Joshua L. Cherry

GENETIC AND PHYLOGENETIC CONSEQUENCES OF ISLAND BIOGEOGRAPHY

Abstract.— Island biogeography theory predicts that the number of species on an island should increase with island size and decrease with island distance to the mainland. These predictions are generally well supported in comparative and experimental studies. These ecological, equilibrium predictions arise as a result of colonization and extinction processes. Because colonization and extinction are also important processes in evolution, we develop methods to test evolutionary predictions of island biogeography. We derive a population genetic model of island biogeography that incorporates island colonization, migration of individuals from the mainland, and extinction of island populations. The model provides a means of estimating the rates of migration and extinction from population genetic data. This model predicts that within an island population the distribution of genetic divergences with respect to the mainland source population should be bimodal, with much of the divergence dating to the colonization event. Across islands, this model predicts that populations on large islands should be on average more genetically divergent from mainland source populations than those on small islands. Likewise, populations on distant islands should be more divergent than those on close islands. Published observations of a larger proportion of endemic species on large and distant islands support these predictions.

DELETERIOUS MUTATION AND THE EVOLUTION OF EUSOCIALITY

Abstract.— Certain arguments concerning the evolution of eusociality form a classic example of the application of the principles of kin selection. These arguments center on the different degrees of relatedness of potential beneficiaries of an individuals efforts, for example a females higher relatedness to her sisters than to her daughters in a haplodiploid system. This type of reasoning is insufficient to account for the evolution and maintainence of sexual reproduction, because parthenogenic females produce offspring that are more closely related to them than are offspring produced sexually. Among the forces invoked to explain sexual reproduction is deleterious mutation. This factor can be shown to favor eusociality as well, because siblings produced by helping carry fewer deleterious alleles on average than would offspring. The strength of this effect depends on the genomewide deleterious mutation rate, U, and on the selection coefficient, s, associated with deleterious alleles. For small s, the effect depends approximately on the product Us. This phenomenon illustrates that an assumption implicit in some analyses–that the relatedness of an individual to an actor is all that matters to its value to that actor–can fail for the evolution of eusociality as it does for the evolution of sex.

3 Speculations on the Origin of Ribosomal Translocation

The discovery of catalytic RNA suggested a way out of a troublesome problem concerning the origin of life (Gilbert 1986). Given the known roles of the various biological informational molecules, it had been unclear how a genetic system could ever have gotten started. The replication of a nucleic acid would require a proteinaceous polymerase, but without replication, nothing, protein synthesis included, could evolve. Put another way, there was no molecule known that could both serve as a template for replication and express its information without some sort of translation, in the loose sense of the word. The discovery of modern catalytic RNA demonstrated in dramatic fashion that RNA could be such a molecule. The first nucleic acid polymerase, it is hypothesized, was an RNA molecule that catalyzed its own replication. Such self-replicating RNAs were the first genetic systems and might well be called the first living things. Once there were replicators, there was selection. Whereas the first selection-driven changes probably were “improvements” to polymerase activity, eventually metabolic innovations arose, including template-directed protein synthesis. The precursor of the modern ribosome, then, was a purely, or mostly, RNA-based system, confirming early suspicions about the origins of rRNA, mRNA, and tRNA (Woese 1967; Crick 1968; Orgel 1968). The fact that the modern translational apparatus contains so much RNA is explained by such an origin of translation. In fact, it is tempting to think of ribosomal RNA as catalytic RNA, and experimental evidence indicates that at least some of the ribosome’s activities are associated...