Sounds easy enough, right? Obviously a penguin is extremely different from, say, a dragonfly. There's no way that you could have a half penguin, half dragonfly animal. Not without some really weird Frankenstein-style 'science,' anyway. And even then, probably not.
But what about two different kinds of penguin? Or a mammoth and a mastodon? Dog and wolf? Or even two different types of seagulls?
And so it gets progressively complicated as you get into closer and closer cousins.
This actually started to really bother me within the past school year. In my ornithology class, we were out identifying birds at one of NC's beaches, and I really began to struggle with the multiple types of seagulls. Referring to my birding book, I came across a page talking about two of the different types having a hybrid with slightly different markings. Not a hybrid like a mule, the mix of a horse and a donkey that can't reproduce, but a hybrid that is a fully functioning animal with slightly different markings. When I asked my professor about this, very confused by the fact that it seemed that everything that I had ever learned about biology was falling apart, I found out that birds frequently get classified this way for the sake of field identification; we identify them based on markings, whether that affects their ability to breed or not. Based on region, such as an eastern and western species of pheobes and blue birds, despite there being areas of overlap where hybrids are common.
Basing it on the frequency of these hybridizations in comparison to when they don't happen.
Biology just got ridiculously complicated and subjective right there.
The trouble is, depending on just what field of biology you go into, the definition of a species may vary. It also may just get tossed in the garbage completely.
Some of the definitions include:
The biological species concept- the idea that if two animals can reproduce and have offspring that can reproduce, they are one species. This may or may not include two animals that would never run into in nature, such as two pigeons on different continents. It depends on how you need to study them. This one poses a problem when you run into fossils, where we can't test this, and when you hit things like wolves and coyotes and dogs that can reproduce together but normally don't.A famous but false example of one of these hybrids is Balto. Though the fictional stories calls him a wolf-dog, history says he was just a very hardy Siberian husky. However, the story does differentiate between the regular dogs and a wolf-dog, because while a mix can function, they're known for having a slightly different build or temperment from either parent. More so than would normally be caused by individual variation. So are dogs and wolves one species?
This is getting to be a bigger problem back home with the rise of red wolves and the increasingly common mix between coyotes and them. Its a conservation problem: red wolves are protected, and coyotes have a bad reputation. Grouping them into one species makes it harder to protect one, but based on the biological concept they are one.
And then you have the problem of animals that don't reproduce together as a result of a difference in mating season, slight differences in courtship, or just because there happens to be a mountain, body of water, or a road in the way. Are these different, even if in captivity they recognize each other?
The cladistic or evolutionary concept: based on a group of animals that are closely related, more or less. This method helps paleontologists, as well as geneticists. The problem here: who has the right to draw that line? If you ask biologists where to draw it, some of them will wave their arms and come up with an arbitrary line. Others will look disturbed and admit to it being arbitrary.
The morphological concept: this species concept is based on shape. This is sort of easy for paleontology, at least when the bones reflect the differences. The idea is that two animals of one 'morphotype' will be one species.
But then... what about big and small dogs, or two dinosaurs that may or may not have a crest? Or two animals that have slightly different markings but are viable if they can just recognize each other as the same?
A good example of this one in the fossil record is that of the neandertals and Homo sapiens (us, in case you need to know!!) The two are very different based on the bones alone. For a long time, they were called two separate species until someone managed to get DNA and found that we actually have some of their DNA in us. Most now consider them a "subspecies," or Homo sapiens neandertalis. Basically, us, but with a slightly different build and face.
And these aren't the only definitions, just the simplest ones.
Personally, the biological concept always made the most sense to me. It was testable, and if the exceptions were really classified based on a simple "yes these work" or "no these don't," it was a good, clean-cut method. Unfortunately there isn't a way to test this with extinct species. In addition, plants are SUPER complicated; you can cross many of them, graft others, combine some that you'd never expect, and them do things that animal chromosomes just can't handle. And bacteria are even weirder; they eat each other, and can frankly pick up and dispose of and reproduce any gene they have a mind to when its available.
Oh yeah. And then you get to viruses. That's another debate entirely.
Basically my conclusion after this semester is that we really don't know what we're doing. There needs to be a good, solid concept of a species for biology to advance, but no one has been able to pose a definition with no exceptions that works. Some fields of biology don't need a definition, but many do. And it needs to be something representative of life, as it was and is and could be, in any form.
Confused yet?
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