EEB214H1 Lecture 14: Genotypes

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University of Toronto St. George
Ecology & Evolutionary Biology
Jennifer Carpenter

EEB214 Lecture 14 • Two-fold costs of sex o your fitness is lower • There are myriad of other costs to sex as well o costs of finding a mate o bed bugs [actually mating] o STIs [costs to interacting with members of own species] ▪ Koalas • There are two major assumptions to the two-fold costs of sex o Asexual and sexual females produce the same number of offspring o the offspring of asexual and sexual females have the same fitness • We know that sex cells are created through meiosis • and a critical part during meiosis is recombination o sometimes when you make gametes, those chromosomes cross over, exchange materials which makes hybrids of mix genotypes o Example: 2 chromosomes ▪ if recombination happens between two genes, we can make a Bt and bT [if crossing over happens] ▪ if no crossing over happens, we can re-create ones we got from parents o if only asexual is happening, then only BT and bt happens o selection will try to increase the frequency of the one with higher fitness ▪ but cannot get B and t without recombination ▪ selection will take the one that will be more important ▪ without sex, we cannot have that happen unless mutation • With recombination beneficial alleles can “group up” and be selected together o can take two good alleles that were separate and bring together to make one better organism o with recombination –new combination of genotype much faster rather than having a lucky mutation on the same genotype • have to have two mutations happen, big B has to happen twice [without recombination] • Recombination can also help good mutations “escape” bad mutations o example: blue and red allele ▪ blue is bad, but mutation occurs on chromosome that is beneficial [without recombination, the beneficial allele cannot spread in population without sex] ▪ if recombination – can have good allele without bad one [mix] and selection cab take the good one and spread to fixation [help gets good alleles together and save from bad ones] • But is making new genotypes beneficial? o Example 1: ▪ in New Zealand there is a species of snail with sexual and asexual lineages ▪ parasites infect both the sexual and asexual snails • eating the snail from the inside out • parasites get carried around and population can get newly infected with these new diseases o populations vary in how much sex they have ▪ pie chart: shows how many males in population ▪ if there are males there, population is having sex, no males –reproducing asexually ▪ we can figure out how much sex by how many males there are ▪ in this chart, more females than male, mate asexually and they can mate faster asexually
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