When the interactions change

4jtJABCi As you think about co-evolution, keep in mind that these interactions aren't always completely fixed in nature. The type of co-evolution between species can change over time. Bees, for example, need nectar; flowers need pollinators. The two species co-evolve in a mutualism: Their interaction is mutually beneficial because when the bee takes the nectar, the flower gets pollinated.

Now imagine a mutation in bees that results in their boring holes in the side of flowers and sucking up the nectar. The system has gone from being mutually beneficial to being parasitic: One species benefits to the detriment of the

The pronghorn antelope

Evolution has an endless number of fascinating stories, and here's another one for your reading pleasure. The fastest land mammal in North America is the pronghorn antelope, which can outrun by a substantial margin absolutely everything that it might ever come across. Why on earth does it run so fast? How could natural selection have caused that speed if nothing is chasing this antelope?

Well, as it turns out, even though no cheetahs exist in North America today, plenty of cheetah fossils turn up on the continent. I can't say for sure that those fossil cheetahs used to chase pronghorns, but they did chase something, and they ran very fast (info gleaned from their bone structure being similar to that of modern cheetahs in Africa).

other. The flowers' nectar is gone, and no pollination has taken place because the bees aren't coming anywhere near where the pollen is or needs to be.

In response, natural selection will favor plants that keep the bees from getting the nectar. Selection may make the flower harder to chew through, in which case plants that happen to have thicker flowers are going to leave more descendents because they still have some nectar left in them to attract other pollinators. The process is still co-evolution, but a transition from one type of interaction to another has occurred.

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