Pangea gone wild

Pangea and the diversity of life. 

Most everyone knows what Pangea was: the largest supercontinent in Earth's history. All land masses united in a large group gathered together for a conference. They also shared all organisms on their surfaces before splitting into individual continents. 

This split didn't magically happen like many people think. It was a very long process. Pangea split into unequal halves forming two supercontinents: Laurasia and Gondwana. Additionally, these two landmasses continued sharing some flora and fauna over a long period of time.

Over millions of years further tectonic activity caused Laurasia and Gonwana to split into smaller landmasses gradually forming the continents we are familiar with today. This long process was a key impact on early evolution of both flora and fauna. 

The expanding distance between continents reduced exchanges of flora and fauna, eventually isolating many groups of life. Some fauna continued dispersal from continent to continent by rafts of islands or ice. Others migrated by air (e.g.seed and flying animals). Our knowledge of the degree of and when continental shifting impacted evolution of flora and fauna is continually evolving (pun intended) in the field of biogeography. There are two theories:

"Do new species come from animals populating new territory (called dispersal), or did populations get separated during Earth’s breakup (called vicariance)?"

We know that both dispersal and vicariance played roles in early evolution of nearly all flora and fauna. And we need to consider that distribution of life occurred over a long periods of time, even during different stages in the evolution of flora and fauna. Local, regional and continental changes in topography or climate can influence dispersal of isolated populations. It can also expand habitats for others enabling mixing of populations where isolation barriers once existed. 

Several approaches can help elucidate the contribution of vicariance or dispersal at different points of an organism's evolution. The most valuable is phylogenic trees, structural diagrams that represent evolutionary relationships among organisms. The pattern of branching in these trees reflects how groups and individuals of  organisms evolved from a series of common ancestors and their predicted evolutionary timing. They are evolutionary 'trees'. 

A group of scientists used the data within a set called the Timetree of Life. It is a phylogenic tree of life scaled to time. Using data for major freshwater and terrestrial vertebrate groups (animals with backbones: fish, amphibians, reptiles, birds and mammals) that were descended from common ancestors and represented on at least two continents, they examined when they diverged. 

Dates of divergence of those groups separated by continents lined up with the continents geographically separating. This supports the theory of vicariance over dispersal as the major cause for speciation. However, this may change as the data sets change. 

There are considerations that may impact this theory. One is the contribution of moving pieces of land, such as land bridges. Another is narrow bodies of water separating the shifting continents and facilitating both flora and land dispersal. 

As the author of the article highlighting the study commented, "this paper is swinging the pendulum between two competing ideas". And, as science is sometimes fraught with binary thinking, the two theories don't have to be a "competition", or mutually exclusive. Life isn't A or B; it is a dynamic collection of events that can happen together or seamlessly flow from one to the other. Generalizations don't always pan(gea) out.