Female Choice in Gray Tree Frogs Female Choice in Gray Tree Frogs Research by Gerhardt et al. (1996). Males call to attract females. Males call to attract females Ribbet! Figure 11.27 Tiny and super cute! Female gray tree frogs are choosy and prefer males with long calls Models of Female Preference Fisher s Runaway Sexual Selection Model Why should females prefer long tails or other such traits? What are the benefits to females of being choosy with respect to a character that appears to be a handicap? The basic goal of any model addressing this problem is to explain: 1. How is the cost of secondary sexual characteristics, that is, their disadvantageous affects on male survival, balanced or compensated for by increased attraction of females and reproductive success? 2. How does female preference for male characters that are disadvantageous for survival evolve in the first place? We'll consider two general models that have been developed to solve this problem. Genetic variation in a female preference and a male trait leads to assortative mating Assortative mating leads to genetic correlation between display and preference
Fisher s Runaway Sexual Selection Model Stabilizing selection leads to equilibrium display and preference values A change in display or preference leads to a correlated response in the other A Positive Feedback Loop Increased preference of male display in females The display optimal for survival Increased display in males Change This feedback loop reinforces the evolution of a more extreme display in males and preference for a more extreme display in females Fisher s Runaway Sexual Selection Model Change A change in preference or display can lead to a positive feedback loop driving the evolution of stronger preference and more extreme display The change (positive or negative) that initiates this positive feedback loop may arise via drift or selection. This general process is called runaway sexual selection. This sexual selection can result in the evolution of preferred displays that are suboptimal for male survival. Runaway Sexual Selection in Stalk-Eyed Flies? Research by Wilkinson and Reillo Teleopsis breviscopium
How Runaway Sexual Selection Works in Theory Variation in eyestalks and preferences should lead to assortative mating Whoa! How Runaway Sexual Selection Works in Theory Assortative mating of parents should produce genetic correlations between son s eyestalk length and daughter s preference within families. In this figure each point represents a family. How Runaway Sexual Selection Works in Theory Does Selection on Stalk Length in Males Produce a Correlated Response in Female Preference? Paired female-choice tests Preference Eyestalks A change in male eyestalks or in female preference due to drift or selection should produce a correlated response in the other trait. This change may initiate a positive feedback loop leading to runaway sexual selection for extreme male eyestalk length and female preference for extreme male eyestalk length. Wilkinson & Reillo s selection experiment 13 generations of selection: Control line: 10 males, 25 random females. Long-selected line: 10 males with the longest stalks from a pool of 50 random males; 25 random females. Short-selected line: 10 males with the shortest stalks from a pool of 50 random males; 25 random females. Figure 11.32
Implications of Wilkinson & Reillo s Results 1. They demonstrate that female stalk-eyed flies are choosy. 2. They demonstrate that both male eyestalk length and female preference are heritable. 3. They are consistent with Fisher s Runaway Sexual Selection prediction that selection on one trait - male eyestalk length - can produce a correlated evolutionary response in another trait - female preference. Other Reasons Why Females May be Choosy Examples from your textbook: Choosy females may have preexisting sensory biases and sexual selection on males to exploit these biases may lead to unusual male characteristics. For example, courtship behavior of male water mites. Choosy females may benefit directly through the acquisition of resources provided by courting males. For example, female hangingflies prefer males who present large prey gifts. And the longer it takes her to eat it, the longer the male gets to copulate with the female. Choosy females may get better genes for their offspring. We will consider this idea in greater detail Choosy Females May Get Better Genes for Their Offspring The Good Genes/Handicapped Male Hypothesis Suppose that in a population some males have multilocus genotypes that confer higher fitness (good genes) while others have combinations of genes that confer lower fitness (bad genes). Question: Good genes for what? The Good Genes/Handicapped Male Hypothesis Now suppose that some of the males in the population possess a handicap as an indicator of overall male quality Species with: Males with bad genes Males with good No male handicap: Live genes Live Male handicap: Die Live If only males with good genes can survive possessing such a handicap then a female who preferentially mates with handicapped males will be mating with the males that otherwise have good genes. Question: What sorts of handicaps may serve this function?
The Handicap Acts as an Indicator of Overall Male Genetic Quality Female choice for handicapped males will be favored by selection if the advantage gained through transmitting good genes to both her sons and daughters outweighs the cost to her sons of the handicap. If this is the case then the net quality of the choosy female's offspring will be higher than that of the randomly mating female and genes for both female choice and male handicap will increase in the population. Why Does the Handicap Have to be Costly? The cost of the handicap guarantees to females that it is a reliable indicator of a male s overall genetic quality. If the handicap were an inexpensive signal to females, then males with low quality genotypes would be under selection to cheat to look like handicapped males with good genes. If, in contrast, the signal favored by females is costly, such as growing a long and ostentatious tail, then selection will less automatically favor cheats. If truly high-quality males are more likely to survive with the handicap, then the handicap will be a reliable indicator to females of a male's overall genetic quality. The Good Genes/ Handicapped Male Hypothesis An experiment in gray tree frogs to test is calling displays given by males indicators of genetic quality. Figure 11.41 Calling Displays and Male Genetic Quality in Gray Tree Frogs Experimental results: Red arrowheads indicate that aspiring of long callers did better. Blanks indicate no significant difference. Dashes indicate no data were taken. Overall, offspring of long-calling males had significantly better performance (p < 0.0008). Offspring of short-calling males never performed better. Does calling signals the genetic quality of males to females? Yes! Figure 11.42
The Nature of Good Genes - The Hamilton-Zuk Hypothesis The Nature of Good Genes - The Hamilton-Zuk Hypothesis Some biographical info... William Hamilton Marlene Zuk Female choice for handicapped males involves a preference for mates with good genes for parasite resistance: Some individuals may be genetically resistant to common parasites and have good genes Other individuals may be more easily parasitized and have bad genes. Selection should favor those females who pick healthy, parasite free males as mates because they would pass on the male's genes for resistance to their offspring. A special and important case of the Good Genes Hypothesis Bird mite in feathers The Hamilton-Zuk Hypothesis Moller Tested the Hamilton-Zuk Hypothesis using Barn Swallows Because plumage is costly to make and maintain it is a handicap that serves as a reliable indicator of overall good male health because a parasitized male cannot produce as attractive a plumage as can an unparasitized male Species with: Males with bad genes Males with good No male handicap: Appear healthy Appear genes healthy Male handicap: Appear unhealthy Appear healthy Again, why does the handicap have to be costly? A male's tail length is indicative of his good genes for resistance to mite parasites!