The Logic of Natural Selection

Charles Darwin was not the first individual to recognize that the characteristics of living organisms had changed over time.  Various notions of change in the living world can be found in the earliest known philosophical writings.  Darwin's name is so often linked to the concept of organic evolution because: (1) he was able to convince the scientific community of his time of the fact of biological evolution; (2) he formulated a clear, materialistic mechanism of evolutionary change - Natural Selection; (3) he saw more clearly than most of his contemporaries the philosophical and scientific implications of both Natural Selection and the overall evolutionary process.

The concept of Natural Selection is based on a logical sequence of observations and deductions that was developed independently by Charles Darwin and Alfred Russel Wallace.  The basic argument is simple, and could have been developed by just about anyone with a logical frame of mind and a basic knowledge of Biology, but putting it together in the proper sequence and realizing the far-reaching implications of the concept required an individual of extraordinary scientific capabilities and experience.  Even Darwin, in spite of his claim that he suddenly developed the idea on September 28, 1838, took some time to formulate the concept and realize the full implications of it as it applied to biological evolution (Ospovat, 1981; Mayr, 1982).  The sequence of Darwin and Wallace's reasoning is outlined below.

Given the simplicity and elegance of the logic of Natural Selection you might assume that the process as a mechanism of evolutionary change was an instant success.  However, while Darwin was able to convince most of his scientific contemporaries of the fact of long term biological change, Natural Selection as an agent of evolution was not accepted widely until the early twentieth century.  The reason for this is that to appreciate fully the implications of Natural Selection requires a fundamental change in some philosophical concepts.  These concepts are as follows:

Variation:  There seems to be a subconscious tendency in humans to place similar entities in a category and to ignore differences among the members of the category.  One of the more serious examples of this is the various forms of racism in human societies, but this attitude can even be found in biologists who know consciously that individuals are different from one another because of genotypic differences.  This viewpoint, which ignores  the reality of individual variability, is known by the formal philosophical concept of Essentialism.  The essentialist view is traceable to Plato's proposal that all entities in this world are


The Logic of the Theory of Natural Selection (Mayr, 1982)

Darwin's theory consists of three inferences based on five facts derived in part from population ecology and in part from phenomena of inheritance.

Fact 1: All species have such great potential fertility that their population size would increase exponentially if all individuals that are born would again reproduce successfully.

Fact 2: Except for minor annual fluctuations and occasional major fluctuations, populations normally display stability.

Fact 3: Natural resources are limited.  In a stable environment they remain relatively constant.

Inference 1: Since more individuals are produced than can be supported by the available resources but population size remains stable, it means that there must be a fierce struggle for existence among individuals of a population, resulting in the survival of a part, often a very small part, of the progeny of each generation.

Fact 4: No two individuals are exactly the same; rather, every population displays enormous variability.

Fact 5: Much of this variation is heritable.

Inference 2: Survival in the struggle for existence is not random but depends in part on the hereditary constitution of the surviving individuals.  This unequal survival constitutes a process of natural selection.

Inference 3: Over the generations this process of natural selection will lead to a continuing gradual change of populations, that is, to evolution and to the production of new species.


imperfect manifestations of some ideal, which means that the variability among members of a category is inconsequential.  Evolution by Natural Selection is impossible in an essentialist system because there is no genetic variability to form the basis of differential survival and/or reproduction.  The recognition of the existence and importance of individual variability is what Ernst Mayr (1982) has called repeatedly Population Thinking.  Without population thinking a full appreciation of the evolutionary process is impossible.

Progress & Perfection:  Another common philosophical attitude that is often applied to the natural world is the belief in inherent progress.  This idea holds that things should become more efficient, more complex, and better adapted.  Natural Selection is a direct challenge to this view because it is a highly opportunistic, "shortsighted" process that leads only to adaptation to local conditions at best.  There is nothing inherent in our current understanding of the evolutionary process that would predict the evolution of an intelligent primate.  If there is an apparent trend in the direction of increasing complexity (assuming that the notion of relative biological complexity can be defined objectively), it is merely the accumulated result of a vast number of population level responses to local selective pressures.  Furthermore, selection is constrained by the available genetic variability, and no amount of selection, regardless of how intense it may be, can bring about a particular adaptation if the genetic variability is not in the population in the beginning.  When you contrast this with other nineteenth century evolutionary theories (e. g., Lamarck) which proposed that evolution was both progressive and had an inevitable result (i. e.,  Homo sapiens), as well as the (still) popular theological argument from design (life appears to be well organized and designed - therefore, there must be a designer; see Dawkins, 1986), you get some idea of the revolutionary view offered by Darwinian evolution.

Randomness:  Even after developing "population thinking" and accepting the opportunistic, non-progressive nature of the evolutionary process, there is one final conceptual barrier that many people face, and that is the inherently statistical nature of evolution through Natural Selection.  First, there is the fact that the genetic variability on which selection acts is random with respect to whatever the current selective pressures may be.  Mutations of a particular type do not occur when they are "needed", but instead they are either in the gene pool of the population or they are not.  Second, relative reproductive success and/or survival of a genotype is expressed in terms of probabilities.  Fitness is a relative concept that depends upon a particular selective context; there are no certainties in the evolutionary process, merely relative probabilities.  As Sewall Wright said in 1967 (quoted in Mayr, 1982): "The Darwinian process of continuous interplay of a random and a selective process is not intermediate between pure chance and pure determinism, but in its consequences qualitatively utterly different from either."