In which we consider the rather exuberant diversity of organisms and introduce the principle that evolutionary mechanisms are responsible for it.
In medieval Europe there was a tradition of books known as bestiaries; these were illustrated catalogs of real and imagined organisms in which it was common for particular organisms to be associated with moral lessons. “Male lions were seen as worthy reflections of God the Father, for example, while the dragon was understood as a representative of Satan on earth.”53 One can see these books as an early version of a natural theology, that is, an attempt to gain an understanding of the supernatural through the study of natural objects. In this case, the presumption was that each type of organism was created for a particular purpose, and that often this purpose was to provide people with a moral lesson. This way of thinking grew more and more problematic as more and more different types of organisms were recognized, many of which had no obvious significance to humans. Currently, scientists have identified approximately 1,500,000 different species of plants, animals, and microbes. The actual number of different types of organisms, referred to as species, may be as high as 10,000,00054. These numbers refer, of course, to the species that currently exist, but we know from the fossil record that many distinct species, which are now extinct, existed in the past. So the obvious question is, why are there so many different types of organisms55? Do they represent multiple independent creation events, and if so, how many such events have occurred?
As the true diversity of organisms was discovered, a number of observations served to undermine the early concept that organisms were created to serve humanity. The first of these was the fact that a number of organisms had very little obvious importance to the human condition. This was particularly obvious in the case of extinct organisms but extended further as a result of newly discovered organisms. At the same time students of nature, known generically as naturalists, discovered many different types of upsetting and cruel behaviors within the natural world. Consider the fungus Ophiocordyceps unilateralis, which infects the ant Camponotus leonardi. The fungus takes control of the ant’s behavior, causing infected ants to migrate to positions 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, which makes it resemble a fruit and increases the likelihood of the infected ant being eaten by birds. The birds transport the worms, which survive in their digestive systems until they are excreted; they are then eaten by new ants to complete the worm’s life cycle56. Perhaps the most famous example of this type of behavior occurs in wasps of the family Ichneumonidae. Female wasps deposit their fertilized eggs into the bodies of various types of caterpillars. The wasp eggs hatch out and produce larvae which then 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 & 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 apparently cruel behaviors, Darwin and others sought alternative, morally neutral naturalistic processes that could both generate biological diversity and explain biological behaviors.
As the diversity of organisms became increasingly apparent and difficult to ignore, another broad and inescapable conclusion began to emerge from anatomical studies: many different organisms displayed remarkable structural similarities. For example, as naturalists characterized various types of animals, they found that they either had an internal skeleton (the vertebrates) or did not (the invertebrates). Comparative studies revealed that there were often many similarities between quite different types of organisms. A classic work, published in 1555, compared the skeletons of a human and a bird, both vertebrates57. While many bones have different shape and relative sizes, what was most striking is how many bones are at least superficially similar between the two organisms. This type of “comparative anatomy” revealed many similarities between disparate organisms. For example, the skeleton of the dugong (a large aquatic mammal) appears quite similar to that of the European mole (a small terrestrial mammal), which tunnels underground on land. In fact, 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 the molecular. So the scientific question is, what explains such similarities? Why build an organism that walks, runs, and climbs, such as humans, 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 initially formed?
- How many species are there on Earth and in the ocean? http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1001127
- As a technical point, which we will return to, we will refer to each distinct type of organism as a species.
- The Life of a Dead Ant: The Expression of an Adaptive Extended Phenotype: http://www.jstor.org/stable/10.1086/603640
- Belon P (1555) L'Histoire de la Nature des Oyseaux. Paris, Guillaume Cavellat