This week’s post is a guest contribution by Gustavo Bravo, who recently finished his Ph.D. at the Louisiana State University Museum of Natural Science , working on the systematics and diversification of the Neotropical bird family Thamnophilidae.
A pervasive goal in evolutionary biology is to elucidate the history of living organisms on Earth. Because we are often interested in knowing when different lineages might have originated, we use different resources to date speciation events as accurately as possible. One of these tools is the “molecular clock”, which is a technique that relies on the rates of nucleotide (DNA) or amino acid (protein) change to infer the timing of events in the distant past. The idea behind the molecular clock is that over time a DNA fragment may accumulate mutations at a constant rate, or in “clock-like” fashion as it is commonly referred to. Therefore, the number of substitutions in a DNA fragment between two different organisms might be proportional to the amount of time since they diverged from each other.
There are two ways in which we can translate the number of substitutions between a pair of lineages into absolute dates. First, we can calibrate the clock against absolute times resulting from independent evidence such as fossil or geological dates. And secondly, we can measure directly the rate of mutation by comparing DNA or protein sequence data in present day organisms. Because the fossil record for some groups is incomplete and the dating of geological events remains controversial, some of those clocks are likely to produce inaccurate estimates of time.