Evolution IV - contents Male competition for promiscuous females Asexual reproduction Reasons for the evolution of sex Sex ratios

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

Promiscuous females Conflict of interest between males and females In species with promiscuous females, males have often “invented” strategies to monopolize females

Dealing with female promiscuity Strategy 1 – guarding females Titi monkeys (Callicebus cupreus) – monogamous Common Shore Tiger Beetle- Cicindela repanda

Dealing with female promiscuity Strategy 2 – removing the sperm of competitors from the female genital tract for example, dragonflies (picture from Alcock) have barbed penises some whales “flush” the vagina of their partners with seawater

Dealing with female promiscuity Strategy 3 – apply unpleasant smell to female (“anti-aphrodisiac”) e.g. some Heliconius butterflies

Dealing with female promiscuity Strategy 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 can’t 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 female’s 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 don’t have sex, or can alternate between sexual and asexual reproduction Asexual reproduction in protozoans (e.g. amoeba)

Asexual reproduction generally is much more common in plants than in animals Offspring typically genetically identical to parent

Budding in animals e.g. in Coelenterates (Hydra)

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

Parthenogenesis Parthenogenesis (“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 reproduction Recombination facilitates repair of damaged DNA Futuyma: “breaks and other lesions in a DNA can be repaired by copying from an intact sequence from a homologous chromosome” Indeed, 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 organism’s 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 1:1 Why? In many species, the 1:1 segregation of X and Y chromosomes (that determine sex) sets a mechanistic constraint But many species have different mechanisms of sex determination There 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 females In such a population, each male mates with (on average) 5 females This 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