Evolutionary Arms Races Essay
“… 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: – 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 have an ability to breach the plant defences. This ability spreads through the plant population, once again through natural selection. – At some point in the future, yet another novel plant defence emerges, and the process of escalation begins again (i. . continuing the arms race). Antagonistic coevolution is not easily studies in nature because it is difficult to determine reciprocal genetic changes within species. However, antagonistic co-evolutionary relationships between pine squirrels (Tamiasciurus sp. ) and coniferous trees have been studies in the pacific Northwest of the U. S. A. The conifer seed constitutes the stable food supply of these particular squirrels, and these squirrels can outstrip a tree of most of its cones.
Hence, over evolutionary time, the trees have reduced the effectiveness of squirrel predation by evolving certain measures: the trees produce cones that are difficult to reach, open or carry; there are fewer seed in each cone; there has been an increase in the “fitness” of the seed coats, requiring the squirrels to spend more time and energy extracting each seed; there has been a trend of putting less energy into each seed, therefore requiring the squirrel to expend more energy; trees have shed their seeds from cones early, before the young squirrels of that year begin foraging, and finally, there have been periodic “cone failures”.
These reduce the squirrel population and the reproductive success drastically, hence reducing the intensity of predation during the next year. The fluctuations in cone crops from year to year have been shown to be an anti-squirrel strategy because even in optimum climatic conditions, this phenomenon still occurs. It is clear, therefore, that squirrels have had a profound evolutionary influence upon various reproductive characteristics of conifers including cone anatomy, location of cones, the number of seeds per cone, the time at which the cones are shed etc.
These restrictions in turn placed on the squirrels have forced them to adapt in various ways, such as choosing cones carefully and stockpiling them. It is possible that the fruit fly (Drosophila pachea) has also entered an arms race with “senita”, a group of cacti. This group of cacti produce an alkaloid that is fatal to larvae of all other fruit flies, but D. pachea has evolved a means of detoxifying this chemical. The degree at which the fly is coevolved with the cacti is still unclear, however it is the only fly that is placing an evolutionary pressure on the plant.
Arms races between species have a strong evolutionary drive, and this in turn may lead to speciation of the different species involved. This is because, for example: – Plants may produce novel secondary compounds through mutation and recombination. – These novel compounds reduce the palatability of these plants to insects, and are therefore favoured by natural selection. – The plants with these new compounds are therefore able to undergo adaptive radiation, where the plants are able to occupy a new “adaptive zone” in which they are free of former herbivores.
Eventual speciation results in a new taxon or group of plants (that share chemical similarity). – However, novel mutations or recombinations appear in certain individuals of an insect population that can overcome the novel plant defences. – These insects then enter their own adaptive zone, where they radiate into a number of species. These species are then capable of radiating onto previously radiated plants containing the novel compounds.
This therefore forms a new taxon of herbivores. The process of escalation may then continue. Ehrlich and Raven (1964) gathered evidence to support this idea. They investigated the associations between major taxa of butterflies and their host plants, and argued that most butterfly taxa are each related to a few families of plants, hence showing “escape and radiative coevolution” after conquering each new adaptive zone. For example, major butterfly-plant host associations include: * Papilionideae on Aristolochiaceae * Pierinae on Capparidaceae * Ithomiinae on Solanaceae
Another striking example of an evolutionary arms race was when Berenbaum investigated the diversification of coumarin-containing plants. Certain plants have altered coumarin molecules. For example, 30 families of plants contained an altered version, furanocoumarin. An additional eight families of plants (mostly within the genera Rutaceae and Umbelliferae) contained linear furanocoumarins, and angular furanocoumarins were found in only 13 genera. He found that the diversity of plant species containing coumarin-based molecules increased as the complexity of the molecule increased.
Hence genera containing angular furanocoumarins were the most speciose, followed by the genera containing linear furanocoumarins etc. This investigation therefore strongly supports the idea that the evolution of phytochemical complexity has shifted plants into a new adaptive zone where speciation can occur. Also, the number of species of insect herbivores associated with each chemical type. This suggests that the insect species have in turn been able to manage each new form of coumarin, and therefore enter into their own new “adaptive zone”.
This sort of adaptive radiation and co-evolutution is referred to as “diffuse coevolution”, where an array of populations provides the selective pressure. Evidence of tight coevolutionary relationships, “paired coevolution”, in which only two species are coevolved , are not common. This is because, when referring to co-evolution between plants and animals, many different organisms can attack plants (including bacteria, fungi, viruses, nematodes, molluscs, mammals, birds and reptiles).
There might therefore not expect such perfect adaptation or coadaptation between insects and plants, when so many organisms are attacking plants. Secondly, different species of insects can exert selection pressures in different directions. For example, tannins deter leaf-chewing insects herbivores on Quercus rubra, while at the same time attracting gall-forming leaf-mining insects. Hence the advantages of leaf tannins may be dependent on the relative densities of leaf chewing or endophagous herbivores.
Hence, in conclusion, predator-prey interactions very often lead to antagonistic evolution, known as evolutionary arms races. These arms races may in turn lead to adaptive radiation and the formation of new species of plants, insects and other organisms. However, whilst looking for any coevolutionary evidence in nature, it is important to consider all factors influencing the evolution of a species. Very often, what might appear to be coevolution between two species may in fact be evolution, but with selection pressures coming from a variety of directions , either abiotic or biotic.