Like diseases that affect humans, parasites can lead an evolutionary “arms race” against their hosts. These evolutionary “wars” depend on their rate of evolution – their ability to “discover” new strategies via random mutations in their genetics – which you can think of as their ability to “upgrade their weapons”.
The more individuals in a species, the greater the likelihood that a beneficial mutation will occur in one of them, which means that species with larger populations should generally win these “wars” scalable.
We can see this problem playing out with COVID-19 in humans, as the virus has a much larger population than us, its host. Thus, it is able to evolve around our defenses when new variants emerge, which helps it to spread.
We would expect to see the disappearance of parasites whose populations are very small compared to their hosts. But this is not the case for Australia’s native social parasitic bee species (genus Inquiline), which rarely infects more than 5% of host bee colonies (genus exonerating).
In fact, results from a new Australian study show that the two species are evolving at similar rates, despite the hosts’ much larger population sizes. The research has been published in Ecology and evolution.
“These parasitic species spend nearly their entire life cycle in the host species’ nest and have extreme adaptations to social parasitism, including greatly reduced mouthparts and pollen-collecting scopae,” says first author Dr. Nahid Shokri Bousjein, from the School of Biology. Sciences at Flinders University in Adelaide, Australia.
Their evolutionary arms race has been going on for about 15 million years, even though the Inquiline the population size is much smaller by at least an order of magnitude than that of their host. The researchers thought this might have been possible due to the fact that the parasites had an accelerated rate of molecular evolution, which would allow them to keep pace with their hosts.
In 2013, researchers took nests in the Dandenong Ranges of Victoria containing the host and parasite bees, and studied their mitochondrial genes that code for proteins.
“We used modern genomics to sequence the DNA of a large number of genes in both the parasite and the host species so that we could directly infer how quickly they were changing in the two groups,” says lead author Michael Schwarz. , associate professor at Flinders University.
“We had already inferred the age of host and parasite lineages, so we could express DNA changes as a rate of change per million years.”
But according to Shokri Bousjein, “surprisingly, our analyzes of the molecular data showed that the rates of evolution were similar between host and parasite.”
Molecular evolution rates are strongly influenced by population size in two different ways: the selection of the previously mentioned beneficial mutations, and also what is known as genetic drift (the random spread or loss of mutations, even whether they are neutral or slightly detrimental).
The effect of positive selection is larger in larger population sizes, but the effect of genetic drift is larger in smaller population sizes, so according to Schwarz, these similar evolutionary rates could represent a balance between both effects.
“We hypothesize that the rates are similar and represent a balance between drift on neutral or mildly deleterious mutations and selection on favorable mutations,” says Schwarz. “Thus, parasites should have high levels of genetic drift but low levels of favorable mutations, while hosts should have lower levels of drift but higher rates of positive mutations.”
This research indicates that evolutionary wars between species and their enemies can be much more complex than we previously thought, and this could have unexpected implications for research.
For example, understanding how very rare species are able to evolve fast enough to avoid extinction could influence zoo practices and how we set aside protected habitat.
“Another possible implication is that parasites (and diseases) have evolved to become less of a threat to their hosts,” adds Schwarz. “If their threat is lower then there is less selection on the hosts to develop strong defences.”
“You could think of it as parasites, or diseases, ‘slipping under the radar’ of their hosts,” he says. “It is a common hope that over time COVID-19 will evolve to be less virulent and therefore our need for a strong defense response will be less.”
We therefore need to know more about the evolutionary dynamics of hosts and their enemies.