Charles Darwin's voyage on the HMS Beagle and his ideas about evolution and natural selection.

Key points:

  • Charles Darwin was a British naturalist who proposed the theory of biological evolution by natural selection.
  • Darwin defined evolution as "descent with modification," the idea that species change over time, give rise to new species, and share a common ancestor.
  • The mechanism that Darwin proposed for evolution is natural selection. Because resources are limited in nature, organisms with heritable traits that favor survival and reproduction will tend to leave more offspring than their peers, causing the traits to increase in frequency over generations.
  • Natural selection causes populations to become adapted, or increasingly well-suited, to their environments over time. Natural selection depends on the environment and requires existing heritable variation in a group.

What is evolution?

The basic idea of biological evolution is that populations and species of organisms change over time. Today, when we think of evolution, we are likely to link this idea with one specific person: the British naturalist Charles Darwin.
In the 1850s, Darwin wrote an influential and controversial book called On the Origin of Species. In it, he proposed that species evolve (or, as he put it, undergo "descent with modification"), and that all living things can trace their descent to a common ancestor.
A species is a group of similar organisms that can interbreed with each other to make healthy, fertile offspring. For instance, all humans belong to the same species, Homo sapiens. Domestic dogs are also a species, Canis familiaris.
Darwin also suggested a mechanism for evolution: natural selection, in which heritable traits that help organisms survive and reproduce become more common in a population over time.
A heritable trait is one that's passed on from parents to children by way of genes. For instance, eye color is a heritable trait: it is determined by the genes you get from your mom and dad. You can substitute the phrase "genetically determined" for "heritable" in this article if that makes more sense to you.
In this article, we'll take a closer look at Darwin's ideas. We'll trace how they emerged from his worldwide travels on the ship HMS Beagle, and we'll also walk through an example of how evolution by natural selection can work.

Early ideas about evolution

Darwin was not the first to come up with the idea of evolution. In fact, even some of the ancient Greek philosophers had evolutionary ideas. However, Plato (among the most famous of these philosophers) believed that species were fixed and unchanging, and his viewpoint was very influential in Western thought for centuries after his death.
Evolutionary ideas began to make a comeback in the eighteenth century. In the early nineteenth century, the French naturalist Jean-Baptiste Lamarck published a book proposing that evolution took place and suggesting a mechanism by which it might occur. Specifically, Lamarck proposed that modifications in an individual caused by its environment, or the use or disuse of a structure during its lifetime, could be inherited by its offspring and lead to a change in a species. (For instance, Lamarck suggested that giraffes have long necks because some of them stretched their necks during their lifetimes, then passed the stretched neck on to their children.)
This mechanism for evolutionary change did not turn out to be correct, and today, we often remember Lamarck as the one who "got the wrong answer." However, before we judge, we should remember that Lamarck was a pioneer despite his incorrect idea. He was one of the very first to seriously engage with the idea of evolution and to actually suggest how it might take place.

Influences on Darwin

In the eighteenth century, James Hutton, a Scottish naturalist, proposed that geological change occurred gradually by the accumulation of small changes from processes operating like they are today over long periods of time. This contrasted with the predominant view that the geology of the planet was a consequence of catastrophic events occurring during a relatively brief past.
Hutton’s view was popularized in the nineteenth century by the geologist Charles Lyell, who became a friend of Darwin's. Lyell’s ideas about gradual geological change would play an important role in shaping Darwin’s thinking about gradual biological evolution.
Thomas Malthus was another thinker who strongly influenced Darwin. Malthus was interested in the growth of human populations and wrote about factors that ultimately limited exponential human population growth, such as diseases and limited foodstart superscript, 1, end superscript. His ideas were crucial in Darwin's realization that most natural populations produced more offspring than their environments could support, such that only a fraction of the offspring could survive and reproduce.

