The sensory-bias model, as Coyne outlines on pages 166-167, suggests that sexual dimorphisms evolved as a result of biases in the female nervous system – whether it be like the theoretical example that he shared with the mutant male's red coloration on his breast stimulating the female's visual preference for red color, or simply, a stimulation of the nervous system as a result of more extravagant displays. Consider why these preferences are so nonspecific; why might it be 'better' to recognize the red coloration rather than a certain chemical secretion? And to what extent do these animals have to recognize and 'appreciate' different behaviors or physical adaptations (such as shiny fur or long mating calls)? Why might females have adaptations to be able to process these different adaptations on males? Why spend more energy, for instance, developing eye spots to visually choose males if that's their only purpose? Is it worth it?
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Preferences, for the female, most likely evolved by selection (as Darwin suggested). The choice the female makes for her mate has to do with natural selection, benefiting her , as Coyne asserts on pg 162 directly (producing healthier young during childcare) and indirectly (choosing a male with “better” genes).
ReplyDeleteWith that established, why red coloration is favorable to a certain species may explain a certain choice an individual has because on certain preferences. According to Coyne, even closely related species (like the New Guinea bird) choose males with very different types of decoration/coloration and mating behavior. While it may seem nonspecific, these certain distinct preferences arose from natural selection and adaptive advantage to the species over time (pg 162).
Coyne uses other examples to corroborate his assertion that females may prefer certain features of males because those males have, over time, displayed mating behavior fit for the female to have successful offspring. For example, the house finch of North America: the female finch is brown while the male finches range from pale yellow to orange to bright red (bright colors on their head and breast area). Research shows that brighter male finches actually feed their young more often than do lighter males; therefore, the female finches have adapted to prefer brighter finches because this places them at an evolutionary advantage and direct/indirect benefits are attained.
For peacocks, males with more eyespots actually produce young that grow faster and survive better (as a result of “good genes” perhaps). Therefore, it is an evolutionary advantage, once again, for the female to spend energy and time to find a suitable mate that will help pass along the genes of that particular species and is completely worth the risk.
Source: Coyne - why evolution is true
The ”sensory bias” model is only one among several theories that attempt to explicate the sexual preferences of females. Typically, mate selection arising from sensory stimulation will be described as nonadaptive. That is, the sensory system of a female will be preadapted to stimulation by certain traits that may not necessarily correlate with genetic viability. Sensory bias, therefore, is inherently distinct from “honest advertisement of male quality” which stimulates female selection of males with costly ornaments that indicate the “quality” of genes. The sensory signals that give rise to “sensory bias” in females are often non-specific since the traits that elicit a “sensory exploitation” response have not evolved by sexual selection but rather a consequence of the preadapted nature of the female sensory system.
ReplyDeleteThe origin of such preadaptation has long been debated among scientists. Dawkins and Guilford of the University of Chicago suggested in 1996 that may have “evolved to respond to features such as color and movement in a nonsexual context” or perhaps because the features in question “resemble eggs or food”. Dawkins and Guilford also claimed that the nonadaptiveness of “sensory bias” was debatable since the female might receive direct advantages such as the “reduction in costs of searching for mates”. Furthermore, it would seem unlikely that sensory preadaptation remain in lineages where such exploitation has been detrimental to the fitness of the species, suggesting that sensory bias may possibly be adaptive to sexual selection.
However, in response to the above post I must emphasize that the “sensory bias” model is not based on adaptation and it is possible that a female responding strongly to certain stimuli “might mate with males that are no different genetically from any other male of their species…[and] would not… be gaining good genes”. (938)
Unlike sexual selection for eyespots in the tails of peacocks and long calls in gray tree frogs, the sensory bias model suggests that a female’s preference for certain traits is arbitrary and often contingent upon “natural selection for some function other than finding mates”. (Coyne, 166)
Studies have shown, for example, that colonies of bumble bees with an innate colour bias for violet will bring in more nectar than colonies with a less stronger bias and may therefore forage more successfully. (Raine & Chittka)
It has also been suggested that guppies may exhibit a preference for carotenoid coloration as a result of a sensory bias for orange objects that may help guppies find “orange-coloured fruits in the rainforest streams of Trinidad”. (Grether, 2181)
It seems therefore that although non-adaptive in a sexual context, sensory bias may in fact be adaptive in a non-sexual context.
Coyne distinguishes between the “sensory bias” model and the “good genes model” which, as indicated by the above post, postulates that sexual selection by females is driven by either indirect benefits such as “good genes” or direct benefits such as resources and increased survival of offspring (164). The good-genes theory is the model often preferred by scientists in explaining the origin and mechanisms of sexual selection rather than ascribing the process to arbitrary bias for nonsexual stimuli.
sources:
Coyne, Jerry. “why evolution is true”:
Gregory F Grether. ”carotenoid availability affects the development of a is genetically linked
colour-based mate preference and the sensory bias to which it”. 2005
Marian Dawkins and Tim Guilford Stamp Sensory Bias and the Adaptiveness of Female Choice. The American Naturalist, Vol. 148, No. 5 (Nov., 1996), pp. 937-942
Raine NE, Chittka L, 2007 The Adaptive Significance of Sensory Bias in a Foraging Context: Floral Colour Preferences in the BumblebeeBombus terrestris. PLoS ONE 2(6): e556. doi:10.1371/journal.pone.0000556