Dinosaurs and cycads - Written assignment 1 PDF

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1. What is the relevant background information that the authors present at the beginning of the article? What did the scientists already know? What are the unanswered questions or assumptions that remain unevaluated? At the beginning of the article, Recent Synchronous Radiation of a Living Fossil , the authors define living fossils as “modern survivors of previously more diverse lineages”. They present an example of a living fossil, the cycad. Using fossil- calibrated molecular phylogenies, they have shown that cycads went through global scale diversification at the beginning of the Miocene period. The scientists’ timetrees indicate that the cycad lineage is about twelve million years old.

2. What is the main question that these authors are trying to address (what is the hypothesis that they are testing)? What was the increase in cycad diversification attributed in the past (morphological, geographical, or taxic diversity), and what is the present day decline in cycad diversity attributed to?

3. What is the general experimental approach taken by the authors? (Just explain broadly – no need to include specific experimental details). To estimate the ages of the living cycad species, the authors used molecular clock analysis. Molecular clock analysis is “the observation of rate constancy in molecular evolution. The extent of genetic divergence at a gene in two taxa is thus a reflection of the time since the taxa last shared a common ancestor” (Foundations of Biological Inquiry). Using this, the ages were evaluated and a phylogeny was created to represent the evolution and divergence of the cycad genera.

4. Examine Fig. 1. What do the colors on the branches of the phylogeny indicate? What conclusion can you draw from the pattern of color across the branches of the tree? The different colors represent the geographic distribution of genera. The different colors of the branches represent the different geographical distribution of the cycad genera that they inhabit(ed). From this pattern shown, we can conclude that the descendants of each ‘color’ have continued to inhabit the same geographical area.

5. What do the branches of the phylogeny indicate in this phylogeny? Do branches of a phylogeny always indicate this? (You might revisit 1 and 22.2 in your book if you are not sure)

The branches show a global diversification event with all of the cycad genera diversifying around the same time, the end of the miocene era. Branches of phylogenies usually do not show global splits occuring at the same time because mutation and the need for adaptation is random and therefore it is not often that an entire genus that inhabit a multitude of different geographical places need to evolve in such an instantaneous and large scale....