A tiny creature that has not had sex for 100 million years has overturned the theory that animals need to mate to create variety.

Analysis of the jaw shapes of bdelloid rotifers, combined with genetic data, revealed that the animals have diversified under pressure of natural selection.

Researchers say that their study “refutes the idea that sex is necessary for diversification into evolutionary species”.

The microscopic animals, less than four times the length of a human sperm, are all female, yet have evolved into different species that fill different ecological niches. Two sister species were found to be living together on the body of a water louse. One of them specialised in living around the louse’s legs and the other stayed close to the chest.

Genetic analysis showed that the two creatures were distinct, a fact backed up by observations that each type had differently shaped jaws.

Asexual animals and plants usually die out quickly in evolutionary terms but the ability of bdelloid rotifers to diversify may explain why they have survived so long.

A specimen trapped in amber has shown that the animals were living at least 40 million years ago and DNA studies have suggested they have been around for 100 million years. Modern Man has notched up about 160,000 years.

It had previously been recognised that asexual animals and plants can evolve through mutations into another species, but only into one species and at the cost of its original form. Bdelloid rotifers have displayed the ability to evolve into many different forms.

The study of several bdelloid rotifers, published in the journal PLoS Biology, was carried out by an international team including researchers from Imperial College London, the University of Cambridge and the Royal Botanic Gardens, Kew. “These really are amazing creatures, whose very existence calls into question scientific understanding,” said Tim Barraclough, of Imperial College.

He added that the two species of bdelloid rotifer almost certainly arrived on the louse as one species and later evolved to take better advantage of the environment.

There are many examples of asexual species of animals and plants, including some dande- lions. Asexuality is most common in invertebrates, such as aphids, but it is also found in a number of fish and frogs.

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