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Isolation, Endemism, and Multiplication of Species in the Darwin Finches

Terrell H. Hamilton and Ira Rubinoff
Evolution
Vol. 17, No. 4 (Dec., 1963), pp. 388-403
DOI: 10.2307/2407090
Stable URL: http://www.jstor.org/stable/2407090
Page Count: 16
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Isolation, Endemism, and Multiplication of Species in the Darwin Finches
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Abstract

1. Directing attention to the inverse association between insular numbers of species and endemic subspecies, the problem of the factors governing insular increases in these numbers is outlined for the Darwin finches in the Galapagos Archipelago. By the method of least squares, partial-regression coefficients and multiple-regression equations are calculated for four factors which conceivably might control insular variations in these numbers. Factors evaluated for their independent or interdependent predictive powers are: (a) insular area, (b) floristic diversity as measured by numbers of land plant species, (c) isolation as measured by distance between nearest islands, and (d) isolation as measured by distance from Indefatigable Island, located near the center of the archipelago. 2. Insular area and insular number of land plant species are found to account (p.r.c. values = 0) for none of the variation in insular numbers of species or endemic subspecies, and geographic isolation seems an overriding factor accounting for such variations. The first observation is surprising; however, numbers of plant species may be a poor index to floristic diversity. About the lack of influence of area, little can be said. The influence of insular area may be different for a monophyletic assemblage than for the total insular fauna of an archipelago. 3. For prediction of insular numbers of endemic subspecies, only geographic isolation (p.r.c. = +0.09) as measured by distance between nearest neighboring islands is of major value. An analysis of variance indicates that variation in this factor may be considered to account positively, and independently of the other factors tested, for the majority of the variation in insular numbers of endemic subspecies. This confirms Lack's conclusion (1947) and negates Bowman's differing conclusion (1961), that the more isolated islands tend to produce more endemics than the less isolated ones. The multiple-regression equation for predicting the number of geospizid endemic subspecies on a given island is 0.09 times the distance from nearest neighboring island minus 0.2. 4. Variation in insular numbers of species is negatively accounted for by variation in geographic isolation (p.r.c. = -0.10) as measured by distance between nearest adjacent islands and, independently of the preceding, by geographic isolation (p.r.c. =-0.02) as measured by distance from Indefatigable Island. The equation predicting numbers of geospizid species for a given island is 9.8 minus 0.10 times the distances from nearest neighboring island minus 0.02 times the distance from Indefatigable. The two measures of geographic isolation leave unexplained a good-sized part of the variation in insular numbers of species, and this is provisionally attributed either to error in a statistical sense or to unknown factors not considered by this analysis. 5. The findings demonstrate the fundamental importance of geographic isolation as a factor which regulates endemism and, to a lesser degree, variations in the size of insular faunae for a presumably monophyletic bird group radiating within an isolated archipelago. Multiple-regression analysis reveals that isolation, measured as linear distance between nearest islands, is the environmental factor which predicts best the empirical observation that the inner islands of the Galapagos Archipelago tend to have more species and fewer endemics, while the outer islands tend to have fewer species but more endemics. The classic explanation for the observation would be that conditions for formation of endemics are unfavorable in the inner islands of the archipelago because of increased dispersal and colonization, interbreeding, and swamping, permitted by the decreased isolation of the islands. 6. If the explanation cited is valid, then the species of the Darwin finches are expected, for the most part, to have their respective origins in the peripheral, or noncentral, islands where reduced swamping would not retard incipient speciation. This hints at the possibility of an adaptive radiation cycle for the Geospizinae: that is, the species originate mostly in the peripheral, more isolated islands, and then disperse to the inner, less isolated islands. Here, where insular sympatry and environmental diversity are maximal, species divergences and specializations might achieve their final manifestations, either in response to related species or to particular insular environments. During the early stages of the cycle, individuals are expected to disperse repeatedly from inner to outer islands, and vice versa. The cycle might thus continue until the outer islands have reached ecological saturation for numbers of species. 7. On general theoretical grounds, it is assumed that, other things equal, opportunity for formation of endemics or incipient species is predicted by a Gaussian distribution for geographic isolation measured by the linear distance between nearest islands, and that opportunity for colonization by dispersors is predicted by such distances varying according to a Poisson distribution. The present analysis of the Darwin finches seems to support this theory, which is not new but classic. Thus, for multiplication of species and the development of endemics, the quantitative differences in geographic isolation assume an importance for individual isolates and species complementing that of the importance of geographical isolation per se.

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