To hear ‘evolution’ is to think of fossils and the history of life. Most people are surprised that evolution can be studied experimentally, just like other areas of science.
Well, the reality is that we routinely do experiments in evolution, and experimental evolution is a scientific area where India really shines. Over half the research groups in the world that do long-term evolutionary experiments are in India, spread across nine different cities.
In 1997, our lab began to ask how organisms evolve when under pressure to become adults rapidly. We decided to use fruit flies (Drosophila) for this work. Fruit flies are easy to rear in large numbers, can turn over a generation in 10-11 days, and have an adult lifespan of 30-40 days. In nature, they mostly feed on rotting plant material, typically fruit, but are not crop pests. They have long been used for studies in genetics, developmental biology, neurobiology, ecology and evolution.
We addressed our question by selecting a batch of flies for rapid development to adulthood. Every generation, we ensured that only the first 15-20 per cent of flies to reach adulthood would get to breed the next generation; late developing flies were discarded. This is painstaking work because adult flies might start appearing at any time of night or day, on a weekend or holiday, and when that happens we have to be in the lab to collect the most rapidly developing ones. Almost 25 years and over 800 fly generations later, these flies have added greatly to our understanding of many evolutionary phenomena.
Earlier, it was generally believed that faster development meant more competitive flies. We were able to show that it was not so. Our fast developing populations were actually less competitive than their ancestors. We were also able to nail down why. The fast developing flies evolved to become poorer than their ancestors in most respects—they fed slowly, were susceptible to bodily wastes, were smaller and less energetic, had less fat, laid fewer eggs, and had shorter lives. What they excelled at was what they had been selected for: to develop fast. Flies from the selected populations became adults in about 150 hours, about 70 hours faster than their ancestors, making them the most precocious flies ever known. They were good at developing fast but poor at everything else, leading a fellow fly researcher, Christian Klingenberg, to label them ‘Formula One flies’.
Males and females need to cooperate to have kids, but there is also sexual conflict. How many kids a female can have is limited by how many costly eggs she can produce. Since sperms are cheap, how many kids a male can sire is limited to how many females he can mate with. This is the root of evolutionary male-female conflict. Males evolve ways to alter female reproductive behaviour in ways that are beneficial to males, but not necessarily to females. This is usually through small proteins in the semen which alter female physiology. Females evolve countermeasures to such manipulation by males, leading to an arms-race like situation.
In an arms race, only the relative strength of the two adversaries matters. Both with guns, or both with swords, lead to a balance. If the one with swords suddenly has to fight with the one with guns, it loses. When females from rapidly developing populations mate with males from ancestral populations, they literally start ‘dying like flies’. This suggested that in the rapidly developing populations, male-female conflict had evolved to settle down at a much lower balance, sort of like both sides having swords. Over many experiments, we were able to establish this and also trace its evolutionary origins to the small size of the rapidly developing males.
We saw that females from rapidly developing populations die soon after mating with ancestral males. Small rapidly developing males find it hard to successfully mate with large ancestral females. Thus, when mixed, these populations largely mate amongst themselves. This is the first step towards the two diverging into different species altogether, and we get to watch it happen before our eyes.
Amitabh Joshi studies and teaches at the Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore
