Live and Let Live, Nature's Message
Both now-discarded Lamarckism (the supposed inheritance of acquired characteristics) and Evolution Theory postulate that function determines form. Natural selection rewards those forms best suited to carry out the function of survival ("survival of the fittest") in each and every habitat (through the mechanism of adaptive radiation). But whose survival is natural selection concerned with? Is it the survival of the individual? Of the species? Of the habitat or ecosystem? These three - individual, species, habitat - are not necessarily compatible or mutually reinforcing in their goals and actions. If we set aside the dewy-eyed arguments of altruism, we are compelled to accept that individual survival sometimes threatens and endangers the survival of the species (for instance, if the individual is sick, weak, or evil). As every environmental scientist can attest, the thriving of some species puts at risk the existence of whole habitats and ecological niches and leads other species to extinction. To prevent the potential excesses of egotistic self-propagation, survival is self-limiting and self-regulating.
Consider epidemics: rather than go on forever, they abate after a certain number of hosts have been infected. It is a kind of Nash equilibrium. Macroevolution (the coordinated emergence of entire groups of organisms) trumps microevolution (the selective dynamics of species, races, and subspecies) every time. This delicate and self-correcting balance between the needs and pressures of competing populations is manifest even in the single organism or species. Different parts of the phenotype invariably develop at different rates, thus preventing an all-out scramble for resources and maladaptive changes.
This is known as "mosaic evolution". It is reminiscent of the "invisible hand of the market" that allegedly allocates resources optimally among various players and agents. Moreover, evolution favors organisms whose rate of reproduction is such that their populations expand to no more than the number of individuals that the habitat can support (the habitat's carrying capacity). These are called K-selection species, or K-strategists and are considered the poster children of adaptation. Live and let live is what evolution is all about - not the law of the jungle. The survival of all the species that are fit to survive is preferred to the hegemony of a few rapacious, highly-adapted, belligerent predators. Nature is about compromise, not about conquest.