Bio 1B: Lec 3: Evolutionary Mechanisms Beyond Selection

How do you measure evolution?

Evolution can be measured as the change in genetic composition of a population

So if ALLELE FREQUENCIES CHANGE OVER GENERATIONS, then it is showing there is evolution

Allele frequency:

Proportion of all gene copies that are a particular allele

If there is no evidence of selection, but allele frequency of A changed from 0.5 to 0.65 over five generations, was there evolution?

Yes, allele frequencies changed

What are the four mechanisms of evolution (that are not natural selection)?

Mutation: adds new alleles, usually changes frequency slowly in one generation

Drift: random sampling, strongest in small populations, often loses variation

Gene Flow: moves alleles between populations, usually reduces differences between populations

Non-random mating: changes genotype frequencies, can alter variation and selection exposure

What type of mutation affect evolution?

Heritable mutations affect evolution, skin cancer you got from the sun does not countHow

How does mutation affect evolution?

Mutations add new alleles to the population through errors in DNA replication

How do mutation rates change allele frequencies?

They change the allele frequencies slowly over one generation, they mostly just introduce genetic variation

High mutation rates and shorter generation times causes most genetic variation change

What is genetic drift?

Survival of the luckiest: change in allele frequency due to a random event

Tends to reduce genetic variation over time

What does an allele being “fixed” mean?

It means that all individuals have this allele, can happen if by chance someone kills all of the yellow beetles in the playground by accidentally falling on all of them, so all beetles in this area will only have green alleles

In a graph, how to tell which allele-frequency line looks most affected by drift?

The one that flat-lines at the end, indicating fixed alleles

How do you know if an allele-frequency line isn’t fixing due to natural selection?

If it is erratic, it indicates there is no adaptive filter

Natural selection should lead to a trend

Genotype Frequencies

the fraction of individuals with a given genotype

Phenotype

Observable features

Population

group of organisms of the same species that are found in the same area and can interbreed

Allele frequency

how frequently a particular allele appears in a population

Allele

A version of a gene

Gene Pool

Set of genes in a population

Bottleneck

A type of drift where population size drops sharply (like half the population falls off a cliff)

Founder Event

a small sample of the ancestral population starts a new population (NOT GENE FLOW bc going into an area that does NOT have the same species there, no gene mixing)

When a population that gets sliced in half is recovering in numbers, why might it have low genetic variation?

Gene pool is now limited to the survivors

What is gene flow?

Migration between two populations, leading to the movement of alleles

What is non-random mating?

Random mating: genotype frequencies follow allele frequencies in predictable proportions, so no restrictions or preferences, following generations have similar genes

Non-random mating: individuals mate based on relatedness, location, phenotype, or behavior (like how humans are usually attracted to more attractive individuals)

Why does non-random mating usually change genotype frequencies?

The three types of non-random mating are:

1. Inbreeding: more homozygotes than expected

2. Assortative Mating: similar phenotypes mate more often

3. Mate Choice: some phenotypes get more mating opportunities

Basically, all of these lead to the same type of category breeding more, leading to that phenotype being picked more often, leading to genotype frequency change

When does non-random mating occur

When the probability that two individuals in a population will mate is not equal for all possible pairs

Mate choice can create sexual selection if some genotypes leave more offspring

Inbreeding

Individuals mate with close relatives more frequently that expected by chance, INCREASING HOMOZYGOSITY (having identical pairs of genes for a specific trait)

Assortative Mating

individuals choose mates based on phenotypic traits (how they look or act), which can maintain differences among phenotypic groups

What does each mechanism of evolution do to genetic variation?

Mutation: adds new alleles, increases potential variation

Drift: samples og population, random frequency change, usually decreases variation

Gene flow: moves alleles back and forth, increases variation WITHIN a population, decreases variation BETWEEN the two populations

Non-random mating: rearranges genotypes, changes heterozygosity