Chapter 1 What Evolution Is

The notion of an ever-changing world is a relatively new idea in Western cultures. Dating back to the teachings of Greek philosophers, the world was long seen as permanent and unchanging. Its inhabitants were thought to be arranged on scale from lower to higher beings by a divine creator, with humans at the very top of the hierarchy. Until the early 1800s, most naturalists were primarily concerned with describing and cataloging life, which led to the modern classification system originally developed by the Swedish biologist Carolus Linnaeus.

By the time Charles Darwin was born in the early 19th century, evidence for a changing world was accumulating. Geologists started to contemplate how slow, gradual processes could carve canyons into stone and change the course of rivers. The discovery of fossils indicated the past existence of organisms vastly different than today’s and suggested a rich history with continuous change. And some scholars, like the French biologist Jean-Baptiste Lamarck (Figure 1.1), started to formulate theories that could potentially explain the diversity of life. Budding evolutionary thought was not limited to Western cultures, and there were a number of pre-Darwinian Muslim scholars who articulated evolutionary ideas as well (Malik et al. 2018).

Jean-Baptiste de Monet Chevalier de Lamarck. Painted by Charles Thévenin, [Public Domain](https://creativecommons.org/share-your-work/public-domain/).

Figure 1.1: Jean-Baptiste de Monet Chevalier de Lamarck. Painted by Charles Thévenin, Public Domain.

It was Lamarck, inspired by the similarities he observed across different species, who formulated the first theory of evolution. Lamarck argued that life was inevitably driven towards increasing complexity, essentially progressing along the scale from lower to higher beings established by the Greeks. He envisioned that simpler life forms, like microbes, continuously arise and eventually develop into higher forms, like plants and animals. Lamarck also believed that progress along this hierarchy allowed organisms to adapt to their environment, which was driven by an “inner need.” He thought that the continuous use of a particular organ allowed for its proliferation and subsequent inheritance to the next generation. Perhaps most famously, Lamarck explained that the long neck of the giraffe was the product of a continuous stretching toward leaves high on the trees. So, across many generations of stretching, giraffe necks gradually reached their current size.

1.1 Darwin and the Conception of a New Idea

When Darwin boarded the British Navy Ship HMS Beagle as a 22-year old, he was already well trained in geology, chemistry, and natural history. Darwin was hired as an unofficial naturalist and companion to the captain of the ship, Robert FitzRoy, and the five-year journey around the world allowed young Darwin to collect evidence of a changing world (Figure 1.2).

Map of the Voyage of the Beagle. Map: Sémhur, [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0/).

Figure 1.2: Map of the Voyage of the Beagle. Map: Sémhur, CC BY-SA 4.0.

Darwin’s visit to the Galapagos Islands, and the specimens of reptiles and birds that he collected there, proved to be particularly inspiring. The Galapagos are an archipelago of 21 small islands, located over 900 kilometers off the coast of Ecuador, and situated right on the equator. Darwin not only noticed that the fauna of these islands was dominated by animals found nowhere else, but there also were differences in the species from one island to the next. Most importantly, bird specimens that Darwin believed to be blackbirds, warblers, wrens, and finches due to their different beak morphologies later all turned out to be just finches—the Darwin’s finches we know today (Figure 1.3). In his own words:

“I have stated, that in the thirteen species of ground-finches, a nearly perfect gradation may be traced, from a beak extraordinarily thick, to one so fine, that it may be compared to that of a warbler. […] Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends.”

— Darwin, 1889

If all the species were created with traits that fit the environments in which they are now found, why would the finches of the Galapagos Islands be so different from one another? Why do closely related species occupy niches that are filled by very different taxa in other regions? Observations like these, which Darwin made at many destinations throughout his travels, led him to suspect that the species we observe today have evolved from a shared ancestor.

Examples of Darwin's finches or Galapagos finches. Note the stark variation in beak sizes among the four species. Drawn by John Gould, [Public Domain](https://creativecommons.org/share-your-work/public-domain/).

Figure 1.3: Examples of Darwin’s finches or Galapagos finches. Note the stark variation in beak sizes among the four species. Drawn by John Gould, Public Domain.

