Evolution and Adaptation

by Thomas Hunt Morgan

In parthenogenetic species the female character is supposed to be uniformly stronger, so that it dominates in every contest, “for the fertilized egg in such species develops invariably into a female.” Under certain circumstances, as Castle points out, the parthenogenetic female produces both males and females, and this is also true in the occasional development of the unfertilized egg of the silkworm moth, and of the gypsy moth, in which both male and female individuals are produced by parthenogenesis. These facts show that even in unfertilized eggs both sexes are potentially present; but this might be interpreted to mean that some eggs are male and some female, rather than that each egg has the possibility of both kinds of development. If, however, one polar body is retained in these parthenogenetic eggs, then ex hypothese each egg would contain the potentialities of both sexes (if the polar body were of the opposite sex character). It seems necessary to make this assumption because in some parthenogenetic forms males and females may be produced later by each individual, as in the aphids, and this could not occur if we assume that some parthenogenetic eggs are purely male and some female.

Castle assumes, in fact, that in animals like daphnids and rotifers one polar body only is extruded, and the other (the second) is retained in the egg, and hence the potentiality of producing males is present. In the honey-bee, on the contrary, Castle assumes that both polar bodies are extruded in the unfertilized egg (and there are some observations that support this idea), and since only males are produced from these, he believes it is the female element that has been sent out into the second polar body. This hypothesis is necessary, because Castle assumes that when both elements are present in the bee’s eggs, the female element dominates. “Hence, if the egg which has formed two polar cells develops without fertilization, it must develop into a male. But if such an egg is fertilized, it invariably forms a parthenogenetic female ♀ (♂), that is, an individual in which the male character is recessive. Accordingly the functional spermatozoon must in such cases invariably bear the female character, and this is invariably dominant over the male character when the two meet in fertilization.”

If it should prove generally true that the size of the egg is one of the factors determining the sex, we have still the further question to consider as to whether some eggs are bigger because they are already female, or whether all eggs that go beyond a certain size are females, and all those that fail to reach this are males. If this is the case, an animal might produce more females if the external conditions were favorable to the growth of the eggs, and if in some cases these large eggs were capable of developing, parthenogenetic races might become established. Should, however, the conditions for nutrition become less favorable, some of the eggs might fall below the former size and produce males. It is not apparent, however, why all the fertilized autumn eggs of the aphids should give rise to females, for although these eggs are known to be larger than the summer eggs, yet they are produced under unfavorable conditions.

The preceding discussion will show how far we still are from knowing what factors determine sex. Castle’s argument well illustrates how many assumptions must be made in order to make possible the view that sex is a predetermined quality of each germ-cell. Even if these assumptions were admissible, we still return to the old idea that the fertilized egg has both possibilities, and something determines which shall dominate. Until we have ascertained definitely by experimental work whether the sex in some forms can be determined by external conditions, it is almost worthless to speculate further. Whatever decision is reached, the conclusion will have an immediate bearing on the question to be next discussed. Meanwhile, we can at least examine some of the theories that have been advanced as to what advantage, if any, has been gained by having the individuals of many classes divided into two kinds, male and female.

Sex as a Phenomenon of Adaptation

Of what advantage is it to have the individuals of many species separated into males and females? It is obviously a disadvantage from the point of view of propagation to have half of the individuals incapable of producing young, and the other half also incapable of doing so, as a rule, unless the eggs are fertilized by the other sex. Is there any compensation gained because each new individual arises from two parents instead of from one? Many answers have been attempted to these questions.

At the outset it should be recognized that we are by no means forced to assume, as is so often done, that because there is this separation of the sexes it must have arisen on account of its advantage to the species. Whether the result may be of some benefit regardless of how it arose, may be an entirely different question. It would be extremely difficult to weigh the relative advantages (if there are any) and disadvantages (that are obvious as pointed out above), nor is it probable that in this way we can hope to get a final answer to our problem. We may begin by examining some of the modern hypotheses that have been advanced in this connection.

Darwin has brought together a large number of facts which appear to show the beneficial effects of the union of germ-cells from two different individuals. Conversely, it is very generally believed, both by breeders and by some experimenters, that self-fertilization in the case of hermaphroditic forms leads, in many cases, though apparently not in all, to the production of less vigorous offspring. Darwin’s general position is that it is an advantage to the offspring to have been derived from two parents rather than to have come from the union of the germ-cells of the same individual, and he sees, in the manifold contrivances in hermaphroditic animals and plants to insure cross-fertilization, an adaptation for this purpose.

This question of whether self-fertilization is less advantageous than cross-fertilization is, however, a different question from that of whether non-sexual methods of reproduction are less advantageous than sexual ones. Since some plants, like the banana, have been propagated for a very long time solely by non-sexual methods without any obvious detriment to them, it is at first sight not easy to see what other advantage could be gained by the sexual method. The case of the banana shows that some forms do not require a sexual method of propagation. Other forms, however, are so constituted, as we find them, that they cannot reproduce at the present time except by the sexual method. In other words, the latter are now adapted, as it were, to the sexual method, and there is no longer any choice between the two methods. The question of whether a non-sexual form might do better if it had another method of propagation is not, perhaps, a profitable question to discuss.

What we really need to know is whether or not the sexual method was once acquired, because it was an advantage to a particular organism, or to the species to reproduce in this way. It is assumed by many writers that this was the case, but whether they have sufficient ground for forming such an opinion is our chief concern here. On the other hand, it is conceivable, at least, that if the sexual method once became established, it might continue without respect to any superiority it gave over other methods, and might finally become a necessary condition for the propagation of particular species. Thus the method would become essential to propagation without respect to whether the species lost more than it gained. Whichever way the balance should turn, it might make little difference, so long as the species was still able to propagate itself.