Form the Base of Our Understanding of
Evolution?
The young Charles Darwin waspassionately interested in geology and
natural science.
In 1831, he was recommended for a
position on the H.M.S. Beagle, for a 5-
year survey voyage around the world.
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Figure 22.1 Darwin and the Voyage of the Beagle (Part 1)
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Figure 22.1 Darwin and the Voyage of the Beagle (Part 2)
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Figure 22.1 Darwin and the Voyage of the Beagle (Part 3)
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22.1 What Facts Form the Base of Our Understanding of
Evolution?
Darwin often went ashore to study rocksand collect specimens, and make
observations about the natural world.
In the Galapagos Islands he observed
that species were similar to, but not the
same as, species on the mainland of
South America. He also realized thatspecies varied from island to island.
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22.1 What Facts Form the Base of Our Understanding of
Evolution?
Darwin postulated that species hadreached the islands from the mainland,
but then had undergone different
changes on different islands.
Part of the puzzle was determining what
could be a mechanism for such
changes.
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22.1 What Facts Form the Base of Our Understanding of
Evolution?
These observations, and many others,led Darwin to propose an explanatory
theory forevolutionary change based on
two propositions:
Species change over time.
The process that produces the change
is natural selection.
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22.1 What Facts Form the Base of Our Understanding of
Evolution?
Darwin continued to amass evidence tosupport his ideas until 1858, when hereceived a letter from another naturalist,Alfred Russel Wallace.
Wallace proposed a theory of naturalselection almost identical to Darwins.
A paper with the work of both men waspresented in 1858 to the LinneanSociety of London.
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22.1 What Facts Form the Base of Our Understanding of
Evolution?
Darwin published his book, The Origin ofSpecies in 1859.
The book provided exhaustive evidence
from many different fields to support
evolution and natural selection.
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22.1 What Facts Form the Base of Our Understanding of
Evolution?
Darwin and Wallace were both influencedby economist Thomas Malthus, who
published An Essay on the Principle of
Population in 1838.
Populations of all species have the
potential for rapid increase.
But this does not occur in nature, so
death rate must also be high.
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22.1 What Facts Form the Base of Our Understanding of
Evolution?
Darwin observed that, though offspringtended to resemble their parents, they
are not identical.
He suggested that slight variations
among individuals affect the chances of
surviving and producing offspring:
natural selection.
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22.1 What Facts Form the Base of Our Understanding of
Evolution?
Natural selection:
Differential contribution of offspring to the
next generation by various genetic
types belonging to the same population.
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22.1 What Facts Form the Base of Our Understanding of
Evolution?
Darwin had observed variation andartificial selection of certain desirable
traits in plants and animals by breeders.
Darwin himself bred pigeons.
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Figure 22.2 Many Types of Pigeons Have Been Produced by Artificial Selection
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22.1 What Facts Form the Base of Our Understanding of
Evolution?
Individuals do not evolve. Populations do.
Population: A group of individuals of the
same species that live and interbreed in
a particular geographic area.
Members of a population become
adapted to the environment in which
they live.
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22.1 What Facts Form the Base of Our Understanding of
Evolution?
Adaptations: Theprocesses by whichuseful characteristics evolve; and the
characteristics themselves.
An organism is considered to be adapted
to a particular environment when it can
be demonstrated that a slightly different
organism survives and reproduces lesswell in that environment.
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22.1 What Facts Form the Base of Our Understanding of
Evolution?
For a population to evolve, its membersmust possess heritable genetic variation.
Thephenotype is the physical expression
of an organisms genes.
Features of a phenotype are the
characters (e.g., eye color), specific form
of a character is a trait(e.g., blue).
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22.1 What Facts Form the Base of Our Understanding of
Evolution?
A heritable trait is at least partlydetermined by genes.
Genetic makeup of an organism is the
genotype.
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22.1 What Facts Form the Base of Our Understanding of
Evolution?
Population genetics has three maingoals:
Explain the origin and maintenance of
genetic variation
Explain patterns and organization of
genetic variation
Understand mechanisms that cause
changes in allele frequencies
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22.1 What Facts Form the Base of Our Understanding of
Evolution?
