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INTRODUCTION

A NESTED HIERARCHY OF LIFE’S GROUPS

 

A great biologist of the early 20 th century, Theodosius Dobszhansky, once said that “Nothing in biology makes sense except in light of evolution.” I am often struck at how profoundly true this statement is. If evolution were not true, I’m not sure I would want to teach biology.

There are more than a million organisms on this planet. If they were all separate creations which popped into existence out of nowhere, how could one begin to study them? It is hard enough to learn the muscles or hormones or cellular structures of one organism. Who would want to study a million unrelated body plans? The human genome has tens of thousands of genes, each with its own sequence, function, and regulatory mechanisms. Why would anyone be interested in studying the genome of any one organism if that study might not produce a single insight into the genome of any other organism? Why should a researcher study natural processes if complex changes can only occur through miraculous intervention? Without evolution, biology is a mass of unrelated facts with no organizational scheme. It simply wouldn’t make any sense.

Evolution, like any idea in science, could be wrong. As such, it must make predictions that can be tested. Evolution does make testable predictions, as do creationism and intelligent design. To illustrate this point, let’s imagine that you decide to study a diversity of organisms. In the evolutionary model, these animals represent modified descendants of common ancestors and, like the members of any family tree, some are more closely related to each other than others. Since the creationist model would predict that these are equally unrelated (having appeared suddenly at creation without common ancestry), there is no reason that they would need to have anything in common. These diverse animals could be equally unrelated in every aspect of their being. Although some have argued that similarities could reflect a “common designer”, these same advocates could just as easily claim that a lack of similarity reflected an “infinitely powerful and creative designer”. Even so, the “common designer” hypothesis does not allow predictions on patterns of similarity. The similarities between modern organisms do not end with reflecting a common design, nor do they end with a set of adaptations to a specific set of environmental conditions. Modern organisms can be classified into a nested hierarchy of groups which organizes the anatomical, physiological, genetic, embryological, and fossil data into predictable patterns.

Based on their evolutionary relationships, one can make astoundingly detailed predictions about an organism’s biology which simply can’t be made if all “kinds” of organisms are equally unrelated to each other. One can make predictions on the leg muscles that each would have, the brain regions that each would have, the genes and gene sequences that each would have, the number of duplicate copies of genes they would have, the types of placenta and extraembryonic membranes that each would have, and even the sequences of non-coding DNA regions and presence of inactivated remnants of viruses. Enormous amounts of scientific evidence have already supported the predictions of the evolutionary model and contradicted predictions of the creationism and intelligent design.

A NESTED HIERARCHY

You are related to all humans, but you are not equally related to them.

You are more closely related to your parents and siblings than to any one else on the planet.

Your parents, siblings, and first cousins are more closely related to you than are those outside this group.

Your second cousins are slightly less related to you.

The rest of humanity can be divided into groups of varying relationship to you. This pattern is referred to as a nested hierarchy.

Relationships between individuals can be depicted as a set of groups within larger, less closely related groups.

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