Quick, close your eyes, and imagine you’re in the forest.

You’re having a nice stroll, taking in the scenery, maybe humming a little tune when suddenly you spot a black bear. Now, what color is the bear in your imagined scenario?

Black? Are you sure about that? I mean, you might be right.

But it might be a blonde black bear, or a chocolate-brown black bear, or even cinnamon-red. In the coastal regions of rainforests in British Columbia, there are even white black bears. These “Spirit Bears” as they are sometimes called were kept secret by the Indigenous First Nations in order to protect them from European fur traders.

The point is, despite their name, the black bear’s coloring comes down to its genetics, which is influenced by the gene pool of its population - or, the group of individuals, living in the same area, that are able to breed with each other. See, at the population level, organisms undergo microevolution – evolution that is expressed through a change in gene frequency usually within a short period of time. Genetic changes in individuals are reflected in the population, which can help us understand why some black bears may actually be a rainbow of colors, and even how some tigers really can change their stripes.

Hi! I'm Dr. Sammy, your friendly neighborhood entomologist, and this is Crash Course Biology.

Now I want you to close your eyes and imagine the funkiest theme music that has ever lived. Did you imagine this: [THEME MUSIC] Whether we’re talking about tigers, tarantulas, or trees, it’s populations that evolve, not individual organisms. But each organism’s fate —whether it lives or dies, whether it reproduces or doesn’t — plays a role in what happens next in its population.

That’s because each organism is a member of their population’s gene pool. That’s the sum total of all the genetic material, spread across the entire population of a given species in a given area. The gene pool includes, as you’d expect, genes: specific regions of DNA that encode instructions for different jobs, like making proteins or growing eyebrows.

But more specifically, the gene pool has alternate versions of genes, called alleles. Diversity is the currency of evolution and alleles are unit of that currency. Like, in the genetic code of cats, the piebald gene has alternate alleles associated with kitty coloring.

Depending on the alleles, they might result in no white spots, or just a pair of mittens, or the look of a permanent tuxedo. Aw, look at you, Mortimer. Always ready for a black-tie event, but never invited to one.

Or the dilute gene, for example, affects how pigmented a cat’s fur is, no matter the color. So, with this set of alleles, an orange cat would be a bright, rich shade of sherbet. But with this set of alleles, an orange cat would be pastel, like peaches and cream.

It’s the same gene - the same region in the genetic code - where the allele variation dictates outcomes that vary within a population. In humans, almost nothing in our appearance gets determined by a single gene. But like cats and many other organisms, we inherit alleles from our parents.

Your inherited alleles decide your eye color or your hair color, but they also influence your risk of developing certain diseases or allergies. Each organism has a set of alleles unique to them, and those alleles are tally marks in the bigger gene pool of their population. Depending on which organisms survive and pass on their DNA, some alleles become more common over generations, while others become more rare.

These fluctuations are the stuff that microevolution is made of. Sometimes, a trait takes hold because it helps individuals that have that trait survive and reproduce successfully in their environment—a process called natural selection. You may have heard the term natural selection before, it’s one of those rare scientific phrases that’s gained mainstream popularity like “chemicals,” or ” climate change,” or “colony collapse disorder,” And, like any science-y word that makes it big, it comes with some misconceptions attached.

We’ll talk more about natural selection, and its less famous sibling artificial selection, in our next episode. But for now, just know that natural selection is the way that species adapt to their environment – it's one of the driving forces behind evolution. But natural selection isn’t the only force driving evolution.

There’s also sheer, random chance. A process called genetic drift happens when chance events cause alleles to spike or decline from one generation to the next. You can think of a population’s gene pool kind of like a boat, rocking back-and-forth in the ocean, holding a cargo of alleles.

A tiny tugboat is more likely to get tossed around, and more vulnerable to losing cargo or capsizing altogether. But a big cruise ship has a better shot at staying steady in the rips and rolls of the ocean. It might lose some deck chairs if a tropical storm blows through — but it still has some reserves.

In a similar way, larger populations aren’t as vulnerable to genetic drift. Their allele frequencies aren’t as likely to change due to chance events — so they keep much of their diversity. But small populations are more likely to see ups and downs in allele frequency, which can sometimes remove a color of fur, for example, from that population’s equation entirely.

Not necessarily by selection - it’s just up to random chance. The founder effect is one powerful way that genetic drift happens. That’s when a small population splits off and becomes isolated from others, founding a new population.

Let’s take a look at the founder effect in the Thought Bubble… In eastern India, you might meet a tiger of a different stripe. Not the cats with the classic orange suit and black pinstripes, featured famously on cereal boxes. But stripes so broad that the pattern almost looks reversed.

Black background, orange stripes. Over one-third of the tigers in India’s Similipal National Park have this pattern—which is exceedingly rare anywhere else. These so-called “black tigers” likely owe their pattern to a special recipe: a dash of random chance, served up in a tiny, isolated population.

