Scales of Ecological Organization Organism Population Community Ecosystem Biosphere

The Evolution of Sexual Reproduction

Reproductive Success In eukaryotes: sexual reproduction is production of gametes by meiosis, and then syngamy Female: the form of the organism that carries the most provisioning for the offspring (i.e., the egg). All other "sexes" are by definition males.

The most important genetic consequence of sex: bringing together in a single cell genes from two parental cells. The process of genetic recombination which occurs during meiosis also ensures that genes from different (but homologous) chromosomes can be combined in a single chromosome Reproductive Success

Asexual reproduction either, fusing of haploid cells within an organism to restores diploid state or, reproduction with no meiosis, cloning

Populations can evolve more rapidly to meet changed circumstances or develop new defenses against disease. Advantage of sex

1. Sexual reproduction speeds up the accumulation of beneficial mutations in a single organism Advantage of sexual reproduction 2. Slowly reproducing organisms can speed up the response to a disease (Red Queen hypothesis) 3. Genomes that accumulate mutations are ‘repaired’

1. Cost of producing males -- halves the productive part of the population. Disadvantage of sexual reproduction 2. Cost resulting from discarding half the genes -- no guarantee that male genes are better (cost of meiosis)

Disadvantage of sexual reproduction

1. Production of sexual mechanisms and devices Other costs of sexual reproduction

2. mating behavior

Other costs of sexual reproduction 3. disease transmission

4. female escape from predator Other costs of sexual reproduction

Prudent predator hypothesis -- to avoid depleting a food resource, an animal develops the behavior of restraint in feeding Group selection 1. A behavior that increases fitness of the population but decreases fitness of the individual. Not usually an evolutionary stable strategy (ESS) Prudent predator hypothesis -- example where group selection is not a stable strategy

Rare in animals (parthenogenesis) Asexual reproduction aphids, weevils very few fish, amphibians, lizards Somewhat more common in plants

Asexual reproduction benefit may be more offspring but the cost may be reduced variation in genome Benefits of sexual reproduction must in general outweigh the costs because it is widespread (also true in past history) Sexual versus asexual reproduction

based on the fact that a female chooses a male Sexual selection sexual dimorphism -- a difference in the outward appearance of males and females 1. different sexual functions shape the body size of males and female 2. male/male contests may favor weapons

Sexual selection 1. different sexual functions shape the body size of males and female 2. male/male contests may favor weapons 3. choice by the opposite sex

Sexual selection

choice by the opposite sex can lead to runaway sexual selection Sexual selection

1. handicap principle -- if a male can have a trait that is not adaptive, it must be that the overall fitness is high 2. parasite-mediated -- non adaptive trait is linked to unapparent adaptive feature

Sexual selection