host-parasite arms race

directional selection

host/parasite accumulate increased resistance/infectivity alleles resulting in hosts being resistant to numerous parasites and parasites ability to infect numerous hosts

• classic arms race

fluctuating selection

• parasite evolved to infect common host genotypes

• so rare host genotypes are favoured and increase in frequency until common

• parasites then evolve to infect those new common genotypes

• results in fluctuating host/parasite allele frequencies so host range or resistance does not increase

red queen hypothesis

• evolution of host and parasite occurrs at same time so no one is ever ahead for long

• created by leigh van valen to explain van valens law stating that extinction occurs randomly in time but non-randomly in ecology

• environment deteriorates for taxa due to interspecific competition

• taxa have to be constantly evolving to not go extinct

effect of parasites on sexual reproduction

no parasites = 20% of population has sex

with parasites = 80-90% of population has sex

sex is maintained through coevolution of parasites and host