Questions: Natural Selection: Types and Contemporary Examples

Question 1

Human birth weight data show that very small babies have high infant mortality from underdevelopment, and very large babies have high mortality from delivery complications. Intermediate-weight babies have the highest survival. This is a classic example of which type of selection?

• A. Directional selection — survival is highest at one end of the distribution
• B. Stabilizing selection — selection against both extremes maintains an intermediate optimum
• C. Disruptive selection — two distinct weight classes are favored over intermediates
• D. Sexual selection — reproductive success is determined by mate choice rather than survival

Answer: (B)

Stabilizing selection is defined by highest fitness at intermediate trait values with selection against both extremes. The birth weight example fits perfectly: too small (underdevelopment) and too large (delivery complications) are both disadvantaged, while intermediate weights are selectively favored. The population mean doesn't shift, but variance is reduced as the tails of the distribution are culled. This is the most common form of selection in nature — it explains why many traits remain stable for long periods rather than continuously shifting.

Question 2

In a bird population, seeds come in only two sizes: very small and very large. Small-beaked birds efficiently crack small seeds, large-beaked birds crack large seeds, but intermediate-beaked birds struggle with both. Over generations, what do you predict will happen to beak size distribution in this population?

• A. The mean beak size will shift toward larger beaks, driven by directional selection
• B. Beak size variance will decrease as stabilizing selection maintains an intermediate optimum
• C. Beak size distribution will become bimodal, with both small and large beaks increasing in frequency
• D. Beak size will not change because the population is already adapted to the food environment

Answer: (C)

This describes disruptive selection: fitness is highest at both extremes and lowest at intermediate trait values. When food availability rewards specialization over generalism, both extreme morphs are favored over intermediates. Over generations, the frequency of intermediate-beaked birds decreases while both small-beaked and large-beaked morphs increase — producing a bimodal distribution. This is the signature outcome of disruptive selection, and it can eventually (especially combined with assortative mating) lead to population divergence and speciation.

Question 3

Stabilizing selection shifts the mean of a trait toward a new optimum while reducing phenotypic variance.

• T. True
• F. False

Answer: False

This confuses stabilizing and directional selection. Stabilizing selection maintains the existing mean by removing individuals at both extremes — it reduces variance without shifting the mean. Directional selection is what shifts the mean, by consistently favoring one extreme. Under stabilizing selection, the population 'stays put' at a well-adapted intermediate value. A common implication is that when we observe stable trait means over many generations, stabilizing selection may be the explanation — the trait is already at its fitness optimum.

Question 4

A population could simultaneously experience directional selection on one trait (e.g., immune gene variants under pathogen pressure) and stabilizing selection on another trait (e.g., body size near an optimal thermal range) at the same time.

• T. True
• F. False

Answer: True

Selection acts independently on different traits, so multiple selection regimes can operate simultaneously in the same population. Each trait has its own fitness landscape, and the shape of selection on each is determined by the ecological pressures relevant to that trait. Industrial melanism affected wing coloration but not body size; antibiotic resistance affects specific biochemical targets but not pigmentation. Understanding that selection operates trait-by-trait is essential for predicting evolutionary responses to complex, multi-dimensional environments.

Question 5

How does disruptive selection differ from directional selection in both mechanism and potential long-term evolutionary consequence?

Model answer: Directional selection favors one extreme of the trait distribution, shifting the population mean toward that extreme over time. Disruptive selection favors both extremes simultaneously while eliminating intermediates, increasing trait variance and potentially creating a bimodal distribution. The long-term consequences differ critically: directional selection produces adaptation toward a new optimum but maintains a unimodal population; disruptive selection can increase polymorphism and, if combined with assortative mating (individuals preferentially mating with similar morphs), can lead to reproductive isolation and ultimately sympatric speciation. Disruptive selection is therefore one of the mechanisms by which a single population can diverge into two distinct lineages without geographic separation.

The African seedcracker finch is a well-documented case: large-billed and small-billed morphs coexist while intermediate birds have significantly lower survival. This illustrates how disruptive selection maintains a polymorphism in a real population. The potential for speciation is what makes disruptive selection evolutionarily consequential despite being the rarest of the three modes.