D.5b: Phylogeny & systematics

30/09/2013 § Leave a comment

What’s kicking, HL bio suckers? How were your weekends? Oh, no, wait – actually, I really don’t care. Because I have to work. I’ll “care” after May of 2014 when the exams are over. For now, let’s talk Cladistics because clearly I have nothing better to do.

You want definitions, yeah, okay, I’ll give you definitions. Cladograms are tree diagrams that visually represent clades, which are groups of organisms that evolved from a common ancestor. On a cladogram, there are branching points (because it’s a tree diagram, get it?) that indicate which groups of organisms are related. This method of taxonomy/classification is special and new from what old, old, old men use, so it has a new name: cladistics, which can be defined as a method of classification of living organisms based on the construction and analysis of cladograms.

But okay, on the topic of cladistics and cladograms. Because cladograms integrate nodes into their structure, branching out can happen at any time, which sort of contradicts traditional classification. Some cladistics suggest completely different phylogenies from traditional classifications. The positive sides of cladistics are objective and not based on only morphologies but on molecular differences in species. On the other hand, molecular differences between species are based mostly on probability. Generally, cladistics can still make errors in its predictions and classifications.


Page 317, inferring evolutionary relationships


a) Using Figure 19 identify a protein that is identical in all three mammals. —> Cytochrome C

b) Using Figure 19 deduce, giving a reason, whether the ancestors of pigs or sheep diverged more recently from those of cows. —> The sheep diverged more recently because it has a lower percentage difference in base sequence to cows than that of the pig.

c) Using Figure 19 explain how the variations in these molecules can indicate the phylogeny of these groups of mammals. —> The differences in these molecules can indicate the phylogeny because the more different the molecules are between either two species, the further back (in the past) the two species probably diverged from each other. The pig and cow, for example, have a larger difference than the pig and sheep, which suggests that the pig diverged before the sheep. This would help pinpoint their locations and when they diverged from each other in comparison to other species.

2. State what additional information would be helpful in constructing a phylogenetic tree for the species in Table 2. —> A phylogeny chart would also need the time since the species diverged.

3. Discuss whether the number of differences is correlated with differences in morphology. —> The number of differences doesn’t directly correlate with the differences in morphology because haemoglobin composition in the species is a molecular measurement and can’t be seen physically immediately (?). However, if the beta chain of haemoglobin controls a physical aspect of species, then the differences could tell us how far apart species are to each other.

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