Speciation - the process by which new species are believed to come into existence - is at the heart of modern understanding of evolution. Unfortunately, most available explanations of speciation are somewhat overloaded with technical language. At the risk of some over-simplification, the explanation which follows tries to avoid this mistake.
What is a Species?
Modern biology teaches that animals divide into about ten phyla, of which the phylum of chordates includes the vertebrates and some others. The next main subdivision is that of class, followed by orders, families and genera. Finally we reach the individual species, which may be divided into sub-species.
The higher-level classifications of classes, orders and so on are of great importance to the biologist but for most of us they are of little practical significance. But the species distinction is vital to us. It doesn’t matter much to us whether we are primates or insectivores. It does matter profoundly to us that we are people and not chimpanzees.
Whereas the other classifications belong to the world of science, the species distinction is a fact of nature. Our own strong feeling that we are people rather than any other animals, leads us to behave quite differently in relation to any member of our own species from the way we behave in relation to other animals.
We are not alone in this. All animals behave quite differently in relation to other members of their own species. In particular, they treat all members of the same species, but of the opposite sex, as potential mates. Mating within the species is encouraged and outside the species is effectively prevented by behavioural, visual, auditory and chemical (smell) signals which attract a potential mate but serve as isolating mechanisms, deterring members of other species, however closely related, from attempting to mate. In our own case it may be significant that the human male, whilst strongly attracted in many ways to the receptive females of his own species, finds the visual signal given by our nearest relative - the swollen sexual organs of the receptive female chimpanzee - particularly uninviting (see link).
A species represents a breeding and genetic unity with isolating mechanisms which separate it from other species. The need to mate binds all members of a particular species together into a breeding population which shares a common gene pool. The basic characteristics of the species are determined by a genetic structure which, despite limited individual variations, is common to all members of the pool.
Within the limits of the common genetic structure of the species, small changes can occur easily - for example, external pigmentation often adapts to the environment. Some members of a species may adopt white protective coloration in winter in places which have a great deal of snow. In warmer climates, other members of the same species may be the same colour at all times of the year. Size and other external characteristics can also change.
Domestic dogs provide an extreme example of the variations in colour, size and shape which can occur (though among wild species, they seldom if ever occur to the same degree) within a single clearly defined species. But despite all their variations, all dogs recognise all other dogs immediately as dogs and behave quite differently towards them from the way they behave towards all other animals. Although each breed represents a partially (artificially) separated gene pool, they are not yet very far apart genetically. All share a fundamental canine identity which comes from the basic genetic structure common to all members of the species.
So that despite this external flexibility, the basic genetic structure of the species changes much less easily. For as long as they still belong to the same breeding population, there appear to be strict limits in the extent to which individuals are able to vary from the common characteristics and structure of the species.
Nevertheless the conservative pull of the large gene pool no longer operates if the pool is divided into two or more units by physical barriers. Gene pools divided by physical barriers tend to develop in different directions.
The speed with which differentiation occurs once the breeding population (the gene pool) is divided, is affected by differing ecological conditions, so that the animals on the two sides of the barrier are subject to different competitive pressures. Climates may be different, feeding habits may change as a result of different foods becoming available; competition from other species may change behaviour, predation may be more or less important and may take different forms.
When a barrier separates the gene pools of the two branches of an existing species, they will (particularly if conditions are different on the two sides of the barrier) gradually become differentiated. If differentiation has gone far enough, the removal of the geographical barrier will not reunite the species; the animals coming from one side will be unable to mate successfully with those from the other. One species will have become two.
The development of many different species over millions of years appears in fact to have been stimulated by the geological and climatic changes of the earth. Animal populations have been divided and reunited at various times as the continents have moved, as new land has been created by volcanic action and as land and sea levels have risen and fallen. The divisions and reunions have enabled new species to come into existence and to radiate over their potential territory.
