Evolution’s tangled web

There are millions of different types of organisms on Earth. Insects, mammals, birds, fish, flowers, trees, and grasses are among the ones we see on a daily basis but there is a multitude of smaller creatures such as bacteria, amoeba, and fungi. Life abounds.

But how do we make sense of all of this? How do we determine which animals constitute a species and how are the species inter-related in the grand scheme of life? How are elephants and daisies related? What differentiates a northern leopard frog from a southern leopard frog?

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These questions have perplexed scholars since the beginning of history. Indeed, one could argue it was far more important for our ancestors to understand these relationships than for us as they were much more dependent on their knowledge of plants and animals for their survival. If you can’t tell the difference between, say, two mushrooms, it could easily be a death sentence.

The classification of animals was certainly a challenge for ancient Greeks. They had a somewhat limited environment in which to work with only a few species from which to generalize. That didn’t stop them from a number of interesting speculations. For example, what do you get when you cross a leopard and a camel? Why, a giraffe of course! (It has the spots of a leopard and the long neck of a camel.)

As animal species were discovered, they were eventually assigned Latin names which were very descriptive but often very long. “Apis pubescens, thorace subgriseo, abdomine fusco, pedibus posticus glabis, untrinque margine ciliates” does not roll off the tongue; it is the full scientific name for the European honey bee.

Publishing in 1735 and 1753, Carl Linnaeus laid out a system for the classification of plants and animals which reduced names to two terms – genus and species. The European honey bee was simplified to “Apis mellifera” and millions of biology students have been grateful ever since.

And while Aristotle classified animals on the basis of whether or not they had red blood (mammals do; insects don’t), Linnaeus shifted the focus to the anatomy and morphology of organisms, particularly the reproductive system. He divided plants and animals into broad kingdoms that he then sub-divided into phyla, classes, orders, families, genera, and finally species. While it isn’t a perfect system, it has served taxonomy well for the past 250 years.

Of course, some animals undergo major changes in morphology during the course of their life span. Consider a butterfly that starts life as a lowly caterpillar. There really isn’t any physical resemblance between the two creatures. And even when creatures – such as the northern and southern leopard frog - look the same, they may still be two separate species.

Classification is a tricky and often exacting art. Understanding the scope of all life on Earth is well beyond the capacity of any single human and so scientists became specialists in their own area. This was parodied in a Big Bang Theory episode where the scientists take a cricket to Professor Crawley for identification.

But the discovery of DNA and our understanding of its fundamental role in the evolution of life have provided a new take on this ancient problem. The relationship between species can now be determined by understanding which genes are present and which are not. More to the point, the relationship between species can be ascertained by examining the base pairs composing genes where variation indicates genetic drift.

That is, the farther apart two organisms are on the evolutionary tree, the more differences appear in the coding for particular genes. A small subset of genes are critical to all life on Earth and are mostly conserved across all species but subtle variations are more indicative of the classification of an organism than simply looking at its morphology. Elephants and daisies are related – as all life is related to a universal common ancestor – but the variations in their genes tell us their branches on the tree life split from each other a very, very, very long time ago.

Paul Hebert of the University of Guelph took classification a step further in 2003 when he introduced DNA barcoding. Like the Universal Product Codes or barcodes that are on every product we buy, DNA can be tagged and sorted to give a chemical equivalent. And just like every product in a grocery store is unique, so is the DNA barcode for each species reflecting genetic differences at the molecular level.

The public collection of DNA barcodes, called Barcode of Life Database (BOLD), has collected and analyzed well over 600,000 named species. The technique has even been used to scan environments where unusual DNA barcodes have led to the discovery of new species.

We have come a long way in our classification of life on Earth but using DNA, we know have a better understanding of the relationships among all living things.

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