Chapter 8: Population structure: genes and phenotypes

population

a group of a single species occupying a given area at the same time

migration

movement of individuals from one population to another

gene flow

movement of alleles from one population to another

combined effects on selection, gene flow, genetic drift on population divergence

divergence : selection and genetic drift

• drives 2 population to be different

convergence : gene flow

• drives 2 population to be same

how to measure gene flow

• difficult to observe and measure

  • potential (dispersal) vs actual (interbreeding)

  • gamete vs individual

1. perform experiments

2. use neutral genetic markers to look for signatures of gene flow

   1. examine polymorphic genetic variants that aren’t targets of selection

   2. neutral markers let us infer non-selective processes affecting genetic diversity of populations

experiment method

question: how much gene flow occurs between geographically separated populations?

experiment:

• establish two populations (homozygotes), fixed for alternative alleles, separated by give distance

• score FS heterozygotes of offspring

• frequency of heterozygotes = estimate of gene flow

example: gene flow between crop and weed sunflowers

most gene flow occurs over a short distance, but a small amount occurs as fast as 1km

what does random mean in evolution?

• stochastic (unpredictable or random) evolutionary forces:

  • mutation

  • recombination

  • genetic drift

• deterministic (predictable or non-random) evolutionary forces:

  • natural selection

stochastic process resulting in loss of diversity

• genetic drift

  • stochastic changes in allele frequency due to random variation in fecundity and mortality

  • most important when populations are small

• population bottle neck

  • single sharp reduction in abundance, usually followed by a rebound

  • causes a loss of diversity

• founder event

  • colonization by a few individuals that start new population

  • colonizing group contains only limited diversity compared to the source population

Random fluctuations in allele frequencies in populations of different size

*genetic drift is more pronounced in small populations

• more drastic fluctuations each generation

• more rapid loss genetic diversity(i.e. faster time to allele fixation or loss)

• less consistency across replicate populations

human genetic variation over space

• human show a loss of genetic variation with increasing distance from east africa

• reflects serial founder events as humans migrated from source population

Two populations: DNA sequences divergence

human population differentiation: one gene with multiple alleles

• across many genes: 93-95% of genetic variation is observable with populations (5-7% between populations)

• human populations experienced recent origins and reasonable high gene flow

*humans are more similar than diff

human population differentiation from east Africa 

• lower gene flow with increasing distance

• isolation by distance = accumulation of local genetic variation due to geographically limited dispersal

Gene flow between humans and neanderthals

• human populations out of Africa:

  • genomes have short stretches of neanderthal - derived DNA

  • averages about 2% of the genome

  • consistent with ancient interbreeding

phenotypic plasticity can contribute to populations looking different

e.g. arrowhead, an aquatic common Ontario wetlands

• terrestrial phenotype

• aquatic phenotype

phenotypic plasticity

the ability of a genotype to modify its phenotype in response to a particular environment

• occurs through modifications to development, growth, and/or behaviour (under genetic control)

• common in sedentary organisms; e.g. plants, corals (also in animal behaviours)

• phenotypic plasticity often is an adaptation to unpredictable environments (but not all phenotypic plasticity results from adaptations)

no plasticity = genotype

plasticity = genotype + environment

highly variable = genotype + environment (many factors)

reciprocal transplant studies

growth of equivalent genotypes in contrasting environments, and comparisons of their relative performance

• can separate phenotypic variation into genetic environmental components

• enables measurement of selection against non-local genotypes

• can provide evidence for/against local adaptation

Clausen-Keck-Hiesey Transect in California

(with Potentilla glandulosa plants)

• differences between population due to BOTH plasticity and genetics

• evidence for widespread local adaptation - local populations had highest fitness

evolution of skin pigmentation: local adaptation associated with UV radiation?

Trade-offs associated with skin pigmentation

• high uv radiation

  • degrades folate = folate critical in highly dividing tissues (e.g. embryos, testes)

  • may have selected for increased pigmentation = strong purifying selection on MC1R on equatorial regions

• low UV radiation

  • reduced vitamin D synthesis - vitD critical for bone development, immunity, etc.

  • may have selected for reduced pigmentation

*no single ‘best’ phenotype across globe due to trade-offs

was there a history of local adaptation in skin pigmentation?

• numerous genes known to affect skin pigmentation

• these genes show higher between-population differentiation than most others

  • evidence supporting a history of local adaptation

• pigmentation genes show evidence for positive selection in regions with distinctive skin colouration

other evidence for local adaptation in human

• disease resistance

• human height

• lactose tolerance