Questions: Sexual Selection as a Driver of Speciation

Question 1

Two populations of cichlids occupy the same lake. Females in population A prefer red males; females in population B prefer blue males. Over generations, males in each group evolve to match their local preference. Which speciation mechanism does this best illustrate?

• A. Allopatric speciation, because the two preference-based groups occupy subtly different microhabitats within the lake
• B. Sympatric speciation through sexual selection, where divergent mate preferences create prezygotic isolation without geographic separation
• C. Parapatric speciation, where selection pressures differ across a continuous habitat gradient
• D. Postzygotic isolation, because hybrid offspring between red-preferring and blue-preferring lineages have reduced fitness

Answer: (B)

Both populations share the same habitat (sympatric). The reproductive barrier is prezygotic: females simply will not mate with males whose coloration doesn't match their preference, preventing gene flow without producing hybrids at all. No geographic barrier is required — the divergence in mate preference itself is the isolating mechanism. This is exactly the mechanism documented in African Great Lakes cichlids.

Question 2

Why is sexual selection predicted to produce speciation faster than most forms of ecological natural selection?

• A. Because sexually selected traits have higher heritability than ecologically adaptive traits
• B. Because sexual selection operates on the traits that directly control who mates with whom, so any divergence immediately reduces gene flow between populations
• C. Because sexual selection only acts in large populations, making fixation of new variants faster
• D. Because female preference divergence requires fewer mutations than ecological niche differentiation

Answer: (B)

Speed advantage is mechanistic: sexual selection targets courtship signals, coloration, and songs — the very traits that determine whether two individuals interbreed. Any divergence in these traits directly reduces interbreeding between populations. Ecological speciation diverges traits related to survival (beak shape, metabolism), which only indirectly affect mating through habitat preference or timing — adding extra steps before reproductive isolation emerges.

Question 3

Sexual selection can primarily produce reproductive isolation between populations that are geographically separated.

• T. True
• F. False

Answer: False

Geographic separation is necessary for allopatric speciation but not for speciation driven by sexual selection. If female preferences diverge between subpopulations living in the same habitat — through genetic drift, differential sensory environments, or slight ecological differences — assortative mating can emerge without any physical barrier. This is the core claim of sexual selection as a sympatric speciation driver.

Question 4

The cichlid radiation in African Great Lakes supports sexual selection as a speciation driver because cichlid species primarily differ in ecologically important traits like body size and feeding morphology.

• T. True
• F. False

Answer: False

The cichlid evidence specifically supports sexual selection because many species differ primarily in male coloration and female color preference — traits directly related to mating, not ecological function. The critical experiment: when researchers manipulated light conditions to make color differences imperceptible, females mated indiscriminately across species. This confirmed that the reproductive barrier was visual mate choice alone, not ecological divergence. If body size and feeding morphology were the primary difference, disrupting color vision would not collapse species boundaries.

Question 5

How does the genetic coevolution between male traits and female preferences create a positive feedback loop that can accelerate population divergence?

Model answer: When females prefer a particular male trait, males with that trait mate more often and pass on both the trait and, through genetic correlation, the preference itself to their offspring. As preference and trait become genetically linked within a population, the preference drives the trait to more extreme expression, and extreme expression reinforces the preference — a runaway dynamic. If two subpopulations start with slightly different preferences, this runaway process amplifies the divergence rapidly in each direction. Each population evolves toward a different extreme, making interbreeding increasingly unlikely as the two lineages diverge.

The key is the genetic correlation: sons inherit the trait, daughters inherit the preference, and these are not independent. This linkage means sexual selection can act faster than standard directional selection — the preference itself evolves, not just the trait being preferred.