Animals live in small isolated populations. It is widely known how salmon swim the great oceans but return at great investment to the brook where they themselves were spawned. The thing they can find there and nowhere else, of course, is salmon they are related to, cousins. The entire effort is a drill in managing population size. They do it because it works. If we want to survive, we must manage to do something much like it.
I once had a friend who enjoyed fishing. It is his story, but I have no reason to doubt it. He lived on a canal that emptied into one of the Great Lakes. Somebody decided that it would be nice to have salmon in the lake for the fishing, so they stocked the lake massively. The salmon flourished, and fishing was good. But then the fish reached the moment when the fish were driven by instinct to spawn. They made for the nearest thing to a river around, which was the canal, and started making heroically for the home they had never had, swimming powerfully and hurling themselves with mighty leaps when it seemed like the right thing to do.
Now one forty pound salmon landing in your back yard might be a good thing if you are there to pick it up, clean it, fillet it and put it in the freezer. You are going to have really great fish dinners many times. But if you miss the moment, dozens of dead salmon rotting in you back yard is less attractive. One wonders what the people thought the salmon were going to do when the mood struck them.
Perhaps you have seen the movie “March of the Penguins.” The birds migrate south for the winter, the Antarctic winter. They make a long journey off the ice shelf to land. Of course the land is nowhere to be seen. It is deep under permanent ice. There they mate, and each female presents her male with an egg, which he holds on his feet while she makes the long trip north back to the sea to fish and fatten up. The males crowd together for warmth, each penguin spending a limited time on the exposed perimeter and most of the time packed in with his fellows. Even with eggs on their feet, they have to keep shuffling about not only to let the cold ones on the edge back in but to keep from melting their way so deep into the ice they would not be able to get out.
For a time the sun goes below the horizon while they nurture the eggs. Eventually the females return, somehow find the right males and take over the eggs while the starving males go back to the sea.
The effort is prodigious. The caloric expenditure is so great it is hard to believe they are able to do it, much less that they should not find a more convenient place. But the reward is that they are able to find at the mating site only penguins that are part of the same community. That makes offspring possible, so that is what is done.
Migrating birds stay in their flocks. I once had a friend who lived in Rochester, Minnesota. One winter day we went down to the power plant where there was a cooling pond for the power plant. The water stayed ice free all winter, and geese had taken to going there instead of making the long dangerous trip south. People would come down to the pond to feed them, and the number of geese around the city was enormous.
There are two kinds of these geese. There is the usual kind and there is a proportion of slightly larger ones. They are different species. For a long time the giant geese were thought to be extinct. They were obviously there in old photographs of hunters displaying dozens of geese they had shot, but the giants had not been seen for many years. Geese were protected and the big ones were not recognized by experts who took an interest in geese. But in the pond they were obvious. Scattered among the others were a few of the larger ones, conspicuous by contrast.
While they were effectively urbanized there in the winter, come summer the geese would fly north to their ancestral breeding grounds, each staying in its own flock as birds do. There may be a trifling aerodynamic advantage to bunching up in a V formation for the long flight. But the real advantage is keeping the community together so they don’t mix up mating groups. That would be fatal in the long run for all of them.
Gregarious animals such as deer typically have a rutting season in which a small minority of the males sire the offspring. They butt heads to decide which male will mate. Or they butt heads and the females decide which males will mate. At all events, the bottom line is that the gene pool size is limited and fertility sustained.
Social insects confine reproduction to a few individuals. A queen bee or aunt will lay all the eggs. If all the bugs did it, the gene pool size of a big nest would be enormous. But again at enormous cost and effort they maintain a system that limits the gene pool size.
Corals release their larvae into the open sea, where one might expect them to be carried by currents anywhere in the world that was a good place for coral. Instead those who have looked at the situation carefully have found that they actually reproduce only locally. So do the fish that live there. (Local Replenishment of Coral Reef Fish In the Marine Reserve. Glen R. Almany, Michael L. Berumen, Simon R. Thorrold, Serge Planes and Geoffery P. Jones SCIENCE: 326 742 May 4, 2007.) And that insight was gained without the benefit of realizing that it was probable if not inevitable.
Even diatoms floating in the sea with no control over their movement except buoyancy have identifiable local populations (Dispersal Limitations Matter for Microbial Morphospecies. Richard J. Telford, Vigdis Vandvik and H. J. B. Birks SCIENCE: 312 1015 19 May 2006).
Some bats mate by having sisters all choose the same male (Same Bat Family. NATURE: 437 xi 15 September 2005).
Sympatric speciation occurs (Evolution Standing in Place. Elizabeth Pennisi SCIENCE: 322: 1372 10 March 2006). In lake Victoria in Africa the cichlid fish, beloved of fish tank owners because of their colorful variety, have evolved into different species while all swimming in the same lake. How they managed to do it is a puzzle, but why they should is no longer a puzzle. It breaks up the population. I understand that agricultural runoff has fouled the lake to the point that the fish have a hard time recognizing which species the other fish are, and the speciation is breaking down. That does not bode well for the fish or for the aquarium keepers.
Some populations – notably lemmings – tend to surge and collapse. Since most of what I remember of lemmings I got from a Disney documentary that has been drawn under serious question, I make to claim to being able to pronounce on them, but I do have a suggestion. At one time the lemmings were kept in check by something, possibly wolves, that kept them in small isolated populations. When the wolves or whatever were eliminated the lemmings numbers could grow. Having no other routine for isolating their communities, they have undergone repeated population growth and collapse.
Animals tend to be territorial. That at least is no mystery. Living wild is dangerous, and if a wild animal is alive it is a good strategy not to change anything. Moving would be a big change. And animals drive others away presumably to defend the resources that are familiar. But there is another effect, which again is to limit gene pool size.
An oddity, however, is something called cryptic species. Two species that are essentially identical can occupy the same environment. An increasing number of these are being discovered. (Body Doubles. Alberto G. Sáez and Encarnación Lozano NATURE: 433 111 13 January 2005). Sometimes there is no way to tell which species of two one is looking at without doing a genetic analysis. Biologists just can’t tell from appearance, behavior or any chemicals the animals may be giving off. But the animals can tell.
The existence of cryptic species is counterintuitive. One would expect that one should eventually wipe the other out since there is no relative advantage on either side. But of course there is an advantage in simply the fact that they are not the same species. They are able to exploit the environment at a higher density that a single species could without exceeding the limit on effective population size.
In fact it is now recognized that most animals live in small isolated communities, even within what is universally recognized as a single species. The advantage is clear. So the animals have responded to the directive, limit your gene pool size or die out.
And humans? We seem to be indifferent. This is costing us dearly and may well cost us everything. Animals have an instinct to make viable choices. We probably do ourselves but ignore it. The way to fix all this may simply be a matter of getting in touch with our inner penguin.
But only 29 have ever visited this site as of 6/30/2008. This is research, not advice. Linton Herbert