University of  Sussex MEDIA RELEASE

The Information Office, University of Sussex, Falmer, Brighton, BN1 9RH.

26 February 1998


A team of evolutionary biologists from the University of Sussex has developed a new theory which may explain a 70 year old mystery. Until now, there has been no adequate explanation for the fact that sexual and asexual forms of certain plants and animals live in different habitats, with asexuals tending to live in colder environments, such as on mountain tops and in the far north. However, in a paper to be published in Nature today (26 February) Drs. Joel Peck, Jonathan Yearsley and David Waxman from the Centre for the Study of Evolution at Sussex put forward a new theory which may account for this geographical pattern in reproductive activity.

There are many animals in which there are two types - one that reproduces in the "normal" sexual manner, (ie. by mating), while the other reproduces asexually, (without mating). One example is the European woodlouse (a small terrestrial crustacean). In northern parts of Europe, most of the woodlice reproduce by asexual means. In more southerly parts of Europe, a substantial fraction of woodlice are produced sexually. Similar patterns have been shown to hold in a wide variety of different animals and plants. Asexuals also tend to occur high on mountains, and they sometimes occur on the edges of sexual populations (often in relatively inhospitable areas).

The new theory to account for these patterns is expressed in terms of mathematical models. The theory assumes that the environment changes from one place to another. This assumption is known to hold in many real environments. For example, some areas may be windy, thus favouring sturdy plants, while others might be sheltered, favouring tall plants than can compete effectively for access to sunlight.

The theoretical work suggests that, in the types of environments where asexuals tend to live, there are often many migrants from other areas. For example, in the North, many migrants move in from the South, where the growing season is longer. This long growing season allows for production of an excess of potential migrants over the course of the year. Many of these migrants from the South carry genes that are good in the South but that are not well adapted for their new northern homes. Therefore, mating in the North is a dangerous affair, because so many migrants are present, and because mating with a mal- adapted migrant is likely to result in mal-adapted offspring. Thus, asexuality is favoured in the North, because it allows the production of offspring without mating.

For further information contact Dr Joel Peck at the University of Sussex on Tel. or Fax. 44 1273 678843, email

This file produced by USIS - 26th February 1998