Figure 15.9 Sexual Selection in Action

Malte Andersson tested the hypothesis that sexual selection is responsible for the evolution of long tails in African long-tailed widowbirds. The researchers captured males and manipulated their tail length by either cutting feathers or gluing on extra feathers. The males were released, and their reproductive success was later measured by counting the nests with eggs or young within each male’s territory. Results showed that males with artificially lengthened tails were approximately twice as successful as normal and control males, while males with artificially shortened tails were about half as successful as normal and control males. Importantly, Andersson found that the modified tail lengths did not impair the birds’ ability to defend their territory, suggesting that the differences in mating success could be directly attributed to tail length. These results were consistent with the hypothesis that sexual selection plays a role in the evolution of long tails in the African long-tailed widowbirds. Additional studies of a number of other bird species have provided similar evidence in support of a role for sexual selection in the evolution of elaborate ornamental displays in males. For example, Marion Petrie studied the impact of the number of eye spots in tails of male peacocks. Although she did not reduce the tail length, she clipped the eye spots out of half of the male’s tails. After observing both normal males and those with clipped eye spots, she found that females preferred males with the most eye spots. Interestingly, a more recent study by M¿ller and Petrie identified a link between the condition of a peacock’s tail and the strength of his immune system. Specifically, the researchers found that the condition and length of the tail was related to B cell production, while the size of the eye spots was related to T cell production. Thus, the male peacock with an elaborate ornamental tail is, in effect, signaling to the female that he is in good health and is a quality mating partner.

 

Original Paper

Andersson, M. 1982. Female choice selects for extreme tail length in a widowbird. Nature 299: 818–820. http://www.nature.com/nature/journal/v299/n5886/abs/299818a0.html

 

Links

Brennan, P. 2010. Sexual Selection. Nature Education Knowledge 1(8): 24
http://www.nature.com/scitable/knowledge/library/sexual-selection-13255240

Stanford University: Sexual Selection
http://www.stanford.edu/group/stanfordbirds/text/essays/Sexual_Selection.html

National Geographic: How Did the Peacock Get His Tail?
http://news.nationalgeographic.com/news/2002/09/0909_peacock.html

Møller, A. P., and M. Petrie. 2002. Condition dependence, multiple sexual signals, and immunocompetence in peacocks. Behavioral Ecology 13: 248–253.
http://beheco.oxfordjournals.org/cgi/content/full/13/2/248

Economist.com: Sexual selection in humans: Mr. Muscle
http://www.economist.com/sciencetechnology/displaystory.cfm?story_id=14302009

Kimball’s Biology Pages: Evolution and Adaptation
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/E/Evolution.html

 

Figure 15.19 A Heterozygote Mating Advantage

Multiallelic polymorphisms of the enzymes at the phosphoglucose isomerase (PGI) locus are common in Colias butterflies in nature. Previous studies demonstrated that flight ability of Colias butterflies depends on the presence of particular PGI alleles and that heterozygotes can fly farther under a broader range of temperatures than homozygotes. These findings led Watt and colleagues to hypothesize that heterozygous male Colias butterflies would have better mating success in nature than homozygous males. To test this prediction, they captured butterflies in the wild, determined their genotypes, allowed females to lay eggs in captivity, and determined the genotypes of their offspring. These data allowed them to determine the genotype of the fathers and, therefore, measure mating success rate. They found that in two different species of butterflies, Colias eurytheme and Colias philodice, the heterozygous males represented only 46 percent and 54 percent, respectively, of the flying population. In contrast, these same males accounted for 72 percent and 80 percent, respectively, of successful matings. These data are consistent with the hypothesis that heterozygous males have a mating advantage over homozygous males. An additional example of a mating advantage attributable to genotype is found in Drosophila melanogaster. J. Rendel (1951) and M. Jacobs (1961) independently described the influence of light on mating behavior of ebony and non-ebony Drosophila melanogaster. Both researchers found that ebony flies showed greater sexual activity in the dark as compared to the light, while non-ebony flies showed greater sexual activity in the light as compared to the dark. In 1978, C. Kyriacou and colleagues demonstrated that flies heterozygous for ebony appear to have a selective advantage over both homozygotes. The ebony gene encodes the enzyme N-β-alanyldopamine (NBAD) synthetase that converts dopamine to NBAD, which is subsequently oxidized to produce a yellowish pigment. In ebony mutants, the body color of the fly varies from black to slightly darker than wild-type, depending on the allele.

 

Original Paper

Watt, W. B., P. A. Carter, and S. M. Blower. 1985. Adaptation at specific loci. IV. Differential mating success among glycolytic allozyme genotypes of Colias butterflies. Genetics 109: 157–175.
http://www.genetics.org/cgi/reprint/109/1/157

 

Links

Rendel, J. M. 1951. Mating of ebony vestigial and wild type Drosophila melanogaster in light and dark. Evolution 5: 226–230.
http://www.jstor.org/stable/2405462

Jacobs, M. E. 1961. The Influence of Light on Gene Frequency Changes in Laboratory Populations of Ebony and Non-Ebony Drosophila Melanogaster. Genetics 46: 1089–1095.
http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1210260

Kyriacou, C. P., B. Burnet, and K. Connolly. 1978. The behavioural basis of overdominance in competitive mating success at the ebony locus of Drosophila melanogaster. Animal Behaviour 26: 1195–1206.
http://dx.doi.org/10.1016/0003-3472(78)90109-4

University of Virginia: Evolution Lab with Drosophila (pdf)
http://www.faculty.virginia.edu/evolutionlabs/DrosophilaEvoBioscenev28-2p3-6.pdf

Miko, I. 2008. Genetic Dominance: Genotype–Phenotype Relationships. Nature Education 1(1)
http://www.nature.com/scitable/topicpage/genetic-dominance-genotype-phenotype-relationships-489