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I am quite comfortable with both Fisher's runaway principle and the pre-existing sensory bias hypothesis.
The runaway principle states that some males have genes that indicate that they are biologically fit. Some females already have (or develop one through a mutation) a gene that allows her to discriminate the male's phenotypic 'health' indicator, as well as a gene that gives her a preference for this trait in the male. The offspring of the female and 'healthy' male will both be 'healthy' and have a preference for other 'healthy' individuals, starting a positive feedback loop in which it favours males to greatly extenuate their indicative traits (leading to, say, the extremely long tail of the collared widowbird). This makes sense.
The pre-existing sensory bias hypothesis can even coexist with the runaway principle, but it differs because it does not have to bestow a direct or indirect benefit to the female with the sensory bias. The hypothesis states that natural selection has previously inbuilt some sort of preference to a female, for instance a preference for the colour red because that is the colour of the, say, berries the female eats. Therefore males that have a bright red colouring are favoured by the females, therefore propagating the preference for, and the possessing of, the trait.
The runaway principle, as far as I know, "suggests that reliable signals must be costly to the signaler, costing the signaler something that could not be afforded by an individual with less of a particular trait. For example, in the case of sexual selection, the theory suggests that animals of greater biological fitness signal this status through handicapping behaviour or morphology that effectively lowers this quality. The central idea is that sexually selected traits function like conspicuous consumption, signalling the ability to afford to squander a resource simply by squandering it. Receivers know that the signal indicates quality because inferior quality signallers cannot afford to produce such wastefully extravagant signals" (taken from Wikipedia). Now here are my problems:
1) If the female has a genetic preference for the handicaps, then surely it is exactly the same as the runaway principle - females that have a preference for males that are more biologically fit create an evolutionary 'virtuous cycle' of sorts? What is the difference?
2) From reading about it, it seems that there is an indication that females can actively make the distinction "this male is handicapping himself but is still functioning well, so he must be a good mate for me" - if this has a genetic basis, it isn't any difference than the Fisherian runaway principle, but if it doesn't, Zahavi is bestowing animals with an intelligent mind I'm pretty sure they don't have.
Thanks for reading, and if there is any fault with any of my understanding here, let me know.
The two thoeries mentioned, Fisher's runaway and fitness indicator /handicap. Attempt to explain extreme sexual displays in two entirely different ways.
Fisher sees these displays as deriving from some preexisting preference in the females that the males adapt to exploit and the females then become more selective (picky) in order to avoid having thier son's passed over in subsequent generations. These selective pressures positively feed back over time resulting in the peacocks tail. Note there is no reference to offspring survival, this is a purely sexual selection process.
Fitness indicator theory uses offspring survival to drive the selection pressure indirectly. The idea is that females can't see "Fitness" directly and must use some sensory input to find the best genes. This theory requires that good genes be linked to the size of the males display. Obviously, delinking the display would allow males to cheat, which males generally do if possible. In order to maintain the linkage handicap theory states that expensive displays assure the females that the display is honest because only males with good genes can pull of the display.
It's not a conscious calculation. Those females that choose lesser displays risk getting bad genes and having weaker and fewer offspring.This is where the selection happens with this theory.
The two theories are in opposition. This means that either one or the other is wrong or they are both wrong.
The fact that neither theory has overcome the other suggests it's the both wrong situation. If you are confortable with either theory I would suggest you are missing the problems with each of them.
Problems with Fisher:
Unlikely to get started and should reverse direction when displays become too expensive. Doesn't explain why some species become extreme and some don't.
Problems with Handicaps:
If you boil down the logic it states that " Bad is so bad it must be good"
It can't predict anything because essentially any display is possible.
A male wtih great genes and no deleterious display should be able to take the genepool over. (fragile linkage)
Again it doesn't predict which species get extreme and which don't.
Both theories also suffer from the same problem. The most extreme examples of male displays occur in species that have the highest mating skews (very few males get a huge proportion of the matings). This leads to very low genetic diversity especially among the males. Genetic diversity is required to maintain the display advantage. The most extreme example should be the easiest to explain not the most difficult.
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