Evolutionary arms race

In evolutionary biology, an evolutionary arms race is an evolutionary struggle between competing sets of co-evolving genes that develop adaptations and counter-adaptations against each other, resembling an arms race, which are also examples of positive feedback. The co-evolving gene sets may be in different species, as in an evolutionary arms race between a predator species 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. One example of an evolutionary arms race is in sexual conflict between the sexes. Thierry Lodé emphasized the role of such antagonist interactions in evolution leading to character displacements and antagonist coevolution. The escalation hypothesis put forward by Geerat Vermeij speaks of more general conflicts and was originally based on his work with marine gastropod fossils.

Co-evolution itself is not necessarily an arms race. For example, mutualism may drive co-operative adaptations in a pair of species. This is the case with certain flowers' ultra-violet color patterns, whose function is to guide bees to the center of the flower and promote pollination. Co-evolution is also interspecific by definition; it excludes intraspecific (within species) arms races such as sexual conflict.

Evolutionary arms races can even be displayed between humans and micro-organisms, where medical researchers make antibiotics, and micro-organisms evolve into new strains which are more resistant.

Symmetrical versus asymmetrical arms races

Arms races may be classified as either symmetrical or asymmetrical. In a symmetrical arms race, selection pressure acts on participants in the same direction. An example of this is trees growing taller as a result of competition for light, where the selective advantage for either species is increased height. An asymmetrical arms race involves contrasting selection pressures, such as the case of cheetahs and gazelles, where cheetahs evolve to be better at hunting and killing while gazelles evolve not to hunt and kill, but rather to evade capture.

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 in 1973.

Examples

The Phytophthora infestans-potato Bintje interaction

Potato Bintje is deriving from a cross between Munstersen and Fransen potato species. It was created in Netherlands and now is mainly cultivated in the North of France and Belgium. Oomycete Phytophthora infestans is an obligatory parasite of its variety and is responsible for the potato blight, in particular during the European famine in 1840. Zoospores (mobile spores, characteristics of oomycetes) are liberated by zoosporangium providing from mycelium and brought by rain or wind before infecting tubers and leafs. Black colours appear on plant because of the infection of cellular system necessary for the multiplication of the oomycete infectious population. The parasite contains virulent-avirulent allelic combinations in several microsatellite loci, likewise the host contains several multiloci resistance genes (or R gene). That interaction is called gene-for-gene relationship and is generally widespread in plant diseases. Expression of genetic patterns in the two species is a combination of resistance and virulence characteristics in order to have the best survival rate.

Bat and moths

Bats have evolved to use echolocation to detect and catch their prey. Moth's ears have in turn evolved to detect the echolocation calls of hunting bats, and evoke evasive flight maneuvers, or reply with their own ultrasonic clicks to confuse the bat's echolocation. Some bats are known to use clicks at frequencies above or below moths' hearing ranges. Barbastelle bats have in turn evolved to use a quieter mode of echolocation, calling at a reduced volume and further reducing the volume of their clicks as they close in on prey moths. The lower volume of clicks reduces the effective successful hunting range, but results in a significantly higher number of moths caught than other, louder bat species. Moths have further evolved the ability to discriminate between high and low echolocation click rates, which indicates whether the bat has just detected their presence or is actively pursuing them. This allows them to decide whether or not defensive ultrasonic clicks are worth the time and energy expenditure.

The rough-skinned newt and the common garter snake

The rough-skinned newt produces tetrodotoxin from skin glands as a defence against predation. This toxin binds reversibly to sodium channels in nerve cells and interferes with the normal flow of sodium ions in and out of the cell. This has the effect of inducing paralysis and death. Throughout much of the newt’s range, the common garter snake has been observed to exhibit resistance to the tetrodotoxin produced in its skin. While in principle the toxin binds to a tube shaped protein that acts as a sodium channel in the snake's nerve cells, researchers have identified a genetic disposition in several snake populations where the protein is configured in such a way as to hamper or prevent binding of the toxin. In each of these populations, the snakes exhibit resistance to the toxin and successfully prey upon the newts. The mutations in the snake’s genes that conferred resistance to the toxin have resulted in a selective pressure that favors newts which produce more potent levels of toxin. Increases in newt toxicity then apply a selective pressure favoring snakes with mutations conferring even greater resistance. This evolutionary arms race has resulted in the newts producing levels of toxin far in excess of what is needed to kill any other conceivable predator. Toxin resistant garter snakes are the only known animals today that can eat a rough-skinned newt and survive.

Introduced species

Cane Toads have experienced a massive population explosion in Australia due to the lack of competition.

When a species has not been subject to an arms race previously, it may be at a severe disadvantage and face extinction well before it could ever hope to adapt to a new predator, competitor, etc. This should not seem surprising, as one species may have been in evolutionary struggles for millions of years while the other might never have faced such pressures. This is a common problem in isolated ecosystems such as Australia or the Hawaiian Islands. In Australia, many invasive species, such as cane toads and rabbits, have spread rapidly due to a lack of competition and a lack of adaptations to cane toad bufotenine on the part of potential predators. Introduced species are a major reason why some indigenous species become endangered or even extinct, as was the case with the dodo.

See also

• Antipredator adaptation
• Parasite-host interactions
• Parent–offspring conflict

References

General

• Dawkins, R. & Krebs, J.R. (1979). Arms races between and within species. Proceedings of the Royal society of London, B 205:489-511.
• Vermeij, G. J., (1987). Evolution and escalation: An ecological history of life. Princeton University Press.
• Leigh Van Valen (1973). A new evolutionary law, Evolutionary Theory 1, 1¬30

External links

• Nature's Eternal Arms Race (PBS Documentary)