Population Structure: Genes & Phenotypes

Lecture 8

Population

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

Migration

The movement of individuals from one population to another

Gene flow

The movement of alleles from one population to another

How to measure gene flow

• Difficult to observe and measure

• Perform experiments

• Use neutral genetic markers to look for signatures of gene flow

  • Examine polymorphic genetic variants that aren’t direct targets of selection

  • Neutral markers let us infer non-selective processes affecting genetic diversity of populations

Types of evolutionary forces

• Stochastic (unpredictable or random) evolutionary forces

  • Mutation, recombination, genetic drift

• Deterministic (predictable or non-random) evolutionary forces

  • Natural selection

Genetic drift

• Stochastic changes in allele frequency due to random variation in fecundity & mortality

• Most important when populations are small

Population Bottlenecks

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

• Causes a loss of diversity

Founder Event

• Colonization by few individuals that start a new population

• Colonizing group contains only limited diversity compared to the source of the population

Random fluctuations in allele frequencies in populations of different sizes

Genetic drift is more pronounced in small populations

• More drastic fluctuations each generations

• More rapid loss of genetic diversity

• Less consistency across replicate populations

Human genetic variation over space

• Humans show a loss of genetic variation with increasing distance from East Africa

• Reflects serial founder events as humans migrated from source populations

DNA sequence divergence

• Low gene flow - unique polymorphism, fixed differences

• High gene flow - shared polymorphism

Isolation by distance

Accumulation of local genetic variation due to geographically limited dispersal

Phenotypical differences between populations within a species

• Genetic differentiation among populations if often observed across a geographical range

  • Difference allele frequencies in different populations

• Phenotypic differentiation may be; adaptive, due to genetic drift or phenotypic plasticity

• Testing fro local adaptation and plasticity

  • Reciprocal transplant studies

  • Genomic analyses

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

• Common in sedentary organisms; e.g. plants, corals

• Phenotypic plasticity often is an adaptation to unpredictable environments

Reciprocal transplant studies

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

• Can separate phenotypic variation into genetic and environmental components

• Enables measurement of selection against non-local genotypes

• Can provide evidence for/against local adaptation

Clausen-Keck-Hiesey Transplant Conclusion

• Differences between populations due to BOTH plasticity and genetics

• Evidence for widespread local adaptation

  • Local population had highest fitness

Tradeoffs associated with skin pigmentation

• High UV Radiation

  • Degrades folate

  • May have selected for increased pigmentation

• Low UV Radiation

  • Reduced vitamin D synthesis

  • May have selected for reduced pigmentation

History of local adaptation in skin pigmentation

• Numerous genes known to affect skin pigmentation

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

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