Evolution IV - contentsMale competition for promiscuous femalesAsexual reproductionReasons for the evolution of sexSex ratios

Adaptive reasons for female promiscuity In many species, females mate (potentially) with more than one malePotential reasons may be related to the fact that males with best genes are not necessarily the best carers, and vice versaPotential benefits are that sperm competition between males results in the genotypically best sperm winningAlso, female avoids putting all eggs in one basket more varied immunocompetence

Promiscuous femalesConflict of interest between males and femalesIn species with promiscuous females, males have often invented strategies to monopolize females

Dealing with female promiscuityStrategy 1 guarding femalesTiti monkeys (Callicebus cupreus) monogamous

Common Shore Tiger Beetle-Cicindela repanda

Dealing with female promiscuityStrategy 2 removing the sperm of competitors from the female genital tractfor example, dragonflies (picture from Alcock) have barbed penises some whales flush the vagina of their partners with seawater

Dealing with female promiscuityStrategy 3 apply unpleasant smell to female (anti-aphrodisiac)e.g. some Heliconius butterflies

Dealing with female promiscuityStrategy 4 seal the females genital tract (chastity belt)e.g. in some bees, and Moniliformes worms

Why sex?In one view, sexual reproduction can be thought of as a tremendous waste of time and resources. Think about how much time you and other animals invest into finding the right person, courting the partner, keeping that partner, etc. Plus, sex is a potentially very dangerous affair when you cant attend to approaching predators while being engaged in it also there is a risk of sexually transmitted diseases. Finally, cloning yourself through asexual reproduction means that something that already thrives (you) does not necessarily need alteration through recombination why destroy adaptive and successful combinations of genes?

The cost of sex fewer copies of a females genome!If a female can produce e.g. 10 offspring, she could produce 10 full copies of her own genome by parthenogenesis but 10 offspring produced through mating with a male would only contain 10 half copies of her genome

Many organisms dont have sex, or can alternate between sexual and asexual reproductionAsexual reproduction in protozoans (e.g. amoeba)

Asexual reproductiongenerally is much more common in plants than in animalsOffspring typically genetically identical to parent

Budding in animalse.g. in Coelenterates (Hydra)

Self-cloning in aphids, mother having just given birth to live offspring (mothers can be pregnant with their own grandchildren!)Photo by Prof Jim Hardie

ParthenogenesisParthenogenesis (virgin birth): females lay unfertilised eggs. Examples: bees, some fish, Cnemidophorus lizards

If so many organisms can do without sex, why do others still engage in it?Conjugation sex in a Ciliate

Why sex? Sex means recombination reshuffling of genes. In the long term, such recombination will allow species to adapt to changing environment conditions, colonise new habitats, etc. But evolution does not plan for the future hence most evolutionary biologists would agree that there must also be a short term advantage

Short term benefits of sexual reproductionRecombination facilitates repair of damaged DNAFutuyma: breaks and other lesions in a DNA can be repaired by copying from an intact sequence from a homologous chromosomeIndeed, some bacteria that have damaged DNA will immediately engage in plenty of sex

The red queen race"in this place it takes all the running you can do, to keep in the same place." Lewis Carroll "Through the Looking Glass"

The red queen race (term coined by Van Valen, U. Chicago)An organisms environment is constantly changing especially parasites / diseases with short generation times may evolve very rapidly (but also predators, competitors may be engaged in an arms race with any given species)Hence, having genetically variable offspring may have immediate, short term benefits

In most species the ratio between males and females is close to 1:1Why? In many species, the 1:1 segregation of X and Y chromosomes (that determine sex) sets a mechanistic constraintBut many species have different mechanisms of sex determinationThere are also adaptive explanations

Theoretically, a population with enough males to fertilise all females would produce most offspring

Adaptive explanation for 1:1 sex ratio (R.A. Fisher)Consider a population with 1 male to 5 femalesIn such a population, each male mates with (on average) 5 femalesThis means that females that produce more sons than daughters will produce more offspring (since sons will be more successful)Thus, when males have an advantage (through being rare), more and more will be produced until sexes are in equilibrium