Like diseases which affect humans, parasites can wage an evolutionary “arms race” against their hosts. These evolutionary “wars” depend on their rates of evolution – their ability to ‘discover’ new strategies via random mutations in their genetics – which you can think of as like their ability to ‘upgrade their weapons’.
The more individuals in a species, the greater the chance that a beneficial mutation will arise in one of them, meaning that species with larger populations should generally win these evolutionary “wars”.
We can see this problem play out with COVID-19 in humans, as the virus has a much bigger population than us, its host. So, it’s able to evolve around our defences when new variants emerge, helping it to spread.
We would expect to see parasites with very small population sizes, compared with their hosts, becoming extinct. But this isn’t the case for the native Australian social parasite bee species (genus Inquilina), which rarely infests more than 5% of host bee colonies (genus Exoneura).
In fact, the results of a new Australian study show both species are actually evolving at similar rates, despite the much larger population size of the hosts. The research was published in Ecology and Evolution.
“These parasitic species spend almost their entire life cycle within the nest of the host species and have extreme adaptations to social parasitism, including strongly reduced mouth parts and pollen-collecting scopae,” says first author Dr Nahid Shokri Bousjein, from the School of Biological Sciences at Flinders University in Adelaide, Australia.
Their evolutionary arms race has been under way for about 15 million years, even though the Inquilina population size is much smaller by at last an order of magnitude than their host’s. The researchers thought this could have been possible due to the parasites having an accelerated rate of molecular evolution, which would allow them to keep up with their hosts.
In 2013 researchers collected from Victoria’s Dandenong Ranges nests containing the host and parasite bees, and studied their mitochondrial genes that code for proteins.
“We used modern genomics to sequence DNA for a large number of genes in both the parasite and host species so we could directly infer how quickly they were changing in the two groups,” says senior author Michael Schwarz, associate professor at Flinders University.
“We had already inferred how old the host and parasite lineages were, so we could express DNA changes as rates of change per million years.”
But according to Shokri Bousjein, “surprisingly, our analyses of molecular data showed that rates of evolution were similar between host and parasite”.
Rates of molecular evolution are strongly influenced by population size in two different ways: the selection for beneficial mutations mentioned previously, and also what’s known as genetic drift (the random spread or loss of mutations, even if they are neutral or slightly detrimental).
The effect of positive selection is greater in larger population sizes, but the effect of genetic drift is greater in smaller population sizes, so according to Schwarz these similar rates of evolution might represent a balance between the two effects.
“We hypothesise that the rates are similar and represent a balance of drift on neutral or slightly deleterious mutations, and selection on favorable ones,” explains Schwarz. “So, parasites should have high levels of genetic drift but low levels of favorable mutations, whilst hosts should have lower levels of drift, but higher rates of positive mutations.”
This research indicates that the evolutionary wars between species and their enemies may be much more complex than we previously thought, and this could have unexpected implications for research.
For instance, understanding how very rare species are able to evolve rapidly 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 hosts to evolve strong defences.”
“You could think of this 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, as a result, our need for a strong defence response will be less.”
So, we need to know more about the evolutionary dynamics of hosts and their enemies.