ii. deviations from hwe a. mutation b. migration c. non-random mating d. genetic drift - sampling...

II. Deviations from HWE

A. MutationB. MigrationC. Non-Random MatingD. Genetic Drift - Sampling ErrorE. Selection

1. Measuring “fitness” – differential reproductive success




a. The mean number of reproducing offspring (or females)/female




a. The mean number of reproducing offspring (or females)/femaleb. Components of fitness:




a. The mean number of reproducing offspring (or females)/femaleb. Components of fitness

- probability of female surviving to reproductive age





- probability of female surviving to reproductive age - number of offspring the female produces





- probability of female surviving to reproductive age - number of offspring the female produces - probability that offspring survive to reproductive age





- probability of female surviving to reproductive age - number of offspring the female produces - probability that offspring survive to reproductive age

c. With a limited energy budget, selection cannot maximize all three components… there will necessarily be TRADE-OFFS.

E. Selection


2. Relationships with Energy Budgets

E. Selection



GROWTH

METABOLISM

REPRODUCTION

E. Selection



GROWTH

METABOLISM

REPRODUCTION

Maximize probability of survival

GROWTH

METABOLISM

REPRODUCTION

Maximize reproduction

E. Selection



METABOLISM

REPRODUCTION METABOLISMREPRODUCTION

Trade-offs within reproduction

Lots of small, low prob of survival

A few large, high prob of survival

E. Selection



3. Modeling Selection


Selection for a Dominant Allele

p = 0.4, q = 0.6 AA Aa aa

Parental "zygotes" 0.16 0.48 0.36 = 1.00

p = 0.4, q = 0.6 AA Aa aa


prob. of survival (fitness) 0.8 0.8 0.2



p = 0.4, q = 0.6 AA Aa aa



Relative Fitness 1 1 0.25



p = 0.4, q = 0.6 AA Aa aa




Survival to Reproduction 0.16 0.48 0.09



p = 0.4, q = 0.6 AA Aa aa




Survival to Reproduction 0.16 0.48 0.09 = 0.73



p = 0.4, q = 0.6 AA Aa aa





Geno. Freq., breeders 0.22 0.66 0.12 = 1.00



p = 0.4, q = 0.6 AA Aa aa






Gene Freq's, gene pool p = 0.55 q = 0.45



p = 0.4, q = 0.6 AA Aa aa







Genotypes, F1 0.3025 0.495 0.2025 = 100





Δp declines with each generation.




BECAUSE: as q declines, a greater proportion of q alleles are present in heterozygotes (and invisible to selection). As q declines, q2 declines more rapidly...




So, in large populations, it is hard for selection to completely eliminate a deleterious allele....




Rate of change depends on the strength of selection; the difference in reproductive success among genotypes.

In this case, a new adaptive mutant allele has been produced in the population. The “selection differential”, s, is selection AGAINST the existing allele that had become ‘fixed’ in the population (f = 1.0)

So, the “better” the new allele is (represented by the greater selective differential against the old allele), the faster the new mutant accumulates in the population.



Selection for an allele where there is not complete dominance:

- Consider incomplete dominance, codominance, or heterosis. In these situations, the heterozygote has a phenotype that differs from either of the homozygotes, and selection can favor one genotype over another:

- Selection might favor one homozygote over the heterozygote and other homozygote (first example), or might favor the heterozygote over the homozygotes (second example), or might favor both homozygotes over the heterozygote (not considered here).

Selection for the homozygote of a ‘non-dominant’ allele

(incomplete dominance, codominance, overdominance)

p = 0.4, q = 0.6 AA Aa aa



Relative Fitness 1 0.5 0.25


Geno. Freq., breeders 0.33 0..50 0.17 = 1.00


Genotypes, F1 0.34 0.48 0.18 = 100

Selection for the homozygote of a non-dominant allele

- deleterious alleles can no longer hide in the heterozygote; its presence always causes a reduction in fitness, and so it can be eliminated from a population (if the heterozygote is less ‘fit’ than the AA).

Selection for the heterozygote

p = 0.4, q = 0.6 AA Aa aa



Relative Fitness 0.5 (1-s) 1 0.25 (1-t)




Genotypes, F1 0.24 0.50 0.26 = 100

AA Aa aa

Maintains both genes in the gene pool

peq = t/s+t

= 0.75/1.25 = 0.6

Maintains both genes in the gene pool

peq = t/s+t

= 0.75/1.25 = 0.6

Selection for the Heterozygote

Sickle cell caused by a SNP of valine for glutamic acid at the 6th position in the beta globin protein in hemoglobin (147 amino acids long).

The malarial parasite (Plasmodium falciparum) cannot complete development in red blood cells with this hemoglobin, because O2 levels are too low in these cells.NN NS SS

E. Selection




4. Types of Selection

- Selection acts on phenotypes, which may be single gene traits, polygenic quantitative traits, and/or effected by epistatic interactions.

- The different effects are measured by changes in the mean phenotype over time.

E. Selection

4. Types of Selection - Directional

E. Selection

4. Types of Selection - Stabilizing

E. Selection

4. Types of Selection - Disruptive

Lab experiment – “bidirectional selection” – create two lines by directionally selecting for extremes. Populations are ‘isolated’ and don’t reproduce.

E. Selection

4. Types of Selection - Disruptive

African Fire-Bellied Seed Crackers

ii. deviations from hwe a. mutation b. migration c. non-random mating d. genetic drift - sampling...

Documents

components of fitness

reproductive age number

reproductive ageii

reproductive agec

nonrandom matingd

female producesii

energy budgetse

limited energy budget