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

Secondary sexual traits frequently have the character of very complicated morphological structures or ornaments on the surface of the body. Their formation is not only the result of a great many genes, but is also affected by a number of various external factors (Beldade & Brakefield 2002). These factors can include the state of health of the individual. The oldest indicator hypothesis assumes that the female selects a male just on the basis of those traits whose expression is most affected by the state of health of the individual.

            If, for example, birds attacked by parasites have shorter feathers, it will certainly be advantageous for females to reproduce with males with longer feathers. To begin with, the female can become infested during copulation and transfer this infestation to her offspring and, in addition, in case of biparental care for the offspring, they can also become infested (Able 1996). In species with biparental care, it must also be recalled that a healthy male is more viable and can devote more energy to care for offspring.

            It has been found experimentally that the degree of expression of secondary sexual traits is correlated with the degree of infestation by parasites in a great many species (Burley, Tidemann, & Halupka 1991; Saino & Moller 1994). Similarly, it has been found that females prefer uninfested males. In birds, females use visual stimuli, which are frequently bound to secondary sexual traits, to determine the degree of infestation by parasites. In mammals and a great many other groups of animals, smell stimuli tend to be more important.

            In order for a trait to function as an indicator of state of health, it is necessary for it to have the smallest possible heritability and react as sensitively as possible to the conditions in the environment, specifically to the momentary state of health of the organism. It has been demonstrated experimentally that secondary sexual traits are frequently extremely sensitive to external effects, for example to parasites or the amount of nutrition (Fitze & Richner 2002; Griffith & Sheldon 2001). It has even been observed that the degree of expression of a secondary sexual trait in birds is not correlated with the degree of expression of this trait in its genetic father, but is highly correlated with the degree of its expression in its adoptive father (Fig. XV.8) (Griffith, Owens, & Burke 1999). It can be assumed that males in good health are apparently capable of providing the offspring with the greatest amount of food and thus favorably affect both their state of health and the degree of expression of their secondary sexual traits.s