Evolution.

Society for Experimental Biology

Date:: 1953

Catalogue details

Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Evolution. Source: Wellcome Collection.

19/484 (page XV)

FOREWORD XV is to say the capacity of individuals for appropriate response to environ¬ mental changes. Thoday is quite aware of the difficulty of assessing the last of these characteristics. Is an organism which achieves the same form in a variety of environments to be regarded as inflexible, or as flexible because it can adjust its developmental processes to give a nearly constant end result? It is perhaps easier to specify the kinds of genetic system which make for long-term fitness, and Thoday of course stresses the importance of outbreeding. So did Darwin; so do 1. But I suspect that all of us may have been influenced by a deep-seated and irrational objection to incest, and that some lines of bacteria or diatoms may have persisted for lo® years without sexual reproduction. Thoday has raised some fundamental ques¬ tions. I do not think that he would claim finality for his answers. Four papers, by Baker, Valentine, Steven and Spurway, dealt with the genetics of closely related species, subspecies or races. If Darwin was right in regarding the origin of species as the fundamental problem of evolution, such work is very important. If Watson is correct in stating ' that the origin of species is a phenomenon which has little to do with the main course of evolution, that it is dependent on accidental and localized occurrences', such work is of less interest. It is at least likely that when enough of such studies has accumulated, our successors will be able to decide whether Darwin or Watson was more nearly right. Baker used the subspecies of Armeria maritima and the species of Limonium to exemplify the principle that obligatorily outbreeding plants are generally more variable in a given locality than self-compatible ones. Apomixis favours the establishment of a great variety of very uniform local races. He goes on to consider the effect of introgressive hybridization on a species as a function of its breeding system. Many of his generalizations are true for animals also. For example, the fact that young geese of several species remain near their parents, while young ducks usually disperse, favours the formation of subspecies in geese. I cannot help wondering whether the very distinct salt-marsh ecotype of Armeria maritima, which at present interbreeds freely with the land form, will ultimately achieve genetic isolation and specific rank. Valentine gave numerous data on the effects of hybridization between five species of the Vernales group of Primula. Many of the differences in their taxonomic characters are controlled by small numbers of genes. But the viability of the hybrid seeds and the fertility of the hybrid plants seem to depend on other sets of genes. P. elatior and P. veris are morphologically similar, but each of them is easier to cross with P. vulgaris, which is morphologically unlike them, than with the other. This incompatibiUty may have arisen by natural selection.