The other day I was reminded of something I worked on years ago, namely what a species is, and how to classify them.
Nowadays, we have a relatively straightforward, "biological" definition of the species based on the capacity to reproduce, which basically says that two life-forms belong to the same species if they can produce offspring that can do the same. The first half of this "definition" seems obvious, the second half is there for cases like mules, which is what you get when you cross a horse with a donkey, but which from a reproductive point of view is a sterile dead-end.
In the not-too-distant past, however, life-forms were often also grouped and classified on the basis of very different criteria. One criterion was mobility: organisms can either be sessile (most plants, some marine animals), passively mobile (plankton), or they can move on their own accord (most higher animals). Aristotle grouped animals according to whether they flew, swam or walked (which basically also coincided with the environment they live in: the air, water or earth). You could also classify lifeforms on the basis of what they eat (plants can absorb nutrients directly from their surroundings plants, but animals are either herbivore, carnivore or omnivore) and/or their place in the food chain.
But by far the most commonly used criterion was morphology. In the beginning, the morphological subcriteria used could be as simple as the number of legs, by which token a life-form with two legs (e.g. Man) could be considered quite different than a life-form with four (cows, dogs, pigs and horses). But it soon became clear that many of these "countables" were not very reliable. If a dog, for example, is born with only three legs, does that make him or her a different species than his siblings? To us, the answer is obvious, but that is because we use the biological species definition, and only use morphology as a quick and easy way to distinguish between species. Not so long ago, before the advent of the biological or reproductive defintition of species, the answer was not obivous at all. And there were other difficulties. Is an arm basically a leg by another name? If so, then the group of legged animals should also include starfish. And what about the neck and head, is that an appendage too? If so, then Man (and cows, dogs, pigs and horses) would have to grouped together with starfish, because they all have five appendages: Man has two legs, two arms, and one head, which makes five. Ah, you say with a mischievous "I got you now" twinkle in your eye, we are different, because upper-echelon animals are all bilaterally symmetrical, and starfish are not. And you would be right in the sense that symmetry is an important difference. So instead of putting humans together with starfish, I would have to put them (and the cows, dogs, pigs and horses of course) together with the millipedes, insects, cockroaches, rats, in a class of bilaterally symmetrical lifeforms. But what about bilaterally symmetrical plants, such as diatoms and some microscopic algae? Do they belong in the same group? Etc. etc. The problems are endless. If you use only morphology, you could easily argue that men and women are not the same species, while in fact (and in spite of the fact that psychologically, they are from different planest - one is from Venus and the other from Mars) we all know that they are not only the same species, but they are both essential to our survival. But that is a different story altogether, and one I will certainly come back to in another entry.
So given that most scientists agree that morphology is not the best way of defining species, what's the big deal? At least one: obtaining proof that two life-forms are the same species can be quite difficult. In most cases, we cannot just put two life-forms in a box and expect them to produce offspring, just because we them to. Even if they are both still alive (99,9% of all species that ever lived are already extinct, so all we have are fossils), they are all kinds of very good reasons that they may not want to perform the deed under our scrutiny. So we use morphology instead, as a sort of shortcut. Speaking of which, I have to cut this entry short.
More later!
Nowadays, we have a relatively straightforward, "biological" definition of the species based on the capacity to reproduce, which basically says that two life-forms belong to the same species if they can produce offspring that can do the same. The first half of this "definition" seems obvious, the second half is there for cases like mules, which is what you get when you cross a horse with a donkey, but which from a reproductive point of view is a sterile dead-end.
In the not-too-distant past, however, life-forms were often also grouped and classified on the basis of very different criteria. One criterion was mobility: organisms can either be sessile (most plants, some marine animals), passively mobile (plankton), or they can move on their own accord (most higher animals). Aristotle grouped animals according to whether they flew, swam or walked (which basically also coincided with the environment they live in: the air, water or earth). You could also classify lifeforms on the basis of what they eat (plants can absorb nutrients directly from their surroundings plants, but animals are either herbivore, carnivore or omnivore) and/or their place in the food chain.
But by far the most commonly used criterion was morphology. In the beginning, the morphological subcriteria used could be as simple as the number of legs, by which token a life-form with two legs (e.g. Man) could be considered quite different than a life-form with four (cows, dogs, pigs and horses). But it soon became clear that many of these "countables" were not very reliable. If a dog, for example, is born with only three legs, does that make him or her a different species than his siblings? To us, the answer is obvious, but that is because we use the biological species definition, and only use morphology as a quick and easy way to distinguish between species. Not so long ago, before the advent of the biological or reproductive defintition of species, the answer was not obivous at all. And there were other difficulties. Is an arm basically a leg by another name? If so, then the group of legged animals should also include starfish. And what about the neck and head, is that an appendage too? If so, then Man (and cows, dogs, pigs and horses) would have to grouped together with starfish, because they all have five appendages: Man has two legs, two arms, and one head, which makes five. Ah, you say with a mischievous "I got you now" twinkle in your eye, we are different, because upper-echelon animals are all bilaterally symmetrical, and starfish are not. And you would be right in the sense that symmetry is an important difference. So instead of putting humans together with starfish, I would have to put them (and the cows, dogs, pigs and horses of course) together with the millipedes, insects, cockroaches, rats, in a class of bilaterally symmetrical lifeforms. But what about bilaterally symmetrical plants, such as diatoms and some microscopic algae? Do they belong in the same group? Etc. etc. The problems are endless. If you use only morphology, you could easily argue that men and women are not the same species, while in fact (and in spite of the fact that psychologically, they are from different planest - one is from Venus and the other from Mars) we all know that they are not only the same species, but they are both essential to our survival. But that is a different story altogether, and one I will certainly come back to in another entry.
So given that most scientists agree that morphology is not the best way of defining species, what's the big deal? At least one: obtaining proof that two life-forms are the same species can be quite difficult. In most cases, we cannot just put two life-forms in a box and expect them to produce offspring, just because we them to. Even if they are both still alive (99,9% of all species that ever lived are already extinct, so all we have are fossils), they are all kinds of very good reasons that they may not want to perform the deed under our scrutiny. So we use morphology instead, as a sort of shortcut. Speaking of which, I have to cut this entry short.
More later!
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