Different forms of a gene are calledalleles.
The gene pool is the sum of all copies of
all alleles at all loci in a population.
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Figure 22.3 A Gene Pool
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22.1 What Facts Form the Base of Our Understanding of
Evolution?
Populations have genetic variation formany characters.
Artificial selection for different characters
in a single species of wild mustardproduced many crop plants.
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Figure 22.4 Many Vegetables from One Species (Part 1)
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Figure 22.4 Many Vegetables from One Species (Part 2)
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22.1 What Facts Form the Base of Our Understanding of
Evolution?
In laboratory experiments withDrosophila, researchers selected for
high or low numbers of body bristles
from an initial population withintermediate numbers.
After 35 generations, numbers for both
high-bristle and low-bristle fell outsidethe range of the original population.
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Figure 22.5 Artificial Selection Reveals Genetic Variation
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22.1 What Facts Form the Base of Our Understanding of
Evolution?
Locally interbreeding groups are calledMendelian populations.
Allele frequencies, or their proportion in
the gene pool, are estimated bycounting alleles in a sample of
individuals.
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22.1 What Facts Form the Base of Our Understanding of
Evolution?
Allele frequency:
If a locus has two alleles, A and a, there
could be three genotypes: AA,Aa, andaa. The population ispolymorphicatthat locus.
populationin theallelesofsum
populationin thealleletheofcopiesofnumber!p
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Figure 22.6 Calculating Allele Frequencies
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22.1 What Facts Form the Base of Our Understanding of
Evolution?
Ifp is the frequency of allele A, and q isthe frequency of allele a,
p + q = 1
q = 1 p
If there is only one allele at a locus, its
frequency = 1. The population ismonomorphicat that locus; the allele is
said to be fixed.
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22.1 What Facts Form the Base of Our Understanding of
Evolution?
Genotype frequencies may not be thesame as allele frequencies.
Frequencies of different alleles at each
locus and the frequencies of genotypesin a Mendelian population make up the
genetic structure of the population.
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22.1 What Facts Form the Base of Our Understanding of
Evolution?
If certain conditions are met, the geneticstructure of a population does not
change over time.
If an allele is not advantageous, itsfrequency remains constant.
The Hardy-Weinberg equilibrium
describes a model situation in which
allele frequencies do not change.
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22.1 What Facts Form the Base of Our Understanding of
Evolution?
Conditions that must be met:
Mating is random.
Population size is infinite. Large populations
arent affected by genetic drift. No gene flowno migration into or out of the
population.
No mutation. Natural selection does not affect survival of
any genotypes.
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22.1 What Facts Form the Base of Our Understanding of
Evolution?
If these conditions hold:
Allele frequencies remain constant;
after one generation, genotypefrequencies occur in these proportions:
Genotype AA Aa aa
Frequency p2 2pq q2
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Figure 22.7 Calculating HardyWeinberg Genotype Frequencies (Part 1)
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Figure 22.7 Calculating HardyWeinberg Genotype Frequencies (Part 2)
22 1 Wh F F h B f O U d di f
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22.1 What Facts Form the Base of Our Understanding of
Evolution?
For generation 1, probability of two Aalleles coming together is:
Probability of two a alleles:
3025.0)55.0(22
!!!v ppp
2025.0)45.0( 22 !!!v qqq
22 1 Wh t F t F th B f O U d t di f
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22.1 What Facts Form the Base of Our Understanding of
Evolution?
There are two ways of producing aheterozygote:
The Hardy-Weinberg equation:
pqpqqp 2or,or vv
1222! qpqp
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22.2 What Are the Mechanisms of Evolutionary Change?
Hardy-Weinberg equilibrium is a nullhypothesis that assumes evolutionaryforces are absent.
Known evolutionary mechanisms: Mutation
Gene flow
Genetic drift Nonrandom mating
Natural selection
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22.2 What Are the Mechanisms of Evolutionary Change?
Mutation is the origin of geneticvariation.
Mutation is any change in DNA; it
appears to be random with respect tothe adaptive needs of an organism.
Most mutations are harmful or neutral,
but if conditions change, could become
advantageous.
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22.2 What Are the Mechanisms of Evolutionary Change?
