My purpose in writing about evolution is not to engage in the controversy with people who find the idea of evolution is incompatible with some core religious beliefs, but because it’s a beautiful theory that illuminates so much about what we see in the system of life on earth — of which we are part.
It’s logic is so clear that it’s a wonder that it didn’t develop more quickly in the history of human understanding. It rests on just a few basic, somewhat obvious points:
1) Only some survive.
2) It’s not random which survive.
3) Characteristics favoring survival are often heritable.
4) With the passage of generations, that process — which Darwin called “natural selection” — can fashion amazing living structures.
To expand on these points:
1) Only some survive: The various forms of life produce way more offspring than can all survive: If every acorn became a mighty oak tree, or every kitten grew up to produce their own litters, etc., it would not take long before any given species took up the whole earth. Therefore, inevitably only some survive and pass along their DNA into the future.
2) It’s not random which survive: Chance can play a role in which organisms survive to reproduce and which do not (who gets struck by lightning might be a matter of chance). But while the race is not always to the swift, the swift are more likely to win the race than the slow. In other words, those organisms that are better equipped to survive in their environment tend to be the ones that get to pass their genetic inheritance into the future.
3) Characteristics favoring survival are often heritable. “Better equipped” often is a genetic thing. Each generation of any species includes genetic variety — some because the genes get shuffled differently in different offspring, some because mutations occur that bring about new possibilities. Some of those variations affect what genes survive into the future.
4) By such means — life continually being chosen over death in a process Darwin called “natural selection” — amazing living structures can be naturally fashioned over the course of many generations. In any species, the non-random success and failure of different genetic structures will tend to move the genetic endowment of that species — over time — in the direction of whatever patterns have proved useful for survival. What is “naturally selected” is whatever works for life.
(Because the system of life on earth contains many potential niches for different kinds of creatures to survive, over time selection leads to branching, as different species arise from common ancestors.)
Given enough time, which means given enough generations in which some genetic blueprints prove viable and others do not, incredible complexity gets crafted by this process of selection.
How much time is required?
Generally a lot more time than is required when human beings deliberately “select” for the characteristics they like among the animals and plants people have domesticated. It is only tens of thousands of years back into the ancestry of our domesticated dogs that we find wolves. And dog breeders can create new breeds within a human lifetime.
The speed of change from such deliberate selection is illustrated by an experiment, begun in the 1950s, in domesticating the fox. People repeatedly chose for breeding whichever young foxes in each generation were most friendly to humans. It took only decades to profoundly transform what had begun as wild and suspicious foxes into animals that acted like pets (and looked like many others of our domesticated animals as well).
But natural selection is not so purposeful, and very long stretches of time have been required for most of what we see around us, and within ourselves. Indeed, it took most of the time — of the more than three and a half billion years that life has been evolving on earth — just to create a single cell of the kind that make up our own bodies.
(Each of those cells being such miracles of complexity, each being indeed a symbiosis of two kinds of organisms that had originally been separate species.)
The evolutionary process can create mind-boggling complexity because there has been so much time, in the history of life on earth, for the winnowing to happen—almost a million times longer than the whole of recorded human history.
Stretches of time whose vastness came gradually as a startling revelation in the history of science. But there really can be no question about it.
(Anyone who still wants to stick with the idea that the universe is only 6,000 or so years old must not only disregard a mountain of evidence from the field of biology, but also the separate evidence — derived from independent methods of dating — from geology, which establishes the earth’s birth some 4.5 billion years ago, and from astronomy, in which the scientists routinely look at light that has been traveling — at 186,000 miles per second — for billions of years.)
Given that the history of life on earth has been ongoing for such vast stretches of time, it becomes comprehensible how — through the operation of natural selection — the richly interdependent system of life on earth, consisting of such incredible variety and complexity of lifeforms could have developed. Natural, almost inevitable, and at the same time a kind of miracle.
Not just an idea in biology. But — because of the huge body of evidence it helps explain — the main organizing principle of that whole scientific field.