The niche parameters of such parasites are much easier to measure than those of free-living organisms, both in ecological and evolutionary time. Permanent parasites often speciate in tandem with their hosts, leading to congruent host-parasite phylogenies. Congruence is seldom perfect, however, because parasites sometimes switch to new host lineages during evolutionary time. This is when things get interesting. My colleagues and I have been addressing these and other questions using a variety of empirical tools, such as molecular systematics, comparative morphological studies and experimental work in both the laboratory and field.
Most species have a unique species of louse, but a few species share lice.
With such experiments we hope to identify factors constraining the hostspecificity and co-speciation of lice with their hosts. We are also measuring the reciprocal effect of foreign parasites on host survival and reproduction. This will allow us to conduct a preliminary test of the hypothesis that parasites reinforce reproductive isolation between host species by reducing the survival of hybrids. This unexplored possibility has major ramifications as it has the potential to explain a significant fraction of the earth's biodiversity. Experimental demonstration of the energetic cost of parasitism in freeranging hosts.