(c) 2001 W.H. Freeman and Company Chapter 16: Population Genetics and Evolution Robert E. Ricklefs...

(c) 2001 W.H. Freeman and Company

Chapter 16: Population Genetics and Evolution

Robert E. RicklefsThe Economy of Nature, Fifth Edition


Chapter Opener


Background: Molecular Basis for Genetic Variation

Genetic information is encoded by DNA.Genetic variation is caused by changes in the

nucleotide sequence of DNA.DNA serves as a template for the

manufacturing of proteins and other nucleic acids: each amino acid 氨基酸 in a protein is encoded by a

sequence of 3 nucleotides, called a codon 密码子 the genetic code contains redundancy 冗余 because

only 20 amino acids need be encoded from 64 possible codons


The source of genetic variation is mutation and recombination.

Mutations are errors in the nucleotide sequence of DNA: Substitutions 置换 (most common) deletions, additions, and rearrangements

重排 also may occur

Causes of mutations: random copying errors highly reactive chemical agents ionizing radiation 电离辐射


Can mutations be beneficial?

Most mutations are harmful: the altered properties of proteins resulting

from mutations are not likely to be beneficial natural selection weeds out most deleterious

genes, leaving only those that suit organisms to their environments

an example is the sickle-cell mutation, which alters the structure of the hemoglobin血红素 molecule with deleterious effects for its carriers


More on MutationMutations are likely to be beneficial when the

relationship of the organism to its environment changes: selection for beneficial mutations is the basis for

evolutionary change, enabling organisms to exploit 开拓 new environmental conditions

Processes that cause mutations are blind 不清楚的 to selective pressures -- mutation is a random force in evolution, producing genetic variation independently of its fitness consequences.


Mutation Rates

The rate of mutation for any nucleotide is low, 1 in 100 million per generation ？ .

Because a complex individual has a trillion or so nucleotides, each individual is likely to sustain 支持 one or more mutations.

Rates of expressed gene mutations average about 1 per 100,000 to 1 per million: rates of expression of phenotypic effects are

often higher because they are controlled by many genes.


Recombination

Variation is introduced during meiosis when parts of the genetic material inherited by an individual from its mother and father recombine with each other: recombination is the exchange of

homologous sections of maternal and paternal chromosomes

recombination produces new genetic variation rapidly

Migration

Migration of individuals within a population or between populations can affect genetic variation in two ways.

On one hand, high migration rates integrate populations into larger units, which tend to retain genetic variation just because of their size.

On the other hand, movement of individuals between habitats with different environmental conditions can mix genes that have been selected under those different conditions and increase genetic variation within the population, both locally and as a whole.


Environmental variation andfrequency-dependent selection

One of the most remarkable cases of such heterozygote superiority involves the sickle-cell allele (S) of the beta-hemoglobin molecule in humans.

Yet, in some parts of tropical Africa, the frequency of the S allele may reach 20% of the gene pool. The reason for this is that in the heterozygous state (AS), the sickle-cell allele confers protection against malaria.



Sources of Genetic Variation

While mutation is the ultimate source of genetic variation: recombination multiplies this variation sexual reproduction produces further

novel combinations of genetic material the result is abundant variation upon

which natural selection can operate


Figure 16.4


The genotypes of all individuals make up the gene pool 基因库 .

The gene pool represents the total genetic variation within the population.

Not all combinations of alleles for a given gene locus will be represented in the gene pool, especially those with low probability.

If a rare combination of alleles confers high fitness, individuals with this combination will produce more offspring, and these alleles will increase in frequency.

Alleles : Alternative forms of the same gene, such as the two forms of the beta-hemoglobin gene, are known as alleles.

Genetic markers can be used to study population processes

Genetic markersAllozymesmicrosatellite MinisatellitesRFLP\AFLPEST (expressed sequence target)SNPsGenome wide association



The Hardy-Weinberg Law 哈文定律 In 1908, Hardy and Weinberg independently

described this fundamental law: the frequencies of both alleles and genotypes will remain constant from generation to generation in a population with: a large number of individuals random mating no selection no mutation no migration between populations


Consequences of Hardy- Weinberg Law

No evolutionary change occurs through the process of sexual reproduction itself.

Changes in allele and genotype frequencies can result only from additional forces 外力 on the gene pool of a species.

Understanding the nature of these forces is one of the goals of evolutionary biology.


Deviations from Hardy-Weinberg Equilibrium 1

For a gene with two alleles, A1 and A2, that occur in proportions p and q, the proportions of the 3 possible genotypes in the gene pool will be: A1A1: p2

A1A2: 2pq

A2A2: q2

Deviations from these proportions are evidence for the presence of selection, nonrandom mating, or other factors that influence the genetic makeup of a population.


Deviations from Hardy-Weinberg Equilibrium 2

Most natural populations deviate from Hardy-Weinberg equilibrium.

