What is Natural Selection?

Natural Selection

Did you know that more than 99% of all species that have ever lived on our planet have died out^[1]? Humans, blue whales, giraffes, mice, bacteria, and everything else we see around us are the exception. We’re the survivors of a process called natural selection. Natural selection is one of the core concepts of the theory of evolution. It explains why some creatures go on to have offspring and spread their genes and why others don’t.

Put simply, natural selection is the biological process where organisms that are most suited to their environment survive and pass on their favorable genetic traits. To understand how this works, we first have to dive into a few other core concepts within evolution:

Descent With Modification

This is the observable fact that when parents have children, they will look mostly the same as their parents but might have slightly different characteristics. They have descended from their parents but have some modifications. These modifications are the result of random mutations within genes.

Our cells are continually dividing. For example, if you fall off a bicycle and skin your knee, your cells need to divide to create new cells to replace the old and damaged cells. Because cell division involves so much genetic material, the chance of mutation is high. The human genome (our genetic blueprint) is around 3 billion letters long, so there are just so many opportunities for mistakes to happen.

Think about it like this. If you were asked to copy one sentence perfectly, for example, “A monkey’s favorite fruit is grapes, not bananas,” you could probably get it right every time. However, if we asked you to copy the entire works of Shakespeare word for word, you’re much more likely to make a mistake – maybe you’ll misspell a word, forget a comma, or lose where you are on the page and accidentally skip a few lines.

Just how common are mutations? It’s estimated that a whopping 120,000 mistakes happen each time one of our cells divides^[2]! When mutations happen in our body cells, it occurs through a process called mitosis. Sometimes these mutations are harmless, having little or no effect, and sometimes they do cause harm. For example, cancer is caused by mutations within cells that cause the cell to divide uncontrollably. Regardless of what effect these mutations have, the results stop with that person – they don’t pass onto the offspring. However, when mutations occur in sex cells (eggs or sperm cells), these mutations are passed on to the next generation, becoming a modification.

Common Descent

This is the theory that all living organisms today have descended from one common ancestor over many generations due to descent with modification. This happens over billions of years.

How Does Random Variation Allow Complex Life to Survive?

Mutations occur all the time, but it’s natural selection that decides which mutations stick around. Selective pressures in the environment determine which mutations are beneficial for survival and which are not. Selective pressures can be split into several categories like biological (predators and pathogens), environmental factors (changes in temperatures, geology, or climate), resource availability (mate selection, food, shelter, habitat), and more. Let’s take a look at an example of natural selection.

Peppered Moths

Before the industrial revolution in the UK, light-colored moths had an advantage over their darker colored counterparts. Light moths could camouflage against lichen-covered trees and avoid being preyed upon by birds (the birds couldn’t see them). When darker colored moths hid against trees, birds would spot them easily and enjoy a good meal. This resulted in lighter-colored moths being more abundant – since more of them survived and passed on their genes, they were higher in numbers. However, during the industrial revolution, the amount of thick smog in the atmosphere sharply rose, and before long, trees were covered in black soot. Suddenly, black moths now had an advantage, and their numbers sharply rose^[3].

Mutations and Genetic Diversity

Mutations provide greater levels of genetic diversity to animals, and this makes them better able to survive shifting pressures. For example, let’s consider skin grafts. Humans can’t accept skin grafts from other people because we are genetically diverse enough that our bodies will reject the foreign skin. However, this isn’t a problem for cheetahs.

A cheetah can accept a skin graft from any other cheetah anywhere in the world. Why? Because there was a bottleneck in their population around 10,000 years ago, and almost all of the species were wiped out. It’s thought that all cheetahs today have descended from the less than seven cheetahs that survived that event^[4]. Cheetahs were lucky to survive, but they are so genetically similar to each other that they potentially lack the genetic diversity to survive a new deadly virus or pathogen.

Do We See Natural Selection Happening Today?

Yes. In 1975, a team of Japanese scientists discovered that a strain of bacteria could break down the human-made material nylon. Nylon is a synthetic material invented in 1939 and commonly used in clothing. A mutation in this bacteria allowed it to create enzymes called nylonase to break down nylon. If this mutation occurred before 1939, it likely would have died out because it gave the bacteria no advantage to survival. But in a nylon-rich environment, suddenly, it’s useful.

^[1] https://www.pbs.org/wgbh/evolution/extinction/massext/statement_03.html

^[2] https://www.nature.com/scitable/topicpage/dna-replication-and-causes-of-mutation-409/#:~:text=In%20humans%2C%20with%20our%206,every%20time%20a%20cell%20divides!

^[3] https://www.bbc.co.uk/bitesize/guides/ztn9y4j/revision/3

^[4] https://www.abc.net.au/science/articles/1999/08/02/40791.htm#:~:text=You%20will%20almost%20always%20reject,genetically%20similar%20to%20each%20other.