Biodiversity increases as one moves closer to the equator—a phenomenon known as the latitudinal diversity gradient. Among possible hypotheses proposed by researchers, the evolutionary speed hypothesis (ESH)—explained by shorter generation times, higher mutation rates and/or faster rate of selection in the tropics—could be the underlying mechanism. But is molecular evolution actually faster in the tropics?
In a new comprehensive study, researchers from Ontario, Canada analyzed and aligned DNA barcode sequences of the mitochondrially-encoded Cytochrome c oxidase subunit I (COI) gene for 8,000 members of six of the largest animal phyla. They then paired closely related lineages exhibiting a difference of at least 20 degrees in median latitude as validated by GBIF-mediated occurrences and compared rates of evolution.
While the researchers only found a weak trend of higher rates in lower-latitude (51.6 per cent) vs higher-latitude (48.4 per cent) lineages, the difference was statistically significant. Some phyla, e.g. Chordata and Echinodermata, displayed stronger associations, significant even when correcting for multiple testing.
So—is molecular evolution fast in the tropics? Yes, but not much.