The effects of geographic barriers which break up the gene pool can be studied in populations which have become cut off from their origins, frequently on islands. This is often found with land-dwelling birds which do not normally cross the sea but may occasionally be blown off course. Thus they may accidentally populate an archipelago (like the Galapagos) off the coast of a continent.
A small flock of continental birds arriving accidentally on an island - particularly if they arrived there before people had boats - would have found themselves in a new environment in which there were both problems and opportunities not present in their original home. Many islands contained no mammals except those which had been able to fly or swim there (e.g. bats and seals). So it would have been less dangerous for a bird to spend time on the ground. Far more fledgelings would perhaps initially have survived. But equally, the bird's most frequent food might not be easily available. Rapid population expansion in the absence of the usual predators might typically have soon led to an urgent need to adapt to other kinds of food.
Perhaps only twenty or so birds would have formed the new island population; perhaps fewer, the minimum number being a single fertilised hen. Thus the gene pool would have been small. Small gene pools are much less stable than big ones - they often produce eccentric variants. These eccentric variants are usually counter-productive, resulting in the extinction of the eccentric line. But eccentricity sometimes works. In the great majority of cases, the immigrant birds would have failed to adapt and soon have died out. But in a few cases, the new population would have survived and adapted. Thus the beginnings of a new species would have come into existence. Separated from the parent species of the mainland, it would have started to develop new isolating mechanisms. These would steadily have reduced the forces of mutual attraction between the island colonists and the original population. Any members of the island sub-species which accidentally found their way back to the continent would nevertheless initially have been re-absorbed into the parent species. But as the new sub-species gradually developed further and further away from their original form, a point would be reached when cross-mating with mainland birds would no longer occur. At that point a new species would exist.
The new species on its island would probably have become less specialised than before, able to eat a wide variety of foods which happened to be available there - perhaps animal food as well as plant food. Let us now suppose that a few of these birds drifted to another island in the archipelago and founded a new population on it. Whilst similar in many respects, the new island offered a slightly different range of food, with perhaps less plant food but more animal food. The new population, whilst still eating plant food also when available, would have become slightly better adapted to eating animal food.
Having passed through the long process of separate speciation, a few of the second island's birds may have found their way back to the first island. Two populations would have been in competition there, one relatively a little better adapted to animal food and the other to plant food. Over time, each would have become progressively more specialised in the food source where there was least competition, and the original stock would have given birth to two new species, a seed-eater and an insect-eater.
Why do we think that this is what happens? The formation of new species (like the formation of rocks) is a slow process and cannot be observed and recorded from start to finish in nature. But the separate stages of development have each been separately observed. Islands close to continents typically contain local variants of those continental bird species which had been able to cross in small numbers, occasionally, from the mainland. Sometimes these island variants have reached the stage of being separate species, sometimes not. And the process of archipelago bird colonisation which I have described is more or less what is believed to have occurred on the Galapagos (see link). Varieties of finch, obviously closely related but separate species, have diversified through the Galapagos archipelago to occupy ecological niches typical elsewhere not of finches but of warblers, woodpeckers and others. The phenomenon of the Galapagos birds influenced Darwin in the thinking that led to The Origin of Species.
Speciation on Islands
Islands are probably the commonest environment in which a small population of animals has been isolated and subjected to conditions demanding new adaptations. Islands have produced a very large number of unique and unusual species. On islands, adaptation to changing conditions is compulsory, if the species is to survive. On the continents, it is optional; there is often an easier option - migration. When the glaciations came and went, continental species well adapted to particular temperatures and vegetation patterns moved north and south with them. But a species isolated on an island which was becoming warmer or colder would have had to adapt to the new conditions or perish. Conversely (as Darwin pointed out) climatic change on an island can open up a range of new ecological niches to be filled by animals that can adapt to them. On the continents, these niches would be filled immediately by inward migrants.
So for a number of reasons, islands have always been full of unusual animals. To give a single example: each of the major islands of the Mediterranean, islands off the coasts of California and Siberia and islands not too far from continents in other parts of the world all once possessed their own species or variety of dwarf elephant (see link). All must have evolved from animals which had occasionally swum across from the mainlands (elephants can swim long distances) or (perhaps more likely) been left behind when sea levels rose.
