Chapter 6 Migration, Genetic Drift and Non-random Mating

7/27/2019 Chapter 6 Migration, Genetic Drift and Non-random Mating

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Chapter 6. Mendelian Genetics in Populations II: Migration, Genetic

Drift and Non-Random Mating

Genetic Drift

Genetic drift - "In small populations, chance events produceoutcomes 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:

o Especially for neutral alleles, frequencies drift to 1 (fixation) or0 (elimination).

o The effect is strongest in small populations, but occurs in allpopulations

o Founder effect" is a special case of genetic drift: the small sizeof a founder population almost guarantees that its allele

frequencies will not be identical to the parent population.

o "Bottleneck effect" occurs when populations undergo periodiccrashes. Allele frequencies after the crash will probably differ

from those before the crash.

Genetic drift causes random fixation of alleles and loss ofheterozygosity. Fig 6.13 shows the trajectories of many populations,

revealing that:

o Each population follows a unique trajectory (evolutionarypath). Some alleles become fixed in some populations, other

alleles become fixed in other populations.

o Change in allele frequency is rapid in small populations,slower in large populations.

o Fixation occurs rapidly in small populations, more slowly inlarger populations, but it eventually occurs no matter the

population size.

o As alleles become fixed, there is an overall decline inheterozygosity as each population become homozygous for oneor the other of the alleles.

o Note that selection can modify these outcomes by eitherfacilitating or preventing fixation/extinction of alleles.

Some real examples:o Buri's experiment withDrosophila (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


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o Relict desert lizard populations in the Ozarks (Templeton, Fig.6.16):. Each population became fixed for a single multilocus

genotype.

o Flowering plants (Young et al., Fig 6.17): There is a positivecorrelation between population size and genetic diversity (twomeasures of).

o Geospiza in the Galapagos: New population established by 3males and 2 females. The new population differs

morphologically from the source population.

Migration

Migration - "In an evolutionary sense, the movement of allelesbetween populations" (p. 157). Naturally, the alleles donut move bythemselves - 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 ofplants, planktonic larvae of intertidal animals, gametes/zygotes of

algae, etc.

Effects of migration on allele frequencies:o In absence of selection (i.e. if alleles are selectively neutral)

migration homogenizes allele frequencies among populations.

o If selection and migration tend to increase the frequencies ofthe same alleles, selection can amplify effect of migration.

o 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:o 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.

o Red bladder campion (Silene dioica) on Swedish islands (Gilesand Goudet, Fig .6.9). This example address the interaction of

drift and migration: Diversity among young populations is due to founder

effect.


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Migrations reduces diversity among populations ofintermediate age.

As populations shrink, chance extinction of allelesgenerates 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:

o Inbreeding - individuals are more likely to mate with closerelatives (e.g. their neighbors) than with distant relatives. Thisis common.

o Outbreeding - individuals are more likely to mate with distantrelatives than with close relatives. This is less common.

Inbreeding changes genotype frequencies, not allele frequencies:o Homozygotes increase in frequency, heterozygotes decrease in

frequency.

o 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.

o Inbreeding in the malarial parasitePlasmodiumfalciparum (Fig. 6.20, Tables 6.2, 6.3).

Strongly female biased sex ratios suggest parasitepopulation 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 :o If mating is random, F=0o If all reproduction is by self fertilization, F=.5o F is between 0 and .5 for most populations.o F can be calculated from genotype frequencies (because

inbreeding depresses frequencies of heterozygotes) or from

pedigrees (Fig. 6.21).


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Since inbreeding increases frequency of homozygotes, if deleteriousrecessive alleles are exposed to natural selection, mean fitness of

population will be reduced. This is inbreeding depression.

Examples:

oHumans (Fig. 6.22): children of first cousins have highermortality rates than children of unrelated parents.

o Plants: inbreeding can be studied experimentally, yieldinginsights into inbreeding in general. Some generalizations:

Inbreeding depression most evident when plants arestressed.

Inbreeding depression most evident as plants age andbecome independent of mother (seed parent), Fig. 6.23.

Inbreeding depression varies among lineages - not allposses deleterious recessive alleles.

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

Inbreeding can be avoided by:o Dispersal (which may also diminish sibling competition)o Separation of sexes (the rule in animals, less common in plants)

to prevent self fertilization

o 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 alsohabitat 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:

o In the small prairie chicken populations, genetic drift results infixation of alleles. Some of these are deleterious, reducing

mean fitness of population.

o As populations shrink, more matings occur among closerelatives. This inbreeding increases frequency of

homozygotes. As deleterious recessive alleles are exposed to

selection, inbreeding depression results in lower mean fitness.

o 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 Countypopulation):


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o From 1963 to 1990 there is a steady decline in hatchingsuccess (Fig. 6.25). This could be due to inbreeding

depression (there are other possibilities).

o If inbreeding depression/genetic drift is the culprit, geneticdiversity (e.g. number of alleles per locus) should be lower inJasper Co. individuals than in individuals from other, larger

populations. It is (table 6.4).

o If inbreeding depression is the culprit, than introducing allelesfrom other populations should increase hatching success. It

did - introducing individuals from from other populations had a

dramatic impact (Fig. 6.25)

Chapter 6 Migration, Genetic Drift and Non-random Mating

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