The Tree of Life Mapping
Evolutionary History
S
cientists understand that over long periods of time, DNA
and amino acid sequences in organisms can change. As the number
of differences between DNA sequences increase, species begin to
diverge from one another. Given
enough time, new species can emerge from the same ancestor.
Scientists trying to piece together segments of evolutionary history
can do this by looking at the number of shared traits between
species. A large number of shared traits are considered strong
evidence that species are closely related.
Although scientists originally grouped organisms using physical
traits, today they are able to group organisms more accurately
using amino acid or nucleotide sequences coded in the genes.
These groupings are used to create branched diagrams called
cladograms. Much like a family tree, cladograms are scientists’
attempt to accurately map out evolutionary history.
Molecular cladograms are created by selecting genes or sequences
of amino acids that are shared by the organisms under study.
Scientists then search for “updates” in the patterns, features known
as characters, that appear in some of the species but are not in the
common ancestor. Organisms with these updated characters are
considered to be more recent arrivals on Earth. Because variations
to the original nucleotide sequence resulted from gene mutations,
they provide clues to how different organisms diverge from shared
ancestors. Comparing patterns of these various characters changing
over time allows scientists to reconstruct a likely evolutionary
history.
When creating cladograms, a branching tree-like diagram must be
constructed. The names of the organisms are placed at the top of
the lines (e.g., A, B, and C). Shared features are placed in solid
boxes along the branches, and the common ancestor is placed in a
circle at the base of the cladogram.
Assume a character will evolve only once, so if different
organisms display that character they should be placed into groups
closer to one another. In other words, the more similar two
organisms are the closer their evolutionary relationship
and the
closer they will be on the cladogram. This also means the two
organisms shared a common ancestor more recently than other
organisms under study. Two examples of cladogram styles are
shown in Figure 1.
Figure 1. Cladograms
Once biologists could view organisms at the molecular level, they
quickly determined physical traits did not provide accurate
evolutionary maps. Sequencing the genome of various organisms
showed changes as small as one nucleotide could indicate even the
most subtle of differences between species at each sequenced gene
site. Biologists began collecting information in an attempt to piece
together the evolutionary pathways. One important comparison
involves the genes for cytochrome c.
Cytochrome c codes for a protein attached to the inner
mitochondrial membrane of eukaryotes. This protein is an electron
carrier in oxidative phosphorylation of cellular respiration. It
moves electrons through the membrane toward the final electron
acceptor. Because most organisms have cytochrome c, protein
sequence variations are often used to determine phylogenic
relationships.
This activity compares sequence variations found in cytochrome c
subunit 1 for several types of dolphins, whales, and other
organisms and then uses the information to depict evolutionary
history using a cladogram.
PURPOSE
This lab will use genomic information to compare the cytochrome
c protein sequences of several organisms. Thousands of gene
sequences for various organisms are stored in a database with the
National Center for Biotechnology Information (NCBI). Although
there are several ways to retrieve sequencing information from this
database, this activity will reference the organism’s scientific
name.