Mutations can also restore alleles thatother processes remove.
Mutation rates are lowabout one per
locus in a million zygotes.
Creates a lot of variation because of thenumber of genes that can mutate,
chromosome rearrangements that canchange many genes simultaneously,and large numbers of individuals.
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22.2 What Are the Mechanisms of Evolutionary Change?
Because mutation rate is low, mutationsin themselves result in only minor
deviations from Hardy-Weinberg
equilibrium.If large deviations are found, it is
appropriate to look for other
mechanisms.
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22.2 What Are the Mechanisms of Evolutionary Change?
Gene flow is a result of the migration ofindividuals and movements of gametes
between populations.
New alleles can be added to the genepool, or allele frequencies changed.
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22.2 What Are the Mechanisms of Evolutionary Change?
Genetic drift results from randomchanges in allele frequencies.
In large populations, genetic drift can
influence frequencies of alleles thatdont affect survival and reproduction.
If populations are reduced to a small
number of individualsa populationbottleneck, genetic drift can reduce thegenetic variation.
Figure 22 8 A Population Bottleneck
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Figure 22.8 A Population Bottleneck
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22.2 What Are the Mechanisms of Evolutionary Change?
A population forced through a bottleneckis likely to lose much genetic variation.
Example: Greater prairie chickens in
Illinois were reduced to about 50 birdsin the 1990s; California fan palms are
now restricted to a few oases in
southern California.
Figure 22 9 Species with Low Genetic Variation (Part 1)
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Figure 22.9 Species with Low Genetic Variation (Part 1)
Figure 22 9 Species with Low Genetic Variation (Part 2)
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Figure 22.9 Species with Low Genetic Variation (Part 2)
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22.2 What Are the Mechanisms of Evolutionary Change?
Genetic drift also effects smallpopulations that colonize a new region.
Colonizing population is unlikely to have
all the alleles present in the wholepopulation.
Founder effectequivalent to a
bottleneck.
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22.2 What Are the Mechanisms of Evolutionary Change?
Example of founder effect:
Populations of European fruit fly D.
subobscura began in Chile, and then in
Washington state.
Both populations grew and expanded their
ranges.
These populations have very similar genetic
structure, and much less variation than the
European populations.
Figure 22.10 A Founder Effect
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Figure 22.10 A Founder Effect
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22.2 What Are the Mechanisms of Evolutionary Change?
Nonrandom mating occurs whenindividuals choose mates with particular
phenotypes.
If individuals choose the same genotypeas themselves, homozygote frequencies
will increase.
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22.2 What Are the Mechanisms of Evolutionary Change?
Nonrandom mating in primroses(Primula):
Two flower typespin and thrum. Pollen
from one type can fertilize only flowersof the other type.
Figure 22.11 Flower Structure Fosters Nonrandom Mating
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g g
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22.2 What Are the Mechanisms of Evolutionary Change?
Selfing, or self-fertilization, is a commonform of nonrandom mating.
Selfing reduces the frequency of
heterozygotes, and increaseshomozygotes, but does not change
allele frequencies in the population.
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22.3 What Evolutionary Mechanisms Result in Adaptation?
Adaptation occurs when some individualsin a population contribute more offspring
to the next generation.
Allele frequencies change in a way thatadapts individuals to the environment
that influenced that reproductive
success.This is natural selection.
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22.3 What Evolutionary Mechanisms Result in Adaptation?
Natural selection acts on phenotype.
Fitness is the reproductive contribution
of a phenotype to subsequent
generations.
Changes in the relative success of
different phenotypes in a population
leads to change in allele frequencies.
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22.3 What Evolutionary Mechanisms Result in Adaptation?
Fitness of a phenotype is determined bythe average rates of survival and
reproduction of individuals with that
phenotype.
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22.3 What Evolutionary Mechanisms Result in Adaptation?
Most characters are influences by allelesat more than one locus.
Such characters often show quantitative
variation instead of qualitative.
Example: The distribution of body size of
individuals in a population is likely to
resemble a bell-shaped curve.
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22.3 What Evolutionary Mechanisms Result in Adaptation?
