War between species: the evolution of matched weaponry

By Megan Widdows

Spe­cies do not evolve in isol­a­tion, but rather in response to an extraordin­ary range of envir­on­ment­al factors. One of these factors is their rela­tion­ship with oth­er spe­cies in their hab­it­at. Each hab­it­at con­sists of an intric­ate web of co-depend­ent spe­cies with dif­fer­ent yet linked rela­tion­ships. Spe­cies that hunt, kill and eat oth­er anim­als are known as pred­at­ors, whilst the hunted spe­cies are known as prey. Often pairs of pred­at­ors and prey are con­sidered nat­ur­al enemies, as they con­stantly com­pete with one anoth­er for their survival. 

Nat­ur­al enemies fre­quently find ways of evolving to out­smart, and out­live, the oth­er. One meth­od of pro­tec­tion for prey is the pro­duc­tion of tox­ins to deter and kill-off pred­at­ors. In response, pred­at­ors can devel­op res­ist­ance to the tox­in, ensur­ing their sur­viv­al. The cycle con­tin­ues, with the prey adapt­ing once again to pro­duce more deadly tox­ins and so on. This pro­cess of co-evol­u­tion is known as an “evol­u­tion­ary arms race” – “arms race” allud­ing to the pro­cess by which mul­tiple coun­tries con­tinu­ously grow their mil­it­ary arm­a­ment to gain superi­or­ity over one-anoth­er. The end res­ult is that nat­ur­al enemies are often closely matched in their abil­ity to kill and avoid being killed by their enemy. This is known as matched weaponry. 

Dif­fer­ent geo­graph­ic­al loc­a­tions often have dif­fer­ent levels of matched weaponry as the evol­u­tion­ary arms race may take dif­fer­ent courses depend­ing on loc­a­tion and peri­od of time. 

An example of a pair of nat­ur­al enemies is the com­mon garter snake and the rough skinned newt, which are found togeth­er in west­ern parts of the USA. The newts pro­duce a deadly neur­o­tox­in called tetr­o­do­tox­in (TTX) and, in response, the snakes have evolved a res­ist­ance mech­an­ism through a muta­tion that dis­rupts the bind­ing of the tox­in to their cells. The level of res­ist­ance in the snakes is closely matched to the tox­icity of TTX in newts in dif­fer­ent loc­a­tions across the USA, cre­at­ing what is known as a geo­graph­ic­al mosa­ic. This mosa­ic is used as evid­ence to sup­port the the­ory of loc­al co-evol­u­tion between pop­u­la­tions in the evol­u­tion­ary arms race. 

How­ever, co-evol­u­tion is just one way of cre­at­ing vari­ation between mem­bers of a spe­cies in dif­fer­ent geo­graph­ic­al loc­a­tions. Oth­er factors that could be influ­en­tial include the loc­al envir­on­ment­al con­di­tions and his­tor­ic­al biogeo­graphy. A recent study from a group of Amer­ic­an uni­ver­sit­ies set out to determ­ine what role, if any, these factors have on the devel­op­ment of matched weaponry. 

First, they found that there was a clear link between the levels of TTX tox­icity and res­ist­ance present in snakes, which sup­ports the the­ory of co-evol­u­tion. There were clear geo­graph­ic pat­terns where areas of high TTX tox­icity saw high levels of res­ist­ance and, con­versely, areas with low TTX tox­icity saw low levels of resistance. 

All geo­graph­ic­al vari­ation in res­ist­ance could be pre­dicted by the level of newt tox­icity, firmly sug­gest­ing that the evol­u­tion of res­ist­ance in snakes is driv­en by their evol­u­tion­ary arms race with the newts. 

In con­trast, the research­ers found that vari­ation in newt TTX tox­icity was more closed linked to theirpop­u­la­tion genet­ic struc­ture and their envir­on­ment­al con­di­tions. In oth­er words, vari­ation in newt tox­icity had not aris­en simply as a res­ult of the co-evol­u­tion­ary arms race. 

Pop­u­la­tion genet­ic struc­ture refers to the total genet­ic diversity and abund­ance of dif­fer­ent genes in a pop­u­la­tion. Unlike res­ist­ance in snakes, which is con­ferred by a single muta­tion, the pro­duc­tion of tox­ins requires a com­plex bio­chem­ic­al path­way, that is coded for by a num­ber of dif­fer­ent genes. This could explain the increased import­ance of pop­u­la­tion struc­ture in the evol­u­tion of tox­icity, as changes to a num­ber of dif­fer­ent genes may be required to induce changes in TTX production. 

Addi­tion­ally, one par­tic­u­larly import­ant envir­on­ment­al factor could be the avail­ab­il­ity of TTX pre­curs­or molecules, which are effect­ively the ingredi­ent that newts need to pro­duce TTX. These are gen­er­ally obtained from food or pro­duced by help­ful, sym­bi­ot­ic bac­teria. The vari­ab­il­ity in the avail­ab­il­ity of these pre­curs­ors in dif­fer­ent geo­graph­ic­al areas could be a major reas­on behind the region­al vari­ation in TTX toxicity. 

Des­pite these find­ings, it is import­ant to note that the co-evol­u­tion is still thought to play an import­ant a part of newt adapt­a­tion towards matched weaponry. This is espe­cially the case in areas where snakes have evolved high levels of res­ist­ance. Here, arms race co-evol­u­tion is likely still present, driv­ing the newts to evolve fur­ther tox­icity to ensure their survival. 

These find­ings show that co-evol­u­tion is not fully respons­ible for the vari­ation in newt tox­icity. Instead, there are a num­ber of genet­ic and envir­on­ment­al factors also at play. This chal­lenges our pre­vi­ous assump­tions that matched weaponry could only arise as a res­ult of an intense co-evol­u­tion­ary battle between pred­at­or and prey. The rev­el­a­tion that oth­er pre­vi­ously unex­plored factors can influ­ence the evol­u­tion of matched weaponry is an import­ant find­ing, fur­ther­ing our under­stand­ing of pred­at­or-prey interactions. 

The study men­tioned in this art­icle can be found here. A gloss­ary of terms can be found below.

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