Kin Selection, Disease Transmission, and Levels of Selection
 

Representative publications:

Pfennig, D. W. 2002. Kin recognition. pp. 592-595. In M. Pagel (ed) Encyclopedia of Evolution, Oxford University Press, Oxford.

Pfennig, D. W. 1999. Cannibalistic tadpoles that pose the greatest threat to kin are most likely to discriminate kin. Proceedings of the Royal Society of London, Series B 266: 57-61.

Pfennig, D. W. & Collins, J. P. 1993. Kinship affects morphogenesis in cannibalistic salamanders. Nature 362: 836-838.

Overview:
Although we often think of natural selection as acting at the level of individual organisms, selection can also operate at other levels of biological organization, such as genes.  Understanding multilevel selection is important, because selection at one level may be in opposition to that at another level.  For example, gene selection helps explain a central paradox of Darwinism: the evolution of altruism.  Altruism is problematic, because it would seem impossible for natural selection to favor an allele that results in behavior benefiting other individuals at the expense of the individual bearing the allele.  Using a gene-centered point of view, William Hamilton showed mathematically that a gene for altruism will spread when rB – C > 0, where r is the coefficient of relatedness between actor and recipient, B is the benefit to the recipient in units of surviving offspring, and C is the cost to the actor.  When Hamilton’s rule holds, kin selection results in altruistic behavior.

Cannibalistic species are ideal for testing this theory.  Kin selection theory predicts that cannibals that avoid destroying kin should often be favored over individuals lacking this ability.  In essence, any gene encoding discriminatory cannibalism should spread because it promotes survival of copies of itself in the cannibal’s kin (Fig. 8), even though such an allele may be personally costly to its bearer by causing the cannibal to occasionally forgo a meal.

Fig. 8 (click to enlarge)

I have been testing the above prediction in spadefoot toads and tiger salamanders.  Tadpoles of both species develop into either a noncannibalistic omnivore morph or a cannibalistic carnivore morph (see the section above on Evolution and Development of Alternative Phenotypes).  In both the field and the lab, when offered smaller tadpoles that differ in relatedness, carnivores preferentially consume more distant relatives (Pfennig et al. 1993, 1994, 1999).  In addition, tadpoles are more likely to express the carnivore phenotype when reared with nonkin than when reared with kin, indicating that kinship environment affects morphological development as well (Pfennig & Collins 1993; Pfennig & Frankino 1997).  This discriminatory cannibalism appears to have evolved via kin selection.  Field experiments reveal that the discriminating behavior of cannibalistic tadpoles satisfies the requirement of Hamilton’s inequality, implying that not eating siblings is favored by kin selection (Pfennig et al. 1999).

I also have been examining additional selective forces that might favor discriminatory behavior.  Although kin selection can explain why cannibalistic animals avoid eating kin, enhanced risk of acquiring disease also may explain why cannibalism is not more common (Pfennig et al. 1991, 1998; Pfennig 2000).  In particular, many parasites are transmitted through ingestion of infected tissue.  Moreover, contagions are often more highly transmissible among phylogenetically similar individuals than among phylogenetically dissimilar individuals.  For example, cannibalistic tiger salamanders are more likely to acquire parasites from conspecifics than from similarly infected heterospecifics (Pfennig et al. 1998).  Can avoidance of such diseases also explain why some cannibals avoid eating kin (Pfennig 1997; Pfennig et al. 1999)?

More generally, I am interested in exploring the principles governing conflict and cooperation among the various levels of biological organization (e.g., genes, cells, and organisms).  I would like to know how these principles are similar or different at the various levels of biological organization.  I would also like to know how interactions at one level influence the evolution of interactions at other levels.

To read more about my research on kinship, cannibalism, and disease, click here.