Why Sex Is Good by Clyde Freeman Herreid

Part III—“Sex and Stress”

There is a snail that lives in New Zealand lakes that has both asexual and sexual individuals. Curtis Lively (currently at Indiana University) and his colleagues decided that the snails could be used to test the hypothesis that a changing or stressful environment would favor sexual reproduction—the logic being that if the environment changes, then variation (sexual reproduction) is a good thing; some of your offspring might have the right genetic constitution to survive.

Here’s the situation the biologists found. The snails live in freshwater habitats and there are over a dozen worm parasites that attack them. The scientists reasoned that there might be a difference in the fitness of the asexual and sexual individuals in ponds where there were different degrees of parasitism.

This is what they found: in ponds where there was a high degree of parasitism there was a much higher percentage (2.5 times more) of sexually reproducing individuals.

Questions

Before carrying out the experiment, why did the scientists expect there would be a difference in fitness between sexual and asexual snails in ponds with different degrees of parasitism?
Are the data they obtained consistent with Weismann’s hypothesis? Explain your thinking.

Go to Part IV—“An Experiment”

Originally published at http://www.sciencecases.org/birds_and_bees/birds_and_bees3.asp