Ch 6. Migration, Genetic Drift, Non-Random Mating

Chapter 6. Mendelian Genetics in Populations II: Migration, Genetic Drift and Non-Random Mating

Genetic Drift

Genetic drift - "In small populations, chance events produce outcomes that differ from theoretical predictions" (p. 165). In any population of finite size, "sampling error" will result in random changes in allele frequency from generation to generation. Consequences:

Especially for neutral alleles, frequencies drift to 1 (fixation) or 0 (elimination).
The effect is strongest in small populations, but occurs in all populations
Founder effect" is a special case of genetic drift: the small size of a founder population almost guarantees that its allele frequencies will not be identical to the parent population.
"Bottleneck effect" occurs when populations undergo periodic crashes. Allele frequencies after the crash will probably differ from those before the crash.

Genetic drift causes random fixation of alleles and loss of heterozygosity. Fig 6.13 shows the trajectories of many populations, revealing that:

Each population follows a unique trajectory (evolutionary path). Some alleles become fixed in some populations, other alleles become fixed in other populations.
Change in allele frequency is rapid in small populations, slower in large populations.
Fixation occurs rapidly in small populations, more slowly in larger populations, but it eventually occurs no matter the population size.
As alleles become fixed, there is an overall decline in heterozygosity as each population become homozygous for one or the other of the alleles.
Note that selection can modify these outcomes by either facilitating or preventing fixation/extinction of alleles.

Some real examples:

Buri's experiment with Drosophila (Fig. 6.14). He began with most populations near p=.5, he ended with most populations near p=1 or p=0. This is exactly the outcome predicted by the computer simulations in Fig. 6.13
Relict desert lizard populations in the Ozarks (Templeton, Fig. 6.16):. Each population became fixed for a single multilocus genotype.
Flowering plants (Young et al., Fig 6.17): There is a positive correlation between population size and genetic diversity (two measures of).
Geospiza in the Galapagos: New population established by 3 males and 2 females. The new population differs morphologically from the source population.

Migration

Migration - "In an evolutionary sense, the movement of alleles between populations" (p. 157). Naturally, the alleles donut move by themselves - they move as organisms disperse from population to population. Note - don't confuse migration in this sense with seasonal migrations, e.g. of birds. "Gene flow" and "migration" are synonymous.
Dispersal can be by adult animal organisms, seeds and spores of plants, planktonic larvae of intertidal animals, gametes/zygotes of algae, etc.
Effects of migration on allele frequencies:

In absence of selection (i.e. if alleles are selectively neutral) migration homogenizes allele frequencies among populations.
If selection and migration tend to increase the frequencies of the same alleles, selection can amplify effect of migration.
If selection and migration are opposed --

If selection is stronger than migration, than differences among populations will be maintained, even in the face of migration.
If migration is stronger than selection, differences among populations will be reduced.

Some real examples:

Water snakes (Nerodia sipedon) on islands in western Lake Erie (Camin, Ehrlich, King; Fig. 6.6, 6.7). Selection on the islands favors unbanded snakes, but dispersal of banded snakes from the mainland results in equilibrium frequencies of the two phenotypes. This is analogous to selection/mutation balance.
Red bladder campion (Silene dioica) on Swedish islands (Giles and Goudet, Fig .6.9). This example address the interaction of drift and migration:

Diversity among young populations is due to founder effect.
Migrations reduces diversity among populations of intermediate age.
As populations shrink, chance extinction of alleles generates diversity among populations of old islands

Nonrandom Mating

Nonrandom mating occurs when the probability that two individuals in a population will mate is not the same for all possible pairs of individuals. When the probability is the same, then individuals are just as likely to mate with distant relatives as with close relatives -- this is random mating. Nonrandom mating can take two forms:

Inbreeding - individuals are more likely to mate with close relatives (e.g. their neighbors) than with distant relatives. This is common.
Outbreeding - individuals are more likely to mate with distant relatives than with close relatives. This is less common.

Inbreeding changes genotype frequencies, not allele frequencies:

Homozygotes increase in frequency, heterozygotes decrease in frequency.
This is most easily seen in extreme case of inbreeding - selfing. When individuals self-fertilize, all of the homozygotes produce homozygotes, and half of the offspring of heterozygotes are homozygotes (only half are heterozygotes). Hence, the frequency of heterozygotes declines by 50% each generation. The same argument applies to sibling matings, half-sib matings, etc.
Inbreeding in the malarial parasite Plasmodium falciparum (Fig. 6.20, Tables 6.2, 6.3).

Strongly female biased sex ratios suggest parasite population is descended from a single foundress. If this is so, than all mating in the parasite population must be between siblings.
The parasite population shows an excess of homozygote relative to Hardy-Weinberg expectation. This is the expected outcome of inbreeding.

F, the coefficient of inbreeding :

If mating is random, F=0
If all reproduction is by self fertilization, F=.5
F is between 0 and .5 for most populations.
F can be calculated from genotype frequencies (because inbreeding depresses frequencies of heterozygotes) or from pedigrees (Fig. 6.21).

Since inbreeding increases frequency of homozygotes, if deleterious recessive alleles are exposed to natural selection, mean fitness of population will be reduced. This is inbreeding depression. Examples:

Humans (Fig. 6.22): children of first cousins have higher mortality rates than children of unrelated parents.
Plants: inbreeding can be studied experimentally, yielding insights into inbreeding in general. Some generalizations:

Inbreeding depression most evident when plants are stressed.
Inbreeding depression most evident as plants age and become independent of mother (seed parent), Fig. 6.23.
Inbreeding depression varies among lineages - not all posses deleterious recessive alleles.

Birds (Great Tit, Parus major): hatching failure increases as function of inbreeding coefficient (F).

Inbreeding can be avoided by:

Dispersal (which may also diminish sibling competition)
Separation of sexes (the rule in animals, less common in plants) to prevent self fertilization
Self incompatibility in bisexual plants.

Conservation Genetics of the Prairie Chicken

Prairie chicken (Tympanuchus cupido pinnatus): a species endangered due to loss of habitat, habitat fragmentation, genetic drift and inbreeding depression.
As prairie habitats shrink, total habitat area diminishes, but also habitat area become fragmented into smaller and smaller "islands" in a sea of farmland.
There is little migration among populations, so these become genetically isolated from each other. Consequences:

In the small prairie chicken populations, genetic drift results in fixation of alleles. Some of these are deleterious, reducing mean fitness of population.
As populations shrink, more matings occur among close relatives. This inbreeding increases frequency of homozygotes. As deleterious recessive alleles are exposed to selection, inbreeding depression results in lower mean fitness.
As population mean fitness is reduced, population size shrinks, exacerbating the problems that caused it to shrink.

Evidence to support this explanation (the case of the Jasper County population):

From 1963 to 1990 there is a steady decline in hatching success (Fig. 6.25). This could be due to inbreeding depression (there are other possibilities).
If inbreeding depression/genetic drift is the culprit, genetic diversity (e.g. number of alleles per locus) should be lower in Jasper Co. individuals than in individuals from other, larger populations. It is (table 6.4).
If inbreeding depression is the culprit, than introducing alleles from other populations should increase hatching success. It did - introducing individuals from from other populations had a dramatic impact (Fig. 6.25)