Эволюциялық биологиядағы «Мүмкіндікті төмендету» гипотезасы

Hypothesis in evolutionary biology
[[File:Peacock Flying. jpg|thumb|300px|right|The peacock tail in flight, a classic example of what Amotz Zahavi proposed was a handicapped signal of male quality The handicap principle suggests that secondary sexual characteristics are costly signals which must be reliable, as they cost the signaller resources that individuals with less of a particular trait could not afford. The handicap principle further proposes that animals of greater biological fitness signal this through handicapping behaviour, or morphology that effectively lowers overall fitness. The central idea is that sexually selected traits function like conspicuous consumption, signalling the ability to afford to squander a resource. Receivers then know that the signal indicates quality, because inferior quality signallers are unable to produce such wastefully extravagant signals. The generality of the phenomenon is a matter of some debate and disagreement, and Zahavi's views on the scope and importance of handicaps in biology have not been accepted by the mainstream. Nevertheless, the idea has been very influential, with most researchers in the field believing that the theory explains some aspects of animal communication.

[[File:Handicap signal of quality. png|thumb|upright=1.5|Graph based on Johnstone's 1997 graphical representation of a Zahavian handicap. Where is cost to a low quality signaller and is cost to a high quality signaller. Optimal signalling levels are for a low quality signaller, and for a high quality signaller. The British biologist John Maynard Smith was a notable early critic of Zahavi's ideas. However, the handicap principle gained wider acceptance because it is supported by game theory models, most notably the Scottish biologist Alan Grafen's 1990 signalling game model. This was essentially a rediscovery of the Canadian American economist Michael Spence's job market signalling model, where the job applicant signals their quality by declaring a costly education. In Grafen's model, the courting male's quality is signalled by investment in an extravagant trait—similar to the peacock's tail. The signal is reliable if the cost to the signaller of producing it is proportionately lower for higher quality signallers than for lower quality ones. This is illustrated in the figures from Johnstone 1997, which show that the optimum signalling levels are different for low and high quality signallers. Further game theoretical models demonstrated the evolutionary stability of handicapped signals in nestlings' begging calls, in predator deterrent signals and in threat displays. In the classic handicap models of begging in game theory, all players are assumed to pay the same amount to produce a signal of a given level of intensity, but differ in the relative value of eliciting the desired response (donation) from the receiver. The hungrier the baby bird, the more food is of value to it, and the higher the optimal signalling level (the louder its chirping). Analysis of some begging models shows that non communication strategies are not only evolutionarily stable, but lead to higher payoffs for both players. In human mate choice, mathematical analyses including Monte Carlo simulations suggest that costly traits ought to be more attractive to the other sex and much rarer than non costly traits. It was soon discovered that honest signals need not be costly at the honest equilibrium, even under conflict of interest. This conclusion was first shown in discrete models and then in continuous models. Similar results were obtained in conflict models: threat displays need not be handicaps to be honest and evolutionarily stable.

In 2015, Simon Huttegger and colleagues wrote that the distinction between "indexes" (unfakable signals) and "fakable signals", crucial to the argument for the handicap principle, is an artefact of signalling models. They demonstrated that absent that dichotomy, cost could not be the only factor controlling signalling behaviours, and that indeed it was "probably not the most important" factor acting against deception. Dustin J. Penn and Szabolcs Számadó stated in 2019 that there was still no empirical evidence for evolutionary pressure for wasteful biology or acts, and proposed that the handicap principle should be abandoned.

[[File:Stotting gazelle. jpg|thumb|Impala stotting, a behavior that may serve as a pursuit deterrence signal to predators However, the mathematical biologist John Maynard Smith commented that other explanations were possible, such as that it was an honest signal of fitness, and it was hard to see how stotting could be a handicap. Another example is provided by larks, some of which discourage merlins by sending a similar message: they sing while being chased, telling their predator that they will be difficult to capture.

The theory of immunocompetence handicaps suggests that androgen mediated traits accurately signal condition due to the immunosuppressive effects of androgens. This immunosuppression may be either because testosterone alters the allocation of limited resources between the development of ornamental traits and other tissues, including the immune system, or because heightened immune system activity has a propensity to launch autoimmune attacks against gametes, such that suppression of the immune system enhances fertility. Healthy individuals can afford to suppress their immune system by raising their testosterone levels, at the same time augmenting secondary sexual traits and displays. A review of empirical studies into the various aspects of this theory found weak support.