D.5a: Phylogeny & systematics

26/09/2013 § Leave a comment

Ah, finally, a proper blog post. This one’s about classification and phylogeny (ooh, bad flashback to lab #1). Why do scientists classify living organisms? Why else! The three primary reasons for classifying organism is

  1. species identification: in order to know which species belong together
  2. predictive value: many members in a group will have similar characteristics
  3. evolutionary links: if the members in a group have similar characteristics, then they probably evolved from a common ancestor

Classification is important because some species share traits with other species that be analogous or homologous structures. An analogous structures have a common function but a different evolutionary origin, while homologous structures have a common evolutionary origin but have different functions (so they’re flip-flopped). Analogous structures are the human and octopus eye, and wings. Homologous structures include the chicken wing and human arm, as well as the oh-so-famous pentadactyl limb in mammals.

Up next is the role of biochemistry in the common ancestry of living organisms. Certain biochemicals are evidence for evolution, such as:

  • all living organisms use DNA/RNA as genetic material
  • ^ all use the same universal genetic code (with a few unimportant alterations)
  • ^ all use the same 20 amino acids for everyone
  • ^ all use left-handed amino acids

And now phylogeny. What is it? Hah, the textbook told me the answer: Phylogeny is the tracing of evolutionary links and origins, like what we did for Ensatina eschscholtzii. Phylogenies are made by the organism’s protein structure and other biochemicals. Another diagram is a cladogram, which looks like a tree that represents a group of an ancestral species and then all the descendants of that species.

And concerning the DNA base sequence that each one of these species hold – I mean, obviously the code between Homo sapiens is going to be different from Australopithecus afarensis. The difference in these sequences are further evidence that differences (evolution) occur constantly. This creates a clock for scientists to determine how long ago a species split away from its ancestor. For example, Europeans and Japanese split from their African ancestors about 70,000 years ago. That’s crazy.

Europeans and Japanese were first, that’s really freaking insane.



Page 316, blue

1. Annotate the cladogram with the clade “aves” (the birds). —> You’re going to have to trust that I did this on my notes. (Basically where the birds are marked, I wrote “aves”.)

2. Discuss whether “reptilia” represents a clade. —> Reptilia could represent a clade because some, if not most of the species in the group hold some of the same physical characteristics. On the other hand, if these characteristics are present in their distant ancestors, then the traits don’t count and reptilia would then not be considered a clade. 

3. Determine from the phylogenetic tree when the non-avian dinosaurs went extinct. —> 35-40 million years ago

4. Determine from the phylogenetic tree whether ancestral crocodiles or dinosaurs appeared first. —> Crocodiles were first, homes.

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