SEGMENT THREE LECTURE FIVE: TREE THINKING
Phylogeny is the study of evolutionary history. Such histories are often
diagrammed as “trees” (Fig. 26.2). The trees consist of branches connected at
nodes. Branches reflect the persistence of species through time. Nodes mark
the point at which new species originated. A tree may have a “root,” which is the
node in a tree that represents the ancestor of the rest of the tree.
Phylogenies are used in different areas of biology:
Origins---how extinct hominids were related to humans;
Medicine---how pathogens evolve;
Agriculture---how insect pests mutate when selected by bioengineered
crops;
Conservation---how to determine if products are being marketed from
illegally harvested species of animals (Fig. 26.6).
Homologies are used to determine the order of nodes in a tree. Homologous
characters are those that are shared between two species because they were
inherited from the common ancestor of those two species.
Convergent Evolution is coincidental similarity between unrelated species due
to natural selection of similar adaptations in similar environments. For
example, the marsupial mole resembles North American moles (which are
placental mammals; Fig. 26.7). However, their smiliarities are not homologies
because these animals are known to be distantly related because of many other
differences between them, such as reproductive and biogeographic differences.
Thus, the coincidental similarities are said to be analogies.
Homology is easier to determine when DNA sequences are used (Fig. 26.8)
because:
1) we know the inheritance pattern of DNAs.
2) we know how DNA functions in gene expression.
3) we know how DNA changes (by mutation).
4) there are thousands of homologous nucleotide characters.
Mon ophyletic Groups - an ancestor and all descendent species (Fig. 26.10a).
The goal of phylogenetics is to identify homologies that define monophyletic
groups. However, when convergent evolution was misinterpreted as homology,
other taxonomic arrangements, such as paraphyletic groups, were mistakenly
recognized (Figs. 26.10b, 26.16).
When DNA is used to infer phylogenetic trees, the DNA must first be aligned.
The process of alignment incorporates an understanding of mutations such as
insertions and deletions that may have changed the length of DNA sequences
between two species (Fig. 26.8). Sequence alignment is essential for the
comparison of homologous nucleotide sites.
