Often, species are identified by their appearance. Why?

Question

A. If two organisms look alike, they must be the same species.

B. This is the criterion used to define a biological species.

C. If two organisms look different, they must be different species.

D. This is the most convenient way of identifying species.

E. Most eukaryotic organisms reproduce only asexually.

Answer 1

Classification on the basis of appearance is only the first step. Taxonomers then use cladistics, numerical phenetics, and genetics, to confirm or reject their initial inference.

The ultimate test of a species, however, is whether or not it is reproductively isolated. If organisms can reproduce with each other over multiple generations, then those organisms are the same species. This rule, naturally, cannot be applied to organisms that reproduce asexually, but such organisms basically produce clones of themselves.