Upon his return to England in 1836, Darwin settled near London and never traveled abroad again. Nonetheless, he remained a prolific scholar writing monographs about various topics, from geology to barnacles. Darwin was also a passionate pigeon breeder, and it was this unlikely hobby that provided additional insights for the formulation of his ideas. It led him to think about the forces that could drive the modification of species from one common ancestor to different descendants:

“It is, therefore, of the highest importance to gain a clear insight into the means of modification. […] At the commencement of my observations it seemed to me probable that a careful study of domesticated animals and of cultivated plants would offer the best chance of making out this obscure problem.”

— Darwin, 1859

In his endeavors of pigeon breeding, Darwin noticed two critical things: (1) Offspring tended to inherit the traits of their parents. (2) If he carefully selected breeders with desired traits generation after generation, he was able to shape the variation of colors, morphologies, and behaviors in his flock. It was his meticulous work as a natural historian, collecting evidence for his emerging ideas for over two decades, that ultimately allowed Darwin to formulate the basic tenets of evolutionary biology that still hold up today, after more than 150 years of scrutiny by the scientific community.

1.2 Two Fundamental Insights

1.2.1 The Pattern: Evolution is Descent with Modification

The first of Darwin’s fundamental insights was the formal description of evolution as we understand it today. He described evolution as descent with modification, postulating the common ancestry of all living things. Hence, different species did not arise independently, but they derived from preexisting form. This insight arose from a careful analysis of patterns of similarities across species. Species that share a recent common ancestor share traits precisely because they inherited them from the shared ancestor. More distantly related taxa exhibit differences in their traits, because they have been on independent evolutionary trajectories for longer periods of time. This perspective describes the observable pattern of evolution.

Given Darwin’s insight, here is a simple definition of evolution that we will rely on for now. As you will see, there are multiple modern definitions of evolution, and we will revisit different definitions throughout the book.

Definition: Evolution

Evolution is the change in the inherited traits of a population across successive generations, ultimately leading to the transformation of species through time (both in terms of changes of traits that occur within species and the origin of new species).

The power of Darwin’s idea of descent with modification is that we can treat it as a scientific hypothesis with empirically testable predictions. If Darwin’s notion of evolution was right, we should be able to uncover evidence that:

  1. Species change through time (microevolution).
  2. Lineages split to form new species (speciation).
  3. Novel forms derive from earlier forms (macroevolution).
  4. Species are not independent but connected by descent from a common ancestor (common ancestry and homology).
  5. Earth and life on Earth are old (deep time).

We will revisit these predictions and examine the evidence for evolution in detail in Chapter 2.

1.2.2 The Process: Natural Selection is a Mechanism of Evolution

The second of Darwin’s fundamental insights was the inception of a mechanism that could produce the observable pattern we call evolution (i.e., change in inherited traits across generations), which Darwin named natural selection.

Definition: Natural Selection

Natural selection is the process in which individuals with a particular trait exhibit higher reproductive success than individuals without that trait.

Natural selection explains how the traits of a population change through time, and why organisms are well suited for their environment. Individuals that exhibit traits that are advantageous under certain environmental conditions have a higher chance of surviving and reproducing, making a disproportional contribution to the offspring of the next generation. If the relevant traits are heritable, their frequency increases across subsequent generations. This process not only leads to change in populations across generations, but said change specifically pertains to traits that are important for survival and reproduction in a given environmental context. In other words, the action of natural selection directly leads to adaptation. We will take a close look at how natural selection works in Chapter 3.

1.2.3 Distinguishing between Pattern and Process

A key misconception is that evolution and natural selection are the same. People often use the two terms interchangeably, conflating patterns and processes of evolution. Distinguishing between the two is critical, because evolution and natural selection do not have to work in unison.

Evolution is a historical pattern of change that can—but does not have to—be caused by natural selection. Evolutionary change can also be driven by other forces that impact the composition of populations across generations. In Part II of this book, we will examine evolutionary forces other than natural selection, which include mutation, genetic drift, and migration. In natural populations, these four forces interact to shape evolutionary change across time.

It is also important to note that the action of natural selection does not necessarily lead to evolution. Natural selection can only cause evolution when it acts on heritable traits that are transmitted from parent to offspring. If that is the case, the offspring of successful individuals will carry the same traits that made their parents successful. However, not all traits are heritable. For example, selection on individuals exhibiting high muscle mass does not translate to evolutionary change if that muscle mass was acquired through exercise and diet. If natural selection acts on non-heritable traits, there is no evolutionary change.

