Biology Reference
In-Depth Information
3.3.2. Time and time again:paleontology
and molecular evolution
There are two basic approaches to measuring evolutionary time. The first
and most straightforward belongs mainly to the colorful realm of pale-
ontology and involves using fossil records to date ancestral forms of life,
and thus reconstruct both cladogenesis and anagenesis. Generally speak-
ing, this approach involves macromutations, which are rare, extraordi-
nary events at the macromolecular level, such as a change in the number
of chromosomes. Often, a macromutation can incur an identifiable
change of phenotype, such as a morphological change, allowing correla-
tion to paleontological time. This is a tremendous advantage leading to
an absolute calibration of the phylogenetic timescale. On the other hand,
macromutations occur at highly irregular intervals, and in the absence of
fossils this renders a quantitative evaluation of the passing of time very
problematic. In addition, this approach is normally not applicable to
paralogous analysis.
The second approach belongs to the realm of molecular evolution
and involves an estimation of the total number of molecular mutations
which took place since a given event of speciation or gene duplication.
It favors the quantification of micromutations, which are small (“point”)
mutations at the DNA level, possibly translating into a change of amino
acid. This approach has gained favor thanks to the massive, quickly grow-
ing amount of genetic sequence data available and to the amenability of
this data to quantitative methods.
3.3.3. Micromutations and the molecular clock
Mutation rates in living cells, at their most basic level, are fairly constant
throughout time and organisms. However, after DNA repair processes
and organism reproduction constraints, the number of mutations that are
actually conserved in a population, and therefore seen in the molecular
data available, can over time present a picture of highly heterogeneous
rates of transformation across the different sections of a phylogram.
Nevertheless, because micromutations are relatively easy to quantify,
their use and modelization in phylogenetics have been developed. There are
