Evolution Basics: An Introduction to Variation, Artificial Selection and Natural Selection
Note: This series of posts is intended as a basic introduction to the science of evolution for non-specialists. You can see the introduction to this series here. In this post, we explore the core ideas of Darwin’s conception of evolution: that variation within species is heritable, that variants do not reproduce at the same rate in nature, and that these features of natural populations can lead to changes over time.
In the last post in this series, we described how Darwin’s studies on biogeography were important in prompting him to consider that species were not fixed, but rather could change over time to become new species. Darwin was at a loss, however, to explain the mechanism by which that gradual change might take place. Since his observations (a) documented variation, or differences, between closely-related species, and (b) suggested that this variation arose through the gradual modification of two separated populations over time, Darwin correctly surmised that understanding variation itself might shed light on the formation of new species. Accordingly, Darwin would undertake a major investigation of heritable variation within domesticated animals. His studies led him to note the importance of selection in forming new domestic breeds, and later he would realize that nature could act as a selective force.
These two concepts, heritable variation and natural selection, remain core ideas within modern evolutionary biology. Given how important these concepts are even today, we’ll examine them briefly before describing how Darwin came to hypothesize about their role in forming new species.
Evolution made simple: heritable variation and differential reproduction
Within any population of organisms, whether domesticated or in the wild, heritable differences exist. We now understand that these heritable differences arise from differences in genetic information (i.e. variation in DNA sequences), but this insight was unknown in Darwin’s time. What Darwin did appreciate, without knowing its molecular basis, was that offspring on average tend to resemble their parents more so than other members of the population at large. From this he inferred, correctly, that much variation was heritable: it was passed down from parent to offspring.
As we will see below, Darwin would also note that if variation is subjected to selection, that average character traits of a population could shift over time. Selection is simply the observation that not all variants within a population reproduce at the same rate. In the case of artificial selection, a human agent decides which variants reproduce at a greater rate than others by enforcing selective breeding. If selection is consistent from generation to generation, over time certain variants will increase in frequency in a population, and other variants will decrease in frequency.
In an nutshell, this is the core of evolutionary theory: that changes in heritable variation over time can shift average characteristics of a population, and that differential reproductive success (selection) is a major mechanism for driving changes in heritable variation from one generation to another. Though our understanding of evolution has greatly increased since Darwin’s time (since evolution is a theory in the scientific sense), these basic principles remain fundamental components of evolutionary biology. Despite their simplicity, it took a significant amount of work over several years for Darwin to piece these ideas together into a coherent framework.
Darwin’s studies on variation, 1837 - 1838
As Darwin recounts in his autobiography, upon his return to England from his voyage on the Beagle, he undertook a systematic effort to accumulate information on variation in “races” of domesticated plants and animals as well as variation in natural populations. (As an aside, in the 1800s, the term “race” was the accepted scientific term for what we could today call breeds or subspecies of plants and animals. “Race” would later come to have its present-day, human-specific connotation, but that was not the case in Darwin’s day, as even a brief perusal of literature of the period will demonstrate. As such, attempts to portray Darwin’s works as racist are unfounded, but perennially tempting for those not knowledgeable of Victorian English, given that the full title of his treatise is On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life.) As Darwin conversed at length with plant and animal breeders (for example, he would spend a great deal of time learning from pigeon fanciers and document their ability to create numerous exotic breeds of pigeons), he was impressed with the power of artificial selection to effect change within a breed over time. Impressed as he was with this power, however, he was yet to conceive of how nature might act as a selective force:
After my return to England it appeared to me that by following the example of Lyell in Geology, and by collecting all facts which bore in any way on the variation of animals and plants under domestication and nature, some light might perhaps be thrown on the whole subject. My first note-book was opened in July 1837. I worked on true Baconian principles, and without any theory collected facts on a wholesale scale, more especially with respect to domesticated productions, by printed enquiries, by conversation with skilful breeders and gardeners, and by extensive reading. When I see the list of books of all kinds which I read and abstracted, including whole series of Journals and Transactions, I am surprised at my industry. I soon perceived that selection was the keystone of man’s success in making useful races of animals and plants. But how selection could be applied to organisms living in a state of nature remained for some time a mystery to me. (Autobiography, pp. 119 -120)
Later he would hit upon his key idea: that nature might apply selection in much the same way that humans could. Rather than a breeder choosing which individuals to mate, the ability of different variants to reproduce in a given natural setting would allow some to reproduce at a greater rate than others. Since their traits would be heritable, this would drive changes in traits over time in the population experiencing “natural selection”, a term Darwin coined as an analogy to human, or artificial, selection. Darwin’s realization was that, in the wild, organisms generate more offspring than are capable of surviving, and this was the last piece of the puzzle (famously supplied by Thomas Malthus) to fall into place for him. As such, there would be competition among the members of a species for limited resources. Competition would also take place between members of different species. These sources of competition would create a “struggle for existence” that would act to select certain variants and promote their reproduction over others:
In October 1838, that is, fifteen months after I had begun my systematic enquiry, I happened to read for amusement Malthus on Population, and being well prepared to appreciate the struggle for existence which everywhere goes on from long-continued observation of the habits of animals and plants, it at once struck me that under these circumstances favourable variations would tend to be preserved, and unfavourable ones to be destroyed. (Autobiography p. 120)
Later, in Origin, he would spell out his ideas in great detail. In the opening chapter, Darwin discusses numerous examples of heritable variation in domestic animals. He then turns to heritable variation in natural populations in the second chapter, again citing numerous examples, and comparing his findings to variation in domestic stocks. In the third chapter he describes the “struggle for existence” in nature, and then ties the three ideas together: if human breeders can use artificial selection to “accumulate” variation over time, then so, too, can natural selection:
Owing to this struggle for life, any variation, however slight and from whatever cause proceeding, if it be in any degree profitable to an individual of any species, in its infinitely complex relations to other organic beings and to external nature, will tend to the preservation of that individual, and will generally be inherited by its offspring. The offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive. I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection, in order to mark its relation to man’s power of selection. We have seen that man by selection can certainly produce great results, and can adapt organic beings to his own uses, through the accumulation of slight but useful variations, given to him by the hand of Nature. But Natural Selection, as we shall hereafter see, is a power incessantly ready for action, and is as immeasurably superior to man’s feeble efforts, as the works of Nature are to those of Art. (Origin, p. 61)
Artificial selection, then and now
Having found his “theory by which to work” , Darwin would spend decades accumulating evidence for his ideas prior to publishing his Origin in 1859. Documenting evidence for natural and artificial selection would be a major focus of his efforts. In the next post in this series, we’ll discuss some observations Darwin made on artificial selection and the domestication of dogs, and examine some recent genomics studies that reveal the molecular details of how artificial selection shaped the dog genome over time.