Does evolution sometimes have to pass through intermediate forms of low fitness?

This is that the hybrids between species are often - indeed, almost always - of low fitness compared to their parents.

Nevertheless, evolution may sometimes pass through intermediates of lower fitness.

"If you are low fitness as an adolescent, you'll probably remain that way as an adult," he said.

Women unable to last six minutes were ranked as "low fitness."

Remember that if the populations have been separated for long enough, it is likely that hybrids between them will have relatively low fitness.

Females largely benefit from this mechanism, but males often can experience low fitness because their paternity is not secure.

The study found that low fitness is a risk factor of the same order as smoking.

It's the first study to show that low fitness is at the top with smoking," Dr. Blair said.

First, low fitness was a strong predictor of all causes of death.