Evolution | Middle school biology | Khan Academy

• [Speaker] How many different species or kinds of birds are there?

Take a guess. 100, 1,000, more? Well, biologists have estimated that there are at least 10,000 different species of birds all around the world, and some biologists think that there are even more, up to 18,000 different species.

Birds are incredibly diverse. Think about the differences between an eagle, a parrot, a hummingbird, a duck, and a penguin. Some of them are small and some are big, and take a look at the differences in their beaks or bills and their feathers.

And not all of these birds can fly, either. Penguins use their wings like flippers to swim underwater. Collectively, these 10,000 plus different species of birds are a product of evolution, which is the change in heritable traits that occurs in a group of organisms over many generations.

But what are these heritable traits exactly? Heritable traits are those that are passed from parents to offspring via genes, and they include things like the colorful patterns and plumage or the webbing or toes and claws of bird feet.

And even though offspring inherit traits from their parents and look pretty similar to them, over many, many generations, these heritable traits can change. Evolution is made possible by genetic variation or differences in genes within a population.

Genes are passed from parents to offspring and they affect how an organism develops, how it functions, and even how it looks. Genes can come in different forms or alleles, which can lead to different versions of a trait.

For example, in city pigeons, also called rock doves, which you might see flying around, different alleles cause different eye colors such as orange, black, and white. Genetic variation causes organisms within a population to have a range of traits like differences in plumage or eye color.

Genetic variation can come from lots of different sources, one of which is through sexual reproduction. This type of reproduction causes offspring to have different sets of traits compared to their parents because they receive genetic material from both of their parents.

In other words, the offspring aren't an exact genetic copy of either parent. In this example of rock doves, parents carry different alleles that affect their plumage, and, in turn, their offspring can inherit different sets of traits and may look quite different from their parents.

Now, let's take a look at that dove example with the plumage differences and think about it at the population level with lots of doves. You could imagine that those plumage traits can change over many, many generations of parents and offspring in a large population.

You might see new traits appear like different colors or different patterns and feathers, or existing traits like dark colors may become more or less common. When this type of change occurs or any change in heritable traits, that group of organisms has evolved.

Over many generations, small differences in traits between generations can add up, and over time, over thousands to millions of years, evolution can give rise to new types of organisms.

For example, over many generations, one population can evolve into two different species, like the rock dove and the hill pigeon. Even though these two types of birds look pretty similar to one another, they are a distinct species that have been changing along independent evolutionary paths for about a million years.

Over the billions of years that life has existed on Earth, evolution has given rise to the amazing diversity of organisms on our planet. We have over 10,000 species of birds alive today, and there are even more bird ancestors species that once existed. Ancestors that were literally dinosaurs, but, that's a topic for another video.