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Area, Climate, and Evolution

Philip J. Darlington, Jr.
Evolution
Vol. 13, No. 4 (Dec., 1959), pp. 488-510
DOI: 10.2307/2406131
Stable URL: http://www.jstor.org/stable/2406131
Page Count: 23
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Abstract

(1) This paper is concerned with general adaptation, which is the kind of evolution that produces dominant animals (p. 488). (2) Area is one important factor in evolution of dominance: the most dominant animals seem usually to evolve in the largest favorable areas (pp. 490-492). How climate affects evolution is the question (fig. 2). (3) The apparent histories of the best known and most recently dominant groups of mammals and the distribution of dominant families of mammals and birds in the Old World tropics and the north temperate zone (fig. 6C) indicate that the most dominant warm-blooded (like cold-blooded) vertebrates usually evolve in the Old World tropics and spread northward, although some countermovements occur (pp. 495-502, summarized p. 502). (4) Apparently neither area alone nor climate alone can make the Old World tropics a unique evolutionary center. The effects of area and climate must be added together (p. 503). (5) Tropical climate might accelerate mutation or reproduction and thus accelerate evolution; but this is unlikely among warm-blooded animals. Large populations presumably have an advantage in evolution by mutation and selection of individuals; but, although large populations occur in large areas, they do not seem to be especially characteristic of tropical climate. Number of populations of both cold- and warm-blooded vertebrates is correlated with both area and climate: the larger the area and the warmer and more stable the climate, the more and more diverse the populations (pp. 503-504). (6) If the relation of area and climate to number of populations is mapped diagrammatically (fig. 7), a world-wide pattern is formed, centered on the Old World tropics, which fits the apparent geographical pattern of evolution and dispersal of dominant vertebrates (fig. 1, B) (pp. 504-506). (7) This suggests that effective evolution-general adaptation-is correlated with number of populations and occurs partly by selection of whole populations, with continual extinctions, replacements, and movements (spreadings) of evolving populations (fig. 8). There are indications that dominant groups do evolve in this way (p. 507). (8) Selection of whole populations tends to avoid individual-versus-species oppositions and makes effective use of recombination-variability. The situation in the tropics-occurrence of many, sparse, perhaps subdivided populations-may be especially favorable to selection of populations and use of recombination-variability. Evolution ought to make situations favorable to itself in the stable tropics (pp. 507-508). (9) Conclusions are presented as a working hypothesis, justified by evidence taken at face value, based as far as possible on observable facts and relationships, and capable of experimental and mathematical testing.

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