Several mechanisms have been put forward to explain mate choice:
1) Direct phenotypic effects
Female preference for a male attractiveness can evolve as a result of direct phenotypic benefits if the morphology reflects the ability of the male to provide material advantages, such as a high-quality territory, nutrition, parental care or protection.
There is considerable empirical support for this mechanism on some animal taxa. Female choice might also evolve as a result of resistance to direct costs imposed by males.
2) Sensory bias.
Female preference favouring a male ornament can initially evolve under natural selection for other reasons. Males evolving traits that exploit this bias then become favoured by mate choice. There is increasing phenotypic evidence that some male traits initially evolved through female sensory biases, but the evolution of female sensory bias itself requiresmore testing.
3) Fisherian sexy sons
If there are genetic components to variance in female preference and male trait, a female choosing a male with a sexy trait bears daughters and sons that can both carry alleles for a sexy trait, and for the preference for it. This genetic coupling might lead to self-reinforcing coevolution between trait and preference.
Direct critical testing of this mechanism is difficult, but molecular genetics offers new possibilities (see main text).
4) Indicator mechanisms (‘good genes’ or ‘handicap mechanisms’)
It suggest that attractive male traits reflect broad genetic quality. Inherent in such mechanisms is the maintenance of genetic variation, the ‘paradox of the lek’, and parasite- and pathogen-mediated mechanisms have been suggested as potential solutions. In addition, other advantageous genes and relative freedom from deleterious mutations might lead to high male condition and expression of sex traits .
Female preference for such traits can provide genetic benefits to those of her offspring that inherit favourable alleles from their father. The resolution of the lek paradox remains a crucial area for sexual selection research.
5) Genetic compatibility mechanisms.
As well as additive genetic benefits reflected by indicator traits, there might be non-additive benefits from choosing a mate with alleles that complement the genome of the chooser. Examples have been found for instance in major histocompatibility complex genes, and compatibility advantages might be one adaptive reason for multiple mating by females.
The evolution of mate choice is based either on direct selection of a preference that gives a fitness advantage (mechanisms 1–2) or on indirect selection of a preference as it becomes genetically correlated with directly selected traits (mechanisms 3-4)
In addition, rather than favouring any particular display trait, mate choice might evolve because it conveys non-additive genetic benefits (mechanism 5).
These mechanisms are mutually compatible and can occur together, rendering the evolution of mating preferences a multiple-causation problem, and calling for estimation of the relative roles of individual mechanisms. Several diagnostic differences among the mechanisms suggest ways in which they can be tested by quantitative genetic analyses.