Biology Reference
In-Depth Information
branches tend to increase over the long term (there are generally more
and more different living organisms), but, furthermore, the organisms
themselves tend to become more complex. With the advent of molecular
biology and genetics, the study of these parallel build-ups of biological
complexity has become an integral part of phylogenetics.
A complex organism, endowed with tens of thousands of genes, does
not evolve overnight. Its intrinsic genetic complexity is the result of
millions of years of successive gene duplications and more complex
(e.g. chromosomal) rearrangements. After every one of these duplicative
events, separate copies of genes had the possibility to evolve and differentiate,
each in its own direction. The most common outcome is the complete
degeneration of all but one copy of the gene. On occasion, however,
some selective advantage is gained by having more than one copy and
they are maintained — it can also be pure chance, of course. Generally,
in this scenario, each copy ends up fulfilling a slightly different function
and continues to evolve. In the end, each copy becomes an individual
gene in its own right, sometimes with a radically different function from
the cenancestor gene.
Therefore, at the molecular level, evolution leads not only to differ-
ences between the genes of one organism and the homologous genes of
another organism, but also to the divergence of multiple copies of a gene
within a single lineage (a gene family). With the present abundance of
molecular data, genes themselves can be studied as evolving objects capa-
ble of duplication, transformation, and disappearance. There is thus
a need to reconstruct not only the evolutionary trees of the families of
living and past organisms, but also the evolutionary trees of present
(and possibly extinct) families of genes.
Since 1970, two terms have been in use to clearly distinguish
between the two types of homology that phylogenetics now addresses.
Related genes that diverged after a speciation event and a gene duplica-
tion event are called orthologs and paralogs, respectively.
The classic example of paralogous genes is the family of oxygen-
transporting globins (Fig. 4). Hemoglobins form a complex family of
proteins, all of them descendants by gene duplication from a single ances-
tor gene which existed some 500 million years ago. Further back, some
