Natural Selection at Work:
A case study
We can make a strong case that natural
selection is operating, even if the details of that selection
are not immediately apparent.
For example, on rocky shores,
animals have ranges that form clear spatial patterns. Some species live
only in deep water, and some only live much higher up the shore.
A snail common on California shores (Tegula funebralis, at right)
can be found in both ranges. In Southern California, Tegula live
high up on the shore, while in Northern California, they live
in deeper water.
Could natural selection explain this pattern? Michael Fawcett1 thought
so and formulated a hypothesis to test. He found that predators,
such as octopi, starfish, and crabs, were more abundant in southern
California than in northern California. Perhaps intense predation
in the south selected for snails that lived higher up the shore,
out of reach of many predators. In the north, selection might not
have been as strong, so the snails were not selected to live high
on the shore.
Fawcett tested this hypothesis by transplanting snails.
He took northern and southern snails, released them in deep water
and watched what happened. If predators were around, all the snails
high-tailed it towards higher ground (snails can probably sense
the chemicals exuded by predators). But southern snails moved further
up the shore faster than northern snails. Because the northern
snails were slower and didn't move high enough, they were more
likely to be eaten by predators.
What did this experiment show?
- There is an innate difference between southern and northern
snails (i.e., some difference that is not merely a function of
being on a southern or northern shore). This difference is probably
genetic (but we would need to do more experiments to be absolutely
- This difference can lead to differential survival. If predation
is intense, snails that move higher faster are more likely to
These results suggest that natural selection has occurred, altering
the predator escape trait. Remember, all you need is
These three features define natural selection. Without them, natural selection does not happen.
- Variation: There is variation in a trait between and
- Heredity: The variation probably has a genetic basis.
- Differential reproduction: The variants of the trait
have different probabilities of surviving to reproduction.