Evolution and Adaptation

by Thomas Hunt Morgan

[34]. Geddes and Thompson’s “The Evolution of Sex” has been freely used in the preparation of this part of this chapter.

2. Hermaphroditic Forms.—There are many species of animals and plants in which each individual contains both the male and the female organs of reproduction, and there are whole groups in which only these hermaphroditic forms occur. Thus in the ctenophors the eggs develop along one side of each radial canal and spermatozoa along the other. The group of flatworms is almost exclusively hermaphroditic. The earthworms and the leeches have only these bisexual forms, and in the mollusks, while a few groups have separate sexes, yet certain groups of gasteropods and of bivalve forms are entirely hermaphroditic.

In the common garden snail, although there are two sets of sexual ducts closely united, yet from the same reproductive sac both eggs and sperm are produced. The barnacles and the ascidians are for the most part hermaphroditic forms. Many other examples might be cited, but these will suffice to show that it is by no means unusual in the animal kingdom for the same individual to produce both male and female germ-cells. However, one of the most striking facts in this connection is that self-fertilization seldom takes place, so that the result is the same in certain respects as though separate sexes existed. This point will come up later for further consideration.

3. Parthenogenetic Reproduction.—It has long been known that, in some cases, eggs that are not fertilized will begin to develop and may even produce new individuals. Tichomiroff showed that by rubbing with a brush the unfertilized eggs of the silkworm moth, a larger percentage would produce caterpillars than if they were not rubbed. During the last few years it has been shown that the development of a non-fertilized egg may be started in a number of ways. Such, for example, as by certain solutions of salt or of sugar, by subjecting the eggs to cold, or by simply shaking them.

There are certain groups of animals in which the males appear only at regular (in others at irregular) intervals. In their absence the females produce eggs that develop without being fertilized, i.e. parthenogenetically. The following examples will serve to show some of the principal ways in which this “virgin reproduction” takes place. In the group of rotifers the males are generally smaller than the females and are usually also degenerate. In some species, although degenerate males are present, they are unnecessary, since parthenogenesis is the rule. In still other species no males exist and the eggs develop, therefore, without being fertilized. In some of the lower crustaceans parthenogenesis occurs in varying degrees. In Apus males may be entirely absent at times in certain localities, and at other times a few, or even very many, males may appear. Some species of ostracod crustaceans seem to be purely parthenogenetic; others reproduce by means of fertilized eggs; and others by an alternation of the two processes. The crustaceans of the genus Daphnia produce two kinds of eggs. The summer eggs are small, and have a thin shell. These eggs develop without being fertilized, but in the autumn both male and female individuals develop from these unfertilized eggs, and the eggs of the female, the so-called winter eggs, are fertilized. These are also larger than the summer eggs, have thicker shells, and are much more resistant to unfavorable conditions. They give rise in the following spring to females only, and these are the parthenogenetic individuals that continue to produce during the summer new parthenogenetic eggs.

It is within the group of insects that some of the most remarkable cases of parthenogenesis that we know are found. In the moth, Psyche helix, only females are present, as a rule, but rarely males have been found. In another moth, Solenobia trinquetrella, the female reproduces by parthenogenesis, but at times males appear and may then be even more numerous than the females. In the gall-wasps parthenogenetic generations may alternate with a sexual generation, and it is interesting to note that the sexual and the parthenogenetic generations are so different that they were supposed to belong to separate species, until it was found that they were only alternate generations of the same species.

The aphids or plant-lice reproduce during the summer by parthenogenesis, but in the autumn winged males and females appear, and fertilized winter eggs are produced. From these eggs there develop, in the following spring, the wingless parthenogenetic summer forms, which produce the successive generations of the wingless forms. As many as fourteen summer broods may be produced. By keeping the aphids in a warm temperature and supplying them with plenty of moist food, it has been possible to continue the parthenogenetic reproduction of the wingless forms for years. As many as fifty successive broods have been produced in this way. It has not been entirely determined whether it is the temperature or a change in the amount, or kind, of food that causes the appearance of the winged males and females, although it seems fairly certain that diminution in the food, or in the amount of water contained in it, is the chief cause of the change.

In the honey-bee the remarkable fact has been well established that fertilized eggs give rise only to females (queens and workers), while unfertilized eggs develop into males. Whether a fertilized egg becomes a queen or a worker (sterile female) depends solely on the kind of food that is given to the young larva, and this is determined, in a sense, entirely by the bees themselves.

In plants also there are many cases of parthenogenesis known. Some species of Chara when kept under certain conditions produce only female organs, and seem to produce new plants parthenogenetically. In this case it appears that the same conditions that caused the plants to produce only female organs may also lead to the development of the egg-cells without fertilization. In fact it is only by a combination of this kind that parthenogenesis could arise. The result is similar when the eggs of insects produce only females whose eggs are capable of parthenogenetic development. If a case should arise in which only females appeared whose eggs did not possess the power of parthenogenetic development, the species would die out.

In the green alga, Spirogyra, it has been found that if conjugation of two cells is prevented, a single cell may become a parthenogenetic cell. In a number of parasitic fungi the male organs appear to be degenerate, and from the female organs parthenogenetic development takes place. A small number of flowering plants are also capable of parthenogenetic reproduction.