VL 4 : The theory of natural selection Flashcards
What are the key processes influencing genetic diversity, and what is evolutionary fitness about?
The key processes are
* mutation,
* selection (natural and sexual), and
* genetic drift.
Evolutionary fitness involves an individual’s ability to survive and reproduce, which determines the number of offspring.
And sometimes there is a coflict between the two.
(Evolutionary fitness has nothing to do with speed or size.)
What are the two main components of fitness, and how can they conflict?
The two main components of fitness are survival and reproduction. Sometimes there is a conflict between these components, such as when traits that enhance survival may reduce reproductive success or vice versa.
What are the three classical types of selection, and what are their effects on phenotypes?
- Directional selection: Favors one extreme phenotype, shifting the population mean.
- Stabilizing selection: Favors intermediate phenotypes, reducing variation.
- Disruptive selection: Favors both extreme phenotypes, potentially leading to speciation.
How does selection act on alleles and phenotypes in the long run?
Selection acts on phenotypes, but only those influenced by genetic factors are subject to evolutionary change.
Over time, selection changes the frequency of different alleles, which in turn changes the frequency of phenotypes.
What are absolute fitness, relative fitness, mean population fitness, and marginal fitness?
- Absolute fitness: Total reproductive success of an individual/ a specific genotype. (largely irrelevant)
- Relative fitness: Performance of a genotype relative to others, with the highest value set to 1, all others < 1: relevant (denoted w11, w12, w22)
- Mean population fitness: Sum of genotype frequencies multiplied by their relative fitness, determining the direction of allele frequency changes. w’ = p^2w11 + 2pqw12 + q^2*w22
- Marginal fitness: average fitness of an allele in a population; depends on its fitness in a certain genotype and the genotype frequencies: interesting (as this determines an allele’s long term fate in a population); denoted wi*
w12: realtive fitness of genotype A1,A2
How is marginal fitness calculated for two alleles?
Where 𝑝 and 𝑞 are the allele frequencies, and
𝑤11, 𝑤12 and 𝑤22 are the fitness values of the genotypes.
Thus, if the marginal fitness of an allele is larger than the average population fitness, its frequency increases.
What is heterozygote advantage (overdominance), and what is its effect on allele frequencies?
Heterozygote advantage occurs when individuals with a heterozygous genotype have higher fitness than those with either homozygous genotype.
This advantage results in a stable equilibrium, where both alleles are maintained in the population, stabilizing allele frequencies and preserving genetic diversity.
What is underdominance, and how does it affect allele frequencies?
Underdominance means the heterozygote has the lowest fitness of all three genotypes.
-> This results in two stable equilibria (p=1 and q =1) where either one of the alleles can become fixed, depending on the starting allele frequencies and the relative fitness of the two homozygotes.
The equilibrium under underdominance is considered unrealistic because it is highly unstable (due to drift). This means that even a slight advantage in the frequency of one allele can cause the population to quickly shift entirely to that allele, eliminating the other.
How does directional selection influence allele frequencies and mean population fitness?
Directional selection favors one homozygote (either alone or with the heterozygote), leading to an increase in the advantageous allele until it becomes fixed.
Mean population fitness is maximized when the advantageous allele is fixed e.g. p=1
What is mutation-selection balance, and how does it explain the persistence of deleterious mutations?
Mutation-selection balance occurs when the rate at which deleterious mutations are introduced by mutation equals the rate at which they are removed by selection.
The equilibrium frequency of a deleterious mutation depends on its fitness effect (s) , mutation rate (μ) , and dominance (h).
Even though deleterious mutations reduce fitness and are subject to selection pressure, they can persist in a population for several reasons:
- Mutation-Selection Equilibrium: If the mutation rate is relatively high and the effects of selection are not strong enough to completely eliminate all deleterious mutations, a balance is reached where the population maintains a stable frequency of these mutations.
- Heterozygote Advantage: In some cases, individuals who are heterozygous for a mutation (carrying one normal and one mutated allele) may have a selective advantage over individuals who are homozygous for the mutation. This can lead to the persistence of the mutation in the population at a stable frequency.
- Environmental Variability: Environmental changes or fluctuations can affect the selective pressures acting on mutations. A mutation that is deleterious in one environment may be less so in another, allowing it to persist at low frequencies.
What is frequency-dependent selection, and how does it affect allele frequencies?
Frequency-dependent selection occurs when the fitness of a genotype depends on its frequency in the population. Negative frequency-dependent selection stabilizes allele frequencies by favoring rarer alleles, while positive frequency-dependent selection leads to unstable equilibria, favoring more common alleles.
e.g. flower without nectar and two flower colours -> negative frequency dependent
Warning colours, e.g. in Helioconus butterflies
-> positive frequency dependent
What is the significance of game theory in understanding evolutionary strategies?
Game theory models interactions between individuals with different strategies (e.g., hawks and doves). An evolutionary stable strategy (ESS) is one that cannot be invaded by an alternative strategy. The fitness payoffs determine which strategies are ESS, influencing allele frequencies and population dynamics.
What is the role of sexual selection in evolutionary fitness?
Sexual selection involves traits that increase mating success. Intrasexual selection involves competition within a sex (e.g., male-male competition), while intersexual selection involves mate choice (e.g., female choice). This can lead to exaggerated traits and behaviors, such as those seen in peacocks and birds of paradise.
Two explanations for sexual selection?
At least two explanations:
1. Co-evolution of male traits and female choice – choosy females transfer father’s traits to sons and choosiness to daughters: Fisherian runaway sexual selection
2. Signaling of “good genes” – only “good” males can produce exaggerated traits and survive:
handicap hypothesis
What is the Hawk-Dove game in game theory?
Hawks: Fight aggressively for resources, risking injury.
Doves: Share or flee to avoid fights.
Hawk vs. Hawk: They fight; each has a 50% chance of winning but also risks injury.
Hawk vs. Dove: Hawk always wins; Dove flees.
Dove vs. Dove: They share the resource equally.
If Hawks are common and the cost of injury (C) is high, Doves can do well by avoiding fights.
If Doves are common, Hawks do well because they face little resistance.
An Evolutionary Stable Strategy (ESS) is when neither strategy can invade the other successfully. For Hawks to be an ESS, the value of winning (V) must be greater than the cost of injury (C).
This model explains why both aggressive (Hawk) and non-aggressive (Dove) behaviors can coexist in a population.
