Speciation in birds: Genes, geography, and sexual selection

1. 

   View larger version:

   • In this window
   • In a new window

   • Download as PowerPoint Slide

   Fig. 1.

   Conflicts between gene and species trees in Poephila finches, and their implications. (a) Frequency distribution of all three possible gene trees encountered in a survey of nuclear DNA sequence variation among three species of Australian grassfinches (Poephila). Branch tips are labeled as follows: a, P. acuticauda; h, P. hecki; c, P. cincta. P. cincta vs. P. acuticauda/hecki is a well established divergence across the Carpentarian barrier in northern Australia and is supported by numerous species-level phenotypic differences; P. hecki vs. acuticauda are allopatric but represent a more recent taxonomic split which is regarded by some (38) as phylogenetic species. In the genetic survey, a single allele was sampled from each of the three taxa for thirty presumably unlinked loci. Twenty-five gene trees exhibiting all three possible topologies were unambiguously reconstructed. Gene trees for five loci could not be resolved. Topology 1 reflects the presumed species tree. (b) Unconditional probabilities of a gene tree being congruent with the species tree over a range of values for T (after ref. 43). The curve is based on the equation for three species [P _{(congruent gene tree)} = 1-2/3e ^-T (42, 43), where T is equal to t/2N_e, t is defined as the time between speciation events (in generations), and 2N_e is twice the size of the effective population size of the basal ancestor. When T = 0, the topology of a gene tree of three sampled alleles is expected to be random with respect to the species tree (i.e., probability of 0.33). Note that T must be at least as large as 2.6 for a 0.95 probability of congruence, yet the empirically derived probability for Poephila finches of 0.56 results in a T value of only 0.4. The Poephila tree congruence probability is based on the fact that 14 of 25 independent gene trees matched the presumed species tree observed by W.B.J. and S.V.E. (unpublished work).

   
   
2. 

   View larger version:

   • In this window
   • In a new window

   • Download as PowerPoint Slide

   Fig. 2.

   Behavioral imprinting maintains host specificity and genetic cohesion of indigobird species but also provides a mechanism for rapid speciation when new hosts are colonized. Male indigobirds mimic the songs of their hosts, whereas females use song cues to choose both their mates and the nests they parasitize. Rarely, females lay in the nest of a novel or alternate host. The resulting offspring imprint on the novel host and are therefore reproductively isolated from their parent population. Indigobird drawing by Karen Klitz.

   
   
3. 

   View larger version:

   • In this window
   • In a new window

   • Download as PowerPoint Slide

   Fig. 3.

   Summary of Haldane's rule in birds. Predictions are based on the presence or absence of faster-male evolution (sexual selection hypothesis), which changes the order of accumulation of incompatibilities in WZ species. The predicted order of accumulation of incompatibilities for the case without sexual selection is based on the pattern in Drosophila. The data observed by Price and Bouvier (11) are boxed. The heterogametic sex is underlined.