S E X Why is sexual reproduction the rule for nearly all living species? Why is it so common, and why did it evolve in the first place?

Reproduction: cloning vs. sex Every organism must reproduce to pass on its genes Reproduction can be asexual, if cells or organisms clone themselves (mitosis) - does not require a mate - produces a perfect copy of the parent Sexual reproduction is the production of male and female gametes (meiosis) - offspring are a genetic mix of two parents

Mitosis: cell duplication In mitosis, a diploid cell replicates each chromosome, then divides into two cells Both daughter cells get identical copies of every chromosome By repeated divisions, one cell of an organism could grow into a whole copy  asexual reproduction

Asexual reproduction: attack of the clones In asexual reproduction, an organism creates an identical twin copy of itself A well-adapted organism can reproduce itself exactly as-is Avoids scrambling of genes, as occurs with sex

Asexual 4 granddaughters per female

Asexual Sexual 4 granddaughters per female 2 granddaughters per female

Population growth advantage Asexual Sexual n = 2 n = 2 n = 8 n = 4 2-fold faster rate of population growth when you don’t waste energy making “useless” males

Benefits of asexuality  Well-adapted organisms can clone themselves, so their good combinations of genes can stay together  2-fold advantage in reproduction when you don’t have to produce males - males consume resources, but cannot make offspring - seem like a waste of space, evolutionarily speaking ...So why do almost all organisms reproduce sexually?

Sex produces genetic diversity A clone of a successful parent would thrive if the clone faced identical challenges from the environment - same predators, diseases, weather, etc. However, conditions change from season to season, meaning children may need different traits to survive Alleles can be viewed as potential solutions to potential problems that may face an organism - sex “shuffles the deck” of alleles, and deals a new hand for each generation

Sex: diversity by meiosis Special mechanisms create genetic diversity, making new combinations of alleles not present in either parent In meiosis, a diploid cell (2 copies of each chromosome) divides twice - produces 4 gametes (sperm or eggs) Gametes are haploid (contain 1 copy of each chromosome)

Sources of diversity #1: independent assortment Homologous chromosomes Each gamete carries a different combination of parental chromosomes One human egg may have 4 chromosomes from grandma and 19 from grandpa... Crossing over Recombination

Sources of diversity #2: recombination From mom From dad Homologous chromosomes Crossing over Recombination Crossing over moves some alleles from one chromosome onto another, creating a new combination of alleles

Sex occurs in nearly all multi-cellular organisms  Very few species only reproduce asexually - a few micro-organisms (rotifers) - parthenogenesis in some fish, amphibians - some, like aphids, start as asexuals & then switch  Fossil record shows most asexual species go extinct  Recombination must therefore confer an advantage on sexually reproducing species, allowing them to out-compete asexual species

Cost of asexual reproduction #1: Muller’s ratchet In an asexual population, once a bad mutation occurs, it is passed on to all offspring of that line Stumpy-arms mutation

Cost of asexual reproduction #1: Muller’s ratchet In an asexual population, once a bad mutation occurs, it is passed on to all offspring of that line Stumpy-arms mutation Square tips mutation

Cost of asexual reproduction #1: Muller’s ratchet In an asexual population, once a bad mutation occurs, it is passed on to all offspring of that line Stumpy-arms mutation Square tips mutation  mutant alleles accumulate every generation

“Turning the ratchet” Sometimes, by chance, all the individuals with no mutations will die off

“Turning the ratchet” Sometimes, by chance, all the individuals with no mutations will die off

“Turning the ratchet” Sometimes, by chance, all the individuals with no mutations will die off All remaining adults will now pass on the mutant alleles Basically, things can only get worse over time

Cost of asexual reproduction #2: Background trapping Consider our mutant starfish carrying 2 bad alleles: Stumpy arms Square tips normal body

Cost of asexual reproduction #2: Background trapping Consider our mutant starfish carrying 2 bad alleles: Stumpy arms Square tips Big strong body Suppose a helpful mutation occurs... This new, beneficial allele is trapped on a chromosome with bad alleles, and can never escape from them

Selfing: 2 ways to get AA offspring once you’ve inherited one A allele AA parent A A A A egg sperm egg sperm AA AA Way #1: get 2nd copy of same A allele Way #2: get copy of other A allele ….but what’s the difference? You end up AA either way

Alleles have a chromosomal context b B A A Alleles are physically linked to neighboring alleles - linkage alters their pattern of inheritance - in other words, alleles aren’t inherited in a vacuum; they usually come with their neighbors In selfing, calculating F = identifying offspring that inherit copies of the same allele from same ancestral chromosome

Offspring inherits 2 copies different chromosomes Alleles have a chromosomal context AA parent A A A A egg sperm egg sperm AA AA Offspring inherits 2 copies of same chromosome Offspring inherits 2 different chromosomes This is why selfing gives a coefficient of inbreeding F = 0.5

Asexual reproduction: pros and cons Advantages: Disadvantages: Well-adapted individuals (1) Muller’s ratchet: can clone their good allele bad mutations keep piling combinations up in every generation (2) 2-fold advantage in (2) Background trapping: reproduction when you don’t new beneficial mutations have to waste effort making get stuck on loser males chromosomes riddled with maladaptive alleles