Natural selection can act on characterswith quantitative variation in three ways:
Stabilizing selection preserves average
phenotype. Directional selection favors individualsthat vary in one direction.
Disruptive selection favors individualsthat vary in opposite directions from theaverage.
Figure 22.12 Natural Selection Can Operate on Quantitative Variation in Several Ways (Part 1)
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Figure 22.12 Natural Selection Can Operate on Quantitative Variation in Several Ways (Part 2)
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Figure 22.12 Natural Selection Can Operate on Quantitative Variation in Several Ways (Part 3)
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22.3 What Evolutionary Mechanisms Result in Adaptation?
Stabilizing selection reduces variationin a population, but does not change the
mean.
Rates of evolution are slow becausenatural selection is usually stabilizing.
Example: human birth weights
Figure 22.13 Human Birth Weight Is Influenced by Stabilizing Selection
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22.3 What Evolutionary Mechanisms Result in Adaptation?
Directional selection occurs whenindividuals at one extreme are more
successful.
If directional selection operates overmany generations, an evolutionary trend
occurs.
Example: resistance to tetrodotoxin(TTX) in garter snakes
Figure 22.14 Resistance to TTX Is Associated with the Presence of Newts
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Figure 22.15 Disruptive Selection Results in a Bimodal Distribution
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22.3 What Evolutionary Mechanisms Result in Adaptation?
Sexual selection is a special type ofnatural selection, which acts oncharacters that determine reproductivesuccess.
If an individual survives but does notreproduce, it makes no contribution tothe next generation.
Sexual selection favors traits thatincrease the chances of reproduction.
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22.3 What Evolutionary Mechanisms Result in Adaptation?
Traits such as bright colors, long horns,and elaborate courtship displays, mayimprove ability to compete for mates(intrasexual selection);
or to be more attractive to the oppositesex (intersexual selection).
Such traits are costly, but reliablydemonstrate the fitness of the bearer tothe choosing sex.
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22.3 What Evolutionary Mechanisms Result in Adaptation?
This has been shown experimentally inlong-tailed widowbirds.
Male tails were shortened or lengthened.
Both were able to successfully defend
their territories, but males with
lengthened tails attracted more females.
Long tails indicate the health and vigor of
the male.
Figure 22.16 The Longer the Tail, the Better the Male (Part 1)
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Figure 22.16 The Longer the Tail, the Better the Male (Part 2)
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Figure 22.17 Bright Bills Signal Good Health (Part 1)
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Figure 22.17 Bright Bills Signal Good Health (Part 3)
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22 4 H I G ti V i ti M i t i d ithi P l ti ?
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22.4 How Is Genetic Variation Maintained within Populations?
Many mutations do not affect the functionof the resulting proteins.
An allele that does not affect fitness is a
neutral allele. They tend to accumulatein a population.
Molecular techniques allow neutralalleles to be identified and used toestimate rates of evolution (see Chapter24).
22 4 H I G ti V i ti M i t i d ithi P l ti ?
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22.4 How Is Genetic Variation Maintained within Populations?
Sexual reproduction results in newcombinations of genes through crossing
over and independent assortment, and
the combination of gametes.
Sexual recombination produces genetic
variety that increases evolutionary
potential.
22 4 Ho Is Genetic Variation Maintained ithin Pop lations?
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22.4 How Is Genetic Variation Maintained within Populations?
But sexual reproduction hasdisadvantages:
Recombination can break up adaptive
combinations of genes.
Reduces rate at which females pass
genes to offspring.
Dividing offspring into genders reduces
the overall reproductive rate.
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22.4 How Is Genetic Variation Maintained within Populations?
How did sexual reproduction arise?
Possible advantages:
Sexual reproduction facilitates repair ofdamaged DNA. Damage on one
chromosome can be repaired by
copying intact sequence on the other
chromosome.
22 4 How Is Genetic Variation Maintained within Populations?
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22.4 How Is Genetic Variation Maintained within Populations?
Permits elimination of deleteriousmutations.
In asexually reproducing species,
deleterious mutations can accumulate;only death of the lineage can eliminate
themMullers ratchet.
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22.4 How Is Genetic Variation Maintained within Populations?
