Question

In some insect species, males are much larger than females. Which one or more of the following evolutionary hypotheses can explain such male-biased size dimorphism?

• A. Body size provides a competitive advantage to females, but not to males
• B. Body size provides a competitive advantage to males, but not to females
• C. Clutch size is positively correlated with body size in females
• D. The mating success of males and females is not related to body size

Hint:

Ask which hypothesis gives specifically males, not females, a reason to grow larger through competition or mate choice.

Answer 1

The Correct Option is B

Male-biased size dimorphism means males have evolved to be bigger than females. Any hypothesis that explains this pattern needs to give males, specifically, a reason to grow larger, usually through some kind of advantage that only applies to male-male competition or mate acquisition.

1. Body size provides a competitive advantage to females, but not to males: If bigger size only helped females compete, selection would push females to grow larger, producing female-biased dimorphism, the opposite of what is described here. This does not explain male-biased dimorphism.
2. Body size provides a competitive advantage to males, but not to females: If larger males win more fights over mates or territories, or are more attractive to females, while size gives females no similar edge, selection favors bigger and bigger males generation after generation, without pushing female size up at the same rate. This produces exactly the male-biased pattern described in the question.
3. Clutch size is positively correlated with body size in females: This is a classic explanation for female-biased dimorphism, since bigger females can carry and lay more eggs, so selection favors larger females. It predicts females growing bigger than males, not the reverse, so it does not fit here.
4. The mating success of males and females is not related to body size: If body size had no bearing on mating success for either sex, there would be no selective pressure pushing either sex to a different size, and no size dimorphism would be expected to evolve at all. This cannot explain the pattern in the question either.

Only the second hypothesis, males gaining a competitive edge from being larger while females gain none, actually predicts males evolving to be bigger than females, matching the male-biased dimorphism described.

Let's summarize:

• Male-biased dimorphism needs a size advantage that applies specifically to males, usually through competition for mates.
• Fecundity-linked size advantages in females, or no size advantage to anyone, both predict the wrong pattern or no dimorphism at all.

So the correct hypothesis is that body size provides a competitive advantage to males but not to females.