Darwin and the voyage of the Beagle

Darwin's seminal book, On the Origin of Species, set forth his ideas about evolution and natural selection. These ideas were largely based on direct observations from Darwin's travels around the globe. From 1831 to 1836, he was part of a survey expedition carried out by the ship HMS Beagle, which included stops in South America, Australia, and the southern tip of Africa. At each of the expedition's stops, Darwin had the opportunity to study and catalog the local plants and animals.
Over the course of his travels, Darwin began to see intriguing patterns in the distribution and features of organisms. We can see some of the most important patterns Darwin noticed in distribution of organisms by looking at his observations of the Galápagos Islands off the coast of Ecuador.
Image credit: "Darwin's finches," by John Gould (public domain).
Darwin found that nearby islands in the Galápagos had similar but nonidentical species of finches living on them. Moreover, he noted that each finch species was well-suited for its environment and role. For instance, species that ate large seeds tended to have large, tough beaks, while those that ate insects had thin, sharp beaks. Finally, he observed that the finches (and other animals) found on the Galápagos Islands were similar to species on the nearby mainland of Ecuador, but different from those found elsewhere in the worldstart superscript, 2, end superscript.
Darwin didn't figure all of this out on his trip. In fact, he didn't even realize all the finches were related but distinct species until he showed his specimens to a skilled ornithologist (bird biologist) years laterstart superscript, 3, end superscript! Gradually, however, he came up with an idea that could explain the pattern of related but different finches.
According to Darwin's idea, this pattern would make sense if the Galápagos Islands had long ago been populated by birds from the neighboring mainland. On each island, the finches might have gradually adapted to local conditions (over many generations and long periods of time). This process could have led to the formation of one or more distinct species on each island.
If this idea was correct, though, why was it correct? What mechanism could explain how each finch population had acquired adaptations, or features that made it well-suited to its immediate environment? During his voyage, and in the years after, Darwin developed and refined a set of ideas that could explain the patterns he had observed during his voyage. In his book, On the Origin of Species, Darwin outlined his two key ideas: evolution and natural selection.
Yes, he did! Although Darwin is remembered as the main architect of the theory of evolution by natural selection, he was not the only thinker of his era to come up with these ideas. In fact, another scientist, Alfred Russel Wallace, independently reached conclusions very similar to Darwin's at roughly the same time. Wallace, like Darwin, had traveled around the world and was influenced by the patterns he'd seen in the distribution of organisms.
This is not an uncommon thing in science. Often, two people (or two teams of researchers) will reach an important conclusion at nearly the same time. This type of "co-discovery" is important and beneficial, because it confirms that the conclusions reached by the groups are well-supported and likely to be correct.

Evolution

Modern-day species appear at the top of the chart, while the ancestors from which they arose are shown lower in the chart. Image credit: "Darwin's tree of life," by Charles Darwin. Photograph by A. Kouprianov, public domain.
Darwin proposed that species can change over time, that new species come from pre-existing species, and that all species share a common ancestor. In this model, each species has its own unique set of heritable (genetic) differences from the common ancestor, which have accumulated gradually over very long time periods. Repeated branching events, in which new species split off from a common ancestor, produce a multi-level "tree" that links all living organisms.
Darwin referred to this process, in which groups of organisms change in their heritable traits over generations, as “descent with modification." Today, we call it evolution. Darwin's sketch above illustrates his idea, showing how one species can branch into two over time, and how this process can repeat multiple times in the "family tree" of a group of related species.

Natural selection

Importantly, Darwin didn't just propose that organisms evolved. If that had been the beginning and end of his theory, he wouldn't be in as many textbooks as he is today! Instead, Darwin also proposed a mechanism for evolution: natural selection. This mechanism was elegant and logical, and it explained how populations could evolve (undergo descent with modification) in such a way that they became better suited to their environments over time.
Darwin's concept of natural selection was based on several key observations:
  • Traits are often heritable. In living organisms, many characteristics are inherited, or passed from parent to offspring. (Darwin knew this was the case, even though he did not know that traits were inherited via genes.)
  • More offspring are produced than can survive. Organisms are capable of producing more offspring than their environments can support. Thus, there is competition for limited resources in each generation.
  • Offspring vary in their heritable traits. The offspring in any generation will be slightly different from one another in their traits (color, size, shape, etc.), and many of these features will be heritable.
Based on these simple observations, Darwin concluded the following:
  • In a population, some individuals will have inherited traits that help them survive and reproduce (given the conditions of the environment, such as the predators and food sources present). The individuals with the helpful traits will leave more offspring in the next generation than their peers, since the traits make them more effective at surviving and reproducing.
  • Because the helpful traits are heritable, and because organisms with these traits leave more offspring, the traits will tend to become more common (present in a larger fraction of the population) in the next generation.
  • Over generations, the population will become adapted to its environment (as individuals with traits helpful in that environment have consistently greater reproductive success than their peers).
Darwin's model of evolution by natural selection allowed him to explain the patterns he had seen during his travels. For instance, if the Galápagos finch species shared a common ancestor, it made sense that they should broadly resemble one another (and mainland finches, who likely shared that common ancestor). If groups of finches had been isolated on separate islands for many generations, however, each group would have been exposed to a different environment in which different heritable traits might have been favored, such as different sizes and shapes of beaks for using different food sources. These factors could have led to the formation of distinct species on each island.
Let's consider a simplified example to see how natural selection, operating on isolated populations of finches in different environments, could have led to a change in beak shape.
If one island had plants that made large seeds, but few other food sources, birds with larger, tougher beaks than average might have been more likely to survive and reproduce there. That's because the big-beaked birds would have been more able to crack open the seeds and eat the contents, and thus less likely to starve.
If another island had many insect species but few other food sources, birds with thinner, sharper beaks than average might have been more likely to survive and reproduce there. That's because the sharp-beaked birds would have been better able to catch insects as prey, and thus less likely to starve.
Over many generations, these patterns of different survival and reproduction based on beak shape (a heritable trait) could have caused a shift in the average beak shape of each population. Specifically, the population on the first island might have shifted towards a larger, tougher beak on average, while the population on the second island might have shifted towards a thinner, sharper beak on average. Eventually, the two populations of finches might have looked different enough from one another (due to this change, and, potentially, other similar changes) to be classified as different species.

