We are into the opening phase of a mass extinction of species. That much is well understood. Hardly understood at all is what the mass extinction will do to the future course of evolution. But we are surely disrupting and impoverishing it in ways that promise to match the greatest set-back to life`s course during the past half billion years - and we are doing it in half a century. So while we are inducing an extinction spasm that threatens, if unchecked, to eliminate half or more of all species, we might consider the full scope of the biological debacle ahead.
Sheer loss of species will likely turn out to be less significant in the long run than our reduction of evolution`s capacity to generate new species. This ultimate upshot will persist, so far as we can gather from recovery periods following mass extinctions in the prehistoric past, for five million years at least, probably several times longer.
It behoves us, then, to appraise the ultimate repercussions of the present mass extinction on basic evolutionary processes such as natural selection and speciation. We appear to be giving it ne`er a thought. No research centre in the world is engaged in a comprehensive and systematic effort to evaluate this key question.
Our Earth is subject to many other environmental assaults such as acid rain, soil erosion, decline of forests and spread of deserts. Fortunately, all these problems are inherently reversible, and they can be rectified within a matter of decades or centuries. Even ozone-layer depletion and global warming can eventually be corrected if we want it to be. Species extinction, by contrast, is final - for the lengthy time being at least. True, evolution will eventually come up with replacement species that will match today`s in numbers and diversity will require a recovery period extending for at least 200,000 human generations into the future - or 20 times more generations that have existed since humankind itself emerged as a species. More specifically, we are almost certainly determining there shall be no new forms of the great cats and apes, rhinos and panders and many other species known amongst biologists as charismatic megavertebrates.
To produce offspring species, they would have to maintain population totals, hence gene pools for natural selection to work on, many times larger than today`s remnant numbers. Zoo`s are no answer. Fine job as they increasingly do for threatened species, they will be geriatric wards at best.
From what we can discern from the geological record, the "bounce back" time following a mass extinction generally requires several million years. After the dinosaur crash 65 million years ago for instance, five to 10 million years elapsed before there were bats in the skies and whales in the sea.
In the wake of the species crash during the late Permian 245 million years ago, when marine invertebrates, being the most numerous categories of species, lost about half their families, it took 20 million years before the survivors cold establish even half as many families as were lost.
But the evolutionary outcome this time around could prove yet more drastic. The critical factor lies with the likely loss of strategic environments. We are set to lose most, if not virtually all, of the tropical forest biome. There is progressive depletion of tropical coral reefs, wetland, estuaries and other ecological zones with exceptional abundance and diversity of species and with unusual complexity of ecological workings. These environments have served in the past as pre-eminent "power-houses" of evolution, meaning they have thrown up more species than other environments.
Virtually every major group of vertebrates and many other large categories of animals and plants appear to have originated in spacious zones with warm equable climates, especially in tropical forest. In addition, tropical species seem to persist for only brief periods of geological time, which implies a high rate of evolution.
Furthermore, the species fall out will surely apply across most if not all, major categories of species. This is axiomatic as extensive environments are eliminated wholesale. So the result will contrast sharply with the "great dying" of the dinosaurs and associated species, when not only placental mammals survived, (leading to the adaptive radioation, eventually including the human species), but also birds, amphibians, and crocodiles and many other non-dinosaurian reptiles.
The impending upheaval in evolution`s course could rank as one of the greatest biological revolutions of palaeontological time. In scale and significance, it could match the development of aerobic respiration, the emergence of flowering plants and the arrival of limbed animals. But whereas these three departures in life`s history rank as advances, the prospective depletion of many evolutionary capacities will rank as a distinctive set back.
These, then, are some ultimate issues for us to bear in mind as we begin to impose a fundamental shift on evolutions course. The biggest factor by far is that, as we proceed on our impoverishing way, we scarcely pause to consider what we are doing. We are "deciding" without even the most superficial reflection - deciding all too unwittingly, but effectively and increasingly.
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