Bird guides once listed the myrtle warbler and the Audubon's warbler as distinct species. Recently, these birds have been classified as eastern and western forms of a single species, the yellow-rumped warbler. Which of the following pieces of evidence, if true, would be cause for this reclassification?
a) The two forms interbreed often in nature, and their offspring have good survival and reproduction
b) The two forms live in similar habitats
c) The two forms have many genes in common
d) The two forms have similar food requirements
e) The two forms are very similar in coloration
Going back to the definition of a species, a species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring - but do not produce viable, fertile offspring with members of other such groups.