Taxonomy, speciation

Poison frogs are a common focus of biologists studying mechanisms of speciation, as they have diverged relatively recently following the formation of the Pliocene land-bridge, connecting Central America to South America. These frogs are phenotypically highly variable, especially in coloration and size, and have been historically categorized as different species based on morphology. More recent genetic analyses have revealed that these morphology-based categories were not always accurate in reflecting actual evolutionary divergence. This figure shows a phylogeny for populations of poison frogs from Central America based upon three sequences from mitochondrial DNA. All frogs are in the genus Dendrobatidae, and classified into three possible species. Based upon this tree and your knowledge of speciation and phylogeny, answer the questions below. Note that reading the associated article is not required for answering the questions, but may provide help in interpretation. However, answers will not be considered complete if simply quoted or copied from the article. (Source: Hagemann, S. and Pröhl, H. (2007) Mitochondrial paraphyly in a polymorphic poison frog species (Dendrobatidae; D. pumilio), Molecular Phylogenetics and Evolution, 45(2): 740-747, doi.org/10.1016/j.ympev.2007.06.010) Adapted figure caption from article: Phylogeny of D. pumilio populations and related species based on combined mitochondrial 16S, Cyt b and COI sequences. Values at nodes are statistical measures of certainty at that branch point, ranging from 0-100. Branch lengths denote degree of genetic divergence. CR= Costa Rica; P= Panama; S= Southern group; N= Northern group; E= Escudo group. D. arborous and D. speciosus are from western Panama. The tree was rooted using D. histrionicus as an outgroup (not shown). 2A. (4 points) The authors of the study conclude that the phylogeny shows paraphyly. Using your own words, explain why this tree is paraphyletic. Refer specifically to the species in the tree that is/are problematic in terms of classification and why this represents paraphyly. (2-4 sentences) 2B. (6 points) The authors propose a new taxonomic scheme with five species groups: The northern, southern, and Escudo groups (shown and N, S, and E on the tree) each as different species, and the other two species kept as is. How does this resolve the paraphyly present in the tree? Also, as an example, explain why designating the northern group (N) as a distinct species is justified by the tree. Refer to branching patterns, length of branches, and use appropriate terminology with explanation in your answer. (2-4 sentences) 2C. (12 points) Determination of reproductive isolation between these frog populations may be useful in determining patterns of divergence and speciation. Suppose you ran some experiments in a lab with frogs from the northern and southern populations and found the results below. Results of mate choice experiments for females presented with a choice of mates from each population. Female frogs are the primary actors in mate choice, thus reverse mate experiments were not conducted. Tadpole survival percentages (up to 100 days) for pairings of frogs of the same or different populations. Southern male Northern male Southern male Northern male Southern female mate choice 11 9 Southern female 52% 23% Northern female mate choice 8 12 Northern female 14% 55% Interpret these results in terms of patterns of reproductive isolation, including pre- vs. post-zygotic isolation. (2-4 sentences). Also, in areas where the southern and northern populations come into contact, sometimes called a hybrid zone, what would you expect to happen over time to these reproductive isolation mechanisms? Why? (3-5 sentences) 2D. (8 points) Using this example of poison frogs, explain why the idea of “species” is in some ways an artificial concept. In other words, when it comes to how evolutionary divergence works, why is the idea of placing organisms into species categories sometimes problematic? Provide at least two examples to illustrate your point, using specifics related to frog phenotypes, the phylogeny, or reproductive data.