Selecting for fast and slow maturing worms.

Parasites, such as viruses, bacteria, protozoa and worms, constitute a constant threat to the survival of free-living organisms. Since parasites are also one of the major limitations of modern food production and an important source ofmortality in humans, amultibillion dollar industry is busy designing new drugs to control infection levels.Most drugs are designed to increase parasite mortality, some by affecting young, infective stages, while others are targeted against the reproductive stage. This massive use of drugs to control parasites can have several negative consequences, of which thedevelopment of drug resistancehas receivedmuch attention. Another consequence, which has largely been ignored, is the potential of drugs to select for changes in parasite life histories. This effect of the drug industry may have alarming consequences, since epidemiological parameters like virulence, disease dynamics and host recovery rates are closely linked to parasite life histories (Anderson & May 1982; Frank 1996; Gandon et al. 2001). A paper in the current issue of Proceedings B (Paterson & Barber 2007) provides new insights into how parasite life histories may evolve under selection regimes that simulate drug treatments. Life-history evolution is a cornerstone in modern ecology, and over the last decades we have seen significant advances in theory as well as a vast number of experimental studies of plants, invertebrates and vertebrates. However, very few of these studies deal with parasites. In 1992, two major textbooks appeared, summarizing the theoretical and empirical state of knowledge of life-history evolution (Roff 1992; Stearns 1992), and these books are still regarded as reference texts in the field. Looking at the index of these books, you will see that parasites are hardly mentioned at all. Since 1992, the situation improved somewhat, but still there are surprisingly few studies on parasite life-history evolution considering the practical importance of the subject. So, what do we know of parasite life-history evolution? The most frequently studied taxon has been nematodes, perhaps not surprisingly, since these worms are a major cause of disease in both humans and livestock and infect most, if not all, wild vertebrate populations. Moreover, nematodes display a staggering variation in life-history traits such as body size, longevity and age at maturity (Skorping et al. 1991; Morand 1996). Comparative studies reveal that life-history patterns of parasitic nematodes are strikingly different from most free-living taxa. Taxa such as mammals, birds and reptiles tend to form a continuum from small, fast species with high rates of somatic growth and high fecundity to larger, slower species with the opposite suit of traits. Within nematodes, small species tend to have low somatic growth rates and low fecundity, and species that are able to postpone their age at maturity have all the advantages: they grow bigger, live longer and reproduce more. Any medical or veterinary