In other species the sexes differ so widely in external appearance that it is difficult to believe that the male and the female belong to the same species. Between these two extremes are a great number of species in which the sexes are more or less dissimilar. Those species in which the sexes differ in appearance are said to be sexually dimorphic. The phenomena of sexual dimorphism are fatal to that form of Neo-Darwinism which sees in natural selection an explanation of all the peculiarities of animal structure and colouration.
It is not easy to understand how natural selection can have caused marked sexual dimorphism in a species where the habits of the sexes are the same, in the Paradise Flycatcher (Terpsiphone paradisi), for example, where the cock and the hen obtain their food in the same way, and share equally the duties of nest-building, incubation, and feeding the young.
Of course, in all species where each individual carries only one of the two kinds of sexual organs, there must of necessity be some slight difference between the individuals that carry the male organ, which performs one function, and those that carry the female organ, which performs another function.
But in many species the sexes display differences which have no direct connection with the generative organs—for example, the deer, where the stag alone has horns.
Those characters which differ with the sex, but are not directly connected with the organs of reproduction, are known as secondary sexual characters.
QUEEN WHYDAH
This species (Tetraenura regia) is a typical example of seasonal sexual dimorphism, the male being long-tailed and conspicuously coloured only during the breeding season, and at other times resembling the sparrow-like female.
Theory of Sexual Selection
In nearly all species where the male and female differ in beauty, it is the male who surpasses the female. Natural selection is, in many cases, not able to explain the origin of these differences, or why, when they occur, the male should be more beautiful than the female. This Darwin saw. In order to account for the phenomena of sexual dimorphism, he formulated the theory of sexual selection. This hypothesis is based on the assumption that there is, in all species of animals, a competition among the males to secure females as mates. It is not difficult to understand how this competition arises in polygamous species. Assuming that approximately equal numbers of males and females are born (an assumption which appears to be justified as regards the majority of species), it is clear that for every male who secures more than one wife, at least one male will be obliged to live in a state of single blessedness.