10 02 2009

Natural Selection and the Malthusian Principle or Competition for Limited Resources

Posted by: Fred Dupont in The Bucky Call

This is the second part of TheBuckyCall Darwin 200 years anniversary special; see the first part: The Reality of Natural Selection.

5.15 Natural Selection and the Malthusian Principle or Competition for Limited Resources

It is a well-known fact that Darwin borrowed heavily from Thomas Malthus’s Essay on Population in his initial conception of natural selection. Malthus believed that the growth of human society tends to outstrip the quantity of available resources: hence, calamities such as war, disease or famine tend to intervene to reduce the overall population to levels that can be supported by the available food supply. Darwin simply took this idea and projected it onto his conception of the natural world.

On this Malthusian view, the various animal species compete for limited resources in the wild. However, since there usually isn’t enough food for all, there is generally a state of ruthless competition for the food that happens to be available, with the result that only the fittest members of each species is able to cat enough to survive. Those individuals who are less fit tend to die off, the immediate consequence of which is that they are less likely to reproduce overall. It is in this manner that those characters that confer the greatest survival value tend to be concentrated in the progeny. This is the essence of natural selection.

It must continually be borne in mind. Though, that even if we assume the validity of this Malthusian principle, the process of natural selection at best can only be used to explain the relative prevalence or certain characters in the population, it cannot be used to explain their ultimate origin, because the selection process clearly does not generate the variations themselves; it only tends to choose the fittest varieties for continued survival.

According to C. Dyke, however, this Malthusian principle or limited resources in the wild may not be accurate after all. While some natural populations may in fact be at or near true Malthusian limits with respect to a given resource, the vast majority of populations throughout history don’t appear to have been anywhere near such a Malthusian limit, for to assume so “would be to assume that the earth is a thermodynamic plenum, a zero- sum game with respect to all biological and ecological accounting systems. But the earth is a thermodynamically open system as a whole, as are all ecological subsystems… it seems probable to me that when Malthusian closure does occur it occurs not as a result of population expansion, but more often as a result of environment contraction as a consequence of climatological and geophysical events”.

Exposing Darwin

But if true Malthusian closure isn’t a reliable feature or life in the wild, how can we possibly believe in a view of natural selection that is based precisely on the presupposition that most populations are at or near this
Malthusian limit? As Dyke explains:

Every selectionist explanation based on the competition for scarce res sources depends on the demonstration that resources are indeed limitingly scarce. Oftentimes this sort of closure is attempted by assuming that populations will naturally expand to the point of Malthusian closure. But this is a bizarre assumption. For in turn it assumes that there are no constraints in any other relevant dimension or at any other relevant level that keep the population below Malthusian limits in the dimension being focused on.

We can accept selectionist explanations based on competition for scarce resources only when Malthusian closure can reasonably be demonstrated. It seldom is. What usually happens is the reverse. We are told that when plausible selectionist explanations can be provided, then we can reasonably assume that Malthusian closure was present. This of course is totally question-begging unless we have an a priori commitment to the exclusiveness and ubiquity of such explanations. The so-called “Just-So Stories” that have become famous in the literature of evolutionary biology, and especially sociobiology (Gould and Lewontin. 1979) are usually explanation guesses without the necessary closure conditions having been established. Somewhere along the line it was apparently decided that Mother Nature is a good frugal bourgeois Hausfrau.

This is a momentous conclusion indeed, for if the Malthusian limit with respect to food supply has not been a dominant feature of the evolutionary process throughout natural history, we can hardly credit natural selection alone for the rise of so many exquisitely adapted creatures in the wild. If a sufficient food supply has been available to the majority or evolving species in times past, then many “less fit” individuals, who would otherwise have been selected against if the food supply had been scarce enough, would have been able to survive long enough to reproduce. This alone would have severely diluted the concentration of significant structural changes in each evolving population, since the accidental appearance of a variation that would confer a greater amount of fitness in a harsher environment would tend to be cancelled by less fit varieties in the gene pool, which were themselves able to gain entrance because of the comparative abundance of the food supply.

Significant evolutionary change (of the random variety) would thus be immensely unlikely on two separate counts: 1- it would be exceedingly unlikely that two separate instances of the same beneficial variant would be randomly produced in a given population (one male and the other female), and 2- it would be more unlikely still for these two individuals to accidentally choose one another as mating partners, especially given the huge number or potential partners in most populations. But if this is true, the odds are vanishingly small that significant numbers of randomly produced variations would be able to be survive intact into subsequent generations, since each beneficial variant would tend to be diluted in the progeny by its less beneficial counterpart. And since these less fit counterparts would presumably abound in the population, due to a more than adequate food supply, significant changes in overall form would be very unlikely indeed, even over many generations. The end result would be relative stasis, not change. The fossil record corroborates this theoretical expectation, as it too records long periods of species fixity, punctuated by relatively short periods or rapid change.

Malthusian limit: The point at which population pressure on the environment starts to be detrimental to the population. That is when the environment pushes back! (Fred’s definition!)

BACK TO DARWIN: The Scientific Case for Deistic Evolution

By Michael Anthony Corey