What Evolution Is by Ernst Mayr

Author:Ernst Mayr
Language: eng
Format: mobi, epub
Published: 2011-10-19T03:34:39+00:00

S P E C I A T I O N

T H E P R O C E S S O F A L L O P A T R I C S P E C I A T I O N

The fundamental question posed by the process of allopatric speciation is: How does the reproductive isolation originate? The

answer is found not by looking at the species as a single population, but by expanding our view of the species to a multidimensional

species taxon.

Not all populations of a species taxon are in contiguous contact with each other, actively exchanging genes. Some populations

actually are geographically isolated from each other through barriers formed by water, mountains, deserts, or any other kind of

terrain unsuitable for this species. These barriers reduce or

prevent gene flow in sexually reproducing species and permit each W H A T E V O L U T I O N I S

isolated population to evolve independently of the other popula-

tions of the parental species. Such a population evolving in

isolation is called an incipient species.

W h a t happens in the isolated population? In such a population, numerous genetic processes take place that may differ from similar processes in the parental species. There may be new mutations,

certain genes may be lost owing to accidents of sampling, recombination results in the production of a diversity of new phenotypes that are different from those of the parent species, and there may be the occasional immigration of different genes from other populations. More importantly, the isolated population lives in a

somewhat different biotic and physical environment from that of

the parental species and is therefore exposed to somewhat different selection pressures. In spite of the continuing activity of normalizing selection, the isolated population will gradually be restructured genetically and diverge increasingly from the parental species. If this process continues long enough, the isolated population may

eventually become sufficiently different genetically to qualify as a different species. During this process it may acquire new isolating mechanisms that will prevent its interbreeding with the parental population when a change in the nature of the barriers permits the newly evolved species to invade the range of the parental one.

When this happens, the incipient species is recognized as a

neospecies. The process described here represents geographical or allopatric speciation. W h a t is the fate of the large number of incipient species that are formed all the time? Most of them reunite

again with the parental species before having reached the species level or else they become extinct. Only a small fraction of such isolated incipient species completes the speciation process. Actually there are two forms of allopatric speciation.

Dichopatric Speciation

In dichopatric speciation the isolation is caused by the rise of a geographical barrier between two previously contiguous portions

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