Evolutionary arms race

In

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.^[3]

Host–parasite dynamic

Selective pressure between two species can include

The Bintje potato is derived from a cross between Munstersen and Fransen potato varieties. It was created in the Netherlands in the early 20th century and now is mainly cultivated in the North of France and Belgium. The oomycete Phytophthora infestans is responsible for the potato blight, in particular during the European famine in 1840. Zoospores (mobile spores, characteristics of oomycetes) are liberated by zoosporangia provided from a mycelium and brought by rain or wind before infecting tubers and leaves. Black colours appear on the plant because of the infection of its 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, in general, 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.^[5]

Bats and moths

The different defense mechanisms have been shown to be directly responsive to bat echolocation through sympatry studies. In places with spatial or temporal isolation between bats and their prey, the moth species hearing mechanism tends to regress. Fullard et al. (2004) compared adventive and endemic Noctiid moth species in a bat-free habitat to ultrasound and found that all of the adventive species reacted to the ultrasound by slowing their flight times, while only one of the endemic species reacted to the ultrasound signal, indicating a loss of hearing over time in the endemic population.^[6] However, the degree of loss or regression depends on the amount of evolutionary time and whether or not the moth species has developed secondary uses for hearing.^[10]

Some bats are known to use clicks at frequencies above or below moths' hearing ranges.

Rough-skinned newts have skin glands that contain a powerful nerve poison, tetrodotoxin, as an anti-predator adaptation. Throughout much of the newt's range, the common garter snake is resistant to the toxin. While in principle the toxin binds to a tube-shaped protein that acts as a sodium channel in the snake's nerve cells, a mutation in several snake populations configures the protein in such a way as to hamper or prevent binding of the toxin, conferring resistance. In turn, resistance creates a selective pressure that favors newts that produce more toxin. That in its turn imposes 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 that needed to kill any other predator.^[14][15][16]

In populations where garter snakes and newts live together, higher levels of tetrodotoxin and resistance to it are observed in the two species respectively. Where the species are separated, the toxin levels and resistance are lower.^[17] While isolated garter snakes have lower resistance, they still demonstrate an ability to resist low levels of the toxin, suggesting an ancestral predisposition to tetrodotoxin resistance.^[18][19] The lower levels of resistance in separated populations suggest a fitness cost of both toxin production and resistance. Snakes with high levels of tetrodotoxin resistance crawl more slowly than isolated populations of snakes, making them more vulnerable to predation.^[17] The same pattern is seen in isolated populations of newts, which have less toxin in their skin.^[20] There are geographic hotspots where levels of tetrodotoxin and resistance are extremely high, showing a close interaction between newts and snakes.^[17]

Predator whelk and the hard-shelled bivalve prey

The whelk predators used their own shell to open the shell of their prey, oftentimes breaking both shells of the predator and prey in the process. This led to the fitness of larger-shelled prey to be higher and then more selected for through generations, however, the predator's population selected for those who were more efficient at opening the larger-shelled prey.^[21] This example is an excellent example of asymmetrical arms race because while the prey is evolving a physical trait, the predators are adapting in a much different way.

Floodplain death adders and separate species of frogs

Floodplain death adders eat three types of frogs: one nontoxic, one producing mucus when taken by the predator, and the highly toxic frogs, however, the snakes have also found if they wait to consume their toxic prey the potency decreases. In this specific case, the asymmetry enabled the snakes to overcome the chemical defenses of the toxic frogs after their death.^[22] The results of the study showed that the snake became accustomed to the differences in the frogs by their hold and release timing, always holding the nontoxic, while always releasing the highly toxic frogs, with the frogs that discharge mucus somewhere in between. The snakes would also spend generously more time gaped between the release of the highly toxic frogs than the short gaped time between the release of the frogs that discharge mucus. Therefore, the snakes have a much higher advantage of being able to cope with the different frogs defensive mechanisms, while the frogs could eventually increase the potency of their toxic knowing the snakes would adapt to that change as well, such as the snakes having venom themselves for the initial attack.^[22] The coevolution is still highly asymmetrical because of the advantage the predators have over their prey.^[22]

Introduced species

When a species has not been subject to an arms race previously, it may be at a severe disadvantage and face

• Anti-predator adaptation
• Parasite–host
  interactions
• Parent–offspring conflict
• Antimicrobial resistance

References