Evolution and Sex I Flashcards
What are the two types of sexual systems?
Dioecious
Hermaphrodite
What are the three dioecious mating systems?
Promiscuity
Polygamy
Monogamy
What are the two hermaphrodite mating systems?
Outcrossing
Selfing
What are the three components of sex?
Fertilization
Recombination
Segregation
What is fertilization?
the union of two gametes to form a zygote, where one gamete is functionally male and the other functionally female
What is recombination
Component of meiosis that alters the association among alleles at different loci on the same chromosome
What is segregation?
affects how alleles associate at a single locus on separate homologous chromosomes
What are the effects of recombination and segregation?
Gene shuffling
How many of the components of sex must be lost for an organism to be asexual?
All three
What is the paradox of sex?
Sex is the dominant mode of reproduction among eukaryotic organisms despite substantial costs associated with sex
What are the three major costs of sex?
Cost of males
Cost of meiosis
Cost of sexual structures and behaviours
What is meant by the cost of males?
o Sexual populations grow slower than asexual populations when everything else is the same between them other than their reproductive system
o In sexual populations with separate male and female sexes, only half of the individuals produce offspring
o In asexual populations all individuals produce offspring
What is meant by the cost of meiosis?
Sexual genotypes pass on half the number of gene copies to each progeny compared to asexual genotypes, which pass on all of their gene copies to progeny
What is meant by the cost of sexual structures and behaviours?
Sexual behaviour and structures are energetically costly, whereas asexual genotypes do not need to invest in sexual structures/behaviour
What are the three major benefits of sex?
Male care
Purging deleterious alleles/Muller’s ratchet
Combining multiple beneficial alleles/Fisher-Muller Model of sex
How is male care a benefit of sex?
o Male care can offset the cost of males
Describe Muller’s ratchet.
Asexual populations accumulate deleterious mutations faster than sexual populations
There can never be fewer deleterious mutations in a population than the minimum number of mutations at the inception of that population
Each time a new mutation is fixed within the population by either selection or drift, it is another click in the ratchet, such that there is a gradual and unstoppable increase in mutation load that can only be purged via extinction
What does Muller’s Ratchet lead to?
A mutational meltdown of asexual populations over time
What does Muller’s Ratchet explain?
The maintenance of sex over long periods of time
What does Muller’s Ratchet not explain?
Why asexual genotypes do not invade sexual populations
What mechanisms enable Muller’s Ratchet to occur?
Recombination and segregation
Describe the Fisher-Muller Model of Sex.
o In asexual individuals, beneficial and deleterious alleles get stuck in the same individuals
o Sex allows individuals to combine beneficial alleles and purge deleterious alleles
o F-M Model: Recombination and segregation in sexual genotypes allow individuals to combine multiple beneficial mutations into the same genome “quickly”, whereas asexual genotypes can only combine them through independent mutations “slowly”
