Running with the Red Queen Why is there sexual reproduction?

Why is there a problem? Loss of adaptive gene combinations –Let us assume that in a particular environment the genotype AaBBCc has the highest fitness. –What happens when two individuals with this genotype mate?

Why is there a problem? Only 25% have the high fitness genotype

Why is there a problem? What happens when an individual that is AaBBCc reproduces asexually? 100% of the offspring have the high fitness genotype!

It gets worse! The asexual individual produces only female offspring. What are the consequences of that? Let’s assume any female (sexual or asexual can produce only 4 offspring) and that on average sexual females produce two males and two females.

It gets worse! Asexual Females leave more offspring

The Cost of Males This large numerical advantage for asexual females is known as the “Cost of Males” or “Cost of Meiosis” When the Cost of Males and the cost of loss of adaptive genotypes are put together there must be some advantage very large advantage to sex for it to exist at all.

Possible Solutions:- The Vicar of Bray Is genetic variation a bad thing? Survival of a species in a changing environment requires lots of flexibility Speed of Evolution

Possible Solutions - The Vicar of Bray

Possible Solutions: - The Vicar of Bray This is very plausible and was the first and most common explanation given for sexual reproduction. However it ignores a basic principle of evolution, natural selection acts on individuals not groups! An individual which varies its offspring will always be at a disadvantage.

Possible Solutions - Muller’s Ratchet When a deleterious mutation arises in an asexual individual it will remain. Over time an asexual population will amass lots of mutations and ultimately all individuals will be carrying deleterious genes In sexual populations recombination gives the possibility of creating mutation free individuals.

Possible Solutions - Muller’s Ratchet Unfortunately this over simplifies the case. In order for it to work we need two assumptions or the advantage of sex is not high enough: –A high mutation rate (no evidence of this) –Interaction between deleterious mutations (no conclusive evidence of this.)

Possible Solutions: - The Red Queen Other organisms are killed by the direct or indirect actions of other organisms more often than they are killed by physical factors. Parasites and their hosts are in a perpetual genetic arms race.

The Red Queen - Our best Explanation “Well in our country,” said Alice, still panting a little. “you’d generally get to somewhere else-if you ran very fast for a long time as we’ve been doing.” “A slow sort of county!” said the Queen. “Now, here, you see, it takes all the running you can do to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that!”

The Battle of the Sexes Once there are two sexes then what are the implications. Since the number of eggs a female can produce is finite and the number of sperm a male can produce is not, their strategies for maximizing fitness differ.

Who has the Babies? The Hermaphrodites Dilemma

Why do some females eat their mate? It’s all about energy

How not to get eaten Help to rear the babies Male investment in offspring –Find food –Build nest –Protection (women and children first)

Choosing a Mate Since the female maximizes her fitness by having high quality offspring she needs to choose a mate that has superior fitness characteristics.

Tail of a Swallow Parasites and tail Length a test of Darwin’s hypothesis –Parasites should reduce the survival of a female's young –Parasite resistance should be heritable –Parasite infection should lead to a visible sign in the male's ornament –Female's should prefer males which show an indicator ornament that establishes his clean bill of health

But it may not be that Simple! In experiments in a maze swallows with long tails performed better than swallows with shorter tails. Females have long tails too.

A convoluted tale(tail)? Are tail streamers of females sexually or naturally selected? –Streamer length may reflect female quality (sexual selection) –Streamers may be selected as flight aids (natural selection) –Females may only have streamers because of a genetically correlated response to their possession by male swallows

A convoluted tale(tail)? Are tail streamers in males: –A measure of male quality (sexual selection) –The result of selection for precision flight (natural selection) –Sexual selection on a pre-existing naturally selected trait.

Why be altruistic? If selection acts at the level of the individual altruistic behavior would seem to reduce fitness and therefore should be selected against.

Manipulation In this case altruism is deleterious e.g. brood parasitism in birds.

Individual Advantage Aggregation - protection against predators

Kin Selection Hamilton’s Rule Where r is relatedness b is benefit and c is cost

Kin Selection Florida Scrub Jays. Pairs may have up to six helpers. Helpers are either full or half-sibs of the the young thy are helping to rear. There are an average of 1.8 helpers per nest. Mumme (1992) experimentally removed helpers from 21 nests.

Kin Selection Helpers RemovedControl Initial Sample Size 4563 % Surv. from egg to hatch 6768 % Surv. from hatch to fledge 3063 % Surv. Fledge + 60 days 3381 % Surv. Egg to Fledge

Kin Selection Using this data we can calculate the benefit, Given 2 helpers on average then the benefit per helper is.

Kin Selection Cost of helping c Lower bound 0 - In the highly competitive environment the Scrub Jay’s live in young birds may not be able to mate Upper bound 7 - Value of reproducing without helpers.

Kin Selection Since the helping behavior is to sibs vs. their own offspring we must modify Hamilton’s rule as follows.

Kin Selection Since the genetic relatedness of full sibs and offspring is the same 1/2 both the lower and upper bound estimate predict that “altruism” will increase fitness. Lower Bound Upper Bound