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.