Wu’s genic view of speciation

Professor Wu deals almost exclusively with the `situation in Drosophila and more speci®cally the melanogaster branch of the genus'. To what extent these ®ndings can be generalized and extended to all animals and all eukaryotes remains a question. I for one am impressed by the extraordinary diversity of speciation phenomena in animals and plants. At one extreme are the African cichlid ®shes in which reproductive communities, that is biological species, are seemingly kept isolated by a few male preference genes of the females; at the other end are such cases of slow speciation as that of two diverse groups of plants. One in eastern North America, the other at the Paci®c coast of eastern Asia which became geographically isolated at least 8 million years ago and yet remain members of a single species without the development of cross sterility or of taxonomic differences. Postmating isolating mechanisms are important in Drosophila but seem to be absent in certain groups of birds. Species of the duck genus Anas seem to be perfectly fertile with each other and in backcrosses although they coexist over much of the northern hemisphere with about one hybrid in 20 000 collected ducks. Considering how many different de®nitions of the species concept we now have, one might expect a number of different speciation processes. I have commented on this problem in two of my recent papers (Mayr, 1988, 1996). Most of the so-called species concepts are simply ways of delimiting species taxa. There really are only two species concepts, the typological one, species status determined by degree of difference, and the biological one, the species denoting a reproductive community. With the typological species concept having been abandoned for sexually reproducing populations by all knowledgeable authors the question becomes, how is a new reproductive community established? The answer is by the acquisition of one or several isolating mechanisms. It seems to depend on the type of organism which is the favoured isolating mechanism. In birds and many other animals it is apparently behaviour. In some groups of grasshoppers it is chromosomal rearrangements (post-zygotic) mechanisms. In Drosophila it is behaviour and sterility genes. Let me now after this preamble, pick up speci®cally some of Wu's statements. If I understand Wu correctly he seems to think that the genic cohesion of a biological species serves as a whole as its isolating mechanism. This is not the traditional concept of the biological species. The reproductive isolation (RI) between two species may be effected by a few genes that prevent interbreeding or by a module of genes doing this. Such genes or modules must ®t harmoniously into the total genotype, but this does not make the genotype as a whole an isolating mechanism. I believe this is where Wu and I would disagree. Wu correctly recognizes the enormous importance of sexual selection in the speciation process of many kinds of animals. This means that an extremely limited number of genes may play a decisive role in establishing a new reproductive community. The recent work on speciation in ®shes (cichlids, sticklebacks, white®shes, etc.) excellently illustrates this phenomenon. It is this which permits sympatric speciation in these groups. All it needs is a strict correlation between a niche preference and a mate preference character. A completely consistent scenario can be inferred without any reference to the genic basis. The sudden occurrence of interspeci®c hybrids among cichlids of Lake Victoria in east Africa when the water became too opaque for the ready recognition of male-speci®c characters illustrates this well. Even within a single genus or family different environmental factors may induce or facilitate speciation. Isolating mechanisms may be merely a by-product of (ecological) adaptation to a new niche or they may be a meiotic accident resulting in an incompatible chromosomal restructuring or be the product of a female preference for a new male variant. For most animal groups, particularly the marine ones, we have no clues whatsoever what facilitates speciation. Here even the nature of the separating barriers between different water masses is unknown. I am not sure that Wu fully understands that the isolating mechanisms are only a small portion of the total genotype. Wu suggests as one possibility `one may insist on the strict application of BSC is complete reproductive isolation across the whole genome'. But why would one postulate `a complete reproductive isolation across the whole genome' when one knows that the isolating mechanism comprises only a very small portion of the genome? But then I do not understand the claim on page 8: `thus the very essence does not have to include RI¢. How important the RI is, is well demonstrated by hybrid zones where two well-balanced well-adapted incipient species meet, produce hybrids of reduced ®tness which are in time eliminated and replaced by new immigrants from the parental semispecies. This shows the importance of RI. In my discussions of the cohesive nature of the genotype of a biological species I have nowhere suggested, so far as I remember, that the genotype as a whole serves as an isolating mechanism. Correspondence: Ernst Mayr, Museum of Comparative Zoology, Harvard University, 26 Oxford Street, Cambridge, MA 02138, USA.