We thus consider some of the forces responsible for such deviations (setting aside mutation and selection): effects of small population size nonrandom mating migration


Genetic Drift

Genetic drift is a change in allele frequencies due to random variations in fecundity and mortality in a a population: genetic drift has its greatest effects in small

populations when all but one allele for a particular gene

disappears from a population because of genetic drift, the remaining allele is said to be fixed 固定


Founder Events

When a small number of individuals found a new population, they carry only a partial sample of the gene pool of the parent population: this phenomenon is called a founder

event founding of a population by ten or fewer

individuals results in a substantially reduced sample of the total genetic variation


Figure 16.5


Figure 16.6


Population Bottlenecks 瓶颈

Continued existence at low population size of a recently founded population results in further loss of genetic variation by genetic drift, referred to as a population bottleneck: such a situation may have occurred in the recent

past for the population of cheetahs in East Africa fragmentation of populations into small

subpopulations may eventually reduce their genetic responsiveness to selective pressures of changing environments


Assortative Mating 先择交配Assortative mating occurs when

individuals select mates nonrandomly with respect to their own genotypes: positive assortative mating pairs like

with like negative assortative mating pairs

mates that differ genetically assortative mating does not change

allele frequencies but does affect frequencies of genotypes


Figure 16.7


Positive assortative mating leads to inbreeding.

Positive assortative mating can lead to an overabundance of homozygotes: one result is the unmasking of deleterious

recessive alleles not expressed in heterozygous condition (inbreeding depression 近交衰退 )

most species have mechanisms that assist them in avoiding mating with close relatives:dispersal, recognition of close relatives, negative

assortative mating, genetic self-incompatibility

The inbreeding coefficient (F)-Fixation index



Is inbreeding always undesirable? 不适合的Inbreeding creates genetic problems,

particularly loss of heterozygosity.In some cases inbreeding may be beneficial:

plants that can self-pollinate are capable of sexual reproduction even when suitable pollinators are absent

when organisms are adapted to local conditions, mating with distant individuals may reduce fitness of progeny


Optimal Outcrossing Distance

Mating with individuals located at intermediate distances (optimal outcrossing distance) may be desirable: nearby individuals are likely to be close

relatives, resulting in inbreeding distant individuals may be adapted to different

conditions:in controlled matings in larkspur 飞燕草 plants,

crosses between individuals 10 m apart enhanced seed set and seedling survival, compared to selfing and mating with distant individuals


Figure 16.8

Genetic drift in small populations causes loss of genetic variation

because of the randomness of births and deaths, all the copies of a particular gene in a population will have descended, just by chance, from a single copy that existed at some time in the past, referred to as the coalescence time.

For nuclear genes in diploid organisms, the average coalescence time is equivalent to 4N generations, where N is the size of the population.

The process by which allele frequencies change and genetic variation is lost due to random variations in fecundity, mortality, and inheritance of gene copies through male and female gametes is called genetic drift


Effective population size

Effective population size can be thought of as the size of an ideal population that undergoes genetic drift at the same rate as an observed population.

Many factors influence effective population size, but variation in population size and the participation of individuals in reproduction are among the most important.


Population growth and decline leave different genetic traces


Coalescence, mutation rate, and time

all the copies of the mitochondrial genome in the present-day human population can be traced back to a single copy that existed roughly 140,000 years ago.

Some people misinterpreted this result to mean that all of us descended from a single woman alive at that time, understandably dubbed “mitochondrial Eve.”

The coalescence time calculated for the Y chromosome suggests that the Y-chromosome “Adam” from whom all our Y chromosomes descended lived only 60,000 years ago.


Loss of variation by genetic drift is balanced by mutation and migration

Mutation–drift balance

What is the “4” doing there? Think of it this way: each individual has two copies of each nuclear gene, and each copy can be inherited through either the mother or the father, so there are four possibilities for inheritance.

In the case of mitochondrial or chloroplast genes, which are present in a single copy inherited through only one parent, the “4” disappears, and F ˆ mitochondrial 1/(Nμ+1).(c) 2001 W.H. Freeman and Com

pany

Migration–drift balance

Just as drift and mutation come into balance, drift and migration also come into balance, at which point



Migration and Deviations from Hardy-Weinberg Equilibrium

Mixing individuals from subpopulations with different allele frequencies can result in deviations from genotypic frequencies under the Hardy-Weinberg equilibrium: mixing results in under-representation of

heterozygotes.this phenomenon is called the Wahlund effect

the Wahlund effect refers to reduction of heterozygosity (that is when an organism has two different alleles at a locus) in a population caused by subpopulation structure.

The Wahlund effect was first documented by the Swedish geneticist Sten Wahlund in 1928


Genotypes vary geographically.

Differences in allelic frequencies between populations can result from: random changes (genetic drift, founder

events) differences in selective factors

Such differences are particularly evident when there are substantial geographic barriers to gene flow.


Ecotypes

The Swedish botanist Göte Turreson used a common garden experiment to show that differences among plants from different localities had a genetic basis: under identical conditions (in the common

garden) plants retained 保持 different forms seen in their original habitats

Turreson called these different forms ecotypes 生态型


Ecotypes may be close to one another or distant.

