species co-evolution

Co-evolution

• in some cases, only one species in interaction had made evolutionary adaptations (e.g., flowers deceive pollinators, parasites evolve effective transmission before host species evolves resistance)

• Natural selection in interspecies interactions differs from selection just imposed by the physical environment

• When species interact ecologically, evolutionary change in one species can evoke reciprocal change in the other species → co-evolution

Specific co-evolution

• two species evolve in response to each other (ex. Predators evolve faster running skills, then predators evolve, etc.)

• Co-evolution in antagonist interactions like an evolutionary arms race

• Organisms in antagonistic interactions must constantly adapt, evolve and proliferate just to survive while pitted against ever-evolving opposing organisms (“Red Queen Hypothesis”)

Myxoma Virus

• Pox virus introduced into Australia to control European rabbits (vectored by mosquitos/fleas, causes skin lesions)

• Initially the virus was extremely virulent (99% mortality) but subsequently a sharp drop in virulence occurred

• Trade-off between virulence and transmission: (highly virulent forms killed too quickly and viruses too mild had fewer lesions and lower viral load, both reduced chances of being picked up by vector)

• “Happy medium” selected for, rather than ever-more benign forms

• Rabbits were evolving against the virus (evolutionary arms race)

Genetic Basis for Myxoma Resistance

• Allele frequencies change (as in evolution) through time

• Part of standing allele is variation in rabbits

• After Myxoma was introduced, a few genes had alleles that illustrated better resistance

• These immune alleles rose to higher frequencies after Myxoma virus over time

Co-evolution & Evolutionary Arms Race Captured in Pond Sediment

• Daphnia magna is a common freshwater flea that occurs in every pond in Europe

• Produces eggs that remain dormant for many years in pond sediment

• Eggs also harbor spores of parasitic bacterium Pasteuria ramosa

• Lake sediment annually deposited & stratified

• Hatch eggs, grow water fleas & bacteria from different years separately in lab

• Experimentally cross-infection Daphnia from several different years with bacteria from the same, preceding, and subsequent years

Daphnia-Pasteuria Co-evolution

• Experimentally exposed Daphnia to bacteria are from the:

  • Same year as Daphnia eggs

  • The previous year

  • The following year and tests infectivity

• Infectivity was higher when Daphnia were exposed to contemporary parasites versus parasites from previous growing seasons

• Parasite adaptation was quickly lost because the average parasite infectivity was often lower when Daphnia clones were confronted with future parasites

• Average virulence of parasite increased over time (virulence alleles were fixed at some loci while infectivity allele frequencies were cycling at other loci)