Mate choice is one of the primary mechanisms under which evolution can occur. It is characterized by a "selective response by animals to particular stimuli" which can be observed as behavior.Bateson, Paul Patrick Gordon. "Mate Choice." Mate Choice, Cambridge University Press, 1985 In other words, before an animal engages with a potential mate, they first evaluate various aspects of that mate which are indicative of quality—such as the resources or phenotypes they have—and evaluate whether or not those particular trait(s) are somehow beneficial to them. The evaluation will then incur a response of some sort.
These mechanisms are a part of evolutionary change because they operate in a way that causes the qualities that are desired in a mate to be more frequently passed on to each generation over time. For example, if female peacocks desire mates who have a colourful plumage, then this trait will increase in frequency over time as male peacocks with a colourful plumage will have more reproductive success. Further investigation of this concept, has found that it is in fact the specific trait of blue and green colour near the eyespot that seems to increase the females likelihood of mating with a specific peacock.
Mate choice is a major component of sexual selection, another being intrasexual selection. Ideas on sexual selection were first introduced in 1871, by Charles Darwin, then expanded on by Ronald Fisher in 1915. At present, there are five sub mechanisms that explain how mate choice has evolved over time. These are direct phenotypic benefits, sensory bias, the Fisherian runaway hypothesis, indicator traits and genetic compatibility.
In the majority of systems where mate choice exists, one sex tends to be competitive with their same-sex members and the other sex is choosy (meaning they are selective when it comes to picking individuals to mate with). There are direct and indirect benefits of being the selective individual. In most species, females are the choosy sex which discriminates among competitive males, but there are several examples of reversed roles (see below). It is preferable for an individual to choose a compatible mate of the same species, in order to maintain reproductive success. Other factors that can influence mate choice include pathogen stress and the major histocompatibility complex (MHC).
Origins and history
Charles Darwin first expressed his ideas on sexual selection and mate choice in his book The Descent of Man, and Selection in Relation to Sex in 1871. He was perplexed by the elaborate ornamentation that males of some species have, because such features appeared to be detrimental to survival and to have negative consequences for reproductive success. Darwin proposed two explanations for the existence of such traits: these traits are useful in male-male combat or they are preferred by females.Darwin, C. (1871). The Descent of Man, and Selection in Relation to Sex. John Murray, London. This article focuses on the latter. Darwin treated natural selection and sexual selection as two different topics, although in the 1930s biologists defined sexual selection as being a part of natural selection.Miller, Geoffrey (2000). The mating mind: how sexual choice shaped the evolution of human nature, London, Heinemann, (also Doubleday, ) p. 8
In 1915, Ronald Fisher wrote a paper on the evolution of female preference and secondary sexual characteristics. Fifteen years later, he expanded this theory in a book called The Genetical Theory of Natural Selection. There he described a scenario, Fisherian runaway, where feedback between mate preference and a trait results in elaborate characters such as the long tail of the male peacock.
In 1948, using Drosophila as a model, Angus John Bateman presented experimental evidence that male reproductive success is limited by the number of mates obtained, while female reproductive success is limited by the number of pregnancies that she can have in her lifetime. Thus a female must be selective when choosing a mate because the quality of her offspring depends on it. Males must fight, in the form of intra-sexual competition, for the opportunity to mate because not all males will be chosen by females. This became known as Bateman's principle, and although this was a major finding that added to the work of Darwin and Fisher, it was overlooked until George C. Williams emphasised its importance in the 1960s and 1970s.Williams, G.C. (1966). Adaptation and Natural Selection. Princeton University Press, Princeton, NJWilliams, G.C. (1975). Sex and evolution. Princeton University Press, Princeton, NJ
In 1972, soon after Williams' revival of the subject, Robert L. Trivers presented his parental investment theory. Trivers defined parental investment as any investment made by the parent that benefits his or her current offspring at the cost of investment in future offspring. These investments include the costs of producing gametes as well as any other care or efforts that parents provide after birth or hatching. Reformulating Bateman's ideas, Trivers argued that the sex which exhibits less parental investment (not necessarily the male) will have to compete for mating opportunities with the sex that invests more. The differences in levels of parental investment create the condition that favours mating biases.
