Ecology & evolution - population genetics

What did population genetics begin with

Taking Mendel’s ideas of heredity (and ‘genes’) and looking at them at a population level within an evolutionary framework

What was one of the first models created in population genetics, use and assumptions under it

The Hardy-Weinberg equation which calculated how the proportions of alleles change throughout a population.

Assumptions: infinite population, random mating, no gene flow, no difference in viability etc.)

Useful as a ‘neutral’ model, when nothing else is going on → very accurate for blood groups, as they are not (usually) a factor when humans are choosing who to reproduce with

Hardy-Weinberg equation

p = proportion of allele A

q = proportion of allele a

p + q = 1

p² + 2pq + q² = 1

What is genetic drift

Changes in allele frequency due to chance (in small populations)

Occurs because populations aren’t infinite and many organisms are killed randomly before reproduction - leading to loss of genetic variation

Strength of genetic drift decreases as population size grows

Hardy-Weinberg equation for average fitness of the population

W = p²(1+s) + 2pq(1+hs) + q²(1)

fitness (W) = reproductive rate of individual

selection coefficient (s) = change in fitness associated with homozygous allele

dominance (h) = extent at which effect of allele on fitness is masked when heterozygous

Hardy-Weinberg equation for the frequency of a specific allele in subsequent generations

p’ = [p²(1 + s)]/W + [pq(1 + hs)]/W

Equation can continue to be iterated

What can population genetics models determine the effect of

Dominance, co-dominance, and recessive interactions

E.g. dominance accelerates adaptation

What is a ‘selective sweep’

A new beneficial allele enters a population, and increases fitness so much that its frequency rapidly increases to 1 and becomes ‘fixed’

What is an example of natural selection studied caused by division of population into two habitats

Loss of body armour in sticklebacks.

Ice age in British Colombia divided sticklebacks into freshwater and marine habitats. Led to the loss of body armour in freshwater sticklebacks, as in this environment that was a selective advantage.

The low body armour allele was a rare deleterious allele that became advantageous in freshwater and was selected for.

What process prevents divergence in large populations split into smaller, localised groups

Gene flow - migration between the small populations

What processes increase genetic diversity

Migration

Mutation

What processes decrease genetic diversity

Selection (deterministic loss of low fitness alleles)

Genetic drift (stochastic loss of diversity by chance)

Features of mutation

Key for variation but quite rare

Most are deleterious and lead to loss of fitness, only some are ‘novel beneficial alleles’ that lead to evolution

Arise from mistakes in DNA replication, can be substitutions, indels, etc.

How are deleterious alleles often hidden in populations

Hidden by heterozygosity, and therefore selection cannot act against them

Method of studying novel, beneficial mutations as they are so rare

Study bacteria and viruses that evolve rapidly - higher frequency of mutations

Short generation times, big population sizes, small genomes, can be cryogenically frozen, fitness can be measured

How to measure / study fitness in bacteria

Genetically tagging them with fluorescent proteins

Can observe fitness evolving in experiments, can also look at evolved and ancestral bacteria competing against one another

What is the problem with sexual reproduction in terms of fitness

Leads to half as much genetic material passed to offspring as asexual reproduction

Why is sex so common as a method of reproduction

Recombination in sexual reproduction allows for genetic variation.

Can accelerate evolution by bringing together beneficial mutations, and prevents beneficial mutations competing against one another

Experiment showing the benefits of sexual reproduction

Experiment done with yeast. Sexual reproduction increased total fitness much more quickly than asexual reproduction.

Led to lots more diversity and polymorphisms.