Bjorn Alfthan

                                                                  Ecology, Behaviour and Evolution Tutorial Essay                 Tutor: Jon Davies

Evolutionary Arms Races

“…Thus I can understand how a flower and a bee might slowly become, either simultaneously or one after the other, modified and adapted in the most perfect manner to each other, by continual preservation of individuals presenting mutual and slightly favourable deviations of structure.” (Origin of Species, 1857)

Darwin was describing the interactions between organisms that result in reciprocal changes in traits (i.e. morphology, behaviour and physiology) over evolutionary time. He termed this phenomenon “co-adaptation”. Over time, however, this term has become known as co-evolution, and the meaning of co-evolution has been refined. It defines an evolutionary change in trait(s) of the individuals in one population in response to a trait in the individuals of a second population, followed by an evolutionary response by the second population to the change in the first.  This distinguishes coevolution from simple adaptations of organisms to their abiotic and biotic environment. For example, an insect herbivore that has the ability to detoxify certain secondary metabolites in the tissues of its host plant may not necessarily be “co-evolved” with that plant:  the secondary metabolite might be present for a variety of reasons (i.e. not just herbivory), or the insect may have had its detoxification mechanisms in place before encountering the host plant in question.   Where two species are co-evolved but have a mutualistic relationship, this is termed mutualistic coevolution. However, where two species are co-evolved but are either competitive or parasitic towards each other, their relationship is termed antagonistic. This is also known as an “evolutionary arms race”, because both groups involved are under selection pressure to out-compete the other.

The extent with which predators and prey will interact with each other is the major determining factor of evolutionary arms races.  Predators are limited to the prey they can consume, due to simple design constraints that prevents, say, a shrew from eating owls.  There are therefore limits of the range of food types eaten by an animal within a habitat.  The “diet-width” model proposed by MacArthur and Pianka (1966) classes predators into two categories, according to their food range:  generalists and specialists.  The generalists pursue a large proportion of the prey encountered, and hence have a minimal effect on each separate species of prey.  The specialists, on the other hand, continue searching a specific prey until found. The prey may then exert evolutionary pressures demanding specialized morphological or physiological responses from the predator/consumer. Due to the restrictions placed on the predator, an evolutionary arms race may follow in which predator and prey will have to compete in order to survive. An evolutionary arms race may be characterised as follows:  

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  • One or more individuals within a plant population develops a new, genetically based defensive trait by random mutation or recombination.
  • Individuals within this trait suffer lower levels of insect herbivory than other individuals within the entire population.
  • The low levels of insect herbivory leads to higher rates of survival or fecundity.  The proportion of individuals carrying the novel defence increases over time by the process of natural selection.
  • Thereafter, within the insect population, one or more individuals develop a genetically based ability to breach the novel plant defence.
  • These insects are therefore at an advantage over their conspecifics, and ...

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