Introduction

In his classic paper on the units of selection Richard Lewontin, following Williams (1966), emphasized that natural selection for characteristics that benefited individual organisms was stronger than selection for characteristics that benefited groups of organisms. One of the most important generalizations from this insight is that natural selection will tend to favor characteristics that benefit the survival and reproduction of individuals (individual selection) over characteristics that benefit group survival or productivity (group selection) when these two benefits are contradictory (Williams 1966, Lewontin 1970). Lewontin went on to suggest that group selection might be particularly strong for parasites because the parasites within a host are a well-defined group and the transmission of such groups may be highly dependent on the characteristics of the group, such as the overall level of harm that the group of parasites imposes on the host; consequently if one wishes to evaluate whether group selection is important in nature, parasites would be appropriate objects for study (Lewontin 1970).

One reason for this expectation derives from the theory of inclusive fitness, which emphasizes how characteristics that benefit other group members can be favored even when they impose a cost on the individual possessing the characteristic as a result of the positive effects of the characteristic on other individuals that harbor the same genetic instructions (Hamilton 1964).