I mentioned a while ago that I’d been invited to write about evolution for Nature’s Scitable blog network. The network finally relaunched this week and my new group blog, Accumulating Glitches, went live earlier today! Together with Sarah Jane Alger, I’ll be writing about how evolution works and the amazing world it has created — “exploring the grandeur of evolution”. We’re planning to post every Monday and I hope you’ll join us there — we’ve got lots of exciting stories to share! For now, here’s a taste of the inaugural post:
Faced with the rich diversity of living beings around us, humans have proven unable to resist the temptation to try to organize and categorize them. We have a natural tendency to classify things, a habit that’s deeply rooted in our cognition and use of language. Our brain excels at recognizing patterns (and thus finding meaning where it doesn’t exist), an ability that allows us to interact with the world using names — like “chair” — that we might be hard-pressed to properly explain. In fact, it’s surprisingly difficult to define even a seemingly straightforward word like “chair” in a way that would let us recognize everything that should be included (from office chairs and recliners to stools and wheelchairs) but nothing that shouldn’t (like tables, tree stumps, or other things we might decide to sit on).
Despite these difficulties, we’ve been classifying organisms throughout the history of human thought, from Aristotle’s division between plants and animals to modern scientific nomenclature. The modern classification system is based on grouping organisms into units called ‘species’; species, in turn, group together into a larger units called genus, family, order, and so on through the nested hierarchy of life. What make a species, though? Why should a particular group of organisms be thought of as a unit and given a distinct name? How do we decide which organisms make up a species?
Read the rest over at Accumulating Glitches…
Already saw it and commented… Congrats!!!!
Thanks! And thanks for all the support and encouragement along the way — it means a lot! 🙂
Right back at you. BTW, did you get a message that I sent you today?
I did. Sorry for not responding earlier — I was juggling too many things yesterday. 🙂
I think the line between one “species” and the next is a bit blurry and certainly arbitrary. It’s just that the human mind likes organization, as you pointed out, and has a need to organize things in tidy boxes. So yeah, species exists but only as an organizational structure. Just my two cents on the matter.
I agree that the line between species can be blurry, but I’m not sure that means they don’t exist. For example, clouds have a pretty fuzzy boundary, yet we still think of them as coherent entities.
I wouldn’t say that species are necessarily arbitrary. Some definitions of species certainly seem arbitrary (eg, a certain percentage difference in DNA), but others are on more solid foundations. It’s certainly the case that there are groups of creatures which cannot interbreed with each other, for example. In plants, a barrier to reproduction can appear in just one or two generations (via autopolyploidy or allopolyploidy). Since these groups can’t swap genes anymore, their evolutionary trajectories will diverge. I don’t think calling those groups ‘species’ is arbitrary; rather, it’s recognizing a separation that actually exists.
Mind you, I’m not arguing that reproductive isolation is the best way to define ‘species’. There are other definitions, some of which might work better for certain types of questions or organisms. I was just using reproductive isolation to show that the divisions aren’t necessarily arbitrary.
I think the species line is necessarily blurry. That’s because evolutionary process is slow. I understand that ants evolved from wasps. In this instance I doubt that a fully evolved ant queen emerged from a wasp nest one sunny day. There had to be many intermediate steps to go from wasp to ant, and each must be successful. So this line between species would have to be blurry during the intermediate steps.
At what point did we stop being ape and start being human? Austrolopithecus? Hardly, this was little more than a walking ape. It isn’t “only” walking that defines humanity. H. Habilus? H. Erectus? H. Heidelbergensis? Each added a little more of what it takes to be human. H. Habilus made tools, but it didn’t make any new tools for over a million years. So it was hardly an innovator. Really, just a clever ape. So at what point did we become more human than ape. One person could say one thing and another something else.
The line, IMHO is blurry because it necessarily requires many steps to get from one species to the next. And we can see this from the human ancestral line. If science says it was at H. Erectus that we became human, then I’ll accept that. One spot is as good as another. At the same time I will insist that this line is arbitrary. That’s not to say there aren’t different species, there are, but a close inspection will always reveal a blurry line at the edges. We are merely deciding at what point the line exists.
I guess part of what I’m saying is that the process isn’t always necessarily gradual. Plants serve as a good example. They can double their chromosomes in a single generation, immediately creating a barrier to reproduction. This isn’t just hypothetical; research has shown that around 15% of flowering plant species originated by such a process.
The question of what we consider “human” in a slightly different issue from what makes a species. Homo habilis, H. erectus, etc are all species of hominid; whether to call them all “human” or reserve that term for some of them depends on what one means by “human”.
I get your point about gradualism, though, and I even mention the similar problem of ring species in the article. It’s difficult to know what to do in cases like that and sometimes (often?) we may make arbitray judgements. But, as I explained above, the division isn’t always artificial. l believe that even when we make “arbitrary” divisions, we’re trying to (or should try to) capture a distinction that actually exists, even if its boundaries may be fuzzy.
Consider this, though: if it’s really just a smooth continuum and species don’t exist except as arbitrary classifications, how can they go extinct? Does extinction then become meaningless?
(And thanks for your thought-provoking comments & discussion!)
I’m looking “species” like I would a gradated color wheel. If you look at the middle of the gradations, green is clearly different from red, red is different from blue, Few would dispute this. It’s only the edges that are blurry. The other day I was in a small shop and my wife asked me to get a drink from the cooler. I asked which flavor. She said “the yellow one.” I corrected her and said “you mean the green one, right?” It looked like a neon-green to me. I asked the customer to my left, and she said it was yellow. I asked the clerk, and she said it was green. I doubt any of the four would dispute that a common leaf is green, but this color lay on the fuzzy border between green and yellow, so a simplistic description becomes disputable.
This is kind of how I see species. Buffalo is clearly not a Wolf. The distinction in species there is clear. A Coyote is not Wolf either, but here the distinctions become fuzzier. An animal in between the two may be more difficult to define.
In regards to why species die out why others do not, the reasons are many as you know. History shows that smaller animals and generalists (omnivores) tend to survive. In Madagascar, The Eye-Eye is type of Lemur, but it is clearly a different species from the Ring-Tailed Lemur. If a disease killing tree-burrowing grubs swept Madagascar, the Eye-Eye would likely die out, while the Ring-Tailed Lemur would not. One is a specialist, the other not, even though they are closely related. With the death of mega-fauna, the Dire Wolves that preyed upon them died out, yet the closely related Grey Wolf survived.
Even though there are blurry edges in speciation, if the design goes far enough in one direction towards specialization the design (animal) becomes vulnerable to environmental stresses. It becomes distinctly different as a species and no longer part of the blurry edge.
I am enjoying this discussion quite a bit too.
Reblogged this on The Science Blog.