Biogeography: Understanding where things live and why

It may not seem intuitive, but biogeography, the study of where organisms live and why they live there, is very consistent with the theory of evolution. However, there is an abundance of uncertainty in understanding how life has spread across the globe, and it may surprise those who accept evolution to learn that in some ways, evolution fares no better than creationism.

Let me start off by describing the two general ideas evolutionary biologists invoke to explain how life has moved around the planet. The first is by dispersal, a rather simple concept in which life moved from point A to point B by, well, dispersing! Some things are really good at dispersing, like birds and bats, so imagining them flying across continents and even oceans should not be difficult to fathom. Similarly, oceanic creatures would not be too hampered from dispersal as they can swim or float to a new coast or sea.

Hypothesis of cichlid dispersal

But what about land-dwelling organisms? How does a lizard from Asia get to a tiny island in the middle of the Pacific? It certainly didn’t fly and it’s very unlikely that it swam. This is where the more sketchy, yet technically plausible, hypothesizing comes in. Now and then, chunks of land can detach and float across great bodies of water. Perhaps, the idea goes, with the right currents, enough such events over time, and the lizard happens to be a female with babies on the way, then our seafaring voyager could land on a different island or continent and start a whole new life.

Though researchers are hard at work understanding what kinds of organisms can survive such transoceanic journeys and estimating the directions of currents in ancient seaways, one has to admit that these ideas are rather difficult to test. Importantly, I do not think such scenarios have a major impact in assessing the validity of evolution, but rather they can be used to understand how life dispersed assuming evolution is true.

The other major mechanism thought to influence the geography of life is a concept called vicariance, whereby a population of organisms is split into two or more populations by the formation of geological features. This idea generally rests on plate tectonic theory, which suggests that the earth is composed of plates that have moved, collided, separated, lifted, and destroyed one another over time. The consequence of this shuffling of plates is thought to have created all sorts of barriers between different forms of life. We think plate tectonics led to the formation of mountains as well as merged and separated continents, and life has cruised along for the ride. Plates do indeed move, a demonstrable fact, especially if you live in an earthquake prone zone, but how well does plate tectonic theory explain the distribution of life on earth?

Plate boundaries

Well, it depends. I give some examples in this blog that are very consistent with plate tectonic theory. However, in many cases, estimates of when species split from each other very frequently post-date the formation of such geological formations. For example, we might estimate continent A split from continent B 90 million years ago, but species X living on continent A split from species Y on continent B 70 million years ago. This leads us to the conclusion that, by default, dispersal must be responsible for where species X and Y live today. Again, this returns us to the problem of trying to figure out how the heck the organisms got to where they now live.

Where does Creationism fit into all of this? Of course dispersal is not an issue for Creationists: organisms move across the earth, and this is clearly demonstrable. For some Creationists, plate tectonic theory is even acceptable [1].

I think the major differences come down to three specific points. First, Creationists believe that all land-dwelling animals started from the same point: Mount Ararat in Turkey. After Noah’s flood, the story goes, animals dispersed from out of the ark and made their way across the world [1,2]. Second, though the dispersal of terrestrial life is in part thought to be natural, Creationists emphasize the importance of humans as agents of dispersal, carrying seeds and moving livestock for purposes such as food [1,2].

Noah's ark

By contrast, evolutionary biologists think that life started in a single unknown, and likely unknowable, location, but as new species were formed over time, they have been dispersing and drifting along with the plates. Furthermore, while humans have undoubtedly aided the dispersal of some organisms, evolutionary biologists believe this has been limited to the very latest part of earth’s history.

The third critical difference is that Creationists believe that all dispersals and/or plate tectonic events happened in just a few thousand years, whereas evolutionary biologists think they’ve occurred over billions of years. I will let the geologists tackle the notion of continents forming in thousands of years or less, but this time difference is key for comparing Creationism to evolution. For example, evolutionary biologists do not think that marsupials had to arrive in Australia 65 million years ago anymore than they could have showed up 25 million years ago. In theory, it could have at whatever point in time after marsupials originated, and it happened when it did because the right situation allowed for it to. Creationists, by contrast, are constrained in that marsupials must have arrived in Australia in just the past few thousand years.

I think you will find in my posts about biogeography that a universal origin of terrestrial life from Mt. Ararat, an extremely short time scale, and the agency of humans in dispersal turn out to be the most difficult details for Creationism to reconcile with the data collected by scientists. This turns out to be especially true when it comes to comparing the fossil record and DNA evidence with biogeography. Where Creationism fails in this respect, I believe you will begin to see that evolution triumphs.





One thought on “Biogeography: Understanding where things live and why

  1. Pingback: Developmental biology: When embryos point to evolution – Evolution For Skeptics

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