Explore More: Misconceptions about Evolution

Despite the broad scientific consensus on the importance of evolutionary theory in understanding the diversity of life, people hold many misconceptions about evolutionary theory and processes, its implications, and its relation to religious beliefs. I encourage you to explore the fantastic resource provided by The University of California Museum of Paleontology, which lists common misconceptions about evolution and clarifies these misconceptions.

1.2.4 Population Thinking

Darwin’s fundamental insights were possible because of a fundamental shift in how he thought about biological entities, from a typological perspective to what Mayr (1982) called “population thinking.” Prior to Darwin, scholars viewed individual organisms as imperfect representations of a central Platonic type (the perfect manifestation of a species). Variation among individuals was considered to be meaningless noise that was either ignored or seen as a nuisance when describing and classifying species. In contrast, population thinking rejects the notion of an ideal representative and instead focus on the variants found within populations. Variation among individuals is not just meaningless noise that represents different degrees of imperfections from a preconceived idea, but it is the raw material for evolutionary change. In other words, what was meaningless noise to naturalists prior to Darwin suddenly became the very focus of evolutionary studies after. Mayr (1982) argued that this paradigm shift was one of Darwin’s most important contributions to modern biology.

1.3 Reflection Questions

  1. How would you define evolution in your own words? How does your definition compare to the one given in this chapter? Are there other definitions of evolution? What are their strengths and weaknesses?
  2. What is the difference between evolution and natural selection?
  3. What do you think of the term “survival of the fittest”? Is it an accurate description of evolution? If not, why?
  4. What would you say to someone who told you that evolution is “just a theory”?
  5. Darwin once wrote (1859): “We see nothing of these slow changes in progress, until the hand of time has marked the lapse of ages.” How long do you think it takes for evolution to take place?
  6. List the causes (or mechanisms) of evolution and describe how each of them impacts the population they are acting upon. Feel free to use examples instead of decription, but make sure to include sufficient detail.
  7. While Darwin understood that some traits are inherited from parents to their offspring, he did not know exactly how that happens. He was largely unaware of the work of his contemporary, Gregor Mendel, who worked out the foundational principles of genetics. Instead Darwin hypothesized that the body continuously emitted small particles he called gemmules that accumulated in the gonads and contributed heritable information to the gametes. How do you think Darwin’s misconceptions about heredity impacted the validity of his evolutionary ideas? How have discoveries in classical genetics and later the discovery of DNA carrying the genetic instructions for the development and functioning of all organisms strengthened or weakened our understanding of evolution as conceived by Darwin?
  8. How does Lamarck’s view of evolution differ from our current understanding? What aspects did he get right, and what did he get wrong?

1.4 Additional Resources

1.4.1 Alternative Evolution Textbooks

If you would like more in-depth reading materials about evolution, there are a number of excellent textbooks available. I particularly recommend the edited volume by Losos et al. with short articles about current topics in evolution, because it is freely available for online reading through most university libraries.

1.4.2 Evolution in the Primary Literature and the News

At times during this course, you may want to consider consulting other resources. You can browse through the list of peer-reviewed journals that publish research related to evolution. If you want to find current research in the field, I also recommend the following websites, many of which provide lay summaries of recent papers:

1.5 References

  • Darwin, C. (1859). On the origin of species based on natural selection, or the preservation of favoured races in the struggle of life. John Murray.

  • Darwin, C. (1868). The Variation of Animals and Plants under Domestication. John Murray.

  • Darwin, C. (1889; original in 1839). Journal of Researches Into the Natural History and Geology of the Countries Visited During the Voyage of H.M.S. “Beagle” Round the World, Under the Command of Capt. Fitz Roy. Ward, Lock and Company.

  • Malik, A. H., Ziermann, J. M., & Diogo, R. (2018). An untold story in biology: the historical continuity of evolutionary ideas of Muslim scholars from the 8th century to Darwin’s time. Journal of Biological Education, 52(1), 3–17.

  • Mayr E. 1982. The Growth of Biological Thought: Diversity, Evolution, and Inheritance. Cambridge (MA): Harvard University Press.