How Evolution works

The story of life on Earth is a story of change. Living things have transformed the atmosphere and the climate. They've survived the movements of the continents, and the rise and fall of the seas. And they've adapted to these changes over the long course of Earth's history, through a process that still continues today: evolution.

Evolution, in the simplest terms, just change over time. And it's responsible for the shape of the tree of life, for creating the diversity that we see in the fossil record as well as in modern ecosystems. It's the very foundation of our understanding of biology, and it continues to help us make sense of the world around us.

Evolution was revolutionary when it was first introduced. The first to put all of the pieces together into a unified explanation that would radically alter our understanding of life on our planet were Charles Darwin and Alfred Russel Wallace. But our understanding of evolutionary theory didn't stop there. In the last 160 years, we've learned what Darwin and Wallace didn't know, and we've figured out a lot about how evolution actually works like how it can produce the incredible array of animals you see here, and how we know they're all related.

Darwin and Wallace were both British naturalists whose thinking about the natural world was deeply shaped by long voyages of exploration. Darwin famously sailed to South America and the Galapagos Islands, and Wallace went to South America and Southeast Asia. Together they observed an unbelievable diversity of life. They observed how very similar organisms seemed to be somewhat restricted in a way that made them ideally suited to their surroundings. In the Galapagos Islands, Darwin observed the different shapes in the beaks of finches on different islands. For Wallace, it was the differences between monkeys living on different riverbanks in the Amazon. And they both recognised that the patterns they observed meant that these species all probably arose from the same place a common ancestor.

They realized the bodies of these animals had been formed over time by the conditions in their environments, occurring in the different forms they found on different islands and riverbanks. Darwin and Wallace's ideas were deeply influenced by other, earlier thinkers, in natural history, geology, and even economics. Scholars like Georges Cuvier, Charles Lyell, Jean-Baptiste Lamarck, and Thomas Malthus helped establish the ideas that were important to evolutionary thinking, that the Earth was very old, that species seemed to change and go extinct over time, and that individuals fought over limited resources. Darwin and Wallace used these insights along with their own observations to both arrive at the same mechanism by which species evolve: natural selection.

In a paper read to a meeting of scientists in London in 1858, their theory of natural selection was presented based on a series of principles: The first key idea was that, in a population of living things, natural variations will occur, and as a result of those changes, some members of the population will survive and reproduce more than others. Then, they posited that those that survive and reproduce will pass on their traits to their offspring. And this meant that traits that give individuals an advantage in a certain environment will get passed on more often. As a result, more members of the population will have that trait. Therefore, gradually and over time, this will result in certain traits showing up more or less often in a population.

Today, when this series of events happen within a species, we call it microevolution. It's how a single species respond to changes in the environment. On a broader scale, we call it macroevolution. This is how these changes accumulate over long periods of time to produce entirely new body plans, new species, and the grander patterns of diversity in the tree of life. One of the most incredible things about the development of the theory of evolution by natural selection was that Darwin and Wallace didn't have a good explanation for how traits were passed from parent to offspring. Genetics as a field was still a long way off, and neither of them was aware of the experiments that were being done on pea plants at the time, by a Czech monk named Gregor Mendel.

In the 1850s, while Darwin and Wallace were putting all the puzzle pieces of natural selection together, Mendel was breeding peas at his monastery to try to figure out how heredity worked. And he figured out that traits didn't simply blend together when living things reproduce. Instead, only some were inherited as discrete traits by different numbers of offspring. Mendel's results were rediscovered around the turn of the 20th century when a new generation of biologists was investigating genetic. And it was a new wave of researchers that brought our understanding of evolution to the next level.

One of these scientists was American biologist Thomas Hunt Morgan. Instead of peas, he bred flies, and in 1910, he bred a fly with an odd trait. It had eyes that were white, instead of red. What's more, he was able to breed that white-eyed trait back into the parent population. Morgen had discovered another key driver of evolution by natural selection: mutation. He realized that the fly had undergone a random change in its genes that made it different from the rest. So Morgan theorized that mutations were a source of variation in living things and that it was the source of the variation that natural selection acted on. Beneficial mutations would be passed on, he thought, and detrimental ones would eventually disappear.

So by early 1900s, we'd already recognized two of the four major force of evolution: Darwin and Wallace gave us natural selection and Morgan brought mutation into the mix. It wasn't until the 1920s that things would really start to come together through the work of three of the founders of the field of population genetics: Ronald Aylmer Fisher, John Burdon Sanderson Haldane, and Sewall Wright. Fisher and Haldane both looked at natural selection mathematically, especially in a large population, using Mendel's ideas about inheritance to figure out how often and how fast natural selection worked on variations. It was Haldane who did the math that explained the transition of England's famous peppered moth, in which a gene for dark colour spread quickly, as pollution darkened the bark of the trees they lived on. Studies like this led Fisher and Haldane to conclude that natural selection acted slowly, but also uniformly, in large populations. Meanwhile, in the US, a geneticist named Sewall Wright was thinking about how evolution worked in smaller, more isolated populations. He did some research breeding animals like cattle and guinea pigs. But it was his mathematical studies of genetics that led him to uncover another key idea: genetic drift.

This is the idea that the frequency at which certain genes appear will sometimes change, totally by chance, and randomly, and Sewell found that this has a greater effect in smaller populations than in larger ones. Another idea that came up around this time, in the late 1930s, is gene flow the movement of genes between populations, by way of migration. So, when members of one population of a species say, panthers from Texas breed with members of another population like panthers in Florida that will change the makeup of the gene pool in the Florida population. And this, too, is a driving force of evolutionary change. Together, the work of Fisher, Haldane, and Wright showed that natural selection acting on genes was the most likely explanation for how evolution works.

And in 1937, another biologist brought together all of the evidence from genetics and natural history to show how evolution by natural selection could produce new species. And this enabled us to make the enormous conceptual jump from microevolution to macroevolution. His name was Theodosius Dobzhansky, and he had worked in Hunt's fly lab. He'd found that fly population from different countries seemed to be genetically different, even though they were considered to be the same species. But, these flies weren't so good at reproducing with each other. So he wondered if they were actually different species. And this took the scientific conversation all the way back to the 1800s, and the once-novel idea that evolution could eventually, gradually produce new species. From his experiments, Dobzhansky produced a theory about how new species originate.

Mutations happen naturally in population, creating variations that can stick around if they're beneficial or just neutral. And if populations are isolated, these variations can remain within a single group, with new mutations popping up. but none of these would spread to the rest of the species. Over time, this would make one group genetically distinct from others, potentially causing problems if it tried to interbreed with others. And given enough time, it would lose the ability to interbreed with other population entirely. It would become a new species.

This was the beginning of "the Modern Synthesis," a collaboration by many evolutionary biologists of the time to explain large-scale patterns of evolution. And while the Modern Synthesis has changed over time, it's still the framework for our current understanding of how evolution works. In 1953, we added a better understanding of how genetics works, through the discovery of the structure of DNA and how it functions. So, now we know that mutations randomly happen when DNA is copied incorrectly during replication. Now we also know that natural selection is only one of the mechanisms of evolution, along with mutation, genetic drift, and gene flow. And it's this knowledge that allows us to witness microevolution taking in studies of bacteria that develop resistance to antibiotics.