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Ecological Factors in Speciation of Peromyscus

W. Frank Blair
Evolution
Vol. 4, No. 3 (Sep., 1950), pp. 253-275
DOI: 10.2307/2405335
Stable URL: http://www.jstor.org/stable/2405335
Page Count: 23
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Ecological Factors in Speciation of Peromyscus
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Abstract

Geographic variation in mice of the genus Peromyscus is attributable to mutations, selection by the environment, and reduced gene flow through partial isolation of large or small segments of the species population. Wild genotypes are difficult of analysis in this group, but some mendelian alleles having important effects on wild populations have been described. Selection by the physical and biological environment is probably the most important agency determining the characteristics of sub-populations within the species range. There is evidence that simple predation is a selective agency of major importance in this group. Partial isolation of sub-populations because of ecological barriers and because of isolation by distance is another important factor in geographic variation. The home-range habit tends to restrict gene flow; dispersal of young on reaching sexual maturity has the opposite effect. The species is always unevenly distributed in its range due to preferences for certain environments, tolerance of others and avoidance of still others. The pattern of distribution varies within the species range, and in some parts of the range it may be more favorable to local differentiation due to isolation than it is in others. Geographic races are ecological phenomena. There is no difference between ecological races, geographic races and subspecies. Species with the greatest geographic ranges, occupying many environments, have the greatest number of geographic races, and the number of races is roughly proportional to the area occupied by the species. There is a correlation, in some species, between certain body dimensions and the vegetational environments occupied. There is a general correlation between pelage color of Peromyscus and the color of the soils on which the mice live. The width of zones of intergradation between geographic races is related to the rate of geographic change in environment. The present distribution and biological relations in four cenospecies of Peromyscus indicate that speciation is commonly initiated in this group by the simple geographic isolation of a segment of the population. There is evidence that new species have originated at the periphery of the range, on coastal islands, and probably on isolated mountain ranges. No marked change in habitat preference has accompanied speciation in Peromyscus, and it is unlikely that invasion of a new environment would have sufficient isolating effect to permit speciation. Opposite ends of chains of races have differentiated morphologically and in habitat preference, and intrinsic isolating mechanisms have possibly developed between some of these end populations. Extinction of intermediate populations would complete speciation in this case. There is no evidence that speciation would go to completion (i.e., breeding discontinuity) without this outside intervention. Geographic variation is favorable to the survival and evolution of the species, without the splitting off of new species. Wide distribution of the species population is favorable for speciation, because the most widely distributed population will have the most opportunities to be split by external factors into separate-breeding populations.

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