Although ecotypes may be geographically isolated and found some distance apart, this is not always the case: if selective pressures between nearby

localities are strong relative to the rate of gene flow, ecotypic differences may arise:plants on mine tailings and uncontaminated soils

nearby may differ greatly in their tolerance to toxic metals (copper 铜 , lead 铅 , zinc, arsenic 砷 )


Clines and Other Geographic Patterns

Some traits may exhibit patterns of gradual change over distance: such patterns are referred to as clines 渐变群 clinal variation usually represents adaptation to

gradually changing conditions of the environment

Other genetic patterns may be found: geographic variation related to random founder

effects differentiation related to abrupt geographic

barriers and spatial/temporal variation in these


Figure 16.11


Figure 16.12


Figure 16.13


Natural Selection

Natural selection occurs when genetic factors influence survival and fecundity: individuals with the highest

reproductive rate are said to be selected, and the proportion of their genotypes increases over time

Natural selection can take various forms depending on the heterogeneity of, and rate of change in, the environment.


Figure 16.14


Stabilizing Selection

Stabilizing selection occurs when individuals with intermediate, or average, phenotypes have higher reproductive success than those with extreme phenotypes: favors an optimum 最优 or intermediate

phenotype, counteracting 抵消 tendency of phenotypic variation to increase from mutation and gene flow

this is the prevailing 流行 mode of selection in unchanging environments


Directional Selection

Under directional selection, the fittest individuals have more extreme phenotypes than the average for the population: individuals producing the most progeny are to

one extreme of the population’s distribution of phenotypes

the distribution of phenotypes in succeeding generations shifts toward a new optimum

runaway sexual selection 失控性选择 is an excellent example of this phenomenon


Disruptive Selection

When individuals at either extreme of the range of phenotypic variation have greater fitness than those near the mean, disruptive selection can take place: tends to increase phenotypic variation in the

population may lead to bimodal distribution 双峰分布 of

phenotypes uncommon, but could result from availability of

diverse resources, benefits associated with alternative life histories, or strong competition for a preferred resource


Figure 16.15


Directional selection changes allele frequencies.

Selection changes the makeup of the gene pool.Selection has several important aspects:

directional selection against a deleterious allele results in a decrease in frequency of that allele, coupled with an increase in frequencies of favorable alleles

the rate of change in the frequencies of alleles is proportional 成正比 to the selective pressure

evolution stops only when there is no longer any genetic variation to act upon; directional selection thus removes genetic variation from populations


Maintenance of Genetic Variation 1

A paradox: natural selection cannot produce

evolutionary change without genetic variation

however, both stabilizing and directional selection tend to reduce genetic variation:how does evolution continue under such

circumstances?does availability of genetic variation ever limit

the rate of evolutionary change?


Maintenance of Genetic Variation 2 Mutation and migration supply populations with

new genetic variation. Spatial and temporal variation tend to

maintain variation by favoring different alleles at different times and places.

When heterozygotes have a higher fitness than homozygotes, the relative fitness of each allele depends on its frequency in the population (frequency-dependent selection 频率依赖的选择 ): alleles are selected for when at low frequency and

against when at high frequency heterozygote superiority is also called heterosis 杂种优势


Figure 16.16


Figure 16.17


How much genetic variation?

About 1/3 of genes that encode enzymes involved in cellular metabolism show variation in most species: about 10% of these are heterozygous in any

given individual however, most genetic variation is apparently

neutral or has negative effects when expressed thus most variation has no fitness consequences

or is subject to stabilizing selection


Genetic Variation is Important

Under changing environmental conditions, the reserve of genetic variation may take on positive survival value.

There seems to be enough genetic variation in most populations so that evolutionary change is a constant presence.


Evolutionary Changes in Natural Populations

Evolutionary changes have been widely documented, particularly in species that have evolved rapidly in the face of environmental changes caused by humans: Cyanide 氰化物 resistance in scale insects (Chapter

9) pesticide and herbicide resistance among

agricultural pests and disease vectors increasing resistance of bacteria to antibiotics 抗生素

In each case, genetic variation in the gene pool allowed these populations to respond to changed conditions.


黄花茅


Figure 16.18


Figure 16.19


Useful Conclusions from Population Genetics Studies

Every population harbors some genetic variation that influences fitness.

Changes in selective factors in the environment are usually met by evolutionary responses.

Rapid environmental changes caused by humans will often exceed the capacity of a population to respond by evolution; the consequence is extinction.


Summary 1

Mutations are the ultimate source of genetic variability.

Recombination and sexual reproduction result in novel genetic combinations.

The Hardy-Weinberg law predicts stable allelic and genotypic frequencies in certain conditions.

Deviations from Hardy-Weinberg equilibrium are caused by mutation, migration, nonrandom mating, small population size, and selection.


Summary 2

Selection pressures may vary geographically, giving rise to variation in gene frequencies within the geographic range of a species.

Selection may be stabilizing, directional, or disruptive.

Selection tends to remove genetic variation, but mutation, gene flow, and varying selective pressures maintain it.

(c) 2001 W.H. Freeman and Company Chapter 16: Population Genetics and Evolution Robert E. Ricklefs...

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