Evolution and natural selection

Definitions & Key takeaways

Darwin's theory of evolution by natural selection is the process by which organisms change over time to become more suited to their environment. The theory of evolution and natural selection explains how life on Earth has changed over time, including the development of new species and how different organisms are related. It also helps us understand the variety of life on Earth today and how it might change in the future.

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Evolution is the process by which populations change over time. Definitions matter - so population refers to a group of organisms within a species that live in the same place, and a species is a group with similar characteristics and are able to breed with one another.
Sometimes there are so many changes that accumulate in a population over time, that it leads to a whole new species, one that’s different from the original species.
This is the process that leads to an incredible variety of living beings in the world —biodiversity— that can come from a common ancestor.
Charles Darwin revolutionized the world of biology when he proposed natural selection as the mechanism by which evolution happens - based on his observations.
First off, he saw that each population contains individuals with traits that are different from one another. Secondly, he noticed that some individuals survived and reproduced while other individuals did not.
Finally, he noticed that some individuals had traits which seemed to confer a better chance of surviving and reproducing — and he called this fitness.
This laid the framework for natural selection, which states that individuals with different traits have differential rates of survival and reproduction.
And in this way, a population slowly changes over time, favoring organisms with reproductive advantages over time. As an example, in 18th century England, most of the trees were covered by lichens - which are a combination of fungi and bacteria - that made the tree trunks look white.
And around those trees, there were peppered moths - just chilling and hanging around. Some of them were white and others were black.
Now this lichen started making it easy for birds to see the black moths, whereas the white moths were camouflaged pretty well on the tree trunks.
As a result, the black moths got eaten up, while more of the white moths would survive and reproduce — this is called differential reproduction.
After a few generations, the frequency of white moths increased in the population, because the white moths were adapted to their environment at that particular moment in time.
Now, a short while later, as the industrial revolution started taking shape, pollution started to destroy the lichen and the soot from the new coal-burning factories started to stain all of the trees black.
All of a sudden, the tables turned and the white moths were getting gobbled up by the birds because they could no longer hide, and the black moths benefitted from camouflage.
The lesson? It’s good to be a bird - because they were eating a steady diet of moths throughout.
The other lesson? This is how natural selection works.
Now keep in mind that these changes are subtle and develop over a long time span. In fact our own lineage as homo sapiens can be traced back to primates and even further back to other mammals, and even further back to even more simple life forms.
The idea is that if moths can change colors over a few generations because of survival advantages, then the same mechanism can explain how we evolved and took advantage of traits like walking upright, using an opposable thumb, and the most important one of all - being able to work in a flexible way with lots and lots of other human beings - creating communities.
But natural selection applies to selection pressure applied by the natural environment. There’s also the idea of artificial selection, which is where people breed a species to select for a particular trait.
Take dogs for example - all of the various species evolved from a common ancestor - the wolf. But over the years, some were bred for having fluffy hair - like a poodle, whereas others were bred to be protective of spaces - like a doberman.
So, to recap. Evolution is the process by which populations change over time to become more suited to the environment.
Natural selection is the main mechanism by which evolution happens. It consists on the differential survival and reproduction of organisms with evolutionary advantages.
These organisms can then pass on their traits to the offspring. Sometimes so many changes accumulate in a population that it ends up turning into a whole new species.
This is how all the variety of living beings in the world —biodiversity— comes from a common ancestor.