Question

An evolutionary arms race occurs when natural selection is acting simultaneously on both sides of a host–pathogen interaction. How does an evolutionary arms race constrain evolution?

Answer 1

In evolutionary biology, an evolutionary arms race is a struggle between competing sets of co-evolving genes, traits, or species, that develop adaptations and counter-adaptations against each other, resembling an arms race. These are often described as examples of positive feedback.
The co-evolving gene sets may be in different species, as in an evolutionary arms race between a predatorspecies and its prey (Vermeij, 1987), or a parasite and its host. Alternatively, the arms race may be between members of the same species, as in the manipulation/sales resistance model of communication (Dawkins & Krebs, 1979) or as in runaway evolution or Red Queen effects

Host–parasite dynamic

Selective pressure between two species can include host-parasite coevolution. This antagonistic relationship leads to the necessity for the pathogen to have the best virulent alleles to infect the organism and for the host to have the best resistant alleles to survive parasitism. As a consequence, allele frequencies vary through time depending on the size of virulent and resistant populations (fluctuation of genetic selection pressure) and generation time (mutation rate) where some genotypes are preferentially selected thanks to the individual fitness gain. Genetic change accumulation in both population explains a constant adaptation to have lower fitness costs and avoid extinction in accordance with the Red Queen's hypothesis suggested by Leigh Van Valen

As these species evolve, other species that are in close symbiotic relationships with those species must also evolve. This is called co-evolution and it is often compared to an evolutionary form of an arms race. As one species evolves, the other species it interacts with must also evolve or they may go extinct.