Section 4 Evolution in Large Populations: Mutation, Migration & Selection

1. Section 4 Evolution in Large Populations: Mutation, Migration & Selection Genetic diversity lost by chance and selection regenerates through mutation. When genetic diversity is lost in small threatened populations, it can be recovered by migration from other genetically distinct populations.

2. Migration often reverses effects of inbreeding. Many rare species are being hybridized out of existence by crossing with common related species. Mutation and migration are often important determinants in the maintenance of genetic diversity.

3. Balance between deleterious mutations & selection results in an ever-present but changing gene pool of rare deleterious mutations (mutation load) in the population. Inbreeding exposes these mutations, resulting in reduced reproduction & survival which in turn increases the extinction risk in threatened species.

4. Genetic diversity is the raw material required for adaptive evolutionary change. However, genetic diversity is lost by chance in small populations and as a result of directional selection. Mutation is the ultimate source of genetic diversity while recombination can produce new combinations of alleles.

5. If genetic diversity is lost, it can be regenerated via mutation, but this is a very slow process. Alternatively, genetic diversity can be restored by natural or artificial immigration between populations with different allelic content.

6. Mutations are sudden changes in an allele or chromosome. All genetic diversity originates from mutations. Patterns of genetic diversity in populations are the result of a variety of forces that act to eliminate or increase & disperse mutations among individuals and populations.

7. Conservation Concerns with regards to mutations: How rapidly mutations add genetic diversity to populations. How mutations affect the adaptive potential and reproductive fitness of populations. How important are the accumulation of deleterious alleles to fitness decline in small populations.

8. The most important mutations are those at loci affecting fitness traits, most notably, lethal or deleterious mutations. G A Transition Substitutions Transversion Substitutions C T

9. Silent Substitution: Base substitution that DOES NOT change an amino acid. These probably have little or no impact on fitness and therefore are also referred to as Neutral Mutations. Neutral mutations are important as molecular markers and clocks that provide valuable information on genetic differences among individuals, populations, & species.

10. Rate of mutation is critical to its role in evolution. Mutation rates differ for different classes of loci. Although spontaneous mutations are considered to be nearly constant over time, mutation rates may be elevated under stressful conditions and by particular environmental agents (radiation, mutagens).

11. Mutation is normally a recurrent process where mutations continue to arise over time.  Mutation Rate: A1 A2 p0 p1 Initial Allele Frequency: p = -p0

12. The time taken to regenerate genetic diversity is a major issue in conservation biology because it may take thousands to millions of generations to regenerate genetic diversity at a single locus. Time to regenerate genetic diversity due to mutation: pt = p0(1 - )t or p0e-t t = (lnp0 - lnpt)/

13. Example: How long will it take a microsatellite locus to regenerate a frequency of 0.5 for an allele that has been lost? p0 = 1.00 pt = 0.5  = 1 X 10-4 t = [ln 1.00 - ln 0.50]/1 X 10-4 = 6,931 generations!

14. Mutations typically occur in both directions and since there are two opposite forces, this usually results in an equilibrium. A1 A2   ˆ Stable Equilibrium: q =  / ( + )

15. Most mutations not occurring in functional loci are expected to be neutral or nearly neutral. Mutations within functional loci will predominantly be deleterious and some are lethal. While selection can remove deleterious alleles from the population, the time taken is so long that new deleterious mutations will arise before previous deleterious mutations have been removed, especially for recessive alleles.

16. Eventually, an equilibrium is reached between the addition of deleterious alleles by mutation and their removal by selection. This is known as mutation - selection balance. Consequently, low frequencies of deleterious alleles are found in all naturally outbreeding populations and this is known as the mutation load.

17. Mutation Loads: Mutational loads are found in essentially ALL species, including several threatened & endangered. Deleterious alleles are normally found only at low frequencies, typically much less than 1% at any locus. Deleterious alleles are found at many loci.

18. Deleterious alleles increase due to mutation rate (p) and are removed by selection at a rate of: (-spq2)/(1-sq2) therefore: q is approximately p - spq2 At equilibrium q = 0, so p ≈ spq2 and q2 ≈ /s Therefore, the equilibrium frequency is: ˆ q ≈ (/s)0.5

19. Migration: Gene pools of populations diverge over time due to chance events and selection. Such divergence may be reduced by migration which can have very large effects on allele frequencies. Change in allele frequency due to migration: q = m(qm - q0) Where m = migration coefficient, qm = allele freq. in migrant population, q0= allele frequency in original population.

20. Example: You have a mainland population of 1,000 bats with an allele frequency (qm) of 0.75. 200 individuals from the mainland migrate to a nearby island that contains a population of 150 individuals with an allele frequency (q0) of 0.40. Of the 200 migrants, only 100 are able to breed. What is the new allele frequency in the island population in the generation following the migration event?

21. n = 1,000 qm = 0.75 200 migrate n = 150 q0 = 0.40 q = m(qm - q0) qm = 0.75 q0 = 0.40 m = migration coefficient = 200 migrate but only 100 breed thus, m = 100/250 = 0.4 q = 0.4(0.75 - 0.4) = 0.14 q1 = q0 + q = 0.4 + 0.14 = 0.54

22. Rearrangement of this equation allows examination ff the effect of Introgression. Example: Ethiopian wolves are genetically distinct from domestic dogs but hybridization occurs in areas where they co-occur, as in Web Valley, Ethiopia. The population for the Sanetti Plateau is relatively pure.

23. Extent of admixture from domestic dogs in the web population can be estimated using allele frequencies at a particular microsatellite locus. Dogs lack the “J” allele while “pure” Ethiopian wolves are homozygous for it. Sanetti population q0 1.00 (“old”) Web population q1 0.78 (“new” -- contains dog) Domestic Dog qm 0.00 (“migrants”)

24. m = (q1 - q0)/(qm - q0) = (0.78 - 1.0)/(0 - 1.0) = 0.22 Based on this, the Web Valley population of Ethiopian wolves contains about 22% of its genetic composition from Domestic dog. It is important to realize that this is an accumulated contribution, not a per generation estimate.

25. Migration-selection equilibrium depends only upon the migration rate (m), the selection coefficient (s) and the allele frequency in the migrants (qm). Thus, equilibrium is NOT dependent upon the allele frequency in the initial population. When migration rates are high and selection is weak, migration dominates the process and can erase local adaptation. Conversely, when migration rates are low and selection is strong, there will be local adaptation.

26. At equilibrium q = 0 and: ˆ q = (2m + s) ± [{2m + s)2 - (8s m qm)}/2s]0.5 Although there are 2 solutions to this equation, because the allele frequency has to be between 0 and 1, only one solution will be correct. Migration-selection balance can arise between wild and captive populations when there is regular movement of wild individuals into captivity or vice versa.