XXI.5.6.1 In species with sexual selection based on female choice, there is a constant tendency towards both phyletic and branching speciation through coevolutionary lift
Sexual selection can basically occur through two different mechanisms. The members of a certain sex, in the vast majority of cases males, can do battle for access to sexual partners or they can be selected as their sexual partners by the members of the opposite sex on the basis of more or less arbitrary criteria.
In the latter case, objective preconditions are formed for the manifestation of a continuously acting tendency towards the creation of new kinds of species by the coevolutionary lift mechanism (autoelection). Amongst females, there is a certain constant variability in the genes determining the traits according to which they will select their sexual partners. As soon as a new trait appears in the population and, simultaneously, in the population of females a gene for preference for this trait emerges, favorable conditions are formed for positive assortative mating of the bearers of this trait and the bearers of the genes for its preference, and thus for branching off and partial reproductive isolation of a certain part of the population. If the presence of the new trait were to reduce the attractiveness of the particular male for the other females and increase his attractiveness only for the subpopulation of females bearing the gene for its preference, the presence of the two complementary genes will constantly increase in the population. The reasons for the increase in the frequency of genes for preferred trait in the population are obvious. However, the genes for preference for the particular trait are also at an advantage. A bearer of the trait (male) is most probably also simultaneously a latent bearer of the gene for preference for this trait. His father was most probably a male who was also a bearer of this trait and therefore his mother was a female that most probably preferred this trait, as she selected a bearer of this trait as her sexual partner. The genes that cause that the female prefer the bearer of a certain trait thus also assist in its preferential spreading, to be more exact spreading of its own copies, which are most probably contained in the genotype of preferred individuals. The fact that there is a simultaneous increase in the frequency of both traits in the population accelerates the coevolutionary lift process because of positive feedback and the gene for the new trait and the gene for its preference are fixed in the population.
The evolution of the system does not end with fixation of the two complementary genes. As soon as all the males in the local population carry the new trait, females that lose the ability to recognize the presence or absence of this trait in their sexual partners cease to be “penalized”. Thus, the gene for preference for the particular trait can gradually disappear from the population. In contrast, males that would lose the particular trait will be selectionally “penalized” as long as at least some females remain in the population that bear a functional gene for preference for the relevant trait. Consequently, males will have a tendency during evolution to gradually accumulate and fix genes for marked secondary sexual traits, while in females the genes for preference for certain sexual traits will tend to periodically fluctuate and accumulate in the population maximally in the form of more or less selectionally neutral polymorphism.
The evolution of male secondary sexual traits and female preference traits can be important from the viewpoint of speciation, for example, when the species has a discontinuous range of occurrence and a number of various traits, according to which the females select their sexual partners, are gradually independently fixed in each individual population. As these traits are fixed in separated populations, some of them can be mutually exclusive or the differences between the males in two populations can be so great that the males of one population can become completely unattractive for the females of the other population. Thus, internal prezygotic reproduction barriers can be created as a consequence of the action of sexual selection through female choice.
If two new sexual traits spread simultaneously in a single population, which can occur quite frequently in species with large unstructured populations, three basic alternatives can occur. If these are traits whose expression in a single individual is mutually excluded, and if the population is otherwise panmictic, it is highly probable that the trait that is preferred by a greater percentage of females will predominate in the population. The bearers of this trait have a greater chance of reproducing than the bearers of the alternative trait. However, the suppression of one of the traits does not mean that its bearers and their progeny, i.e. all its alleles located in other loci, would be eliminated from the population by selection. As the gene flow barriers formed by preference for sexual partners are mostly permeable, i.e. individuals sometimes make a mistake in selection of the right sexual partner, and the individual genes can “move” between the chromosomes bearing various alleles for the preferential and preferred genes through genetic recombination, probably only the genes for the particular trait, genes for its preference and also alleles in immediately neighboring loci will be eliminated.
The second possibility occurs if the traits are also mutually exclusive, but the population is spatially structured and new traits spread towards one another from two different areas. A more or less sharp boundary in male phenotypes will tend to exist at places where the two populations meet. If a male passes into the territory in which females preferring bearers of the opposite trait predominate, he will be at a disadvantage in sexual selection and will leave fewer progeny. Simultaneously, there need not be practically any barrier between the two forms for the passage of other genes than genes for the sexual trait and genes for its preference. As soon as a male in the territory of the other form crosses with the local female form, some of its sons will inherit from the female the genes for the alternative form of the sexual trait and some of his daughters will inherit the gene for preference for this trait, so that they will not be at a disadvantage in competition with the other members of the population and will be able to freely pass their genes down in the second part of the population.
The third possibility occurs when the expression of both traits is not mutually exclusive. In this case, both traits will probably be fixed.
The importance of sexual selection in speciation is not entirely clear at the present time. However, meta-studies have indicated that, of 15 formerly published comparative studies, twelve demonstrated a positive correlation between the level of sexual dimorphism and species diversity in the relevant taxon (Panhuis et al. 2001). This could mean that species in which this mechanism is important also actually exhibit a greater rate of speciation. On the other hand, from the viewpoint of anagenesis, the coevolutionary lift mechanism could explain the fact that there are very great differences between related species in traits visible on the surface, i.e. in coloration, patterns or structure of the surface. This mechanism could, in fact, be responsible for “address phenomena”, whose existence was pointed out in the mid-twentieth century by A. Portmann (Portmann 1960). It is very striking that most organisms have a tendency towards “self presentation”, i.e. have various ornaments, bright colors or structures on their surfaces. In contrast, most internal organs are usually uniform in color, aesthetically uninteresting or even ugly. It is frequently very difficult to imagine a biological function that these surface structures, which are part of the self presentation of the members of a certain species, could fulfill for their bearers, e.g. some marine turbellarians or opistobrancial gastropods. It is quite possible that, in actual fact, these traits do not provide any advantage for their bearers, but only provide an advantage for those genes that encode their formation, and the genes that enable the members of the other sex to recognize these traits. Any mutation can be fixed by genetic drift. However, the probability of fixation of a new mutation by drift is very low. Mutations advantageous for the aspect of survival can become fixed by natural selection; however, this mechanism is also not very efficient in polymorphous populations of species with sexual reproduction (see IV.9). In contrast, practically any mutation that is capable of sufficiently obvious manifestation in the appearance of its bearer can be very efficiently fixed in sexually reproducing species through the coevolutionary lift mechanism. Thus, coevolutionary lift could be responsible for a large portion of self-presentation phenomena manifested in living organisms and thus for a large part of the aesthetics of living nature.