Hank: Sometimes, the way we use words in conversation is different from the way scientists use them in a formal setting. Which is fine, but it can cause confusion when you’re trying to talk about science.

Evolution as a subject is especially vulnerable to these kinds of misunderstandings. Like, you might have heard someone dismiss evolution as “only a theory.” But in science, there’s nothing “only” about theories. So here are some of the most common misconceptions about evolution — and why they are so wrong.

You’re probably familiar with the basics of evolution: It’s a change in the genetic makeup of a population of organisms. Different organisms have different genetic mutations, which create the variation that leads to evolution. If an organism dies without leaving offspring, its genetic material is gone from the population, so the gene pool has changed. If another organism has lots of offspring, its genes are over-represented in the next generation, which is also a change in the gene pool. Natural selection can cause those kinds of changes, but so can other factors, including random chance.

One of the most common misconceptions about evolution is that it is, quote, “just a theory.” And it’s true — evolution is a theory. But in science, the word “theory” means a lot more than it does in everyday conversation.

Often, when someone- like when I say I’ve got a theory, it means that I’ve got a hunch or I’ve got a guess, but when scientists talk about theories, that’s not what they mean. In science, a hypothesis is a prediction of what might happen based on available evidence, and a theory is a whole collection of hypotheses.

Theories fit all the facts. They’re a sort of framework for thought, and you use them to make new predictions. A strong scientific theory can expand to fit new evidence -- it can accommodate things that aren’t necessarily even known yet.

For example, Darwin knew practically nothing about genetics. He understood that variation among individual organisms could be inherited, but he didn’t know how. Even so, as knowledge of genetics grew in the late 19th and 20th centuries, that knowledge slotted neatly into our understanding of evolution. So, yes, evolution is a theory. It’s a rigorous framework of testable predictions that accounts for all known evidence, and can account for more evidence that we don’t even have yet.

Speaking of Darwin, another common misconception is that he invented the theory of evolution. It is true that in its current form, evolution traces its academic roots to Darwin’s book On the Origin of Species, but it’s totally unfair to say he came up with the whole idea.

Darwin didn’t “invent” evolution. Lots of people before him had noticed that groups of organisms can change over time, and many scholars proposed their own theories of evolution before Darwin. One of the best known is Jean-Baptiste Lamarck, whose work Darwin knew about and referenced.

Lamarck got the “what” of evolution right, but he got the “how” wrong. The “what”, that species were changing gradually over time, was totally true. As for the how, though, he thought organisms could pass on characteristics acquired during their lifetimes, which doesn’t quite gel with our understanding of genetics.

We do now know that a parent’s environment can have effects on its offspring through epigenetics, but those changes don’t seem to stick around in the genome and drive evolution the way Lamarck was imagining. So, Darwin’s contribution wasn’t the idea of evolution, it was the mechanism, the “how.” It was natural selection.

He even borrowed the phrase “struggle for existence” from an economic thinker, Thomas Malthus. By that, both of them meant that populations grow faster than the resources that support them, which leads to competition for limited food, space, and other needs.

Darwin also knew that not every individual is the same. Some individuals might be taller, or faster, or have stronger teeth. Those variations give them an edge when they compete for resources, and can also be passed down to offspring. Organisms with more advantageous variations tend to have more successful offspring, so those variations -- which we now recognize as genetic traits -- show up more and more in later generations.

Even the idea of natural selection wasn’t totally original to Darwin. An eighth century Arab scholar called al-Jahiz made similar observations about competition and predation. al-Jahiz still credited a creator with the ultimate origin of species, whereas Darwin’s framing called for only mechanisms that can be observed in the natural world. So it wasn’t like he was just ripping off that other guy.

But the evidence has been there for a long time for people other than Darwin to observe and think about. Scholarship never originates with just one person, in other words. But Darwin developed the foundation of the theory of evolution that we know today, and popularized it among other scientists.

