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Darwin, evolution, & natural selection

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.
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.
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.

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.
Four drawings of the heads of finches. Drawing 1, labeled Geospiza magnirostris, has a very large, wide beak. Drawing 2, labeled Geospiza fortis, has a large beak that is not as wide as that of drawing 1. Drawing 3, labeled Geospiza parvula, has a small beak that is not as wide as that of drawing 2. Drawing 4, labeled Certhidea olivasea, has a long, narrow beak.
_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 world2.
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 later3! 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.

Evolution

A speciation chart with modern-day species along the top, level 14, and the ancestors from which they arose at the bottom, level 0. There are 11 ancestor species at level 0. Six of the ancestor species end before reaching modern day. One ancestor species does not branch at all before reaching modern day. The remaining 4 ancestor species show branching at many steps, resulting in many separate species. There are 15 species at modern-day, level 14.
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.)
    A diagram with text reading parents pass on heritable traits to their offspring. On the left a dark blue and a light blue butterfly are crossed to produce offspring with wings of varying shades of blue. On the right a dark red and a light red butterfly are crossed to produce offspring with wings of varying shades of red.
  • 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.
    A diagram with a box reading limited resources. Arrows point away from the box to bubbles reading lack of food, lack of habitat, and lack of mates. Text below reads …not all individuals will survive and reproduce. A group of 16 butterflies with wings of varying shades of blue and red is shown. A text bubble reading gleep! comes from 4 of the butterflies.
  • 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.
    A group of 16 butterflies with wings of varying shades of blue and red is shown. A text bubble reading Hey, are you red? That's pretty sweet! comes from one of the blue butterflies. A text bubble reading Whoa! Love that blue wing color comes from one of the red butterflies. Text at the bottom reads Butterflies do not actually talk! Cartoon for cute illustration purposes only. A smiley face is shown next to the text.
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.

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.
A 3-panel cartoon, each showing a hawk flying above a group of mice. In the first panel there are 3 black mice and 6 tan mice. The black mice match the black ground. Text reads A population of mice has moved into a new area where the rocks are very dark. Due to natural genetic variation, some mice are black, while others are tan. An arrow points from the first panel to the second with text reading Some mice are eaten by birds. In the second panel there are 3 black mice and 2 tan mice. Text reads Tan mice are more visible to predatory birds than black mice. Thus, tan mice are eaten at higher frequency than black mice. Only the surviving mice reach reproductive age and leave offspring. An arrow points from the second panel to the third with text reading Mice reproduce, giving next generation. In the third panel there are 7 black mice and 2 tan mice. Text reads Because black mice had a higher chance of leaving offspring than tan mice, the next generation contains a higher fraction of black mice than the previous generation.
_Schematic based on similar schematic in Reece et al. 4. 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.

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.

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.

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.

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.

Want to join the conversation?

  • leafers ultimate style avatar for user Sunit
    In the example of the mice and hawks, what if due to natural selection the hawk's ability to spot out black mice increases? Will that cause the mice to 'counter evolve' and will this cycle of evolutions on the prey and predators' part continue?
    (20 votes)
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  • mr pink red style avatar for user Abhijithlal
    How would have homo sapiens evolved from the apes , why did the characteristics of standing erect dominate over bending forward. would it be a transmission of the lifestyle of apes eating food from the ground to hunting ?
    (13 votes)
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    • male robot donald style avatar for user Quang Luong
      From the Homo Sapiens book, Yuval shared a very simple but reasonable concept that standing on 2 legs on the ground will give more broad and wide of view instead of 4 legs.

      Climbing on to the tree will give more advantage but more effort just to check the surroundings. So standing on 2 legs dominated over time for our species to become.
      (2 votes)
  • blobby green style avatar for user Sasha Scarlet Scott
    If natural selection is favouring traits which are beneficial for the environment then couldn't it be stated that as a human choosing to have less children is a trait which is favourable to pass onto your children?
    (12 votes)
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    • aqualine ultimate style avatar for user Talos
      Natural selection favors traits that are better for the ORGANISM'S survival and reproduction not for the environment. So actually the inverse would be true. A human who has a gene (not conscious choice, as genes - not thought - are passed onto offspring) that makes him susceptible to having more children will pass on that gene (as it is likely that more of his children will survive than someone who has, say 1 child) to more offspring and will thus be selected for.
      (27 votes)
  • starky tree style avatar for user i learn and that's it
    I have a doubt, is it possible for natural selection to reduce the size of an organism? If possible, are there documented cases of this?
    (4 votes)
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    • female robot grace style avatar for user tyersome
      Of course, natural selection just selects for the organisms that will be the most fit (have the most offspring) in a particular environment.


