Genetics - evolution

What are Darwin’s 4 postulates of evolution through natural selection?

• Individuals within species are variable

• Some variations are passed on to offspring

• In most generations, more offspring are produced than can survive

• Survival and reproduction are not random

How is Neo-Darwinian (Modern Synthesis) evolution different to classic Darwinian evolution?

• Understanding of mechanisms of inheritance combined with Darwin’s concept of natural selection

• Evolution is considered in terms of changes in allele and gene frequencies over time and average action of selection on genotypes

How does genotype-phenotype variation occur?

• Genotypic variation promotes phenotypic variation

• Selection will operate to remove less fit variants or increase the abundance of variants with greater fitness

How do rare alleles manage to invade populations?

• New alleles are constantly created at a rate by mutation

• New mutations are initially rare and most are recessive

• Rare, recessive alleles are mostly present in heterozygotes which makes selection against them negligible

• Rare alleles tend to persist

• Dominant alleles are always expressed and so if frequent selection against them will remove recessive alleles

• Frequency of rare, recessive alleles in the population is set by mutation rates

How can genetic drift change allele frequencies?

• Chance difference in transmission of alleles, leading to fluctuations in allele frequency

• Most strongly affect rare alleles

• Has a greater influence on rare alleles than selection

• Primary mechanism for increasing rare recessive allele frequencies

• Responsible for changing frequencies

What two types of selection can select for rare alleles?

• Frequency dependent selection

• Fluctuating selection

How does frequency dependent selection select for rare alleles?

• Colouration in snails may determine predation

• Predators develop a search image - ‘recognise’ prey

• Rare allele producing unusual phenotypes may be predated on less

• Alleles can become common and predators learn to recognise them

• Alleles only have an advantage when rare

How can fluctuating selection select for rare alleles?

• In unstable and changeable environments

• Selection may favour opposing phenotypes/alleles over short timescales

• Caused different beak sizes in different years for African fire birds depending on weather

• Host-pathogen arms race

• Selection may vary in space - heterogenous environments can favour different alleles

What is fitness used for?

Measure reproductive success of a given genotype relative to the optimum genotype

What are the two components of fitness?

• Survivorship

• Fecundity

How do you calculate fitness?

W = 1 - s

• W - fitness

• s - selection

How does selection affect fitness?

• Determines how fast allele frequencies change

• Allele frequencies change rapidly when s is high

• Can calculate the selection coefficient in wild populations

• Conduct mark and recapture experiments in different environments to calculate survivorship

• Can use the fitness calculation to work out the fitness and selection coefficient for either form

What are the three modes of selection on quantitative traits?

• Stabilising selection

• Directional selection

• Disruptive selection

How does stabilising selection change quantitative trait frequencies?

• Intermediate variants are selected for

• Reduces variance of a trait

• Arguably the most common mode of selection

• Eg birth weight in humans

How does directional selection change quantitative trait frequencies?

• Individuals at one extreme are selected for

• Shifts the mean value of a trait

• Tends to be associated with changing environments

• Eg colouration change in peppered moths

How does disruptive selection change the frequencies of quantitative traits?

• Individuals at both extremes are selected for

• Leads to a bimodal distribution

• Associated with sympatric speciation (within a population)

• Eg African fire birds

How does selection between individuals in the same generation change quantitative traits?

• Competition for mates

• Maximises reproductive success of the species

• Even at a potential cost to survival

How does kin selection change quantitative traits?

• Refers to changes in gene frequency across generations driven by interactions between related individuals

• Helping relatives (even at a cost to yourself) you may promote the success and transmission of genes that are commonly shared

What are the two measures of relatedness?

• Coefficient of relatedness - r = (1/2)^n

• n - connection removed from self

• Hamilton’s rule - rB > C

• r - genetic relatedness to the recipient of the altruistic act

• B - additional reproductive benefit gained by the recipient of the altruistic act

• C - reproductive cost to the individual performing the act

What are the two types of speciation possible in a population?

• Allopatric - separated geographically so evolved separately

• Sympatric - separated genetically so evolved separately

What types of isolation reinforce speciation?

• Pre-mating isolation - behavioural/spatial/temporal/mechanical

• Post-zygotic isolation - sterility/unviability