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POSSIBLE PREZYGOTIC REPRODUCTIVE ISOLATION IN SEA URCHINS SEPARATED BY THE ISTHMUS OF PANAMA

New species are formed when populations acquire reproductive isolation from other, previously conspecific, populations. How and when in the process of speciation prezygotic reproductive isolation arises has yet to be established. Two views are predominant (Littlejohn, 1969; Dobzhansky, 1970; Mayr, 1970; Dobzhansky et al., 1977). The "sympatric hypothesis" or "speciation by reinforcement" holds that populations, after they have been separated by a geographic barrier, diverge genetically, and if they should come in contact again, produce hybrids oflower fitness due to the incompatibility oftheir genomes. Prezygotic reproductive isolation, according to this view, appears in syrnpatry through natural selection to prevent investment of gametes in substandard hybrids (Dobzhansky, 1940, 1970). The "allopatric hypothesis," on the other hand, asserts that prezygotic isolation mechanisms can arise in allopatry either as the result of overall genetic divergence stemming from adaptation to local environments (Moore, 1957; Mayr, 1970; Paterson, 1978; Templeton, 1981), or due to a slight amount of genetic change in a few loci that control characteristics of the organism vital for reproductive isolation (Muller, 1942; Lewontin, 1974). Though speciation by reinforcement is believed by some authors to be unsupported by either theory or evidence (Moore, 1957; Paterson 1978; Templeton, 1981; Futuyma, 1983), it is still accepted by many evolutionary biologists as a reasonable hypothesis for the evolution of prezygotic reproductive isolation (Dobzhansky et al., 1977; also see Futuyma, 1983). Obtaining evidence either in favor or against it from natural populations has been difficult, because the history of geographic barriers to gene flow is usually unknown. Observed cases of prezygotic reproductive isolation between allopatric populations, which otherwise would constitute evidence against the sympatric hypothesis, have thus been explained as the result of previous (and unknown) invasions by one isolate into the range of the other (Dobzhansky, 1940; Dobzhansky et al., 1968). To distinguish between predictions of the sympatric and allopatric hypotheses one has to find populations separated by a known geographic barrier that appeared at a known time and has remained continuously in place to the present day. The Isthmus ofPanama is a geographic feature that fulfills these conditions. Connections between the tropical Atlantic and Pacific oceans were severed in the Pliocene, 3.1 to 3.5 million years ago (Saito, 1976; Keigwin, 1978, 1982). The separation between previously continuous marine populations has resulted in a host of "geminate species," i.e., closely related species, one on each side of Central America (Jordan, 1908; Ekman, 1953). One genus containing a geminate pair is the sea urchin Diadema. Diadema antillarum occurs throughout the tropical Atlantic, while D. mexicanum is found along the Pacific coast of Central America. Multivariate morphometries have shown that populations from the two sides of the Isthmus of Panama are on the average more similar to each other than to populations from the same coast (Lessios, 1981a), and echinoid taxonomists have recognized that if D. antillarum and D. mexicanum had not been spatially separated, they would not have been described as different species (Mortensen, 1940; Mayr, 1954). Other pairs of echinoid geminate species exhibit transisthmian morphological divergence, which, though still small, is larger than that of Diadema (Lessios, 1981a). Electrophoresis has revealed a great deal of biochemical similarity between the geminate species ofDiadema (Lessios, 1979, 1981a). Enzymatic proteins of Atlantic and Pacific populations are as similar to each other as those of populations from the same ocean. Other echinoid genera assayed with the same techniques displayed transisthmian molecular differentiation up to 20 times greater than that of Diadema. The similarity in Diadema, therefore, is not an artifact of the limited resolving power of electrophoresis. Thus, D. antillarum and D. mexicanum both by the standard of intraversus inter-specific differentiation and by comparison to other echinoid geminate species are genetically very similar. Whether they have acquired prezygotic reproductive isolation in the three million years that they have been separated is important for our understanding of the process of geographic speciation. One possible form ofprezygotic reproductive isolation between populations is asynchronous reproductive cycles (Littlejohn, 1969; Dobzhansky, 1970; Mayr, 1970). The annual reproductive cycles of D. antillarum and D. mexicanum were studied during 1976-1977 and found to be unlikely candidates as mechanisms of reproductive isolation, because they overlap (Lessios, 1981b). Many diadematoid sea urchins, however, also have a lunar spawning cycle