      One situation where selection can often favor being smaller is on islands and other isolated ecosystems where limitations in food supply and the inability to migrate can mean that smaller organisms are better able to survive famine.

      This phenomenon is known as "insular dwarfism" — there is an entire wikipedia article on this subject including numerous examples here:
      https://en.wikipedia.org/wiki/Insular_dwarfism

      Perhaps the most dramatic example of this are the numerous (extinct) species of dwarf elephants:
      https://en.wikipedia.org/wiki/Dwarf_elephant


      Does that help?
      (12 votes)
  • blobby green style avatar for user 19kmloomis
    How does natural selection occur in both plants and animals?
    (4 votes)
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    • leafers ultimate style avatar for user Stefan Vriend
      Natural selection is a mechanism that makes any species - be it a plant, animal, fungus, bacteria, you name it - better adapted to their environment. For herbivores (i.e. plant eaters), the plants are indeed part of their environment. In such a situation, the evolution of the plant may affect the evolution of the plant eater (which may in turn affect the evolution of the plant). This process (or feedback) is called coevolution.

      Coevolution may also occur with predator and prey, parasite and host, and many other cases where two species are dependent on each other.
      (5 votes)
  • purple pi teal style avatar for user StellaRaymond
    All theories must be testable with many pieces of evidence to back it up. Is there any examples of famous experiments used to test this theory? Or any sources the support this theory?
    (3 votes)
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    • starky ultimate style avatar for user ++§ Αλεκσανδαρ
      That's not how science works. You can come up with any ridiculous theory and find as many evidences for it you please. What you should do is to create an experiment that can overthrow the theory. If your theory falls, it's dead. If it survives, it lives as long as some experiment doesn't overthrow it.

      As for the theory of evolution, Darwin and Wallace (and some people before them who are usually not given any credit whatsoever) came up with it after observations in the nature. Since then, theory of evolution has survived every attempt of overthrowing. Evolution is a long process, so it's not easy to come up with an experiment to test it. That's why there are still many people who are really skeptical about it.

      Probably one of the most famous experiments (or projects...) that has anything to do with evolution is the domestication of red fox in Siberia. Check it out:
      https://en.wikipedia.org/wiki/Russian_Domesticated_Red_Fox
      (5 votes)
  • blobby green style avatar for user steven_demmons
    What are the three observations that Darwin's concept of natural selection was based on?
    (2 votes)
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    • piceratops sapling style avatar for user funkyhedgehog
      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 (colour, size, shape, etc.), and many of these features will be heritable.
      (8 votes)
  • blobby green style avatar for user icsHlucie
    on the second to the last paragraph,i would like to ask if such heritable differences occur to the point that the groups can have different numbers of chromosomes? If so, how?
    (3 votes)
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    • winston baby style avatar for user Ivana - Science trainee
      Not sure about animal species.

      In-plant kingdom it occurs and helps the creation of new species or more 'potent' for agriculture, such as watermelons ets- Polyploidy (multiplying sets of chromosomes) do exist in plant kingdom.


      As for animals, polyploidy is incompatible with life. Aneuploidy, on the other hand, is compatible but always leads to errors and diseases in animals.


      You propose some error/event occurring where two chromosomes fuse and it results in one less or 2n-2 chromosome garniture resulting in evolving species?

      Not sure about genetic mechanisms, maybe that is possible in unicellular organisms.
      (4 votes)
  • starky sapling style avatar for user Pavit Philip
    Is there other theory that doesn't agree with Darwin's theory of evolution
    (2 votes)
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  • leaf orange style avatar for user dkh5pie
    In the example of the mice and predatory birds, why wouldn't the birds also evolve--better eyesight, have to fly lower, etc. to make catching the black mice easier? At least as far as predators go shouldn't natural selection even things out over long periods of time?
    (3 votes)
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