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al 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 9 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 10 in 1915 ronald fisher wrote a paper on the evolution of female preference and secondary sexual characteristics 11 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 12 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 13 14 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 15 direct and indirect benefits edit 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 5 and this approach offers the least controversial model to explain discriminate mating 6 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 7 inbreeding avoidance edit usually animal biologists assume that mate choice is biased against relatives because of the negative consequences of inbreeding 16 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 16 inclusive fitness appears to be maximized in matings of intermediately related individuals 17 mechanisms edit as of 2018 update 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 4 direct phenotypic benefits edit 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 18 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 19 20 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 21 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 22 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 22 sensory bias edit see also sensory trap hypothesis 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 23 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 24 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 another example of sensory exploitation is the case of the water mite neumania papillator an ambush predator which hunts copepods small crustaceans passing by in the water column 25 when hunting n papillator adopts a characteristic stance termed the net stance it holds its first four legs out into the water column with its four hind legs resting on aquatic vegetation this allows it to detect vibrational stimuli produced by swimming prey and to use this to orient towards and clutch at prey 26 during courtship males actively search for females 27 if a male finds a female he slowly circles around the female whilst trembling his first and second leg near her 25 26 male leg trembling causes females who were in the net stance to orient towards and often to clutch the male 25 this does not damage the male or deter further courtship the male then deposits spermatophores and begins to vigorously fan and jerk his fourth pair of legs over the spermatophore generating a current of water that passes over the spermatophores and towards the female 25 sperm packet uptake by the female would sometimes follow 25 heather proctor hypothesised that the vibrations made by trembling male legs mimic the vibrations that females detect from swimming prey this would trigger the female prey detection responses causing females to orient and then clutch at males mediating courtship 25 28 if this was true and males were exploiting female predation responses then hungry females should be more receptive to male trembling proctor found that unfed captive females did orient and clutch at males significantly more than fed captive females did consistent with the sensory exploitation hypothesis 25 other examples of the sensory bias mechanism include traits in auklets 29 wolf spiders 30 and manakins 31 further experimental work is required to reach a fuller understanding of the prevalence and mechanisms of sensory bias 32 fisherian runaway and sexy son hypothesis edit main articles fisherian runaway and sexy son hypothesis this creates a positive feedback loop in which a particular trait is desired by a female and present in a male and that desire for and presence of that particular trait are then reflected in their offspring 22 if this mechanism is strong enough it can lead to a type of self reinforcing coevolution 22 if runaway selection is strong enough it may incur significant costs such as increased visibility to predators and energetic costs to maintain the trait s full expression hence peacocks extravagant feathers or any number of lek mating displays this model does not predict a genetic benefit rather the reward is more mates in a study done on great reed warblers models based on the polygyny threshold and sexy son hypotheses predict that females should gain evolutionary advantage in either short term or long term in this mating system although the importance of female choice was demonstrated the study did not support the hypotheses 33 other studies such as those conducted on long tailed widowbirds have demonstrated the existence of female choice 34 here females chose males with long tails and even preferred those males with experimentally lengthened tails over shortened tails and those of naturally occurring length such a process shows how female choice could give rise to exaggerated sexual traits through fisherian runaway selection indicator traits edit this section needs more citations please help improve this section by adding citations to reliable sources unsourced material may be challenged and removed january 2017 learn how and when to remove this message indicator traits signal good overall quality of the individual traits perceived as attractive must reliably indicate broad genetic quality in order for selection to favor them and for preference to evolve this is an example of indirect genetic benefits received by the choosy sex because mating with such individuals will result in high quality offspring the indicator traits hypothesis is split into three highly related subtopics the handicap theory of sexual selection the good genes hypothesis and the hamilton zuk hypothesis people rate the importance of certain traits differently when referring to their own or to others ideal long term partners research suggests that women consider traits indicating genetic fitness as more important for their own partner while prioritising traits that provide benefits to others for their sister s ideal partner 35 indicator traits are condition dependent and have associated costs therefore individuals which can handle these costs well cf i can do x here survive with one hand tied behind my back should be desired by the choosy sex for their superior genetic quality this is known as the handicap theory of sexual selection 36 the good genes hypothesis states that the choosy sex will mate with individuals who possess traits that signify overall genetic quality in doing so they gain an evolutionary advantage for their offspring through indirect benefit the hamilton zuk hypothesis posits that sexual ornaments are indicators of parasite and disease resistance 37 to test this hypothesis red jungle fowl males were infected with a parasitic roundworm and monitored for growth and developmental changes female preference was also evaluated the researchers found that parasites affected the development and final appearance of ornamental traits and that females preferred males who were not infected this supports the idea that parasites are an important factor in sexual selection and mate choice 38 one of many examples of indicator traits is the condition dependent patch of red feathers around the face and shoulders of the male house finch this patch varies in brightness among individuals because the pigments that produce the red color carotenoids are limited in the environment thus males who have a high quality diet will have brighter red plumage in a much cited manipulation experiment female house finches were shown to prefer males with brighter red patches also males with naturally brighter patches proved better fathers and exhibited higher offspring feeding rates than duller males 20 genetic compatibility edit further information genetic incompatibility genetic compatibility refers to how well the genes of two parents function together in their offspring choosing genetically compatible mates could result in optimally fit offspring and notably affect reproductive fitness however the genetic compatibility model is limited to specific traits due to complex genetic interactions e g major histocompatibility complex in humans and mice the choosy sex must know their own genotype as well as the genotypes of potential mates in order to select the appropriate partner 39 this makes testing components of genetic compatibility difficult and controversial role of major histocompatibility complex edit main article major histocompatibility complex and sexual selection a controversial but well known experiment suggests that human females use body odor as an indicator of genetic compatibility in this study males were given a plain t shirt to sleep in for two nights in order to provide a scent sample college women were then asked to rate odors from several men some with similar mhc major histocompatibility complex genes to their own and others with dissimilar genes mhc ...
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