Sexual recombination produces someindividuals with many deleterious
mutations, some with few. The
individuals with few deleterious
mutations are more likely to survive.
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22.4 How Is Genetic Variation Maintained within Populations?
The variety of genetic combinationspossible in sexually reproducing species
may be especially valuable in defense
against pathogens and parasites.
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22.4 How Is Genetic Variation Maintained within Populations?
Sexual recombination does not affect thefrequency of alleles, but generates new
combinations of alleles on which natural
selection can act.
22 4 How Is Genetic Variation Maintained within Populations?
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22.4 How Is Genetic Variation Maintained within Populations?
Frequency-dependent selection: Apolymorphism can be maintained when
fitness depends on its frequency in the
population.
Example: a scale-eating fish in Lake
Tanganyika. Left-mouthed and right-
mouthed individuals are both favored;the host fish can be attacked from either
side.
Figure 22.18 A Stable Polymorphism
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22.4 How Is Genetic Variation Maintained within Populations?
Environmental variation also helps topreserve genetic variation.
Example: Colias butterflies live in an
environment with temperature extremes.The population is polymorphic for anenzyme that influences flight at differenttemperatures.
Heterozygotes are favored because theycan fly over a larger temperature range.
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Figure 22.19 A Heterozygote Mating Advantage (Part 2)
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22.4 How Is Genetic Variation Maintained within Populations?
Subpopulations in different geographicregions maintain genetic variation.
The subpopulations may be subjected to
different environmental conditions andselective pressures.
Example: populations of white clover thatproduce cyanide as defense againstherbivores. Plants that produce cyanideare more likely to be killed by frost.
Figure 22.20 Geographic Variation in a Defensive Chemical
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22.5 What Are the Constraints on Evolution?
Lack of genetic variation can preventevolution of potentially favorable traits.
If the allele for a given trait does not exist
in a population, that trait cannot evolve,even if it would be favored by natural
selection.
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22.5 What Are the Constraints on Evolution?
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22.5 What Are the Constraints on Evolution?
Developmental processes also constrainevolution.
All evolutionary innovations are
modifications of previously existingstructures.
22.5 What Are the Constraints on Evolution?
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22.5 What Are the Constraints on Evolution?
Example: two lineages of bottom-dwellingfishes
Skates and rays evolved from a common
ancestor with sharks. They started with aflattened body plan, and can swim alongthe ocean floor.
Sole and flounder evolved from laterallyflattened bony fishes. They cant swimwell, but lie still on the bottom. The eyesgradually shifted to one side.
Figure 22.21 Two Solutions to a Single Problem (Part 1)
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Figure 22.21 Two Solutions to a Single Problem (Part 2)
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22.5 What Are the Constraints on Evolution?
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Adaptations involve both fitness costs andbenefits.
Benefit must outweigh cost if adaptation is
to evolvethe trade-offs must beworthwhile.
Example: Garter snake resistance to TTX
only occurs where poisonous newts arecommon.
22.5 What Are the Constraints on Evolution?
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Conspicuous features used by somemales to compete with other males are atrade-off with reproductive success.
Species in which males have multiplemates arepolygynous. Males are usuallylarger than females and often haveweaponshorns, antlers, etc.
Dramatic differences between sexes areknown as sexual dimorphism.
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In polygynous species, males mustdefend their mates against other males.
Defensive structures require a lot of
energy, but the male has a lot ofreproductive success.
22.5 What Are the Constraints on Evolution?
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Short-term changes in allele frequenciescan be observed and manipulated todemonstrate the processes by whichevolution occurs.
But patterns of long-term evolutionarychange can be influenced by infrequentevents (e.g., meteorites) or very slow
processes (e.g., continental drift). Othertypes of evidence are used to studythese long-term changes.
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22.6 How Have Humans Influenced Evolution?
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Humans have also influenced theevolution of other species:
Efforts to control populations of pests
make humans agents of evolutionarychange.
Sport hunters that seek large trophy
animals are removing the biggest andhealthiest animals from populations.
Figure 22.22 Trophy Hunting Selects for Smaller Males
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Humans also move species around theglobe, and modify species through
breeding and biotechnology.
Humans are also changing the climate;and have caused the extinction of many
species.