There are more specific misconceptions about evolution, too — like the idea that there’s a missing link between humans and apes out there that we just haven’t discovered yet. Basically, people figure that apes are a lower or more primitive form of life, so there must be a link that’s somewhere in between more primitive apes and humans. But that’s not quite how evolution works, because there is no single, unbroken chain between our ancestors and us.

Evolution is about diversification, not linear progression. Far in the distant past, we humans share an ancestor with apes. Some of the offspring of that ancestor developed an interest in walking upright, banging rocks together, and eventually barbershop music. Others stuck with foraging for fruit and grooming insects off of each other. But for a while, those two populations weren’t all that different--they could still interbreed and swap genetic material.

For example, modern humans once interbred with other species of hominids -- the Neandertals and the Denisovans. You can still find their DNA in humans alive today. We know Neandertals and Denisovans aren’t our direct ancestors, but we also know they branched off from our common ancestor and continued to trade genes with us for a while.

Evolution tinkers around with organisms, but it makes branching trees, not long chains. So, there is no need for a “missing link” because there’s no one chain. It’s more of a bush, and we’re one leaf waving at the other leaves over there that look a little bit like us.

Then there’s the phrase “survival of the fittest.” It’s another thing that’s often associated with Darwin’s thinking, but it wasn’t original to him either. It was coined by sociologist Herbert Spencer. This phrase is considered... unfortunate by many evolutionary biologists because of how easily it lends itself to misunderstanding.

It conjures up an image of the most ruthless, cutthroat, strongest organisms trampling over their peers to, like, “win” at evolution. But “fittest” doesn’t necessarily mean strongest. If it did, you’d expect the most successful dinosaurs alive today to be like the T. rex. But instead, it’s, like, chickens. Good job, chickens!

“Fitness” is another one of those words whose rigorous scientific definition is different from its everyday one. In evolutionary biology, fitness refers to an organism’s capacity to thrive in its environment well enough to have offspring. And that can mean very different things in different contexts. In some environments, a plant with nice, broad green leaves is probably able to harvest more sunlight than its competitors, making it more fit and better able to reproduce. Whereas if you had broad green leaves, then you should probably get that looked at.

In other words, there isn’t any one adaptation that’s a magic bullet, and there isn’t one ultimate form for an organism. Something incredibly beneficial to one scenario could be useless or even detrimental in others. So “survival of the fittest” doesn’t always mean what people think it means.

Even if you totally understand natural selection and survival of the fittest, it’s easy to think that evolutionary change is always good for the organisms it’s happening to. Because natural selection does favor organisms that are fitter or more successful than others. But we defined evolution as a genetic change in a population, and sometimes that happens for literally no reason. Genetic mutations can be helpful or harmful, but sometimes they’re just trade-offs that are sorta good and sorta bad. And a whole bunch of the time, they don’t make any difference at all. That’s because a lot of our DNA doesn’t do much, and even the important regions usually have a little flexibility. So, sometimes a mutation doesn’t do anything except sit there, being a little different than it was before.

The laws of statistics have a kind of surprising effect on these neutral mutations — they contribute to a type of evolutionary change called genetic drift. Among all the organisms in the population, there might be ten different versions of the same gene that do the same thing. But since these trivial mutations don’t affect an organism’s ability to reproduce, natural selection doesn’t control whether a mutation is passed on.

Every time a parent reproduces, it’s just random chance which version of the gene gets passed on. So you might think that eventually, you’d just end up with more and more neutral variants of the gene within the population. But because statistics are kind of strange, that’s not what happens. Instead, over a long time and many generations, only one neutral variant of a gene will stick around, all the others vanish because of pure chance.

Genetic drift doesn’t have as much of an effect on larger populations, because there’s more room for the effects of random chance to equal out. But in smaller populations, it can lead to pretty significant genetic changes, just by chance. So the idea that evolution is always beneficial is just another misconception.

All of these misunderstandings stem from old, confusing phrasing. But the next time you meet someone who’s confused about them, hopefully you’ll be able to explain why they have been misled.

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