Thomas Dudley Cabot Professor of the Natural Sciences Matthew S. Meselson has devoted his career to studying questions in molecular biology and fighting biological warfare.

His current research focuses on the mechanisms of molecular evolution, or more simply, why there are two sexes.

Meselson is now researching whether species can proliferate and survive without sexual reproduction.

Of the two million species of plants and animals alive today that have been named, 2,000 are purely female and reproduce asexually, Meselson says.

However, these species appear to become extinct every few thousand years. But entirely new asexual species then appear, Meselson says.

The prevalence of sexual reproduction in 99.9 percent of known species indicates that there must be some type of strong selection for it, he says.

However, scientists are unsure what the exact biological basis for sexual reproduction is.

"It is a very deep, fundamental and ancient question in biology," Meselson says.

Meselson is currently trying to unravel that mystery.

One way to study the question, Meselson says, is to determine whether a species has managed to proliferate despite remaining asexual.

Bdelloid rotifers-asexual organisms that are believed to have survived millions of years-are the focus of Meselson's studies.

These freshwater invertebrates, comprising more than 350 species, are believed to reproduce without sex. Meselson's group studies these organisms to determine whether sexual reproduction is even necessary.

Such a study is a complicated and manifold process.

Scientists must first ensure that bdelloid rotifers are and always have been asexual.

Meselson is using DNA sequence analysis and chromosome mapping to confirm the absence of sexual reproduction in their evolution.