Every now and then I’ll hear someone say something like: “This [insert trait here] makes perfect evolutionary sense! It prevents overpopulation!” But it shouldn’t take much thought to realize that this doesn’t make much sense.
Firstly, any given trait of any given animal cannot be said to exist only by having provided an evolutionary advantage to past generations. That is, some traits can be passed on from one generation to the next despite being a burden to the quality of that organism’s life, as long as it does not too greatly hinder the reproduction of the population as a whole.
Secondly, how would overpopulation reduce reproduction of the population as a whole anyway? Overpopulation comes about when the ability to breed is easier than the ability for all members of a population to access needed resources to live long enough to continue breeding. This will prevent the reproduction of some members of the population, but it would not affect the reproduction of the population as a whole. Therefore no evolutionary traits could possibly be passed on to prevent overpopulation. If a trait by itself hinders reproduction, it won’t be passed on. Overpopulation does not hinder the reproduction of the species as a whole — therefore it causes no evolutionary effect in and of itself. (The fight for needed resources may have evolutionary effects if those without the necessary access to the resources die off, but the cause of the scarcity of the resources is irrelevant; it doesn’t matter if the scarcity of a resource is caused by overpopulation, by competition from other populations, or if the resource is just naturally scarce.)
Every population will continue to reproduce until it reaches the limits of its needed resources, or until the limits of the resources change or the population is gobbled up by some other population (or controlled by humans). In this way, you could say it is natural for every population to breed until overpopulation occurs. The only population that can escape this nature is the human population, because we can make the conscious decision to not breed.
So, if you ever find yourself asking: “Hmmm, I wonder why [insert trait here] is passed on from generation to generation even though it does not aid reproduction?” and then find yourself answering: “Oh, to prevent overpopulation!” — please take a moment to consider your lack of logic and amend your thinking ways.
1 Comment
S P Hannifin · February 6, 2012 at 3:28 AM
Looking at my Google Analytics, I saw that I was getting some links through reddit, where this post got some comments:
Link here
My response to the comments:
Firstly, I certainly don’t claim to be an expert on biology or genetics. And I certainly don’t mean to claim that evolution is a simple matter made understandable only by reading a few paragraphs from a high school text book about it.
But my post above was (obviously) not about the science of evolution, or how traits are expressed in genes and passed on, but of the logic of it.
For example, let’s say that we think of hands as a trait. I know it’s not that simple or scientific. I’m not claiming that the genetic material that somehow causes a hand to develop is some specific set of genes that are either passed on or not. I’m not claiming that hand-less people would always have hand-less children. That’s not my point.
But, considering a hand as a trait, we can clearly understand why having hands provides an advantage to living, and thus breeding; the agility of our hands allow us to manipulate the world around us in ways that directly affect our ability to breed.
I therefore see no problem in saying: “OK, so a hand is for holding things, moving things, manipulating the world, etc.” because those are the actions that made hands emerge into existence in the first place. If you would argue that considering a hand to be for something is illogical because it is only the outcome of deterministic biological systems, however complex, that’s fine. That’s a philosophical discussion I’m not considering here, and my entire post above is completely pointless from that point of view anyway, so we’d be in agreement. (If no traits are considered to be for anything, including population control, the issue is settled.)
Now let us consider some other (non-scientific) trait that inhibits breeding. For the sake of simplicity, let’s say that some organisms in a population are born without reproductive organs.
I am not arguing that such a trait will simply disappear because those who are born with it cannot breed.
I’m arguing that this trait cannot be said to exist because it helps prevent overpopulation. That isn’t to say it won’t help prevent overpopulation. Perhaps it will. It’ll certainly affect the population in some way if organisms without breeding organs are impacting resource levels without the possibility of breeding. I’m arguing that we can’t claim that that’s what the trait is for. That is, a population can’t have a trait somewhere in it that is for preventing breeding. Such a trait can’t come about out of a population’s need for population control. That might be what it ends up causing, but that can’t be why it came about, or why it presents itself every now and then.
Does that make any sense?
And, again, I’m not claiming that overpopulation has no affect on evolution. Surely it does.
Even if we could claim that overpopulation was the only thing that could cause environmental stresses and lack of resources (because perhaps we humans are controlling some other population’s environment), and if we observed that breeding-inhibiting traits show up more often when these stresses or lack of resources are introduced, and we see that population dwindles until those stresses subside, we still can’t say that those breeding-inhibiting traits are for population control. They’re not for anything.
“But,” you argue, “by that logic, how can a hand be said to be for holding something?”
Because that’s what a hand allows the organism to do that positively affects breeding. Hands can also be used to gauge out each other’s eyes, but we cannot say that that is what hands are for simply because hands makes it possible, unless it was often necessary to gauge out someone’s eyes to breed.
A breeding-inhibiting trait cannot positively affect breeding. It might show up in cycles in which a population as a whole overpopulates and shrinks and overpopulates and shrinks, but to say that the breeding-inhibiting trait is thus for population control is akin to saying that limited resources are for population control. No, that’s just the way the resources are: limited. (Well, unless we humans are purposefully limiting them.) And that’s just the way the trait is: breeding-inhibiting. (Again, unless we humans purposefully designed the trait.) It has no way to positively reinforce the effects of its existence, so it cannot be for anything.
After all, what would happen to an overpopulating population if not for breeding-inhibiting traits? Well… nothing! It’s impossible for any population to reproduce over the limits of its needed resources. The population must either continuously live at the fringe of its population limit (as determined by limited resources) or be growing to it or shrinking from it. There is no other possibility.
A comment says: “The authors summation that its impossible that species develop traits that specifically deal with overpopulation is incorrect.”
No, that is not my summation, nor the point of my original post. My summation is: It’s impossible that species develop traits to specifically deal with overpopulation by negatively impacting breeding. Of course traits that can positively deal with the negative effects of overpopulation may develop; such traits would positively reinforce themselves, because they’d be dealing with the effects of overpopulation. But no traits would develop to directly counter overpopulation by completely preventing breeding in the first place. (Again, only humans can do this by consciously deciding not to breed even when we can and want to, or by preventing others from doing so.)
Anyway, I suppose this all is an opinion in the sense that it’s more of a philosophical point about assigning purposes-to-be-fulfilled to traits that emerge through evolution, but that’s the point-of-view of the idea I’m arguing against, so it certainly can’t be a wrong opinion. It’s either a right opinion, or just completely irrelevant.