Like all men, all species must eventually die. Just as some men perish with neither issue nor close kin and others achieve partial immortality through the transmission of some of their genes to their offspring, or more remotely, by the survival and reproduction of their brothers and sisters—so some species become utterly extinct whereas others live on, in a shadowy way, through one or both of two evolutionary mechanisms, succession and branching. Succession is also called phyletic evolution or anagenesis; the technical word for branching is kladogenesis. Evolution through succession occurs when a genetically iso-ted population acquires a new and favorable hereditary trait at is controlled by a single gene or by a complex of genes operating in concert. Then the new trait gradually replaces the old one through natural selection.
Evolution through branching occurs when two or more geographically separate populations of a single, polytypic species become genetically isolated from one another and then evolve into species of their own.
Succession tends to favor a process known as general adaptation whereas branching works rather through special adaptation, but the two are not mutually exclusive.
General adaptation involves the acquisition of a new trait or trait complex that is useful in more than one environment and under various different circumstances. Warm-bloodedness in birds and mammals is one example. Another is an increasing intelligence, which many forms of animal life have developed throughout geological history. A more limited example is the power of speech, which is useful to all men.
Special adaptation involves the acquisition of a new trait or trait complex that is useful in a single environment under special circumstances. It is the process which enables an animal to resist heat, cold, or bright light, to see well in dim light, to run faster or to swim better than its fellows, or to live without water in deserts, and which gives it many other such specializations. Special adaptation led the ancestors of the whales from the land back into the sea, and general adaptation gave them the intelligence needed to communicate with one another, by a system similar to sonar, and to survive, as mammalian populations, in their aqueous medium.
General adaptation tends to lead a species into evolution by succession because most species are polytypic, and a polytypic,,-species includes several populations living in different environments. Each of these populations becomes adapted to its special environment to a certain degree, but it cannot speciate by branching as long as it remains in genetic contact with its sister populations, since new traits involved in local specialization cannot completely replace old ones while genes continue to flow back and forth. If, however, in one or more populations a new trait appears which is equally favorable to all the populations and in all the environments occupied by the species, then the existing gene flow will help the new trait replace its predecessor in all the component populations, including that or those in which it started. By this process the old species evolves as a unit into a new species. At the same time speciation need not prevent the component populations from carrying their old, partial specializations, such as to heat and cold, from one species into another.
If however, a single population of a polytypic species becomes physically isolated from its fellows, so that gene flow is completely interrupted, then that population can evolve by branching. Now special traits that have no general value can completely replace the old ones that used to flow in over the border. If such a population happens to be confined to a small space, such as an island, and has no natural enemies, it can become a monotypic species as specialized as the dodo, the classic example of this process.
Fig. 1 How One Polytypic Species Can Evolve Into Another. Above: Five subspecies, in peripheral contact with each other, are illustrated by five circles, numbered l through 5. A mutation favorable to all five arises in No. 3. It spreads to Nos. 2 and 4, and is carried by further peripheral gene flow to Nos. 1 and 5. When all five subspecies have it, the species has begun to evolve into a new one by anagenesis—evolution through succession. Below: In this example the favorable mutation arises independently in Nos. 3 and 5, and, except for the direction of gene flow between Nos. 4 and 5, speciation takes place as in the first example.
Although the component populations of a polytypic species evolve as a unit, they cannot do so simultaneously since it takes time for a mutation to spread from one population to another. If we measure time on the broad scale of tens of millions of years used by paleontologists, these changes may appear simultaneous, but if we measure it on the geologically microscopic scale of the last 700,000 years, which is the age of man, we will see that related populations, which in our case are subspecies, passed from species A, which is Homo erectus, to species B, Homo sapiens, at different times, and the time at which each one crossed the line depended on who got the new trait first, who lived next to whom, and the rates of gene flow between neighboring populations.
Whether a new species is polytypic or monotypic, whenever it arises the evolutionary process is essentially the same. The new, critical trait responsible for speciation first appears in a few individuals, and its presence makes little difference to the population in which it arises. It may even appear and disappear several times before it takes hold. But after it has begun to spread, a point is reached when those who have it begin to outnumber those who don't. This point is marked by a rapid growth in population. The particular population has gained an advantage over competing species in its own lehensraum, and in the process it has become a new species of its own.
It need not, however, have completely lost the gene or genes for the old trait that is on the way out. After the new species has established itself, become stable in numbers, and reached a new equilibrium with the other species of plants and animals in its environment, the old trait may completely disappear. At that point a second and final threshold of speciation has been crossed. One may say that a new species has come into existence when it has acquired a new and more favorable ecological position, and that/ it has reached maturity when the traits responsible for these changes have completely replaced their predecessors. By the time the second threshold has been crossed, as likely as not a new species-forming mutation shall have begun to appear, and the cycle has started over again.
It is easy to understand, then, why some populations within any polytypic species have come closer, at any given time, to the second threshold of speciation than other populations. In man some groups of people alive today have preserved archaic traits, diagnostic of Homo erectus, in a higher percentage of individuals than other populations. For example, more natives of New Caledonia have big teeth and heavy browridges than a corresponding percentage of Japanese.
This and similar disparities can be explained in two ways.
(1) The more archaic population acquired the new trait complex that led to speciation later than the more modern population did.
(2) After crossing the first threshold of speciation, the more archaic population has been discarding its old traits at a slower rate than the more modern population:
Both explanations can be true at the same time. There is no necessary correlation between the time at which a threshold was crossed and the rate of change that follows the crossing. In either case, the critical mutation may have been original to the population concerned, or it may have been acquired by gene flow from a neighboring population. The older the trait the more likely that it was original; the younger the trait the more likely that it was derived from outside.
In any event, once a species has come into being, the old species from which it evolved is extinct. There are several kinds of extinction: utter extinction without issue, which is commonest among monotypic species; extinction through absorption, by which a subspecies ceases to exist as a separate entity when its remaining members are taken into the body of another; and extinction through successive evolution, which is the process we have just described.7
In the case of man, only the second and the third kinds of extinction can be traced. The Tasmanian aborigines who died out in the nineteenth century have living survivors among the racially mixed inhabitants of the islands between Tasmania and Australia, and the Fuegian Indians of South America are disappearing into the mixed population of that continent. But neither Tasmanians nor Fuegians were whole subspecies. The Australoid and Mongoloid divisions of man to which they belong survive in large numbers elsewhere. There are also, in a sense, degrees of extinc-ion, for it takes a long time, on our human time scale, for one species to replace another completely, and in that sense some human races are more nearly extinct than others.