Surviving with low genetic diversity: the case of albatrosses

Abstract

Low genetic diversity is predicted to negatively impact species viability and has been a central concern for conservation. In contrast, the possibility that some species may thrive in spite of a relatively poor diversity has received little attention. The wandering and Amsterdam albatrosses (Diomedea exulans and Diomedea amsterdamensis) are long-lived seabirds standing at an extreme along the gradient of life strategies, having traits that may favour inbreeding and low genetic diversity. Divergence time of the two species is estimated at 0.84 Myr ago from cytochrome b data. We tested the hypothesis that both albatrosses inherited poor genetic diversity from their common ancestor. Within the wandering albatross, per cent polymorphic loci and expected heterozygosity at amplified fragment length polymorphisms were approximately one-third of the minimal values reported in other vertebrates. Genetic diversity in the Amsterdam albatross, which is recovering from a severe bottleneck, was about twice as low as in the wandering albatross. Simulations supported the hypothesis that genetic diversity in albatrosses was already depleted prior to their divergence. Given the generally high breeding success of these species, it is likely that they are not suffering much from their impoverished diversity. Whether albatrosses are unique in this regard is unknown, but they appear to challenge the classical view about the negative consequences of genetic depletion on species survival.

Figure 1

Likelihood that a set AL_total of ancestral AFLP loci, among which a number AL_fixed are fixed, leads to the observed pattern of fixation in the two albatross species after their divergence. The current pattern is 184 loci fixed in both species and one locus fixed differentially (i.e. present in the wandering Albatross and absent in the Amsterdam Albatross). Results are from 10 000 simulations for each starting values of AL_total and AL_fixed. Note that the maximum number of fixed ancestral loci cannot exceed its current number (184) since pure drift will maintain monomorphic loci as monomorphic. In addition, the number of ancestral loci has to be at least as large as the number of loci found in the dataset (185) since loci subjected to pure drift can only disappear through drift, i.e. after fixating for absence in both species (see the worked out example in Appendix A).