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

2. 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

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

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

5. 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

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

7. Dealing with female promiscuity
Strategy 4 – seal the females genital tract (“chastity belt”)
e.g. in some bees, and Moniliformes worms

8. 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?

9. 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

10. Many organisms don’t have sex, or can alternate between sexual and asexual reproduction
Asexual reproduction in protozoans (e.g. amoeba)

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

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

13. 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!)

14. Parthenogenesis
Parthenogenesis (“virgin birth”): females lay unfertilised eggs. Examples: bees, some fish, Cnemidophorus lizards

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

16. 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

17. 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

18. 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"

19. 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

20. 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

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

22. 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