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The Evolution of Evolutionary Thought: Part 6, Natural Selection

Darwin fish symbol

The theory of natural selection—you’ve probably heard of it, but what exactly does it say? Learn what Charles Darwin’s theory involves in this post!


In a previous post, you learned that evolution is a theory, and a theory is something that all the scientific evidence supports. In another post, you learned about the theory of evolution. But, how did the theory of evolution come about? What was the evolution of evolutionary thought? Well, in order for the theory of evolution to be developed, some ideas needed to change.

In the first post in this series, we looked at ideas about the relationship between humans and other forms of life. In the second post, we looked at ideas about the age of the Earth. Next, in the third post, we looked at ideas about fossils and what they implied about life. Then in the fourth post, we looked at ideas about adaptation to the environment. And in the fifth post, we explored Charles Darwin’s ideas, and how he integrated all these ideas we discussed into his theory. In this post, we will explore Darwin’s theory of natural selection.

The Theory of Natural Selection

In each generation, there are more offspring than can survive, due to limited resources. So, there is competition for these resources, and organisms that have certain traits will have an advantage over those who don’t.

Natural Selection. Image of a green moth and a brown moth on a green background.

What traits are an advantage or not is determined by the environment. You can see that in this environment in the image above, being the color green has the advantage, because it’s hard for predators to see you against the green background.

Natural Selection. Image of a green moth and a brown moth on a brown background.

What is beneficial in one environment may be detrimental in another environment. You can see that now in this other environment, being brown has the advantage, because it’s harder for predators to see you.

Traits are passed down to the next generation. Organisms with beneficial traits create more offspring than others, and so over time, the beneficial traits are more common. Detrimental traits aren’t passed on as much, and so they eventually get weeded out.

Over long periods of time, the population’s later generations may be so different from the earlier generations, that a new species is formed. New species can also be formed by geographic isolation. If populations get separated from each other by something like a mountain range or river, then each group adapts over time to a different environment. Over long periods of time they may become different species. 

Natural Selection in Action

Natural Selection. Image of light grey trees with light grey moths and dark grey moths.

So, that’s the theory of natural selection. Now, here’s a well-known example of natural selection in action. There are 2 kinds of a certain moth in England and one is speckled light gray and one is a dark gray. Back in England’s past, the light-colored moths rested on trees covered in lichen. They were hard to see so they survived more than the dark grey ones, which were highly visible and eaten by birds. So over time, there were more light gray moths than dark grey ones. 

Natural Selection. Image of dark grey trees with dark grey moths and light grey moths.

But then the environment changed, and England became industrialized. Coal dust from factories covered the tree trunks, which turned them dark grey. Now, the light-colored moths were easier for birds to see, and they were eaten, but the dark-colored moths could remain hidden. So, then there was a lot more dark gray colored moths compared to the light gray ones. Later in time, clean air acts were passed, and the air became cleaner. Then, the light gray moths became more frequent again. So, this is a real-life example of natural selection!


In summary, the theory of natural selection says that there is competition for resources, and some organisms with certain traits will have an advantage over other organisms. These organisms will live and reproduce, and pass their beneficial traits on to the next generation. Over time, the population will contain more of the beneficial traits. Then, over a long period of time, the later generations may be so different from the original ones that a new species has been created.

Want to learn more about evolution?

Just take my Udemy course, “Exploring Genetics and Evolution Through Physical Anthropology: Anthropology 4U.”

Thanks for reading!