Sexual Selection - Evolutionary Biology - Lecture Notes, Study notes of Evolutionary biology

Download Sexual Selection - Evolutionary Biology - Lecture Notes and more Study notes Evolutionary biology in PDF only on Docsity! 1 Lecture 10 Sexual Selection I. Introduction. 1. Sexual dimorphism – a difference in form between male and female individuals – is not unusual in most phyla of the animal kingdom. 2. Charles Darwin was the first to recognize that sexual dimorphisms might be linked to getting enough sex. 3. For asexual organisms, passing your genes on to the next generation consists of two major steps (or hurdles): a. Living long enough to attain reproductive age. b. Reproducing. 4. Sex adds a slight twist – or a major complication – finding a cooperative (or willing) mate. 5. This adds another component to which variation can be added that impacts overall fitness. a. First, individuals vary in their ability to survive. b. Second, individuals vary in their reproductive output. c. Third, there is variation in the success of organisms in finding, attracting a mate and persuading him/her to copulate. II. The selection pressures of sexual reproduction – Men are from mars and women are from Venus. 1. This is dictated in part by the energetic investment by each sex towards reproduction, an argument articulated by A.J. Bateman in 1948 and refined by Robert Trivers in 1972. B. Sexual reproduction for females is expensive and cheap for males. 1. The energy required to produce eggs is greater than the energy to produce sperm. a. In addition, females generally make a greater parental investment than males. 2. Recall from ecology, Vx, the reproductive value. a. Vx = bx + RRV. b. Once current reproduction kicks in, future reproduction (RRV) declines. c. This is because any female carrying an embryo to term, and any parental investment in the offspring, incurs a cost. d. This cost limits the females future reproductive output because she may not survive, or will produce fewer offspring in the future. 3. On the other hand, males have only to produce sperm and the semen with which to transport it. III. Reproductive success will be defined differently for males and females. 1. A females reproductive success is limited by the number of eggs she can make and carry to term. a. Her reproductive output will be limited by the number of eggs she can make, not by the number of males she can mate with. 2. Males on the other hand, are not limited by their ability to make sperm. a. Their reproductive output will be limited by the number of mating they can have with different females. A. Example of measures of reproductive success in Drosophila melanogaster. 1. A. J. Bateman places 6 flies in a bottle. a. 3 virgin females, and 3 virgin males. b. Each fly is heterozygous for a particular trait that can be tracked phenotypically. Results: 2. For males, reproductive success increased in direct proportion to the number of mates. 3. For females, reproductive success did not increase in proportion to the number of mates. 4. The results of this experiment demonstrate that: for males, access to mates is a limitation for reproductive success, but not for females. Docsity.com 2 B. The asymmetric limits on reproductive success should lead to different predictions on behavior. 1. Males should be competitive. If fitness to males is limited by access to females, then males should compete among themselves for opportunities to mate. 2. Females should be choosy. If the fitness of females is not limited by opportunities to mate, but any given mating will involve the commitment by the female to a large investment in offspring, the females should be selective about whom they mate with. 3. There are two forms of sexual selection, depending upon the males’ ability to monopilize access to females. a. Intrasexual selection - Where males can monopolize access to many females (e.g. a harem for groups), males will fight to win opportunities to mate with females. b. Intersexual selection - When males cannot control access to females, males will advertise for mates. The females choose among the advertisers. IV. Intrasexual Selection - Male-Male competition for mates. 1. Males monopolize females through direct control of the females themselves, or through control of some resource required by females. A. Combat – very much like interference competition. There is selection for larger body size, weaaponary, and body armor. In addition more effective tactics are also selected for. 1. The marine iguana (Amblyrhynchus cristatus), native to Galapagos. 2. What’s the selection pressure in terms of reproductive success? a. The limit on the maximum potential reproductive success of males will be access to females. b. For females, limits to reproductive success is the number eggs they can produce. c. The iguana mating behavior is consistent with these expectations. 3. Not surprisingly, larger males claim and hold onto more territory, and hence command more matings. 4. Assuming that body size is heritable, then Wikelski have demonstrated: a. Variation b. Heritability c. Differential mating success. d. The elements for evolution by sexual selection – and thus an explanation for why marine iguanas are larger than the optimal size for survival. B. Alternatives to combat – Besides selecting for larger size, weapon-like appendages, and more heavily armored bodies, combat competition can also select for more effective, or sneaky strategies. 1. Small male iguanas may be ousted from mating territories, but they don’t give up! C. Cheaters – In coho salmon, there are two types of salmon males that return to the spawning grounds. a. Hooknoses return at 18 months, are larger and armed with enlarged hooked jaws and protected by cartilaginous deposits on their backs. b. Jacks return at 6 months, are small and poorly armed and defended. c. The relative fitness of hooknoses vs. jacks depends in part upon the frequency of hooknoses versus jacks. D. Sperm Competition – it ain’t over until it’s over. 1. Copulations are not the true measure of reproductive success, but the number of offspring sired is. 2. For any animal, it is important that their genes be passed onto the next generation. a. Since females produce and carry their own eggs, they can be assured that their genes are indeed represented in the next generation. b. But for males that compete to copulate with females, there are sometimes few assurances that their sperm was used to fertilize the female’s eggs. 3. One way that males might compete is to produce ejaculates with much more sperm than is necessary. 4. Males may also prolong copulation – thus preventing access by other males. a. This is common among scorpion flies, damsel flies, some Heliconius butterflies (zebra) and some Lygaeid bugs (i.e. milkweed bugs). b. Others may deposit a copulatory plug, thus preventing insemination by other males. c. However, in some species of damsel flies, males can not only scoop out the copulatory plug, but also the sperm left by the previous male. Docsity.com