ch22 lecture

8/3/2019 Ch22 Lecture

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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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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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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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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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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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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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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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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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Evolution?

Natural selection:

Differential contribution of offspring to the

next generation by various genetic

types belonging to the same population.


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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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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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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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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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Evolution?

A heritable trait is at least partlydetermined by genes.

Genetic makeup of an organism is the

genotype.


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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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Nonrandom mating occurs whenindividuals choose mates with particular

phenotypes.

If individuals choose the same genotypeas themselves, homozygote frequencies

will increase.


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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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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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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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Fitness of a phenotype is determined bythe average rates of survival and

reproduction of individuals with that

phenotype.


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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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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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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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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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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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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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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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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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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.

22 4 How Is Genetic Variation Maintained within Populations?


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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.



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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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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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The variety of genetic combinationspossible in sexually reproducing species

may be especially valuable in defense

against pathogens and parasites.



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Sexual recombination does not affect thefrequency of alleles, but generates new

combinations of alleles on which natural

selection can act.



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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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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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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?


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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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Developmental processes also constrainevolution.

All evolutionary innovations are

modifications of previously existingstructures.



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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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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.



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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.



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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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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.

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