Direct and indirect benefits The act of being choosy was likely selected for as a way to assess whether or not a potential partner's contribution(s) would be capable of producing and/or maintaining the viability of an offspring. Utilizing these behaviors usually results in two types of benefits to the individual who is being choosy:
Direct benefits increase the fitness of the choosy sex through direct material advantages or resources. These benefits include but are not limited to increased territory quality, increased parental care, and protection from predators. There is much support for maintenance of mate choice by direct benefits and this approach offers the least controversial model to explain discriminate mating. Indirect benefits increase genetic fitness for the offspring, and thereby increase the parents' inclusive fitness. When it appears that the choosy sex does not receive direct benefits from his or her mate, indirect benefits may be the payoff for being selective. These indirect benefits may include high-quality genes for their offspring (known as adaptive indirect benefits) or genes that make their offspring more attractive (known as arbitrary indirect benefits).
Inbreeding avoidance
Usually, animal biologists assume that mate choice is biased against relatives because of the negative consequences of inbreeding.Dorsey OC, Rosenthal GG. A taste for the familiar: explaining the inbreeding paradox. Trends Ecol Evol. 2023 Feb;38(2):132-142. doi: 10.1016/j.tree.2022.09.007. Epub 2022 Oct 11. PMID 36241551 However certain natural constraints act to limit the evolution of inbreeding avoidance, particularly when there is a risk of mating with a partner of a different species (heterospecific mating) and losing fitness through hybridization. Inclusive fitness appears to be maximized in matings of intermediately related individuals.Puurtinen M. Mate choice for optimal (k)inbreeding. Evolution. 2011 May;65(5):1501-5. doi: 10.1111/j.1558-5646.2010.01217.x. Epub 2011 Jan 5. PMID 21521199
Mechanisms , five proposed mechanisms address the evolution of mate choice: Direct phenotypic benefits Sensory bias Fisherian runaway Indicator traits Genetic compatibility
Direct and/or indirect benefits drive the mating biases described in each mechanism. It is possible that these mechanisms co-occur, although the relative roles of each have not been evaluated adequately.
Direct phenotypic benefits A choosy mate tends to have preferences for certain types of traits—also known as phenotypes—which would benefit them to have in a potential partner. These traits must be reliable, and communicative of something that directly benefits the choosy partner in some way. Having a mating preference is advantageous in this situation because it directly affects reproductive fitness. Direct benefits are widespread and empirical studies provide evidence for this mechanism of evolution.Moller, A.P. (1994). Sexual selection and the barn swallow. Oxford University Press, Oxford.
One example of a sexually selected trait with direct benefits is the bright plumage of the northern cardinal, a common backyard bird in the eastern United States. Male northern cardinals have conspicuous red feathers, while the females have a more cryptic coloration. In this example, the females are the choosy sex and will use male plumage brightness as a signal when picking a mate — research suggests that males with brighter plumage feed their young more frequently than males with duller plumage. This increased help in caring for the young lifts some of the burden from the mother so that she can raise more offspring than she could without help.
Though this particular mechanism operates on the premise that all phenotypes must communicate something that benefits the choosy mate directly, such selected phenotypes can also have additional indirect benefits for the mother by benefiting the offspring. For example, with the increased help in feeding their young seen in Northern Cardinals with more plumage-brightness, comes an increase in the overall amount of food that is likely to be given to the offspring - even if the mother has more children. Though females may choose this trait with the presumed directly advantageous aim of allowing them more time and energy to allocate to producing more offspring, it also benefits the offspring in that two parents provide food instead of one, thereby increasing the likelihood of the overall amount of food available to the offspring despite a possible increase in the amount of offspring siblings.
Sensory bias
The sensory-bias hypothesis states that the preference for a trait evolves in a non-mating context and is then exploited by the less choosy sex in order to obtain more mating opportunities. The competitive sex evolves traits that exploit a pre-existing bias that the choosy sex already possesses. Following this hypothesis, increased selectivity for one of these specific traits can explain remarkable trait differences in closely related species because it produces a divergence in signaling systems which leads to reproductive isolation.
Sensory bias has been demonstrated in guppies, freshwater fish from Trinidad and Tobago. In this mating system, female guppies prefer to mate with males with more orange body-coloration. However, outside of a mating context, both sexes prefer animate orange objects, which suggests that preference originally evolved in another context, like foraging. Orange fruits are a rare treat that fall into streams where the guppies live. The ability to find these fruits quickly is an adaptive quality that has evolved outside of a mating context. Sometime after the affinity for orange objects arose, male guppies exploited this preference by incorporating large orange spots to attract females.