How do shared taxa help scientists compare the similarity of two geographic regions?

To compare the similarity of two geographic regions, scientists quantify the overlap in their biological composition. While early biogeographic classification relied on mapping the boundaries of endemic taxa—where regions were defined by having the most unique species—modern approaches often utilize mathematical models to measure similarity.1

One common method is the use of Jaccard’s coefficient of biotic similarity. In this calculation, the coefficient of similarity ($s$) is determined by the following equation:

$$s=\frac{c}{c+a+b}$$

In this formula:
* $c$ represents the number of taxa shared between the two areas.
* $a$ and $b$ represent the number of taxa peculiar to each area alone.1

By analyzing these shared distributions, scientists can better understand evolutionary processes, such as the impact of geographic barriers, climatic factors, and historical events like continental drift. These comparisons help researchers move beyond simple description to investigate the causative factors behind the evolution and dispersal of life across the globe.234