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Until the mid-1800s, many questions about the human species, the age of the Earth, and other basic inquiries were answered by looking to one document: the Christian Bible. People argued that it contained all the answers humans would ever need, so no further investigation was necessary. The age of the Earth? An Irish archbishop calculated it as about 6,000 years, based on biblical chronologies. The origins of humanity? Clearly laid out in the first pages of Genesis: God created humanity in a moment of divine inspiration. Whatever one thinks of the morality prescribed by the Bible (and plenty of scientists use its messages as guides to their moral life), it’s clear today that these so-called facts are simply incorrect, dating from an age in which little was empirically known about the age of the Earth, the origins of humanity, or even that our own planet wasn’t at the center of the universe, but only one of many. For science, the interpretation of the universe could not proceed just as interpretations of biblical passages. New ways to investigate the world had to be invented. And one of the things they discovered was the evolutionary process.

Yes, the evolutionary process. Evolution is process, not a thing. In fact, it’s a single word used to describe the cumulative effects of three independent facts. Importantly, these attributes of evolution can be (and are) observed in nature, and the laboratory, every day. They are

 Replication: The fact that life forms have offspring

 Variation: The fact that each offspring is slightly different from its parents and siblings

 Selection: The fact that not all offspring survive, and those that do tend to be the ones best suited to their environment

Figure 3-2 shows these characteristics in more detail.

Illustration courtesy of Cameron M. Smith, PhD

FIGURE 3-2: Evolution as the result of replication, variation, and selection.

Regardless of your personal views on the topic of evolution, the three processes of evolution aren’t arguable. Whether it’s in the form of zebra calves, salmon fry, or human infants, life forms replicate. Also, all offspring aren’t clones; variation occurs in small ways and significant ways, but it occurs. And if it weren’t for selection, the world would be swarming with every mosquito, beetle, and tadpole ever born; the fact that it isn’t verifies that not all of these creatures born survive into adulthood. Finally, it’s not arguable that the offspring best suited to their environment tend to pass their genes on to the next generation. And it is simply the cumulative effect of these processes that we call evolution.

When replication happens, the variable offspring are born into an environment that basically either selects for or against them; if two dragonflies are pursued by predators (like birds), the one with a better build for its environment is most likely to survive. It’s been selected for rather than against, and it’s therefore more likely than its less-fit sibling to pass on the genes that made it. Now the genes that made a fit dragonfly go on to make the next generation of dragonflies, which are slightly fitter than the parent generation. Essentially, that’s evolution: selection acting on the variable offspring, leading to the change through time of the characteristics of the organism.

It is the genes that direct the building of the life form that are passed on in the genetic code of sperm and egg cells. The whole body, of course, is not transmitted, but the instructions for building it are (in the incredible DNA molecule).

Keep in mind that the term selection implies that someone is making a decision, or selecting. But really, it just refers to the survival probability of a given life form. One way to think of selection with less implication of a deliberate “selector” is in the phrase “the organism proposes, the environment disposes.”

Groups of living things that can interbreed and have healthy offspring are called members of a single species. Groups of similar species diverge into further groups, forming a biological classification hierarchy that I discuss in Chapter 4. In this chapter, just remember that a genus is the level above species. Humanity is in the genus Homo and the species sapiens, yielding the scientific name Homo sapiens.