Example: How natural selection can work

To make natural selection more concrete, let's consider a simplified, hypothetical example. In this example, a group of mice with heritable variation in fur color (black vs. tan) has just moved into a new area where the rocks are black. This environment features hawks, which like to eat mice and can see the tan ones more easily than the black ones against the black rock.
Because the hawks can see and catch the tan mice more easily, a relatively large fraction of the tan mice are eaten, while a much smaller fraction of the black mice are eaten. If we look at the ratio of black mice to tan mice in the surviving ("not-eaten") group, it will be higher than in the starting population.
Schematic based on similar schematic in Reece et al. start superscript, 4, end superscript. Hawk outline traced from "Black and white line art drawing of Swainson hawk bird in flight," by Kerris Paul (public domain).
Fur color is a heritable trait (one that can be passed from parent to child). So, the increased fraction of black mice in the surviving group means an increased fraction of black baby mice in the next generation. After several generations of selection, the population might be made up almost entirely of black mice. This change in the heritable features of the population is an example of evolution.
You don't actually need to think in terms of genes and alleles to get this concept, as long as you accept that a tan mouse is more likely to leave tan offspring than a black mouse, and vice versa.
However, if the question of inheritance pattern is bothering you, here is one way you can think about it:
If we see only black and tan mice in the population, then a simple explanation is that the fur color trait is controlled by a single gene whose two alleles have a complete dominance relationship. Let's say, for the sake of argument, that tan is dominant (T) and black is recessive (t). This means that a tan mouse could be either Tt or TT, while a black mouse must be tt.
In the extreme case where all tan mice are eaten by predators before reproductive age, the only mice who will leave any offspring are black (tt) mice, who will mate with one another and produce more black (tt) offspring.
In reality, selection probably would not be that strong. Some tan mice would make it to mating season, and when they mated with the black mice, some tan baby mice would be born along with black baby mice. However, the more tan mice that got siphoned out of the gene pool by the predators, the higher the fraction of black-furred baby mice we'd expect to see in the next generation.
The genetics would be a little less obvious if we flipped the dominance relationships (making black dominant to tan), but the same principle would hold: the more tan mice that got removed from the population by predators, the larger the fraction of baby mice that would have black fur in the next generation.
You can learn more evolution at the level of alleles and genes in the population genetics tutorial.

Key points about natural selection

When I was first learning about natural selection, I had some questions (and misconceptions!) about how it worked. Here are explanations about some potentially confusing points, which may help you get a better sense of how, when, and why natural selection takes place.

Natural selection depends on the environment

Natural selection doesn't favor traits that are somehow inherently superior. Instead, it favors traits that are beneficial (that is, help an organism survive and reproduce more effectively than its peers) in a specific environment. Traits that are helpful in one environment might actually be harmful in another.
For example, in the simplified scenario above, the black mice don't become more common over generations because they are inherently "better" or "more evolved" than tan mice. Instead, they become more common because they have a heritable feature that makes them better able to survive and reproduce in a specific setting, one that happens to include black rocks. In a setting with light-colored rocks, the helpful and harmful traits would be reversed.

Natural selection acts on existing heritable variation

Natural selection needs some starting material, and that starting material is heritable variation. For natural selection to act on a feature, there must already be variation (differences among individuals) for that feature. Also, the differences have to be heritable, determined by the organisms' genes.
For instance, in the mouse example, there was heritable variation in fur color: certain gene variants made a mouse black, while others made it tan. If all the mice had been tan, the population would have had no way to adapt to its new environment by natural selection and might instead have been wiped out.
What if there had been non-heritable variation (e.g., because we dyed some tan mice black)? This might have affected the survival of the individuals, but it wouldn't have changed the composition of the population over generations (because the babies of the dyed tan mice would have inherited the tan-mouse gene variants present in their parents, not black-mouse ones).

