Lecture 8 - Population Structure, Gene Flow, and Genetic Drift

Define population.

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

Define migration.

Movement of individuals from one population to another

Define gene flow.

Movement of alleles from one population to another

Define genetic drift.

Random fluctuations in allele frequencies caused by chance variation in survival and reproduction

What is phenotypic plasticity?

The ability of a genotype to alter its phenotype in response to the environment, via developmental/growth/behavioral modifications

Why might two populations look different but not be genetically differentiated?

Differences may be due to phenotypic plasticity rather than genetic divergence

What are the main questions in studying population structure?

What forces influence genetic differentiation? How is diversity distributed within vs. between populations? What forces drive phenotypic differentiation? How to separate genetic vs. environmental effects on phenotypes?

Why is gene flow important?

It homogenizes populations, reducing genetic differentiation across space

How can gene flow be measured?

Experiments (introduce separated pops with different alleles, measure heterozygosity in offspring); Genetic markers (neutral variants not under selection)

Why use neutral markers to measure gene flow?

They reveal migration patterns without confounding effects of natural selection

Why do crop and weed sunflowers exchange alleles despite being distinct populations?

Because pollen (gametes) and seeds disperse, enabling gene flow

What type of evolutionary force is drift?

Stochastic (random), unlike natural selection which is deterministic

When is drift most powerful?

In small populations

What are two events that cause drastic loss of diversity?

Bottleneck: Sharp population reduction followed by rebound; Founder effect: New population started by few individuals, limiting diversity

How does population size affect drift?

Small pops → faster allele fixation/loss, stronger fluctuations, more divergence between replicates

Why do endangered species often lose genetic diversity quickly?

Small effective population size intensifies drift

What geographic trend exists in human diversity?

Declines with increasing distance from East Africa (serial founder events during migrations)

How much human genetic variation is within vs. between populations?

~93-95% within populations; only ~5-7% between populations

What does this suggest about human history?

Recent common origin in Africa and high ongoing gene flow

What evidence exists for ancient interbreeding with Neanderthals?

Modern non-African genomes contain ~2% Neanderthal-derived DNA

What are the three causes of phenotypic differences among populations?

Local adaptation (selection); Genetic drift; Phenotypic plasticity

How can we test for local adaptation vs. plasticity?

Reciprocal transplant experiments + genomic analysis

Example of phenotypic plasticity?

Arrowhead plants grow aerial leaves on land, submerged leaves in water

Why might Arctic and tropical frogs differ in body size even if they share genes?

Environmental influences → plasticity, not necessarily genetic adaptation

What do reciprocal transplant studies measure?

Growth/performance of the same genotypes in different environments to partition genetic vs. environmental influences

Classic example?

Clausen-Keck-Hiesey transplant of Potentilla glandulosa in California: showed both plasticity and genetic local adaptation

What pattern usually emerges?

Local populations typically have highest fitness in their native environment

How does skin pigmentation illustrate local adaptation?

High UV → darker skin protects folate (critical for reproduction); Low UV → lighter skin enhances vitamin D synthesis; No universal "best" phenotype; tradeoffs exist

What genetic evidence supports selection on pigmentation?

Pigmentation genes show stronger population differentiation and signatures of positive selection

Other examples of human local adaptation?

Disease resistance, height variation, lactose tolerance

Why is lactose tolerance prevalent in European & some African populations but rare in East Asia?

Gene-culture coevolution: selection favored persistence of lactase enzyme where dairy farming evolved