Tigers with this pattern have two copies of an uncommon allele, associated with a gene also linked to dark, splashy patterns in cheetahs and domestic cats. The rare allele has a high frequency in the Similipal tiger population, comprising an estimated 58% of the alleles in the gene pool. And that’s because the population in this area probably began with just two or three founders.

And they’ve been isolated from other tiger populations for at least ten generations, so there are no new potential mates to mix things up by introducing new genes through reproduction. With each generation, genetic drift has happened —increasing the chances that a tiger inherits dark alleles from their parents. That’s the power of the founder effect.

Given the right ingredients, even a rare allele can drift into a common one. And even a tiger can change its stripes – well, a population of them at least. Thanks, Thought Bubble!

So, the founder effect happens when a smaller population becomes isolated from the original. But there are other events that can cause changes in a population. Catastrophic loss from an outside force such as a fire or a hurricane, can cause alleles to randomly drift because the population loses individuals regardless of how well they were adapted to the environment.

This drastic depletion of the gene pool is called the bottleneck effect because it’s kind of like shaking just a few marbles out of a bottle. The original bottle might have a wide variety of colors: yellow, blue, orange, whatever the heck “chartreuse” is. But just a few marbles tumble out onto your hand, and in a narrower range of colors.

It’s the same story with alleles. The proportion of alleles in the surviving population will be different than it was before the population loss that led to the bottleneck. And in a newly tiny population, those alleles will be more likely to drift for the generations to come.

Today’s northern elephant seals, for example, number at over 200,000-strong, but they’re all descended from the same twenty or so individuals that survived being hunted to near-extinction in the 19th century, so there’s not a ton of variation in their alleles. These genetic dips and dives can happen to any population of living things: whether that’s tiny bacteria or an endangered fungus. And there’s yet another force that can nudge allele frequencies, even counteracting the effects of drift: gene flow.

Gene flow happens when individuals move and breed between different populations, bringing new alleles in or out. Like, if a separate population of tigers did somehow find the isolated black tigers of Similipal, they might breed with the locals, infusing the gene pool with new alleles. But new alleles can also arise spontaneously, without any movement between populations at all.

Like mutations, which are random changes in genes that can happen because of mistakes when copying DNA. Or there can be random changes in chromosomes — the DNA molecules that hold the genetic information — resulting in genes switching locations or even duplicating. And that can fall on a spectrum between actively harmful, fairly neutral, or even pretty useful.

Like, we humans and other mammals share a long-ago ancestor that probably only had one gene for detecting odor. But this gene has duplicated many times since then. So now we’ve got around 400 odor-distinguishing genes.

All the better to select the right porta-potty at that outdoor concert. Hope that toilet humor wasn’t too on-the-nose…for you …you get it. On the nose…no.

I’m sorry. So, yeah, there are multiple forces that can affect the microevolution of a population. Like, the rare allele responsible for white coats of Spirit Bears probably arose due to genetic drift, but it might also have an advantage in their environment.

Spirit Bears might have an edge at salmon fishing, for example, making them harder to spot from a fish’s perspective. Like, maybe they look like clouds. Biologists need a way of measuring and modeling all these changes.

And for that, they have the Hardy-Weinberg equation. This equation assumes a hypothetical scenario where a population isn’t evolving and its allele frequencies are totally steady. A population like that would have to be enormous, with no gene flow and no mutations.

All mating would be random, and no organism would be better at reproducing than any other. This sort of population would likely never arise in nature, but biologists use this imaginary, never-evolving equilibrium as a baseline to measure and compare genetic changes. By plugging in actual allele frequencies, which can be measured by counting how many times the allele appears in a population and dividing by the total number of copies for the gene, the Hardy-Weinberg equation can reveal whether real-life populations are evolving.

In fact, the Hardy-Weinberg equation is so important, that it is the theoretical bedrock of a whole field of biology, called population genetics. Which is all about nerding out over the genetic composition of populations and their changes over time. But hey, is there any scientific field that doesn’t include nerding out over something cool?

I mean, that’s kind of our whole thing. It’s thanks to the enthusiasm of scientists over things like genes, natural selection, and microevolution that we understand why evolution doesn’t occur with the individual, but with the population. And why a tiger might not be able to change its stripes – but a population of them can!

With each new generation, different forces can push some alleles to become more common and some alleles to become more rare. These spikes and declines can happen due to chance. But alleles can also become more common because they help organisms survive and reproduce in their environment.

In our next episode, we’ll talk more about how selection — not just random chance — plays a role in evolution. I’ll see you then! This series was produced in collaboration with HHMI BioInteractive.

If you’re an educator, visit BioInteractive.org/CrashCourse for classroom resources and professional development related to the topics covered in this course. Thanks for watching this episode of Crash Course, which was filmed in Indianapolis, Indiana and was made with the help of all these nice people. If you want to help keep Crash Course free for everyone, forever, you can join our community on Patreon.