This blog post is about how the Phyletic Gradualism model of evolution is disconfirmed and false, and why the Punctuated Equilibrium model is right. Who cares? If you use (or try to use) the conclusions of evolutionary science to improve your health and your life, you ought to care.
Most people (even scientists) who believe in a God or Gods, and also "believe in evolution," even if they deny the explicit interventionist versions of "intelligent design" still think of evolution as design by other means. In other words, they believe that God's creation is perfect; that God set up and used the laws of nature to result in organisms capable of optimal life and optimal health. The reason for this blog post is to make it easier for my fellow Atheists to separate the science from the beliefs of scientists.
2. The Intellectual Origin of Phyletic Gradualism
It is not the custom of scientists to challenge cultural preconceptions without first having confronted and assembled overwhelming evidence. The religious belief in the perfection of God's creation was not seen by Darwin and his contemporaries as challenged by the theory of evolution. In their day, to contradict, from evidence, the scriptural account of creation, in favor the operation of natural laws that might or might not have been created by a God, was challenge enough. As late as 1970, the phyletic gradualism model was generally accepted by evolutionary biologists, in part because it did not contradict the notion of evolution leading to, or at least moving in the direction of, organs and organisms optimally suited to an optimally healthy existence in their natural environment.
Since the course of evolution is set by a random process of mutations followed by natural selection through differences in reproductive success, its Archimedean derivation is necessarily based on measure theory, probability theory, and mathematical and computational statistics. (Reader, do not be intimidated. You do not need to be a mathematician to understand the essence of the derivation; I will give pictorial hints so that you can let your visual imagination do most of the work.) In measure-theoretic representations of evolution, the probability of reproductive success can be visualized as the height of a variable surface, above the multi-dimensional space representing the state of the genome. A peak at which this probability is higher than it is at all points around it, is called a local optimum. The global optimum, corresponding to the highest possible likelihood of reproductive success, is the highest peak.
Up until the identification, in 1953, of DNA as the genetic material of life, biologists thought of "genes" as direct blueprints for all the tissues, organs and structures of the organism. Assuming this correspondence between the genes, and the traits of the organism, led the scientists of the time to think of the probability-of-reproductive-success surface as having a single optimum only: the location at which the genetic "blueprint" corresponds to the optimal, rational design for the given structure, organ or tissue. Then natural selection selects those mutations that move up the upward slope, rather than down the downward slope, from the present spot on this surface. Gradual evolution to the single, global optimum: this is the mathematical expression of the "Phyletic Gradualism" model.
It took two decades, roughly from 1971 to 1992, for this model to be overturned.
(continued in Part II)