Isolation by distance (IBD) has been a common measure of genetic structure among populations and is based on Euclidean distances among populations. Whereas IBD does not incorporate geographic complexity (e.g. dispersal barriers, corridors) that may better predict genetic structure, a new approach (landscape genetics) joins landscape ecology with population genetics to better model genetic structure. Should IBD be set aside or should it persist as the most simple model in landscape genetics? We evaluated the status of IBD by collecting and analyzing results of 240 IBD data sets among diverse taxa and study systems. IBD typically represented a low proportion of variance in genetic structure (mean r 2 00.22) in part because many studies included relatively few populations (mean 011). The number of populations studied (N) was asymptotically related to IBD significance; a study with 9 populations has only 50% probability of significance, while one with!23 populations will have 90% probability of significance. Surprisingly, ectothermic animals were significantly (p00.0018) more likely to have significant IBD than endotherms, which suggests a metabolic basis underlying gene flow rates. We also observed marginally significant effects on IBD significance for a) taxa in general and b) dispersal modes within actively-dispersing endotherms. Other factors analyzed (genetic markers, genetic distances, habitats, active or passive dispersal, plant growth form) did not significantly affect IBD, likely related to typical N. For multiple reasons we conclude that IBD should continue as the simplest reference standard against which all other, more complex models should be compared in landscape genetics research.

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Abstract-Why do bilingual language teachers and students switch between the two languages in their language classrooms? On the evidence of current research findings in relation to English-Vietnamese codeswitching in the educational contexts of Vietnam, this article identifies that classroom code-switching between the second language and the first language has its own pedagogic functions and it can be a valuable language classroom resource to both teachers and learners. In Vietnam, the implementation of the monolingual approach of teaching English-through-English-only faces many challenges such as inadequate classroom resources, students' low levels of English competence, motivation and autonomy, teachers' limited English abilities, and inappropriate teaching methods. Many Vietnamese teachers of English support code-switching in the classroom and they teach English through the bilingual approach. English-Vietnamese code-switching is reported not to be a restriction on the acquisition of English; rather, it can facilitate the teaching and learning of general English in Vietnam. This practice of code-switching is not just due to a lack of sufficient proficiency to maintain a conversation in English; rather, it serves a number of pedagogic functions such as explaining new words and grammatical rules, giving feedback, checking comprehension, making comparison between English and Vietnamese, establishing good rapport between teachers and students, creating a friendly classroom atmosphere and supporting group dynamics.