BIO220H1 Lecture 2: Lecture 2 The Evolution of Sexual Differences

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11 Feb 2012

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Lecture 2 bio220 the evolution of sexual differences (dimorphisms) Sexually selected traits function solely to enhance mating success. It is a subset of natural selection but it can be useful to consider them separately. Fitness depends on: survival (both males and females, reproductive rate (mostly females, number of mates/mating success (mostly males) The origin of sexual differences: anisogamy (unequal sized gametes) Parental investment theory (trivers, 1972: sexes differ in reproductive investment, members o the sex that invest little in offspring compete to mate with members that invest more in offspring, large gametes are limiting. Males: linear correlation between number of mates and number of offspring. Increased number of mates produces increased offspring but plateaus due to the limited gametes, resources. Males: higher number of maximum number of offspring produced in lifetime. Reproductive potential: high for males, low for females. Intrasexual selection: male-male competition (precopulatory; # of copulations, post- Manifestations of male-male competition: fighting behavior, territoriality, social status, sperm competition.

Related Questions

1.Why would you predict that females in species with internal fertilization and long periods of gestation would be choosier in selecting their mates than females in other species that lack these traits?

2.As we talked about in elephant seals, in many species, there is much greater variance in male reproductive success than in female reproductive success. This could result from a few dominant males controlling the access to the breeding grounds the females need to give birth. There are other ways, however, to get the same pattern of variance in male reproductive success WITHOUT males controlling access to a resource or to females. How might you get the same variance in male reproductive success via intersexual selection?

there could be more than one answer or no answer at all

1. _____ Females that reproduce sexually versus asexually are said to incur a âtwo-fold cost of sexual reproduction.â Which of the following costs are associated with sexual reproduction for females?

A) MALES! That do-nothing lot really does not directly contribute offspring to the next generation. Females would be better off making all daughters.

B) FEMALES! In most natural settings, those daughters compete with their mothers for potential mates and resources needed to successfully reproduce.

C) MITOSIS! Since sexual reproduction can produce many more offspring than asexual reproduction, there are energetic costs associated with the mitotic divisions required to produce those offspring.

D) MEIOSIS! Every time a gamete is produced, the parent loses a little bit of itself. Half to be precise since only half of its genes are passed on to sexually produced offspring.

E) DIVERSITY! Mothers maximize fitness if all offspring are genetically just like them. Sexual reproduction causes genetic diversity, which ultimately lowers the fitness of both parents.

What underlying difference in the reproductive biology of males and females drives sexual selection? (In other words, why is it usually males whose morphologies are conspicuously shaped by sexual selection, rather than females) What are some of the possible outcomes of sexual selection?

BIO220H1 Lecture 2: Lecture 2 The Evolution of Sexual Differences

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