Today is the 200th anniversary of the birth of one Charles Darwin, a would-be-cleric-turned-naturalist whose seminal work On The Origin of Species turned the study of nature into a science. Although Darwin was by no means the first to propose that all life descended from a common ancestor, and although his theory of evolution by natural selection has been challenged and augmented since its initial publication in 1859, his name remains synonymous with evolution because his was the first and most enduring theory to propose a solidly scientific explanation for the remarkable diversity of life on Earth.
Let me explain what I mean when I say that the theory of evolution made biology a science. The scientific use of the word theory is very different from the colloquial use. You might casually say that you have a theory about why your friend is late for your movie date or why your car is making a funny noise. In this usage, “theory” means “guess” or “conjecture”. Not so a scientific theory. According to the U.S. National Academy of Sciences, “the word theory refers to a comprehensive explanation of an important feature of nature supported by facts gathered over time. Theories also allow scientists to make predictions about as yet unobserved phenomena.” The cause of gravity, the wave and/or particle nature of light, relativity – these are scientific theories, treated as fact. A scientific theory has a very important defining characteristic: the ability to make predictions. The rigorous testing of the predictions made by a theory is science. Prior to the theory of evolution, biology was basically the practice of collecting and cataloging Earth’s life forms. Naturalists could tell us what sorts of species were out there, but not why there were species. Think about it. Why should there be species at all? Why does every species seem to be uniquely adapted to its environment? Why do some species thrive while others don’t? Why are there fossils of strange creatures that don’t exist anymore? Scientists are able to answer questions like these with the theory of evolution by natural selection.
I doubt that you could find anyone in the English-speaking world who has never heard of evolution, but I doubt that many of them could explain what Darwin’s theory actually claims. For their benefit, I present to you here the truncated version (with many thanks to my professor Eduardo Wilner for his insightful classes on evolutionary theory and philosophy of biology).
- Observed: Living things reproduce, and when they reproduce they tend to make more than one offspring. If this were not the case, their species would die out as a matter of simple arithmetic.
- Observed: Resources are limited. The planet is finite in size. Even if there were only one type of reproducing animal and only one type of food, the planet would eventually be overrun.
- Observed: Animals will die.
- Conclusion: Given limited resources, staying alive is a competition. If one animal is more suited to staying alive than another, it will outlive its competitor (this is what we call fitness).
- Observed: Offspring tend to resemble their parents.
- Conclusion: There must be some manner in which the traits of the parent are passed on to the offspring. (Keep in mind that Darwin wrote “On the Origin of Species” before Mendel’s discovery of genes had been published, and nearly a century before Watson, Crick, and Franklin elucidated the structure of DNA).
- Conclusion: More animals that are fit will live to reproduce than will less fit animals. The traits of surviving animals, including traits of fitness, will be passed on to their offspring. Eventually, these traits of fitness will become widespread in a population and less fit traits will disappear.
- Observation (and this one’s the clincher): Creatures change with time. Darwin didn’t know about DNA or genes or mutation, but he observed (as other naturalists had been observing for centuries) that creatures are remarkably capable of changing with their environment, and that entirely new traits can spring into existence. He proposed that their must be random changes in creatures and that these changes can be passed on to offspring (that they are heritable).
- Conclusion: Changes that increase fitness will be retained; changes that decrease fitness will be weeded out. Over time, this will give rise to creatures that don’t resemble each other at all, even though they descended from the same ancestor.
So there you have it: evolution by natural selection can be summed up as heritable variation in fitness.
How does evolution by natural selection hold up as a scientific theory? Let’s look at the criteria:
- Does it explain the world around us? Yes. It offers a plausible explanation for why there are many different types of life on earth, and why each type of creature is remarkably adapted to its environment.
- Does it make predictions? Yes. Since the whole theory is based on logical conclusions drawn from observations, we can draw further conclusions that should be true and check whether these conclusions hold up with what we see in the world.
- Are the predictions testable? Yes. For example: if all life on earth has a common ancestor, we would expect to see common traits present in distinct species. Do we observe this? Certainly. Mammals from mice to bats to whales to people have remarkably similar skeletons (i.e. fingers and toes). All life on earth uses the same 4 molecules to make up their DNA. There is remarkable similarity in the DNA codes for making proteins, and there is often further similarity between species in the sequence and structure of specialized proteins.
An important point here: nothing can ever be proven in science. Every time an observation matches a prediction, we say that the hypothesis (and thus the theory) has been supported. If the observation does not match the hypothesis, then the hypothesis is proved wrong. If hypotheses are disproved enough times, one should begin to doubt the theory. If, however, hypothesis after hypothesis and prediction after prediction are supported by the observations, the theory grows in strength until it is accepted as practical fact.
I am well aware that many people do not find Darwin’s story compelling; some find it downright offensive. Future posts will discuss why that is. But in honor of Darwin’s birthday, I give you the above with the hopes that you will think twice before ever saying “Evolution is just a theory”.