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Evolution and natural selection





Population genetics
Genetic disorders
Down syndrome (Trisomy 21)
Edwards syndrome (Trisomy 18)
Patau syndrome (Trisomy 13)
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Autosomal trisomies: Pathology review
Muscular dystrophies and mitochondrial myopathies: Pathology review
Miscellaneous genetic disorders: Pathology review

Evolution and natural selection


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High Yield Notes
11 pages

Evolution and natural selection

15 flashcards
External References

Content Reviewers:

Rishi Desai, MD, MPH

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.


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.