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Evolutionary Analysis

Fourth Edition

Chapter 7

Mendelian Genetics in Populations II: Migration, Genetic Drift, and Nonrandom Mating

Scott Freeman • Jon C. Herron

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A greater prairie chicken

This male has inflated his air sacs and fanned his feathers as part of his courtship display

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Habitat destruction (the introduction of the steel plow) and the shrinking range of Illinois greater prairie chickens

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A greater prairie chicken population in danger of extinction

Crash again?

Year 1994, 5-6 males

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7.1 Migration

To evolutionary biologists, migration means gene flow: the transfer of alleles from the gene pool of one population to the gene pool of another population.

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The one-island model of migration

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Migration can alter allele and genotype frequencies

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Migration

Migration is a potent mechanism of evolution. In practice, migration is most important in preventing populations from diverging.

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Migration

Empirical Research on Migration as a Mechanism of Evolution

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Water snakes and where they live

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Among very young snakes un-banded individuals are more cryptic on island rocks than are banded individuals

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Category A snakes are unbanded; category B and C snakes are intermediate; category D snakes are strongly banded. Snakes on the mainland tend to be banded; snakes on the islands tend to be unbanded or intermediate

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Migration

Migration of individuals from the mainland to islands appears to be preventing the divergence of island versus mainland populations of Lakke Erie snakes.

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Migration

In this example, migration is acting as an evolutionary mechanism in opposition to natural selection, preventing the island population from becoming fixed for the un-banded allele.

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Migration as a Homogenizing Evolutionary Process across Populations

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Variation in allele frequencies among populations of red bladder campion Silene dioica

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Skeppsvik �Archipelago, Sweden

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7.2 Genetic Drift

Genetic Drift = Sampling Error

Selection is differential reproductive success that happens for a reason; genetic drift is differential reproductive success that just happens.

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An imaginary population of 10 mice from one generation's gene pool

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The range of possible outcomes in our model population of ten mice

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Genetic Drift & Population Size

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A simulation of drawing alleles from a gene pool, run three times

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Founder Effect

When a new population is founded by a small number of individuals, it is likely that chance alone will cause the allele frequencies in the new population to be different from those in the source population. This is the founder effect.

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Random Fixation of Alleles & Loss of Heterozygosity

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A simulation of the cumulative effects of genetic drift

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Genetic Drift

Under genetic drift, every population follows a unique evolutionary path. Genetic drift is rapid in small populations and slow in large population.

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Random Fixation of Alleles

If genetic drift is the only evolutionary process at work, eventually one allele will drift to a frequency of 1 (that is, to fixation) and all other alleles will be lost.

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Nineteen generations of genetic drift in 107 populations of 16 fruit flies

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Loss of Heterozygosity

As alleles drift to fixation or loss, the frequency of heterozygosity (0.5 – 0) in the population declines.

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The frequency of heterozygotes declined with time in Buri's experimental populations (9 flies?)

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Effective Population Size

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Genetic variation in Ozark glade populations of the collared lizard

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Small Populations

Emperical data from a natural experiment confirm that small isolated populations lose their genetic diversity as a result of drift.

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Population size and genetic diversity in flowering plants

Figure 7.19 Population size and genetic diversity

Each data point on these scatterplots represents a population of flowering plants. Polymorphism, plotted on the vertical axis of the graphs at left, is the proportion of allozyme loci at which the frequency of the most common allele in the population is less than 0.99. In other words, polymorphism is the fraction of alleles that are substantially polymorphic. Allelic richness, plotted on the vertical axis of the graphs at right, is the average number of alleles per locus. The statistic r is a measure of association, called the Pearson correlation coefficient, which varies from 0 (no association between variables) to 1 (perfect correlation). P specifies the probability that the correlation coefficient is significantly different from zero. Salvia pratensis, Scabiosa columbaria, and Gentiana pneumonanthe are all flowering herbs; Eucalyptus albens is a tree. [The blue dots in (c) right represent small populations isolated by less than 250 m.] Reprinted from Young et al. (1996).

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The Rate of Evolution by Genetic Drift

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Mutation is the creation of a new allele; Substitution is the fixation of the new allele, with or without additional mutational change

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Molecular Evolution

When mutation, genetic drift, and selection interact, 3 processes occur:
(1) Deleterious alleles appear and are eliminated by selection.
(2) Neural mutations appear and are fixed or lost by chance
(3) Advantageous alleles appear and are swept to fixation by selection.
The relative importance of (2) and (3) in determining the overall substitutions rate is a matter of debate.

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The Neutral Theory of Molecular Evolution

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The neutral theory predicts that both kinds of substitution will accumulate in populations by genetic drift, but that synonymous, or silent, substitutions will accumulate faster, as happens in flu viruses

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Negative Selection & Positive Selection

Natural selection against deleterious mutations is called negative selection (or purifying selection).
Natural selection favoring beneficial mutations is called positive selection.
In most coding sequences, substitution rates are higher at silent sites than at replacement sites. This is consistent with the notion that molecular evolution is dominated by drift and negative selection.

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Problems with Neutral Theory of Molecular Evolution

DNA sequence data
The neutral mutation rate should vary among species as a function of generation time.
Over any given time interval, more neutral mutations should occur in species with short generation times than in species with long generation times.

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“Nearly” Neutral Model�by Kimura

Drift & selection would affect mutations that are slightly deleterious.
Mutations are effectively neutral, when
s < 1 / 2Ne
s = selection coefficient, Ne = effective population size (the number of breeding adults)

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Generation time, population size, and nearly neutral mutations

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Selection on Replacement Mutations

dN/dS < 1, deleterious mutations, negative selection

dN/dS = 1, neutral mutations, no selection

dN/dS >1, advantageous mutations, positive selection

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Positive selection on the BRCA1 gene in humans and chimpanzees

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Selection on “Silent” Mutations

Codon Bias: Condon usage is highly nonrandom

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Codon bias correlates with the relative frequencies of tRNA species

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Hitchhiking & Selection Sweep

Hitchhiking can occur when strong positive selection acts on a particular amino acid change. As a favorable mutation increases in frequency, neutral or even slightly deleterious mutations closely linked to the favored site will increase in frequency along with the beneficial locus.
Only occurs when recombination fails to break up the linkage.

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Nonrandom Mating

Inbreeding alters genotype frequencies

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Every individual reproduces by selfing

Figure 7.25 Inbreeding alters genotype frequencies

(a) This figure follows the genotype frequencies in an imaginary population of 1,000 snails from one generation's adults (lower left) to the next generation's zygotes (upper right). The frequencies of both allele A₁ and A₁ are 0.5. The colored bars show the number of individuals with each genotype. Every individual reproduces by selfing. Homozygotes produce homozygous offspring and heterozygotes produce both heterozygous and homozygous offspring, so the frequency of homozygotes goes up, and the frequency of heterozygotes goes down. (b) These bar charts show what will happen to the genotype frequencies if this population continues to self for two more generations. The orange portions of the bars show the decrease in heterozygosity and the increase in homozygosity due to inbreeding.

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Empirical Research on Inbreeding

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A sea otter feeding in a kelp bed off Monterey, California

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Coefficient of Inbreeding F

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Calculating F from a pedigree

Half Sibling

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Inbreeding depression in humans

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Inbreeding depression in flowering plants increases as individuals age, later in the life

The open bars show data from the first year of growth; the filled bars indicate traits expressed in the second year (when the plants mature, flower, and die).

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Inbreeding increases egg failure in great tits

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Declining hatching success in a greater prairie chicken population

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End