Species are very important in biology and in fact Darwin's book, the title is about species, ostensibly the book is about species, although actually Darwin really talked very little, if at all, about speciation. These are two different species of birds. This is a lazuli bunting and a indigo bunting and this is identified in your text. book is two species of birds that can hybridize.
We typically define species as organisms that interbreed, but that's not strictly true. Of course, horses and donkeys are considered to be different species and they can interbreed and they can produce a mule, although a mule is sterile. So under certain circumstances, even though different populations, different taxa can interbreed.
breed, we still consider them to be separate species. So species, and we've seen this slide before, species we have said are very difficult to define and and there isn't one species definition that we can all agree on, that every biologist agrees on, and that's applicable in every circumstances. It's very difficult to determine the boundaries between species in a given case. We may not be able to determine whether two organisms belong to the same species. And there are many, many different species concepts as we've discussed.
We've mentioned the biological species concept, for example. And we've discussed these particular criteria that members of the same species share a common descent. It's the smallest distinct grouping, the twig on the evolutionary tree. forms a reproductive community, at least among sexual species.
These are the organisms that reproduce with each other. A species forms a gene pool. They're genetically similar. They share genes.
Species form an ecological unit. They share a common niche in the ecosystem. Here on the left you see a raven, Corvus corvus. Korax and then two different species of crows.
And the two crows hybridize in a hybrid zone in Europe. And the two crows look very different. One of them is all black, the other one is black with white markings, and they're very distinct in most of their range.
But if you go in that hybrid zone, you can find lots of different, lots of variation between the two crows morphologically and genetically. We've mentioned the biological species concept and the endobiological species concept is widely used in biology. If you refer to the biological species concept or the BSC, biologists will know what you're talking about and Ernst Mayr was the originator of this.
We mentioned species are groups of interbreeding. natural populations that are reproductively isolated from other such groups. So, they are reproductively isolated. Reproductive isolation is what's important. And this, of course, depends on this being a sexually reproducing species.
It's difficult to apply this to species that reproduce strictly asexually. But there are a number of other species concepts in addition to the biological species concept, which you see on top. There's the evolutionary species concept. The phylogenetic species concept is very important.
It's defined in a couple of different ways. An irreducible basal cluster of organisms that is diagnosably distinct from other such clusters. And when with within which there is a parental pattern of ancestry and descent, or described in another way, it's the smallest monophyletic group of common ancestry.
It's basically defined as the end branch, in other words, of the evolutionary tree, the phylogenetic species concept. So the genealogical species concept, the recognition or cohesion species concepts, and I don't think we need to go into all of these, but you can you can read through these. Another species concept that I would like to is the genetic species concept and this was put forth actually by a couple of colleagues of mine at Texas Tech University. Robert Baker was my PhD advisor and Robert Bradley was a graduate student at the same time I was.
They define a genetic species as a group of genetically compatible interbreeding natural populations that is genetically isolated from other such groups. So this is very similar to the biological species concept, but instead of saying reproductively isolated, they are saying genetically isolated. This focus on genetic isolation rather than reproductive isolation distinguishes the genetic species concept from the biological species concept.
So the idea here in the genetic species concept is that we're looking at a gene pool. There is a pool of alleles that defines a species. Even though there may be some reproduction going on, there may be some hybridization, there may be some alleles that get into a hybrid zone, if the general gene pool remains distinct between the species, then these are considered to be two distinct species, even though there may be some reproduction going on.