Cracking the AP Biology Exam

12

Evolution

SPECIES

A dog and a bumblebee obviously cannot come together to produce offspring. They are therefore different species. However, a poodle and a Great Dane could reproduce (at least in theory). We would not say that they are different species; they are merely different breeds.

Let’s get back to our moths. We said above that evolution occurred when they could no longer reproduce. In fact, this is simply the endpoint of that particular cycle of evolution: speciation. Speciation refers to the emergence of new species. The type of evolution that our peppered moths underwent is known as divergent evolution.

Divergent evolution results in closely related species with different behaviors and traits. As with our example, these species often originate from a common ancestor. More often than not, the “engine” of evolution is cataclysmic environmental change, such as pollution in the case of the moths. Geographical barriers, new stresses, disease, and dwindling resources are all factors in the process of evolution.

Convergent evolution is the process in which two unrelated and dissimilar species come to have similar (analogous) traits, often because they have been exposed to similar selective pressures. Examples of convergent evolution include aardvarks, anteaters, and pangolins. They all have strong, sharp claws and long snouts with sticky tongues to catch insects, yet they evolved from three completely different mammals.

There are two types of speciation: allopatric speciation and sympatric speciation. Allopatric speciation simply means that a population becomes separated from the rest of the species by a geographical barrier so that they can’t interbreed. An example would be a mountain that separates two populations of ants. In time, the two populations might evolve into different species. If, however, new species form without any geographic barrier, it is called sympatric speciation. This type of speciation is common in plants. Two species of plants may evolve in the same area without any geographic barrier.