Random Thoughts About Evolution by James Coors (September 1999)

I have been a member of the Plant Breeding and Plant Genetics faculty at the University of Wisconsin for the past 16 years, and I teach a course in selection theory for students who intend to become plant breeders. Plant breeders are merely directing evolution for their chosen plant species, and the ever-increasing productivity of food crops throughout the world is excellent testimony to their success. The same is true for animal breeders. So it’s interesting to listen to all the arguments about whether there’s any evidence supporting evolution in general when its processes have been routinely and productively employed by humans for thousands of years.

But perhaps the most bizarre argument against evolution is the probability argument. Creationists believe that the biological complexity on our planet could never have arisen through random mechanisms alone. The probability of such complexity is so small that divine intervention is required. In fact the Kansas Board of Education recently decided, based on arguments such as this, that evolution should be deleted from required school curricula.

Creationists might want to revisit a bit of probability theory to see if it actually works the way they claim. A very simple exercise highlights a major weakness in the argument. Next time you are debating someone about this matter ask the person to start flipping a coin. You record on a piece of paper whether each successive coin toss results in heads or tails. Keep a running tally, e.g., HTHTTH . . . If each coin toss takes two seconds, and the coin is tossed for one minute, the coin tosser will then create a specific sequence of 30 heads and tails. At the end of this exercise you will have the exact sequence on a piece of paper for all to view.

There are about one billion different sequences of heads and tails that could arise in 30 coin tosses. So the probability that the particular sequence of 30 heads and tails on your piece of paper occurred by random chance alone is about one in a billion. (To be more accurate, the probability is (1/2)30.) Your sequence, then, seems highly unlikely, but perhaps not completely out of the realm of possibility. But if the coin tosser continues for five minutes, the resulting sequence of 150 H and Ts would have a probability of about one in 1045. And if the tosser is very patient and tosses the coin for thirty minutes, the resulting sequence would have a probability of about one in 10271. You would be witness to an astounding miracle! If all humans started tossing coins for the rest of their lives, and then their children, grandchildren, great grandchildren, and so on, did the same for the next billion years, it is still extremely unlikely that the same sequence would ever occur again.

This example shows that it is very easy to create an event that, in retrospect, is highly unlikely. In the case of the coin toss, something has to occur after thirty minutes. A sequence of heads and tails will be produced. All it takes is a patient coin tosser. But once a sequence is produced, one can’t then turn around and claim, as a creationist would, that the particular sequence is so rare that it could not have happened by chance alone. Retrospective probability arguments like this are obviously flawed.

At first glance, the coin tossing exercise may seem somewhat peripheral to biological complexity and evolution. Just the opposite is true. Many readers may recall learning about the events that control cell division and sexual reproduction, mitosis and meiosis, from their first high school biology course. Maybe mitosis and meiosis are now quite dim concepts to most people, and this is certainly understandable. High school itself is a dim concept to most of us in advanced stages of middle age. But most of us did learn at some point that each of our genes has two functional elements called alleles, and we inherit one allele from each of our parents. The allele inherited from your father may be different from that inherited from your mother, and in this case, you carry two distinct alleles for that gene. Your father’s allele was carried in his sperm, and your mother’s in her egg cell, which then unite. After some cell division and differentiation, welcome to planet earth.

Somewhere in the process of sperm development, a “choice” had to be made about which one of your father’s two alleles at the gene was to be passed on in the sperm, and the same is true for the allele in the egg. This is what meiosis is all about. There are two possible alleles for the gene contributed from your father, of which one was “chosen.” The same is true for the alleles from your mother. The choice of each allele is essentially equivalent to the coin toss, heads = allele 1, tails = allele 2. In other words, each allele has a probability of 1/2 of being passed on to the progeny, and the process is essentially random.

Estimates are that humans may have up to 100,000 different genes, so without going into tedious detail, it would seem that the outcome of each mating would produce an extremely unlikely combination of alleles. Nonetheless, there are lots of children around. It is quite obvious from both probability arguments, and by just looking around, that the vast majority of new children represent combinations of alleles that are very different for each individual. In fact, most of the combinations have never been produced before in human history, and they never will occur again in the future. Each of us is so genetically unique that, in retrospect, the odds of your or my existence are practically nil. But, nonetheless, each of us does exist, and we need not evoke anything other than a series of meiotic “coin tosses” to explain our presence as well as all the variation around us.

Life on earth began over three billion years ago. Quite a long time relative to a 30-minute coin toss or even a human generation. It’s unclear just how old meiosis is, but for much of life’s history, meiosis or meiotic-like sampling processes have been creating immense genetic diversity. This is precisely what makes meiosis so powerful. Innumerable types of unique organisms can be produced through random meiotic processes, and the current biological state of our planet is but one among many possible assemblages of organisms. The current form of life is only superior to any of the other possibilities in the sense that the current form exists and the others don’t. The notion of “value” of outcomes, though, is another tough question for creationists, and it leads to what function natural selection plays in shaping biology. A topic for another time.

Without doubt, though, the combination of a random process such as meiosis that creates virtually infinite genetic variation, and natural selection, which chooses among variants at any given time, is a most potent creative force.

James G. Coors is a Professor of Agronomy and a member of the Plant Breeding and Plant Genetics program at the University of Wisconsin-Madison. He teaches courses on selection theory and the processes involved with the domestication of crop plants. He also conducts a breeding program to develop new varieties of corn. He is a Life Member of the Foundation.

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