It is generally assumed that the Quaternary was a period of heightened diversification in temperate vertebrate organisms. Previous molecular systematics studies have challenged this assertion. We re-examined this issue in north temperate birds using log-lineage plots and distributions of sister-taxon distances. Loglineage plots support earlier conclusions that avian diversification slowed during the Quaternary. To test plots of empirical sister-taxon distances we simulated three sets of phylogenies: constant speciation and extinction, a pulse of recent speciation, and a pulse of recent extinction. Previous opinions favour the model of recent speciation although our empirical dataset on 74 avian comparisons failed to reject a distribution derived from the constant and extinction models. Hence, it does not appear that the Quaternary was a period of exceptional rates of diversification, relative to the background rate.

Understanding the evolutionary history of the species in a particular region provides insights into how that fauna was formed. Of particular interest to biogeographers is examining the impact a geographical barrier had in generating temporal genetic diversity among codistributed species. We examined the impact a major New World barrier, the Isthmus of Tehuantepec (IT) in southern Mexico, had on a regional bird fauna. Specifically, genetic data from 10 montane-forest bird taxa were analysed using approximate Bayesian computation (ABC) to test the hypothesis of simultaneous intraspecific diversification at the IT. Because effective population size (N e ) has the greatest impact on coalescent times, thereby affecting tests of divergence among codistributed taxa, we chose priors for both current and ancestral N e using empirical estimates of theta. The ABC method detected two discrete diversification events. Subsequent analysis with the number of diversification events constrained to two suggests that four taxa diverged in an older event, with the remaining six diverging more recently. Application of a range of mutation rates from 2.0 to 5.0% Myr 21 places both events within the Pleistocene or Late Pliocene, suggesting that fluctuations in montane habitat induced by climate cycles and a late Pliocene seaway may have fractured this montane bird fauna. The results presented here suggest this avian fauna responded in a relatively concerted fashion over the last several million years.

An alternating treatments design was used to investigate the effects of three types of opportunities to respond (i.e., individual, choral, and mixed responding) on sight words and syllable practice in six elementary students with behavioral problems. During the mixed responding condition, five out of six students demonstrated a lower rate of disruptive behavior, and four out of six students had fewer intervals of off-task behavior. Results of the three types of opportunities to respond on participants' active student responding were less clear. A discussion of limitations, implications, and future research directions is included.

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Integration of molecular methods, ecological modeling, and statistical hypothesis testing are increasing our understanding of differentiation within species and phylogenetic relationships among species by revealing environmental connections to evolutionary processes. Within mammals, novel diversity is being discovered and characterized as more complete geographic sampling is coupled with newer multi-disciplinary approaches. North American red squirrels exemplify a forest obligate genus whose species are monitored as indicators of forest ecosystem condition, yet phylogenetic relationships reflecting evolutionary history within this genus remain tentative. Through testing of competing systematic and niche-based divergence hypotheses, we recognize three species, Tamiasciurus douglasii, T. hudsonicus, and T. fremonti. Our data provide evidence of regional differences in evolutionary dynamics and continental gradients of complexity that are important both for future management and for investigating multiple pathways that can lead to the formation of new species.