Modes of Selection on Quantitative Traits

Flashcards covering directional, stabilizing, and disruptive modes of selection on quantitative traits, including selection differential, response to selection, and biological examples.

What relationship does Figure 8.7 illustrate between selection differential ($S$), narrow-sense heritability, and response to selection ($R$)?

It shows that the response to selection ($R$) is proportional to the product of the selection differential ($S$) and narrow-sense heritability, represented by the slope of the best-fit line.

What defines directional selection and what are its key impacts on a population's trait distribution?

Under directional selection, fitness continuously increases or decreases with trait value, giving extreme phenotypes at one end a fitness advantage. This changes the population's average trait value and slightly reduces trait variation across generations.

What shape can fitness functions under directional selection take in practice besides a linear relationship?

Nonlinear relationships, such as accelerating or decelerating curves, can also describe fitness variation under directional selection.

Which mode of selection on quantitative traits is illustrated by the linear fitness function and shifted trait mean shown in this figure?

Directional selection, where individuals at one end of the phenotypic spectrum have a fitness advantage, resulting in a higher (or lower) trait mean and slightly reduced variation after selection.

What are two specific examples of directional selection provided in the text?

1. The change in beak size of Darwin's finches during drought. 2. The decrease in eye size in some cave fish populations.

What defines stabilizing selection and how does it affect a population's mean trait value and trait variation?

Under stabilizing selection, intermediate traits have the highest fitness while extreme phenotypes are counter-selected. It does not change the average trait value in a population, but it reduces trait variation across generations.

Which mode of selection is represented by this hump-shaped fitness function and reduced trait variation?

Stabilizing selection, where intermediate individuals exhibit a fitness advantage, leaving the population trait average unchanged while decreasing trait variation across generations.

Why is stabilizing selection believed to be common for most traits in natural populations?

Once a trait reaches an optimum in a population, selection keeps the trait at that optimum by removing variants that deviate from it.

What defines disruptive selection and how does it alter trait mean and trait variation?

Under disruptive selection, intermediate individuals have a fitness disadvantage compared to extreme phenotypes at either end of the spectrum. It does not change the average trait value, but it increases trait variation across generations.

Which mode of selection on quantitative traits is characterized by this U-shaped fitness function?

Disruptive selection, where intermediate individuals have a selective disadvantage, increasing trait variation across generations while keeping the mean trait value constant.

What distribution pattern can form if disruptive selection persists over multiple generations, and what role does it play in speciation?

If disruptive selection persists for multiple generations, populations can exhibit bimodal trait distributions. Disruptive selection can also cause speciation, such as the stark beak size differences among species of Darwin's finches.

What two examples of intraspecific polymorphisms resulting from disruptive selection are given in the text?

1. Light and dark forms of the peppered moth camouflaged on light or dark backgrounds. 2. Different beak morphs in seedcrackers exploiting different food resources.