Heritable variation comes from random mutations

The original source of the new gene variants that produce new heritable traits, such as fur colors, is random mutation (changes in DNA sequence). Random mutations that are passed on to offspring typically occur in the germline, or sperm and egg cell lineage, of organisms. Sexual reproduction "mixes and matches" gene variants to make more variation.
An important point here is that mutation and genetic variation are random, not directed. That is, a mouse can't intentionally mutate to make itself (or its offspring) a different color. Instead, if there by chance happens to be a mutation that changes mouse fur color, the variation produced by that mutation may be acted on by natural selection.

Natural selection and the evolution of species

Let's take a step back and consider how natural selection fits in with Darwin's broader vision of evolution, one in which all living things share a common ancestor and are descended from that ancestor in a huge, branching tree. What is happening at each of those branch points?
In the example of Darwin's finches, we saw that groups in a single population may become isolated from one another by geographical barriers, such as ocean surrounding islands, or by other mechanisms. Once isolated, the groups can no longer interbreed and are exposed to different environments. In each environment, natural selection is likely to favor different traits (and other evolutionary forces, such as random drift, may also operate separately on the groups). Over many generations, differences in heritable traits can accumulate between the groups, to the extent that they are considered separate species.
Based on various lines of evidence, scientists think that this type of process has repeated many, many times during the history of life on Earth. Evolution by natural selection and other mechanisms underlies the incredible diversity of present-day life forms, and the action of natural selection can explain the fit between present-day organisms and their environments.

Attribution:

This article is a modified derivative of "Understanding evolution," by OpenStax College, Biology, CC BY 4.0. Download the original article for free at http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8.
The modified article is licensed under a CC BY-NC-SA 4.0 license.

Works cited:

  1. Wilkin, D. and Akre, B. (2016, March 23). Influences on Darwin - Advanced. In CK-12 biology advanced concepts. Retrieved from http://www.ck12.org/book/CK-12-Biology-Advanced-Concepts/section/10.18/.
  2. Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., and Jackson, R. B. (2011). The voyage of the Beagle. In Campbell Biology (10th ed., p. 466). San Francisco, CA: Pearson.
  3. Darwin's finches. (2016, April 25). Retrieved March 16, 2016 from Wikipedia: https://en.wikipedia.org/wiki/Darwin%27s_finches.
  4. Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., and Jackson, R. B. (2011). Figure 1.18. Natural selection. In Campbell biology (10th ed., p. 14). San Francisco, CA: Pearson.

Additional references:

Jean-Baptiste Lamarck. (2016, May 9). Retrieved May 16, 2016 from Wikipedia: https://en.wikipedia.org/wiki/Jean-Baptiste_Lamarck.
Lamarckism. (2016, May 13). Retrieved May 16, 2016 from Wikipedia: https://en.wikipedia.org/wiki/Lamarckism.
McClean, P. (1997). Darwin's theory of evolution by natural selection. In Population and evolutionary genetics. Retrieved from https://www.ndsu.edu/pubweb/~mcclean/plsc431/popgen/popgen5.htm.
Raven, P. H., and Johnson, G. B. (2002). Darwin’s theory of evolution illustrates how science works. In Biology (6th ed., pp. 10-16). Boston, MA: McGraw-Hill.
Raven, P. H., Johnson, G. B., Mason, K. A., Losos, J. B., and Singer, S. R. (2014). The science of AP biology. In Biology (10th ed., AP ed., pp. 1-16). New York, NY: McGraw-Hill.
Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., and Jackson, R. B. (2011). Descent with modification by natural selection explains the adaptation of organisms and the unity and diversity of life. In Campbell Biology (10th ed., pp. 465-470). San Francisco, CA: Pearson.
Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., and Jackson, R. B. (2011). The core theme: Evolution accounts for the unity and diversity of life. In Campbell Biology (10th ed., pp. 10-15). San Francisco, CA: Pearson.
Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., and Jackson, R. B. (2011). The Darwinian revolution challenged traditional views of a young Earth and unchanging species. In Campbell Biology (10th ed., pp. 463-465). San Francisco, CA: Pearson.
Than, K. (2015, May 13). What is Darwin's theory of evolution? In Live Science. Retrieved from http://www.livescience.com/474-controversy-evolution-works.html.
University of California Museum of Paleontology. (2016). Early concepts of evolution: Jean Baptiste Lamarck. In Understanding evolution. Retrieved from http://evolution.berkeley.edu/evolibrary/article/history_09.
Wilkin, D. and Akre, B. (2016, March 23). The theory of evolution - Advanced. In CK-12 biology advanced concepts. Retrieved from http://www.ck12.org/book/CK-12-Biology-Advanced-Concepts/section/10.19/.
Wilkin, D., and Akre, B. (2016, March 23). The voyage of the Beagle - Advanced. In CK-12 biology advanced concepts. Retrieved from http://www.ck12.org/book/CK-12-Biology-Advanced-Concepts/section/10.17/.