It's not just human beings that divide into early rising `larks' and late rising `owls'. Flies too are governed by biological, or circadian, clocks that dictate when they emerge from the pupa and when they are active. But how did these clocks evolve? To investigate, Vijay Kumar Sharma and his colleagues at the Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, India, imposed a selection pressure on flies (Drosophila melanogaster) as they emerged from the pupa. By selecting flies that emerged in the morning or evening, they wanted to see if they could change the timing of the flies' clocks over many generations (p. 906).

Keeping flies in a regime of 12 h dark then 12 h light every 24 h, the team chose larks that emerged from their pupae in the morning between 5 am and 9 am, or owls that emerged in the evening between 5 pm and 9 pm. The team kept these early and late risers separate, using them to breed the next generation, before selecting the early and late risers from the offspring for breeding, and so on for 55 generations.

Interested to know how the flies' clocks were changing through the generations, the team carried out a series of tests on the 5^th, 10^th, 25^th, 40^th and 55^th generations. Measuring when flies in the early or late groups emerged from their pupae, they found that the percentage of flies in the early group emerging in the morning increased, but evening risers decreased. In the late populations the reverse occurred; more flies emerged in the evening, and fewer in the morning. This showed that they were successfully breeding lark and owl populations of flies.

Next, the team looked at the pattern of emergence, noting how many flies emerged during set time periods, so that they could calculate the time when the greatest number of flies emerged relative to when the lights came on. They found that in early groups, the peak emergence occurred close to lights on, with more flies emerging at this peak through the generations. They observed a similar pattern in the late groups, only the peak occurred close to lights off.

While the team had selected flies according to when they emerged, they also wanted to see if any of the flies' other behaviours had changed. By measuring when the adult flies were active, they found that adults from early groups were more active in the morning, and those from late groups were more active in the evening, mirroring the emergence patterns. The implication of this result is that `rhythmic processes in the body are controlled by multiple clocks, but the clocks talk to each other' says Sharma, so selecting for emergence also influences when flies are active.

Finally, to find out how the flies' clocks would operate without any light cues, they plunged the flies into darkness and measured their emergence patterns and activity. Previous studies had shown that animals with early clocks have a slightly faster circadian rhythm than animals with late clocks, and the team found the same with their flies. The flies' emergence patterns and activity followed the same patterns as when the lights were on and off, although the early flies had a slightly faster daily rhythm of 23.6 h compared to the late flies' rhythm of 24.3 h. This shows that `clocks evolve through selection pressure on the timing of rhythmic behaviour,' Sharma says.