Module 8: Evolution and Sexual Reproduction

pathenogenesis

virgin birth - identical clones from females with no male contribution

obligate parthenogenesis

entire species consists of females only, which reproduce by making identical clones of themselves

cyclical parthenogenesis

populations consist of females that reproduce clonally throughout most of the year, but at certain times, males are produced and sexual reproduction occurs

hermaphrodites

organisms that have both male and female sex organs and are capable of self fertilization

the offspring of hermaphrodites are genetically

less diverse than the offspring of individuals that outcross

maynard smith model assumptions

1. a female’s reproductive mode does not affect the number of offspring that she can make

2. a female’s reproductive mode does not affec tthe probability that her offspring will survive

outcrossing results in new combinations of

genes in the offspring

recombination crossing over in the process of meiosis during gametogenesis

creates new gentic variance

two factors create genetically diverse offspring and

they allow for the purging of deleterious mutations

self fertilization is still a form of sexual reproduction because

chromosomal crossing over and recombination cna still occur during gametogenesis

the offspring resulting from self-fertilization are

genetically less diverse than normal sexual reproduction involving outcrossing, because of the loss of diversity resulting from mating between different individuals

outcrossing results in

increased fitness, and the outcrossing rate itself can increase in response to stress

crossing over and recombination during gametogenesis is a very important component of sexual reproduction, and has two important effects

1. it breaks down linkage disequilibrium

2. it increases genetic diversity in offspring by creating new combinations of alleles

linkage disequilibrium constitutes a violation of Mendel’s second law

it can be defined as non-random associations between alleles at different loci, it can arise when different loci are situated close together on the same chromosome, resulting in them being inherited as a single unit, instead of segregating independently, this will result in less genetic diversity in offspring

1. first law: random segregation

alleles at a single locus segregate randomly, offspring have a 50% chance of inheriting each of a parent’s alleles (this is the basis of the hardy-weinberg principle)

2. second law: independent segregation

alleles at different loci segregate independently, so that the inheritance of alleles at locus A is independent of the inheritance of alleles at locus B

linked

if genes are located close together on the same chromosome

linkage disequilibrium

linked genes dont segregate independently, but inherited as a single unit, the genotypes at two different linked loci will not be randomly associated with each other

benefits of recombination during meiosis

breaks up linked genes and breaks down linkage disequilibrium

sexual recombination creates new

combinations of alleles and destroys existing combinations

the point is that with recombination

you will always ahce at least some of the favourable genotypes

Red Queen hypothesis

host/parasite and predator/prey dynamics: both must continually evolve better means for coping with the other. Sex is adaptive in such circumstances because it provides new and/or different allelic combinations across loci

sex is beneficial indefinitely in

situations where the environment is continuously change