Homology is the study of likeness, the similarity between
species that results from inheritance of traits from a common ancestor. The
study of similarities is broken up into three main categories: structural, developmental,
and molecular homology.
Structural homology is looking at a particular part of the
body and comparing structures. So for example, forelimbs in vertebrates. The
vertebrate forelimbs are used for different functions but have the same
arrangement of bones. Looking at structural homologies can prove that a group
on species evolved from a common ancestor. When comparing the structural
components of a human, a horse, and a dolphin we can see that they have the
same sequence. These three vertebrates all have an ulna and a radius, followed
by carpals, metacarpals, and then phalanges. The structural similarities of
these vertebrates suggest they evolved from a common ancestor. The theory of
evolution helps to express this. The previous theory, theory of special
creation, has a very hard time explaining these similarities. The second kind
of homology is developmental homology. Developmental homology looks and
compares embryos of various species. Using the vertebrate example again we can
look at the embryos of snake, cat, bat, and human. These four vertebrates look
very similar to each other during early development. Two key characteristics to
look at is the pharyngeal pouches and the tail. Humans who don’t have gills or
a tail develop these two characteristics while still in the womb. The gills
interestingly enough contribute to the development of the lower jaw. These
developmental traits connect these vertebrates. As development continues the
vertebrates look extremely different. The third type of homology is molecular
homology. One version of this is shared flaws in the genome. Shared flaws in
the genome suggest that they developed from a common ancestor. It is an
important concept because it can test the relationships between modern taxa.
Molecular homology looks at the similarity of species on the molecular level.
They look at the species DNA and compare it to another’s DNA. These three forms
of homology help scientists to make connections between different species and
to help prove the Theory of Evolution.
http://www.bio.miami.edu/dana/160/160S11_3.html
It is; however, interesting to note how the three categories of homologies are interrelated. The molecular sequence determines/guides the development of the organism and in turn this ontogeny results in the overall structure. So, here again we see how information flows through a living system…starting with DNA. Since genes are inherited and assuming that we are looking at groups that split from a common ancestor (at one point) then it should not be too surprising that we would see similar molecular, developmental, and structural homologies. Of course, the waters begin to get a little murky when one starts to consider convergent evolution and analogous structures (homoplasy).
ReplyDelete