PUNCTUATED EQUILIBRIUM
A MODE OF SPECIATION
How does speciation occur? This is a perennial hot topic in evolutionary theory, but most biologists would subscribe to the allopatric theory (the debate centers on the admissibility of other modes; nearly everyone agrees that allopatric speciation is the most common mode.) Allopatric means "in another place'. In the allopatric theory, popularized by Ernst Mayr, new species arise in very small populations that become isolated from their parental group at the periphery of their ancestral range. Speciation in these small isolates is very rapid by evolutionary standards, hundreds or thousands of years (a geological millisecond).
Major evolutionary change may occur in these small, isolated populations. Favorable genetic variation can quickly spread through them. Moreover, natural selection tends to be intense in geographically marginal areas where the species barely maintains a foothold. In large central populations, on the other hand, favorable variations spread very slowly, and most change is steadfastly resisted by the well-adapted population. Small changes occur to meet the requirements of slowly altering climates, but major genetic reorganizations almost always take place in the small, peripherally isolated populations that form new species.
If evolution almost always occurs by rapid speciation in small, peripheral isolates, rather than by slow change in large, central populations, then what would the fossil record look like? We are not likely to detect the event of speciation itself. It happens too fast, in too small a group, isolated too far from the ancestral range. We will first meet the new species as a fossil when it reinvades the ancestral range and becomes a large central population its own right. During its recorded history in the fossil record, we should expect no major change; for we know it only as a successful, central papulation. It will participate in the process of organic change only when some of its peripheral isolates speciate to become new branches on the evolutionary bush. But it, itself, will appear "suddenly" in the fossil record and become extinct later with equal speed and little perceptible change in form.
Gould, S.J., 1977,Ever Since Darwin
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