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Robert E. Ricklefs The Economy of Nature, Fifth Edition

Chapter 16: Population Genetics and Evolution

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Background: Molecular Basis for Genetic Variation
 Genetic  Genetic  DNA

information is encoded by DNA.

variation is caused by changes in the nucleotide sequence of DNA. serves as a template for the manufacturing of proteins and other nucleic acids:
each amino acid in a protein is encoded by a sequence of 3 nucleotides, called a codon  the genetic code contains redundancy because only 20 amino acids need be encoded from 64 possible codons


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The source of genetic variation is mutation and recombination.
 Mutations

are errors in the nucleotide sequence of DNA:
 substitutions

(most common)  deletions, additions, and rearrangements also may occur
 Causes

of mutations:

 random

copying errors  highly reactive chemical agents  ionizing radiation

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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 molecule with deleterious effects for its carriers Insert figure 16.2

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More on Mutation
 Mutations

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 new environmental conditions

 Processes

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

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Mutation Rates
 The

rate of mutation for any nucleotide is low, 1 in 100 million per generation. it and it changes ….) a complex individual has a trillion or so nucleotides, each individual is likely to sustain one or more mutations. of expressed gene mutations average about 1 per 100,000 to 1 per million:


 (Contextualize  Because

 Rates

rates of expression of phenotypic effects are often higher because they are controlled by many genes.

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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  to know: ‘evolution of body size in Galapagos marine iguanas. Natural and sexual selection have opposing influences on the size of males. Read ‘more on the web’
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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

+The genotypes of all individuals make up the gene pool.
 The

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gene pool represents the total genetic variation within the population. (all the genes in all the individuals in a population)
all combinations of alleles for a given gene will be represented in the gene pool, especially those with low probability. a rare combination of alleles confers high fitness, individuals with this combination will produce more offspring, and these alleles will increase in frequency.

 Not

 If

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The Hardy-Weinberg Law
 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
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Consequences of Hardy- Weinberg Law (what does it mean?)
 No

evolutionary change occurs through the process of sexual reproduction itself.
in allele and genotype frequencies can result only from additional forces on the gene pool of a species. the nature of these forces is one of the goals of evolutionary biology.

 Changes

 Understanding

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

 Deviations

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Deviations from Hardy-Weinberg Equilibrium 2
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Most natural populations deviate from Hardy-Weinberg equilibrium. We thus consider some of the forces responsible for such deviations (setting aside mutation and selection):
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effects of small population size nonrandom mating migration

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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

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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

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Population Bottlenecks
 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
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Assortative Mating


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

+ Positive assortative mating leads to inbreeding.
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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) most species have mechanisms that assist them in avoiding matings with close relatives:
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dispersal, recognition of close relatives, negative assortative mating, genetic self-incompatibility

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Is inbreeding always undesirable?
Inbreeding creates genetic problems, particularly loss of heterozygosity. In some cases inbreeding may be beneficial:
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plants that can self-pollinate are capable of sexual reproduction even when suitable pollinators are absent when organisms are adapted to local conditions, matings with distant individuals may reduce fitness of progeny

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

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Migration and Deviations from Hardy-Weinberg Equilibrium


Mixing individuals from populations with different allele frequencies can result in deviations from genotypic frequencies under the Hardy-Weinberg equilibrium:
  

such movement of genes is called gene flow mixing results in under-representation of heterozygotes this phenomenon is called the Wahlund effect

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Genotypes vary geographically.
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Differences in allelic frequencies between populations can result from:
 

random changes (genetic drift, founder events) differences in selective factors

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

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Ecotypes


The Swedish botanist Göte Turreson used a common garden experiment to show that differences among plants from different localities had a genetic basis:
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under identical conditions (in the common garden) plants retained different forms seen in their original habitats Turreson called these different forms ecotypes

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Ecotypes may be close to one another or distant.
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Although ecotypes may be geographically isolated and found some distance apart, this is not always the case:
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if selective pressures between nearby localities are strong relative to the rate of gene flow, ecotypic differences may arise:
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plants on mine tailings and uncontaminated soils nearby may differ greatly in their tolerance to toxic metals (copper, lead, zinc, arsenic)

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Clines and Other Geographic Patterns
 Some
 such

traits may exhibit patterns of gradual change over distance:
patterns are referred to as clines  clinal variation usually represents adaptation to gradually changing conditions of the environment

 Other

genetic patterns may be found:
variation related to random founder

 geographic

effects  differentiation related to abrupt geographic barriers and spatial/temporal variation in these

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Natural Selection


Natural selection occurs when genetic factors influence survival and fecundity:
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individuals with the highest reproductive rate are said to be selected, and the proportion of their genotypes increases over time

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

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Stabilizing Selection
 Stabilizing

selection occurs when individuals with intermediate, or average, phenotypes have higher reproductive success than those with extreme phenotypes:
an optimum or intermediate phenotype, counteracting tendency of phenotypic variation to increase from mutation and gene flow  this is the prevailing mode of selection in unchanging environments
 favors

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Directional Selection
 Under

directional selection, the fittest individuals have more extreme phenotypes than the average for the population:
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 is an excellent example of this phenomenon
 individuals

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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:
to increase phenotypic variation in the population  may lead to bimodal distribution of phenotypes  uncommon, but could result from availability of diverse resources, benefits associated with alternative life histories, or strong competition for a preferred resource
 tends

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Directional selection changes allele frequencies.
changes the makeup of the gene pool.

 Selection

 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 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

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Maintenance of Genetic Variation 1


A paradox:
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natural selection cannot produce evolutionary change without genetic variation however, both stabilizing and directional selection tend to reduce genetic variation:
 

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how does evolution continue under such circumstances? does availability of genetic variation ever limit the rate of evolutionary change?

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Maintenance of Genetic Variation 2
 Mutation

and migration supply populations with new genetic variation.
and temporal variation tend to maintain variation by favoring different alleles at different times and places. heterozygotes have a higher fitness than homozygotes, the relative fitness of each allele depends on its frequency in the population (frequency-dependent selection):
 

 Spatial

 When

alleles are selected for when at low frequency and against when at high frequency heterozygote superiority is also called heterosis

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How much genetic variation?
 About

1/3 of genes that encode enzymes involved in cellular metabolism show variation in most species:
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
 about

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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.

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+ Evolutionary Changes in Natural Populations
 Evolutionary

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changes have been widely documented, particularly in species that have evolved rapidly in the face of environmental changes caused by humans:
 cyanide

resistance in scale insects (Chapter 9)  pesticide and herbicide resistance among agricultural pests and disease vectors  increasing resistance of bacteria to antibiotics
 In

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

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Useful Conclusions from Population Genetics Studies
 Every

population harbors some genetic variation that influences fitness.
in selective factors in the environment are usually met by evolutionary responses. environmental changes caused by humans will often exceed the capacity of a population to respond by evolution; the consequence is extinction.

 Changes

 Rapid

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Summary 1
 Mutations

are the ultimate source of genetic variability. and sexual reproduction result in novel genetic combinations.

 Recombination

 The

Hardy-Weinberg law predicts stable allelic and genotypic frequencies in certain conditions. from Hardy-Weinberg equilibrium are caused by mutation, migration, nonrandom mating, small population size, and

 Deviations

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Summary 2
 Selection

pressures may vary geographically, giving rise to variation in gene frequencies within the geographic range of a species. may be stabilizing, directional, or disruptive.
tends to remove genetic variation, but mutation, gene flow, and varying selective pressures maintain it.

 Selection

 Selection


				
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posted:11/25/2009
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