Genetic drift Flashcards
‘The differential survival of genotypes drives evolution’. What does this say about alleles?
Frequency is key to diversity and population structure.
‘1-s’ is the selection coefficient. What does it say about a particular genotype?
That it will survive by proportion less well than others by proportion s.
Is it possible for an unfit genotype to survive purely down to luck?
Yes, ‘there are always expected deviations from fitness values’.
Loci can be either selectively advantageous, disadvantageous or neutral. When they are neutral, what changes in allelic frequency do we expect to see?
No changes between generations.
Define genetic drift.
The chance disappearance of alleles in a small population caused when individuals die or fail to reproduce.
Genetic drift drives evolution by creating variation. True or false?
True.
Why does genetic drift have a bigger effect in small populations?
It leads to a loss of diversity or a fixation of genotypes.
Consider a single, self-fertilising organism, the smallest population size possible. Explain why small population size leads to a loss of diversity.
- If the organism is a heterozygote, it will produce 2 different types of gamete.
- When the gametes come together, 1/2 will be heterozygous, 1/4 will be homozygous dominant and homozygous recessive.
- Thus in one generation, 1/2 of all offspring (the homozygotes) have already lost an allele.
If there is no variation there is nothing for selection to act on. True or false?
True.
What happens when genetic drift acts on large populations? How does this occur?
It creates divergence which leads to speciation. Isolated sub-sets of an original population are all exposed to different environments, meaning selection acts on them independently and produces different results.
What do genetic drift models assume about mating?
It is random.
What 2 factors increase genetic drift in a population?
- Small population size
2. Fewer individuals contributing to reproduction, i.e. species in which males have harems so not all males get to mate
What 2 factors reduce drift in a population?
- Large population size
2. Monogamy, every male gets to mate with a female
Is it important to understand the mating systems of organisms when calculating genetic drift?
Yes.
What is Ne?
The ‘effective population size’ coefficient. It refers to the number of organisms in a population that are reproductively active.
Ne values are an estimate and can be misleading. Why?
Because the Ne values for a small, monogamous population might be the same as a large, random-mating population. Both monogamy and large size reduce drift but small size and random-mating increase it.
What is genetic identity?
The probability that alleles are identical.
What is ‘F’?
The inbreeding coefficient, it is a measure of homozygosity WITHIN an individual.
What is ‘F’ used to assess?
‘Cosanguity’ or the degree of inbreeding in an individual.
What is ‘θ’?
A measure of coancestry between individuals.
What is ‘α’?
A measure of coancestry between populations.
Fixation indices are used to compare heterozygosity. What is F(IS)?
The fixation indice WITHIN a population. F(IS) gives information about inbreeding and assortative mating.
How do you calculate F(IS)?
F(IS) = [(1- θ) – (1 – F)]/(1- θ)
Fixation indices are used to compare heterozygosity. What is F(ST)?
The fixation indice BETWEEN populations. F(ST) gives looks at whether there has been differentiation from an orginal population.
What does it mean if F(IS) > 0?
Homozygosity is greater than expected, there has been inbreeding or assortative mating.
What is meant by assortative mating?
When individuals preferentially mate with others of a particular genotype.
How do you calculate F(ST)?
F(ST) = [(1- α) – (1 – θ )]/(1- α)
What does it mean if F(ST) > 0?
There has been differentiation from a single, original population.
What does ‘1 - F’ mean?
Observed heterozygosity.
What does ‘1 – θ ‘ mean?
The average expected heterozygosity within a population.
What does ‘1 – α’ mean?
The average expected heterozygosity between populations.
What can we look at to observe variation within populations? Give 3 examples
- SNPs
- Genetic markers
- Mutations in microsatellite regions
How can the principles of genetic drift be used to track migration?
When a sub-set of the original population leaves and migrates to a new location, diversity is greatly reduced as they carry a low percentage of the total alleles. With each migration there is a reduction in diversity. The least genetically diverse populations are the most recent ones.
