HYBRIDIZATION AS A SOURCE OF VARIATION FOR ADAPTATION TO NEW ENVIRONMENTS.

Abstract

We generally tend to think of a species as being characterized by particular geographical and ecological ranges, ranges which, except for minimum fluctuations at the margins, are as much characteristic of the species as morphology. Of course, we have many examples of radical expansion of the geographic range of a species. By and large such expansions are the result of the introduction, by man, of the species into an area that was previously too remote for propagules to arrive in the normal course of events or else a radical remaking of the environment, again by man usually, so that it is now hospitable to a new species. But both of these kinds of events are geographical expansions without being ecological expansions. Many such cases are documented in Elton (1958). A little reflection makes it obvious that real ecological expansions involving adaptation to new environments must be the usual occurrence in speciation and that the ecological range of any species is the result of an evolutionary process involving more or less profound genetic changes. The rarity with which such a change has been observed is undoubtedly a reflection of the fact that this process occurs only occasionally in the evolution of a given species, perhaps only once. Moreover, when it does occur, it may be a very slow process, thus difficult to observe. It is, nevertheless, a process of supreme importance both to the species and the community, and if a case could be found of a species rapidly expanding its ecological range, caught in flagrante delictu, it might be possible to study the genetic basis of such a change. It is necessary to distinguish cases of new adaptation from cases of preadaptation since both will appear as ecological expansions, but the latter involves little or no genetic reorganization. Many cases of insect pests attacking and decimating cultivated plants are cases of preadaptation as are instances of diseases spreading to new hosts. If a phytophagous insect is placed suddenly in contact with a new host and is not excluded by other phytophagous animals or predators, we might expect quite often a successful invasion of the new host and even a shift from the old host without major genetic change. How are we to distinguish such preadapted ecological shifts from cases of real change in the adaptive norm of a species? We might suspect the latter was happening if long established boundaries of its distribution began to move despite lack of change in relevant components of of the environment in the invaded area. Of course, we may never be certain that some subtle but relevant change has not occurred. However, this is a limitation that all studies of ecology and genetics of natural populations suffer from. One can never be certain he has exhausted the relevant aspects of the environment. One can only be critical, and hope. It is the purpose of this paper to describe a case of a species rapidly expanding its ecological tolerance and geographical range, and to give the results of some experiments that show how hybridization between species or near-species can provide the genetic variation necessary for natural selection to produce a new adaptive norm.