The (probably outdated) definition of a species that I learned was that two creatures were considered to be in the same species if they could naturally interbreed and produce fertile young. So, a beagle and a great dane are both in the same species, for example.
My question is, how does this work for asexual creatures? As far as I know, ameobas, for instance, only reproduce by splitting in half…there’s no breeding involved at all. But, ameobas are a phylum and divided into species. There’s Amoeba proteus, Pelomyxa palustris, etc. What does it mean when we say “these two amoebas are of a different species”? Or is my definition of species just hopelessly wrong and inaccurate?
First, a small correction about the definition of species. According to the Biological Species Concept, two populations belong to different species if they do not normally produce fertile hybrids in nature. Many species are considered to be valid, even though they are capable of producing completely fertile offspring in captivity, just as long as this does not happen often under natural conditions. So that just because two organisms can interbreed doesn’t automatically make them members of the same species.
The Biological Species Concept cannot be applied to organisms that do not reproduce sexually (or in a number of other situations I won’t go into here). Other concepts must be used for such organisms. Usually some kind of morphological Species Concept is used (which was a concept frequently used for other organisms before the BSC was developed in the 1930s.) Groups of organisms are defined as species on the basis of how similar they are in appearance to other organisms. Usually there are morphological gaps between different lineages of asexually reproducing organisms that allow “species” to be described and defined. Nowadays, genetic data is allowing biologists to get a handle on how closely such lineages are actually related, and allow species to be defined on this basis.
Some of us would say that this “quandry” illustrates that the BS concept has been overemphasized (and how is it more biological than any other, anyway?). Sexuality is not fundamental to life, and most life has been approximately asexual for most of the history of life on earth.
What Colibri said. When I was back getting my microbiology degree, I was told that two bacteria are said to be in the same species if at least 80% of their DNA was identical. It’s a basically arbitrary cutoff point. Of course, one could argue that the concept of “species” is completely artificial to begin with, but that’s another thread.