Darwin’s On the Origins of Species provided a vivid and practical mechanism to explain how variations within a species can be selected for. This mechanism became known as natural selection or, perhaps less accurately, survival of the fittest. Although Darwin’s natural selection mechanism was not novel—Malthus described a similar mechanism as actively limiting a population’s growth through competition for limited resources—his application of such a concept to species showed to be fruitful. In the Origins, much time is spent in demonstrating the evidence for species-wide change over time, a change brought about by natural selection as the natural variation is pruned over many generations. In this area Darwin was revolutionary, thorough, and applicable, yet is that the whole of biological evolution?
A fully developed theory of biological evolution needs to be able to scientifically explain why variation occurs within a species, how individual variation is inheritable, and how individual variation can drive species-wide changes. Only the last aspect falls under the purview of Darwin’s natural selection mechanism; and in the end, Darwin left the other aspects of biological evolution to his scientific progeny.
The first new development in a theory of biological evolution came through the study of inheritance. Understanding how a variation or novelty in a parent can manifest and present itself in the parent’s offspring was critical to permitting natural selection the ability of progressive change over countless generations. While Darwin did propose a possible construction to explain this inheritance through gemmules, the model did not mirror inheritance in nature. Darwin’s gemmules failed to explain how change could propagate through a population. Being conceptually similar to a bag of colored sand, the theory lead to watered down characteristics with no means to benefit survival. Yet, where Darwin failed, others took up the challenge.
Mendel provided the first insight into a scientific understanding of inheritance. Although a physical mechanism underpinning inheritance was not yet forthcoming for Mendel, he did manage to study populations and traits in such a way as to mathematically model the laws of inheritance. By leveraging the Boolean traits of pea plants, such as yellow/green or tall/short, Mendel was able to control and manipulate populations of plants to directly interrogate the inheritance of traits over many generations. The numbers supported an allele based system of inheritance where traits were passed on from parents to offspring. Each parent contributed one allele for a trait and the offspring’s phenotype was then controlled by the presence or absence of the dominant allele. While Mendel’s work was certainly a step in the right direction, it not only failed to provide a physical mechanism of inheritance but it also restricted itself to the least Darwinian type of traits: traits of an extreme and Boolean nature.
The ravine between Darwin’s gradual change and Mendel’s decisive traits was bridged through the work of T. H. Morgan. Morgan developed a well-supported model of heredity which found that the “chromosomes are the bearers of the hereditary characters and that the known chromosomal behavior suffices as a mechanism to explain Mendel’s law” (Zoology 421). These hereditary characters, latter called genes, were a physical manifestation of an individual’s traits—traits that could be mutated by some as-of-yet unknown mechanism and could be passed on. The smallest unit of a trait was found to be “different states in the same locus on one chromosome” and was known as allele. “Mechanistic manifestation of Mendelian heredity” (Zoology 429) finally seemed possible with the rate that technology was advancing. Thus, one more aspect to the biological theory of evolution was well on its way to being explained, but it would still be some time before a conclusive understanding of individual variation would coalesce.
The answer to where individual variation stems from could not be accurately understood until a molecular basis for genetics was developed in the 50s. Once the source of genetic material was found to be DNA—and it’s structure was elucidated—a wide range of mechanisms for individual novelty in variation presented themselves.
Published in 1953, Watson and Crick wrote a series of reports on the structure and make up of DNA based on a wide array of evidence. As early as 1928, the DNA molecule was associated as a ‘hereditary molecule’ and that it provided the molecular basis of Morgan’s heredity characters. Much speculation as to how traits were contained within the genetic material and how these traits were realized into a biological organism, yet the evidence from experimental techniques was lagging. Until the structure of DNA was formally understood there was little support for any theory which sought to explain how variation arose—let alone how the alleles were converted into traits. When it turned out that the DNA molecule was a double strand of complementary sequence, the basis for variation became clear. As a linear sequence of ordered bases, any alteration in the order could yield genetic change. This genetic change would then be converted to phenotypic change through the mysterious workings of the cell. With the molecular basis for inheritance and variation establish and without contradiction with each other or Darwin’s mechanism of selection, a cohesive theory of biologic evolution was realized.
Even with a practical theory of biologic evolution pieced together, much was left unexplained and unknown in biology. Biological evolution merely strove to understand a simple, possible mechanism for the progressive changes in a species while ultimately remaining silent on a range of other aspects. This development is intriguing for the logic of the progression seems simple, and even inevitable, in hindsight, yet science is not often that direct. From one step of this development to the next, questions changed from ‘how are traits stored in the cell’ to ‘how does DNA structure record the traits’ and so on and so forth. This foundational change in narrative over time is a defining characteristic of a paradigm change and it clearly took several revolutions in science to get to where we are in Biological Evolution today.