Phylogeography and the origin of species

Phylogeographic research on Dall's porpoise has provided information concerning the history and origin of present-day populations of Dall's porpoise. These data are used to determine the "big picture", the stage or background on which more recent population genetic processes arise. The estimated time to the hypothetical ancestral mtDNA haplotype of the Dall's porpoise, from which the haplotypes of present-day Dall's porpoises derived, existed at least between 600,000 and 1,000,000 years ago, based on coaslescent theory. Results further suggest that Dall's porpoise experienced a rapid divergence from an ancestral population in the western North Pacific overlain with high levels of historical gene flow (Escorza-Treviño and Dizon, 2000).

Taxonomic Status. Does Morphology Predict Taxonomy?

The taxonomic status of the two morphologically distinct forms of Dall’s porpoises (dalli-type and truei-type) is unsettled. Different investigators have considered these forms as separate species, separate subspecies, or morphological extremes of a single species. To help resolve this uncertainty, mitochondrial DNA (mtDNA) sequences were used to estimate phylogenetic relationships between the two types and, in conjunction with microsatellite markers, the degree of genetic differentiation between truei-type and currently recognized dalli-type populations was tested. Results showed two distinctive clades of mtDNA haplotypes (Escorza-Treviño et al., 2004). However, both were composed of individuals from both types, with shared mtDNA haplotypes between morphotypes. These results suggest that truei- and dalli-types are forms of the same species. The truei-type animals were also typed for six microsatellite loci. F-statistics of mitochondrial and nuclear data, however, revealed statistically significant genetic differentiation between truei-type and sympatric and adjacent dalli-type populations. These results argue that the truei-type, currently subjected to direct commercial harvesting, has demographically insignificant exchange with the much larger dalli-type populations and should be considered a separate management unit.

Population structure. What are the management units?

Successful conservation of living resources is based upon the accurate designation of "management units". A major goal of our research effort is to define these units so that management efforts can be appropriately placed. In Dall's porpoise, animals from several geographic strata were sequenced for a hypervariable region of the mitochondrial genome and typed for variation at 6 microsatellite loci. Analysis using FST values (based on haplotype frequencies) and FST values (based on frequencies and genetic distances between haplotypes) yielded statistically significant separation among most of the stocks currently used for management purposes by the International Whaling Commission. The analyses also revealed two additional populations, requiring further conservation attention (Escorza-Treviño and Dizon, 2000). Overall, mtDNA analysis supported more population subdivision than microsatellite analysis. This initially unusual result led to the question of sex-biased dispersal patterns (see below).

Similar markers were used to investigate genetic differentiation in spotted dolphins (Stenella attenuata) in the Eastern Tropical Pacific and to determine the taxonomic status of the coastal subspecies (S. a. graffmani). Analysis of mitochondrial control region sequences, from animals present in several coastal areas and offshore, using FST and FST yielded statistically significant separation among several different coastal and the offshore strata (Escorza-Treviño et al., 2002). Analysis using FST values for seven nuclear microsatellite loci yielded statistically significant separation between each of the coastal strata and offshore animals, but no coastal populations were distinguished. These results argue for the existence of at least four distinct coastal populations that should be treated as separate units for management purposes (Escorza-Treviño et al., 2005) and suggest the existence of male-biased dispersal among the coastal strata.

Sex-biased dispersal patterns: philopatry, dispersal, and the power to homogenize populations

In mammals, female philopatry and male dispersal is the rule. This may be especially true among marine mammals that are widely distributed and mobile. Yet, the extent of a bias in dispersal between the sexes, or of its effects on population structure, is little known for any marine mammal species. For both Dall's porpoises and spotted dolphins, highly male-biased migration was detected for several population comparisons (Escorza-Treviño and Dizon, 2000). The sex-biased dispersal of Dall’s porpoises explained discrepancies in the population structure data as well. It seems possible that site fidelity may be especially high for females of both these species and, while males disperse and homogenize the populations, different maternal lineages separate and define populations.

Other research

Our immediate plans are to continue molecular studies of the spotted dolphin complex, investigating how sex-biased dispersal and social structure affect population structure. Through similar molecular methodologies, we are investigating the genetic relatedness among individuals sampled in the same group to establish if relationships are based on kinship, gender, and/or a combination of these. In addition, we are comparing these relatedness results between coastal and offshore spotted dolphins in order to establish a connection between social structure and population differentiation.

We also intend to expand our research efforts to forensic applications of molecular techniques. In the past, we have worked on the identification of tissue samples suspected of being from protected species. We are planning on establishing fast and replicable methods for species identification to curb the illegal catch, trade, and transport of protected species. In addition, we expect to incorporate knowledge of population differentiation to identify samples to a greater geographic detail as a means to enhance conservation efforts. These methodologies are easily applicable to other protected species, be they marine or terrestrial.