Most of the varied and unusual species living on islands - including the dwarf elephants - have now disappeared. Since people began to visit and colonise islands in boats, hunting and bringing domestic animals and small rodents, very widespread extinctions have occurred. But if we go back to the period before people became able to cross the sea, we have to imagine a world in which every island had its own specialised animal life.
Until people arrived in boats, the only opportunities which island species had of colonising the continents arose from changes in sea levels or underlying geology which destroyed their geographical isolation. These changes did not happen very often. When an isolated island species became exposed to continental competition in this way, the result was probably most often the rapid extinction of the island species. But not always. Occasionally, an eccentric island variant turned out to have developed characteristics which enabled it to compete successfully elsewhere.
The Importance of "Peripheral Isolates"
Island origin answers a number of questions. It goes some distance towards explaining why paleontology has so far seldom found the direct ancestors of modern forms. This would not be surprising if many new species - including perhaps our own - had first started to differentiate themselves in a micro-population on an island. The micro-population might be too small to leave much trace, and the island might now be under the sea.
Of course not all new species appear on islands. Island location is only one possible form of geographic isolation. For fish, rivers and lakes perform a similar function. The crucial point is that new species typically appear, initially, as isolated small populations. In most cases, the reintegration of the isolated population into a larger environment results in its extinction. But new species, when they do appear, spring from the small minority which survive and spread successfully in the larger environment. In the words of Ernst Mayr:
"The widespread occurrence of geographic speciation is not now seriously questioned by anyone....Most isolates are ephemeral; they become extinct before they have had the opportunity to function as full species.. Peripheral isolates are produced 50 or 100 or 500 times as often as new species, but when a new species evolves, it almost invariably evolves from a peripheral isolate."
(Ernst Mayr Populations, Species and Evolution p294)
Successful new species normally arise not from the gradual large-scale transformation of successful existing species but from small-scale origins, from which they emerge to challenge the dominance of existing forms of life, sometimes expanding very rapidly when the existing species fail to compete successfully with them; and thus giving the impression of very abrupt change when we look at the fossil record. Abrupt change of this kind does not result from a sudden genetic mutation in the continuing population, but from an incoming species displacing the previous species from its ecological niche; in the way that the immigrant grey squirrel has recently and within less than a hundred years displaced the indigenous red squirrel (see link) in the U.K.
Life has evolved over very long periods of time, during which mountains and sea levels have risen and fallen many times, islands have separated from continents and been reconnected; and living populations have been ceaselessly divided up by natural barriers and later brought together again, perhaps in different combinations. Geological and climatic change and the occasional dramatic meteorite arrival have served as the engines of evolution. Changes in the land and freshwater environments have over and over again exposed isolated populations to the challenge of revolutionary adaptation to new conditions. Most have failed to adapt. Those who succeeded sometimes (very occasionally) later conquered the bigger areas. Responding to the new wealth of opportunities now available to them, they have increased in numbers, variety and perhaps in physical size.
By contrast the oceans, the greatest and most stable environment in the world, containing innumerable different forms of life but with very few natural barriers, have for a long time now produced relatively little which is new. The most important events in the evolution of the sea in the last two hundred million years have all come from the land: first the very successful marine reptiles; then the marine mammals and birds who displaced them; most recently the fisheries and pollution brought by man.
The land and its rivers and lakes are where innovative evolution has been happening. The amphibians - the first vertebrates to colonise the land - are not likely to have emerged on the shores of the sea; they are typically a freshwater group. Other evolutionary developments - the first bony fishes, the first vertebrates - may have originated in rivers and lakes also; this is currently controversial. But natural selection operates in the sea as elsewhere; the recent evolutionary infertility of the sea is in itself a good enough reason to doubt whether natural selection alone is enough to account for evolution.