Sex, Cooperation and conflict

The occurrence of meiosis

Sex does not require separate sexes Sex is not necessary for reproduction Sex is not the same as reproduction

Sex must confer some sort of fitness benefit or it wouldn’t be so pervasive

Takes a long time Risk of reproducing maladapted offspring Cost of mating (finding a mate, sexually transmitted diseases) Loss of half of reproductive output (2-fold cost of sex)

When producing sexually a female must combine her offspring with a male and thus loses 50% of her gene copies. when producing asexually she has a 2-fold advantage of passing of two copies of her genes all asexually produced offspring are female

Asexually - 90mins Sexually - several days

The clonal offspring of an asexual female multiplies at twice the rate of the progeny descended from a sexual female a sexual female has only 50% of the fitness of an asexual female

Sex is costly

Selection against strongly deleterious mutations Fixation of weakly deleterious mutations Accumulation of deleterious mutations

The population gets weaker in time due to lack of chromosome mixing

Random loss

Stochastic loss of mutation-free chromosomes within a population leads to an accumulation of deleterious mutations

Cannot be revered within an asexual populations Leads to fitness decline over generations Accumulation of deleterious mutations

Deleterious mutations accumulate because they are linked to beneficial mutations Selection for strongly beneficial mutations Hitchhiking of linked deleterious mutations

Recombination could de-couple but this cannot happen in asexual reproduction beneficial and linked deleterious mutations are linked and cannot be separated Accumulation of deleterious mutations

Selection against strongly deleterious mutations Elimination of linked beneficial mutations Linked beneficial mutations cannot be de-coupled and so are lost Less adaptation

Without recombination, fixation of beneficial mutations is much slower. Selection is weaker without sex and recombination.

Benefit of the group not the individual This does not fit with how we understand evolution Evolution doesn’t have foresight

Advantageous in a variable environment Red queen hypothesis - running to stand still

Organisms that can switch between sexual and asexual reproduction Favours environmental change

Parasite causes castration of snail (complete fitness loss) High levels of parasite numbers correlate with high levels of sexual reproduction

Parasitic form reproduce sexually to compete with changing host behaviour Free living are much more stable

Same sized gametes (ancestral sexual state)

Different sized gametes Balanced polymorphism