The theory of punctuated equilibrium has developed
The fast evolution during speciation was first thought to be caused by allopatric occurrences in geographically distant isolated populations, as proposed by the punctuated equilibrium model. How evolution may occur in a tiny, geographically isolated population has been speculated about in many ways.
If a population is isolated from the rest of the world, it may develop in an unusual way due to the unusual circumstances it faces. However, evolution generally continues normally under selection and drift, which is sufficient to form the punctuated equilibrium pattern. The development of homeostatic systems is favored by stabilizing selection; but, when these mechanisms fail, an unexpected phenotypic repertoire may be manifested. Changing environmental circumstances may allow for the expression of latent genetic variation that was previously suppressed by homeostatic processes. This, of course, would need the introduction of new genotypes. Macromutation may be responsible for major evolutionary advances.
It has been hypothesized that new species and large groupings originate via regulatory macromutation rather than through changes in structural genes.
Therefore, non-adaptive forces would drive macroevolution, whereas adaptive evolution would be limited to causing only very small changes within individual populations.
The essential speculation of these ideas is the separation of macro- and microevolutionary changes; specifically, that natural selection and ordinary variation may only result in incremental microevolutionary change. Only in small populations subjected to very harsh circumstances can completely novel genotypes emerge.
Figure: two models of allopatric speciation. (a) The dumb-bell model in which the
ancestor species is divided into two roughly equal halves, each of which forms a
new species. (b) The peripheral isolate model, in which the new species forms
from a population isolated at the edge of the ancestral species range.
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