3.0: Introduction

In which we consider the exuberant diversity of organisms and how they came to be. To understand these processes requires an introduction to core evolutionary mechanisms, both adaptive (natural, sexual, and social selection) and non-adaptive (genetic drift and bottlenecks). As part of our discussion we consider the history of how people considered the diversity and meaning of life.

In medieval Europe there was a tradition of books known as bestiaries, illustrated catalogs of real and imagined organisms. Often each organism was associated with a moral lesson. “Male lions were seen as worthy reflections of God the Father ... while the dragon was understood as a representative of Satan.”^[82] Bestiaries are an example of a natural theology, an attempt to gain an understanding of the supernatural through the study of natural objects.^[83] The presumption was that each type of organism was created for a particular purpose, and that this purpose was to provide a moral lesson. Natural theology grew problematic as more and more different types of organisms were discovered, many with no obvious significance to humans. Scientists have identified approximately ~1,500,000 different types of plants, animals, and microbes. The actual number of different types of organisms (species) may be much higher.^[84] These numbers refer to species that currently exist, but as we know from the fossil record many species that once existed are now extinct. So why are there so many different types of organisms?^[85] Given how different types of organisms look and behave, it seemed possible that trees, mushrooms, spiders, whales, and humans represent distinct lineages and separate creation events.

As the number and diversity of organisms was recognized, a number of observations undermined the assumption that organisms were created to serve or instruct humanity. The first was the fact that many organisms had little obvious relevance to humans. While obvious in the case of extinct organisms, this extended to newly discovered (by Europeans) organisms; panda bears, potatoes, and maize (corn). At the same time students of nature, known as naturalists, discovered different types of upsetting and "cruel" behaviors in the natural world. Consider the fungus Ophiocordyceps unilateralis; it infects the ant Camponotus leonardi and takes control of the ant’s behavior, causing it to migrate to environments that favor fungal growth before killing the infected ant. Similarly, the nematode worm Myrmeconema neotropicum infects the ant Cephalotes atratus, leading to dramatic changes in the infected ant's morphology and behavior. The infected ant’s abdomen turns red and is held raised up, making it resemble a fruit and increases the likelihood that it will be eaten by a bird (→). The bird then transports the worms, which survive in its digestive system; excreted worms can be eaten by, and so infect new ants thus completing the worm’s life cycle.^[86] Perhaps the most famous example of "natural cruelty" involves wasps of the family Ichneumonidae. Female wasps deposit fertilized eggs into the bodies of various types of caterpillars. The wasp's eggs hatch and produce larvae that feed on the living caterpillar, consuming it from the inside out. Charles Darwin, in a letter to the American naturalist Asa Gray, remarked “There seems to me too much misery in the world. I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of caterpillars, or that a cat should play with mice.” Rather than presume that a supernatural creator was responsible for such "cruel" behaviors, Darwin and others sought alternative, morally neutral "natural" processes that could generate biological diversity and explain biological behaviors.

As the amazing diversity of organisms became apparent and difficult to ignore, another broad and inescapable conclusion emerged. Different organisms displayed detailed structural similarities. Comparative studies revealed similarities between different types of organisms. A classic work, published in 1555, compared the skeletons of a human and a bird, both vertebrates.^[87] While many bones have different shapes and relative sizes, many bones were at least superficially similar to one another (top →). In another example, the skeleton of the dugong, a large aquatic mammal, appears similar to that of the European mole (bottom →), a small terrestrial mammal that tunnels underground. There are general skeletal similarities between all vertebrates. The closer we look, the more similarities we find. These similarities run deeper than the anatomical, they extend to the cellular and molecular levels as well and apply to both vertebrates and invertebrates. So the question remains, what explains these similarities? Why build an organism that walks, runs, and climbs, such as a human, with a skeleton similar to that of a organism that flies (birds), swims (dugongs), or tunnels (moles). Are these anatomical similarities just flukes or do they imply something deeper about how organisms were formed?

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