If life evolves steadily from one species to another, then why do homo sapiens and chimpanzees still co-exist ? That’s a classic question, and one which goes right to the heart of evolution.
The point is that whilst evolution is a slow process, the mechanism which allows change to happen is not a gradual one at all. We might see Darwin’s drawings of Galapagos finches as a continuous spectrum of evolutionary development, but perhaps that sketch gives quite a false impression of how evolution really works.
When evolutionary change takes place, it does so rapidly and abruptly.
The qualities of any large and connected gene pool drift more or less together – all things being equal, then most adaptations will typically spread only very slowly. The gene pool is in equilibrium, most of the time.
Whilst variation between individuals is inherent, and random mutations are constantly occurring, systematic drift within a species will most likely be ruthlessly denied by constant re-equilibration with the wider population.
That’s clear to everyone – if you stable your sleek and shiny thoroughbred with a humble carthorse, then the offspring will never win the Derby. Likewise let your pedigree poodle razzle with a mongrel from down the road, and the puppies will never show at Cruft’s. Those analogies serve to prove the point – under ordinary circumstances, the scope for quite advanced stages of adaptation to survive contact with the wider gene pool is surprisingly limited.
So how is evolutionary change possible at all ?
The answer is that evolution proceeds much more effectively within smaller populations. We all know that breeding strains of beans, racehorses, fruit flies or pedigree dogs works best in tightly-controlled conditions. Keep your poodle firmly on a lead at breeding time.
Within the natural world, subsets of the gene pool will develop in any setting where the breeding population is restricted. In geographically disconnected communities within remote valleys, impenetrable forest and on isolated islands, then mutationary adaptations which are found to be favourable within that setting have a far greater chance of establishing themselves.
Eldredge and Gould termed such environmental subsets ‘peripheral isolates’.
Over time, the successful spread of mutations within a peripheral isolate can (and inevitably will) lead to the emergence of a population which really is genetically different from the main gene pool. This is especially true where the environmental conditions begin to differ from those experienced by the larger population – for example a wetter climate on a windward island might foster a substantially different vegetation and food supply for the animals living there.
Depending upon the length of time, the degree of environmental separation, and the size of the population, this process can lead progressively through the development of different races and subspecies, eventually to the evolution of new and entirely distinct species and genera.
This is the process which is reflected today within the distinct faunas of isolated islands – both on the small scale of an individual Galapagos island, and on the relatively larger scale of Australia.
It was the observation and mapping of such faunal differences across the island chains of Indonesia which led Alfred Russel Wallace to propose the theory of natural selection, in turn encouraging Darwin belatedly to publish his own ideas within The Origin of Species.
But isolation does not always last for ever. Onshore, sea level falls may reconnect communities through the emergence of new land and ice bridges, whilst forest fires and flash floods may link once separate savannahs and lakes. Offshore, plate tectonics is expert at fragmenting and re-linking isolated marine shelves. The eternal dance of the continents around the globe has efficiently driven the formation and destruction of new environments for three billion years – including 2 700 million of them before life first dared to grace the land.
Following reconnection of distinct communities, there will be competition between the different populations as they mix and spread across the reconnected area. If divergence has proceeded far enough, beyond the point of dangerous competition, then the two communities may be able to co-exist successfully in parallel (as was the case, at least until recently, with chimpanzees and humans).
Alternatively, if the gene pools have not separated too far, then the different populations may still be able to re-equilibrate once more (as is happening in our cities today between the once-isolated Chinese, Japanese, African, Caucasian and aboriginal Australian peoples).
Often, divergence has gone far enough to prevent inter-breeding, but not far enough to prevent competition for food and living space. Reconnection may see one population now facing a selective disadvantage. The new adaptations may die out, proving to offer only a short-lived evolutionary dead-end. But sometimes, the newcomers will succeed in taking over the wider realm. In the blink of an eye, long-established lifeforms can be obliterated, with the new species replacing them seeming to have appeared from nowhere.
That begs another famous question – if evolution occurs gradually, over many generations, why do we rarely find intermediate forms ?
And now we can appreciate the answer. New forms typically evolve within geographically isolated communities, often across tiny areal distributions and populations which are numerically almost insignificant compared to the whole.
It’s the small size and local scale of isolated populations which allows them to diverge. Equally it’s those same factors which limit their preservation potential within the fossil record to almost nil. And it’s the sudden reconnection of isolated communities which inevitably results in rapid (geologically instantaneous) bursts of evolutionary change – since discrete evolutionary ‘jumps’ are just as certain to be recorded whether the new population supplants the old, or even if both forms can distinctly co-exist.
And far from being a slow process, taking place via slow drift across the gene pool, now we can envisage evolution as a fundamentally episodic process of rapid change. Evolution progresses not strictly through gradualism, but through an inexorable series of punctuated equilibria. Put simply, there are long periods when nothing much happens in evolutionary terms, and then suddenly new forms will appear and old ones disappear, all within a short space of time.
So that’s how evolution works.
It’s a simple but devastatingly effective mechanism which has operated on this planet, continuously and relentlessly for over three billion years.
A conventional view is that biology, genetics and zoology tell us how evolution proceeds today. Some would say that geology and palaeontology merely augment that story, with the rocks and fossils recording how evolution progressed throughout the past.
They’re right, of course. And yet, I’d like to go much further still. Because an appreciation of geological and plate tectonic processes really does provide the key to how evolution ever worked at all.
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