AN ANALYSIS OF A MORPH RATIO CLINE IN THE BANANAQUIT (COEREBA FLAVEOLA) ON GRENADA, WEST INDIES.

Abstract

Polymorphic populations provide opportunities in which evolutionary change can be studied by both observation and experimentation (Ford, 1965). Geographic gradients in the frequencies of different morphs, such as morph ratio clines, are subject to the interacting forces of gene flow and natural selection as well as other biological and geographic factors influencing geographic variation. A theoretical framework for understanding the mathematical relationships between gene flow and selection in a cline has been provided by Haldane (1948), Fisher (1950), Slatkin (1973), May et al. (1975), Nagylaki (1975), and Endler (1977), but field measurements of the appropriate evolutionary forces are few (e.g., Bishop, 1972). Microgeographic variation within islands might be expected in small sedentary animals, but is unusual in mobile organisms such as birds (Mayr, 1963; Gill, 1973). A notable exception is the complex variation in Zosterops borbonica on Reunion Island (2,590 kM2) in the Indian Ocean (Gill, 1973), which involves clines in color morph frequencies over short distances and geographically distinct races. Grenada, a small oceanic island with an area of 310 km2 and maximum elevation of 840 m, is considerably smaller than Reunion and is inhabited by two color morphs of the Bananaquit (Coereba flaveola), an abundant passerine bird. The purpose of this work is to report the distribution of the color morphs and to consider the factors responsible for the existing steep morph ratio cline on Grenada.