Evolution
Evolution: Defined
When most people hear about evolution, they picture this image to the left as kind of the "go-to" depiction of it. And a lot of people have a general idea of what evolution is, but people have general ideas about things they don't fully understand all the time. What actually is evolution?
Charles Darwin defined evolution as "descent with modification." The fuller, more detailed definition is that evolution is the change in the genotype of a population over generations. To fully explain what this means, we will go through the requirements of evolution and an example of it occurring, using the Pokémon sandshrew as an illustrative example.
Why is "change over time" a TERRIBLE definition for evolution?
One of the most commonly used definitions for evolution is "change over time" because it is short, simple, and easy to remember. However, it is a really bad definition for the concept. Think about what you look like right now compared to ten years ago. Think about a tadpole becoming a frog. Think about what happens to a piece of metal when left out in a very wet environment for years. Think about the formation of something like the Grand Canyon. All of those things have changed over time, but none of them evolved. So, as much as this pains me to say, a slowpoke in Pokémon turning into a slowbro is not actually evolution.
Populations evolve. Individuals develop.
Definitions like "change over time," while simple and easy, are inaccurate and can lead to misconceptions and a lack of understanding about what evolution really is.
The Three Requirements for Evolution
While he is far from the only evolutionary biologist, and was not even the first scientist to come up with the idea of evolution, Charles Darwin is by far the most famous evolutionary biologist. He is particularly well known for his work on finches on the Galapagos Islands and his book, On the Origin of Species. As he developed his ideas behind how evolution worked, he came up with four postulates, which can be simplified into the following three requirements for evolution:
Variation
There must be variation within the population for it to evolve. In other words, there must be a range of characteristics and differences among the members of a species within the particular population.
While Darwin did not know what the mechanism was behind these differences, we now have a strong understanding behind these with the advancements in genetics that have been made since Darwin's time.
Heritability
Along with differences existing within the population, this variation must also be able to be passed on from the parents to the offspring.
Variations can exist without being heritable. For example, is an animal is injured and loses an eye, its offspring will still be born with both eyes. The injury is a variation, but not one that will be passed on. This again was an example where Darwin did not know the mechanism behind it like we do now, but he knew that there were traits that would be passed on to offspring and traits that would not be.
Differential Reproductive Success
In nature, there is a struggle for survival. Only some individuals and offspring will survive and make it to adulthood. Since individuals have different traits, those that have more favorable traits are more likely to survive and reproduce than others. This is also known as differential fitness - in the evolutionary sense, an organism's fitness refers to how much that individual is contributing to the gene pool of the next generation. In other words, how many kids is the individual having; more kids = greater fitness.
So, let's walk through our example using the images above.
There is a population of sandshrew that live on a snowy mountaintop. Within this population, there are white sandshrew and yellow sandshrew. This difference in phenotype is a variation that is present.
Whether a sandshrew will be white or yellow is determined by its genes. If two "pure" (homozygous) white sandshrew mate, they will have white babies. If two "pure" yellow sandshrew mate, they will have yellow babies. This variation in their hide color is heritable.
The sandshrew are not the only species that live on this mountaintop. Beartic, their predator, are another species that lives there. When hunting, it is easier for the beartic to spot and capture the yellow sandshrew, as the white ones blend in with the snow. As more white sandshrew are surviving, an individual white sandshrew is more likely to have kids than an individual yellow sandshrew. In other words, white sandshrew have greater reproductive success.
1 - intro.mp4Over generations, the population will have a greater and greater increase of white sandshrew as a result. This population of sandshrew is evolving because the prevalence of the genotype for being white is increasing from one generation to the next.
An important thing to note is that evolution did not create the white trait in this example. Evolution does not create needed traits, it simply makes an existing trait more common.