Population Genetics and Natural Selection Flashcards
What is evolution and what factors can cause evolutionary change?
Evolution is the change in the form and/or behavior of organisms between generations, also known as descent with modification. Evolutionary change can occur due to various factors including chance bursts of reproduction, deaths, and mutation.
What are the three main conditions required for evolution by natural selection to occur?
Variance: Individuals must vary in some trait.
Selection: Individuals with certain trait values are more likely to survive and reproduce.
Heritability: Parents pass on similar trait values to their offspring.
In a haploid organism, how do allele frequencies change under natural selection?
In a haploid organism, the allele frequencies (e.g., for alleles A and a) are denoted by p[t] and q[t], where p[t] + q[t] = 1. If alleles have different fitness levels (WA for allele A, Wa for allele a), the frequency of alleles will change over time based on their fitness. The allele with higher fitness increases in frequency.
How is fitness defined in the context of natural selection, and how does it affect allele frequencies?
Fitness is defined as the average contribution of an individual to the next generation, including survival and reproduction. It affects allele frequencies by increasing the frequency of alleles with higher fitness over generations.
What happens to allele frequencies in a population without selection?
Without selection, allele frequencies remain constant over generations. The combination and separation of alleles during fertilization and meiosis are random, and there is no change in allele frequencies across generations.
How does evolution by natural selection differ in sexual versus asexual reproduction?
In asexual reproduction, allele frequencies follow a similar equation to sexual reproduction, provided there is no selection in the diploid phase. Evolution still depends on the relative fitness of alleles, and selection can lead to changes in allele frequencies.
What is the “selection coefficient” (s) and how does it influence the spread of a beneficial allele?
The selection coefficient (s) is the proportional increase in fitness caused by replacing allele a with allele A. It measures how much more frequent the beneficial allele becomes compared to the wildtype. A larger selection coefficient leads to faster spread of the beneficial allele.
How does the mean fitness of a population change due to natural selection?
The mean fitness of a population tends to rise (or at least stay the same) as natural selection favors alleles with higher fitness, causing those alleles to increase in frequency. Over time, the average fitness of the population improves.
What are some processes that lead to evolutionary change but are not considered natural selection?
Evolutionary change can also result from:
Mutations
Random chance (genetic drift)
Sex and recombination
Pleiotropy (where one gene affects multiple traits)
Hitchhiking (where neighboring alleles are carried along with selected alleles)
Describe how the spread of a beneficial allele follows an S-shaped curve.
The spread of a beneficial allele follows an S-shaped curve, increasing slowly when the allele is rare (low frequency), then accelerating as it becomes more common, and finally slowing again as it approaches fixation (high frequency). This happens because selection is weaker at the extremes (when the allele is very rare or very common) and stronger in the middle.
What happens to an allele if its fitness is equal, greater, or less than another allele?
If WA = Wa, the frequency of allele A remains constant (neutral).
If WA > Wa, allele A increases in frequency and moves toward fixation (p = 1).
If WA < Wa, allele A decreases in frequency and moves toward loss (p = 0).
How long does it typically take for a beneficial allele to spread through a population, based on the selection coefficient?
For a beneficial allele to rise from low to high frequency:
It takes ~100 generations if s = 0.1
It takes ~1,000 generations if s = 0.01
It takes ~10,000 generations if s = 0.001
As a general rule, if the selection coefficient is 10 times smaller, it takes ~10 times longer to observe the same frequency change.
