Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

There are two basic types of genetic sex determination, the Drosophila and Abraxas types (See Chap. I). In species of the Drosophila type, which also includes humans, males have two kinds of  sex chromosomes (the X –chromosome and the Y-chromosome)  and they are thus heterogametic, i.e. produce two types of gametes, while females have two copies of the same sex chromosome (the X-chromosome).  In the Abraxas type, which includes birds and butterflies, the males are homogametic, and the females are heterogametic, i.e. they have two kinds of sex chromosomes (the W-chromosome and the Z-chromosome) and produce two types of gametes.

            In species with homogametic females, there is an evolutionary obstacle to the formation of genes controlling preference for sex partners with traits reducing the fitness (Hastings 1994). In subsequent generations, such a gene occurs in the genome of the males, i.e. individuals bearing the relevant disadvantageous trait. Thus, genes for preference of the unsuitable trait are “penalized” in the following generation. In contrast, in species with heterogametic females, the genes for preference for males with disadvantageous (excessive) traits can occur on an unpaired sex chromosome (on a W-chromosome). This can never get into the genome of sons, in which the particular disadvantageous secondary sexual trait is expressed, but will always be only in the genome of daughters.

            If this principle is actually valid in evolution, then it can be expected that a preference for males with disadvantageous traits will be found especially in species with heterogametic females, e.g. birds and butterflies and that the genes for preference for these traits will mostly be encoded by unpaired sex chromosomes (W-chromosomes). The first assumption has been confirmed to a certain degree; striking colors and structures that are a disadvantage for their bearers occur frequently in birds, while traits useful in battles between males tend to predominate in mammals. The second precondition, i.e. more common occurrence of genes for preference for a certain trait on W-chromosomes has apparently not yet been systematically tested.