University:
California State University, Northridge
Course:
BIOL 322 | Evolutionary Biology
Academic year:

2006
Views:

243

Pages:

27
Author:

azaria93c7h

Evolution at multiple loci • Linkage • Sex • Quantitative genetics Quantitative genetics • • • • Heritability Selection Evolutionary response to selection Modes of selection Requirements of natural selection • Individuals vary • Some of that variation is genetic • More offspring are produced that can survive (reproduce) • Survival (reproduction) not random Selection on quantitative traits • Most traits controlled by many many genes • Phenotype produced also depends on environment • Quantitative traits continuously distributed – Qualitative traits, either yes or no Continuous variation Phenotype due to • Interaction of genes with environment – No trait is entirely genetic or entirely environmental • But within a population, differences among individuals can be due to genes, environment, or both Causes of variation • Individuals vary, they are not all the same • Total phenotypic variation in the population, symbolized Vp, is due to genetic variation, symbolized Vg, and variation due to the environment, symbolized Ve • Vp = Vg + Ve Heritability • The fraction of phenotypic variation that is due to genetic variation • Symbolized h2 • h2 = Vg/Vp • It is a number, ranges from 0 to 1 Intuition • Suppose that differences among individuals are mostly genetic • Then offspring will strongly resemble parents • Suppose that differences are mostly due to environment • If offspring environment not similar to parental environment, then offspring do not closely resemble parents Heritability graphs Human height heritability Environmental correlation • offspring might resemble their parents because they have similar environments, not similar genes • Important to eliminate effects of common environment • Which is a better genetic estimator, – Identical twins reared together – Or identical twins reared apart Cross fostering Measuring selection • Selection differential = S – The difference between the average survivor (or reproducer) and the entire population • Selection gradient – Slope of relative fitness on the trait value Example Selection gradient = selection differential/trait variance Response to selection • Recall that evolution by natural selection happens if – – – – Individuals vary Variation is genetic (at least partially) Not everyone reproduces Reproduction not random • The evolutionary change requires genetic variation Calculating response to selection • R = h2S • Response = heritability*selection differential • If heritability = 1, response to selection = selection differential • If heritability = 0, response to selection = 0 • Usually heritability intermediate, response proportional Modes of selection Net stabilizing selection example Net stabilizing selection Selection and genetic variation • Change in mean fitness is a function of genetic variation in fitness • “Rate of increase in fitness of any [population of] organism[s] at any time is equal to its genetic variance in fitness at that time” • Fisher’s fundamental theorem of natural selection • Selection removes genetic variation Scaling up to quantitative traits • R = h2S • But selection depletes genetic variation so h2 changes…because Va changes • A response to selection continues initially because of recombination, and allele frequency change Example: Q-trait 5 loci, 10 alleles LOCUS A A B B C C D D E E ALLELE EFFECT FREQUENCY A1 -1 0.5 A2 1 0.5 B1 -1 0.5 B2 1 0.5 C1 -1 0.5 C2 1 0.5 D1 -1 0.5 D2 1 0.5 E1 -1 0.5 E2 1 0.5 Example Cont. PHENOTYPES GENOTYPES -5 11111 0.031 -3 11112 11121 0.031 0.031 -1 11122 11212 0.031 0.031 1 11222 12122 0.031 0.031 3 12222 21222 0.031 0.031 5 22222 0.031 11211 0.031 12112 0.031 21122 0.031 22122 0.031 12111 0.031 21112 0.031 12212 0.031 22212 0.031 21111 0.031 11221 0.031 21212 0.031 22221 0.031 12121 21121 12211 21211 0.031 0.031 0.031 0.031 22112 12221 21221 22121 0.031 0.031 0.031 0.031 22111 0.031 22211 0.031 Phenotypic frequencies 0.35 0.4 0.3 0.35 0.3 0.25 0.25 0.2 0.2 0.15 0.15 0.1 0.1 0.05 0.05 0 0 -5 -3 -1 1 3 5 -5 All loci p = q = 0.5 -3 -1 1 3 5 All loci p = 0.7 q = 0.3 0.4 0.45 0.35 0.4 0.35 0.3 0.3 0.25 0.25 0.2 0.2 0.15 0.15 0.1 0.1 0.05 0.05 0 0 -5 -3 -1 1 3 5 All loci p = 0.6 q = 0.4 -5 -3 -1 1 3 5 All loci p = 0.8 q = 0.2 Alleles at loci reach fixation 0.7 0.45 0.4 0.6 0.35 0.5 0.3 0.4 0.25 0.2 0.3 0.15 0.2 0.1 0.1 0.05 0 0 -5 -3 -1 1 3 5 -5 1 locus fixed -3 -1 1 3 5 3 loci fixed 0.9 0.6 0.8 0.5 0.7 0.6 0.4 0.5 0.3 0.4 0.2 0.3 0.2 0.1 0.1 0 0 -5 -3 -1 1 2 loci fixed 3 5 -5 -3 -1 1 4 loci fixed 3 5 Maintenance of genetic variation • Direction and stabilizing selection reduce genetic variation – Therefore Vg/Vp decreases, – R=h2*S, response decreases • Mutation, mutation-selection balance • Disruptive selection • Fluctuating selection – Random – Cyclic

Evolution at Multiple Loci

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