Mutations and natural selection
How mutations and natural selection could give rise to such creepy powers is a particularly intriguing puzzle for evolutionary biologists.
One useful concept for thinking about it comes from biologist Richard Dawkins, author of the landmark book The Selfish Gene.
In that book Dawkins argued that genes evolve to make copies of themselves more successfully. Our bodies may be important to us, but from our genes’ point of view, they are nothing more than vehicles to get themselves intact into the next generation.
The entire collection of the genes that make up you or me is called our genotype. The sum total of all the bodily parts and functions that our genotype creates to advance its cause—you or me—is called our phenotype.
It occurred to Dawkins that we don’t have to limit phenotypes to the boundaries of our bodies. They also include the behaviours brought about by our genes.
A beaver’s genes encode its bones and muscles and fur. But they also encode the brain circuits that lead the beaver to gnaw at trees to build dams.
The beaver benefits from the pond created by the dam in many ways. It’s harder for predators to attack the beaver’s lodge, for example, because of deeper surrounding water.
If a gene mutation gives rise to a beaver that builds even better dams, that particular beaver phenotype may stand a better chance of survival and, on average, have more baby beavers itself.
As a result, the mutation will become more common over the course of many generations. From an evolutionary perspective, the dam—and even the pond that it creates—is as much an extension of the beaver’s genes as its own body is.
If the power of a gene can extend to manipulation of the physical world, Dawkins wondered, could it not extend as well to the manipulation of another living creature?
Dawkins argued that it could, and he pointed to parasites as his prime example. The ability of a parasite to control the behaviour of a host is encoded in its genes. If one of those genes mutated, the host’s behaviour would change.
Depending on how it changed, the mutation might help or harm the parasite. If a flu virus mutates so that its victims lock themselves away and starve to death, the virus will be unlikely to spread to other hosts, and it will disappear from the population of viruses.
A mutation in a parasite that influences a host’s behaviour for the better will become more common.
If a wasp acquires a mutation that compels its ladybug host to begin to act as a bodyguard, for example, its offspring carrying that trait will thrive, because fewer of them will be killed by predators.
Dawkins first developed these ideas in his 1982 book The Extended Phenotype. In many respects it was a book far ahead of its time.
In the 1980s scientists had carefully studied only a few examples of parasites manipulating their hosts’ behaviour. But if the hypothesis was correct, there had to be genes within the parasites that trumped the genes in the hosts themselves that normally controlled their actions.
Understanding parasite mind control (continues next month)
For more information, see Tree Fruit January 2015
