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.

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Over 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.

Natural Selection

The most well-known type of selection, natural selection is when the individuals who are most suited to survive in an environment will survive and reproduce. "Survival of the fittest" is often used to describe this, where evolutionary fitness refers to those who are the most reproductively successful.

Natural selection is what was observed with the classic example of evolution, Darwin's finches, where the birds evolved different beak size/shapes based on the food that was available to them.

Adaptations

Natural selection is adaptive. Adaptations are variations that provide an advantage in a particular environment, increasing the organism's fitness.

Species are not perfect, however, and neither are adaptations.

Adaptations arise by modifying existing structures. If you look at the image to the left, the recurrent laryngeal nerve loops around the aorta in both humans and giraffes, despite it not remotely being the most efficient path for us, let alone the giraffe.

Traits that are adaptive in one environment are not necessarily in a different environment, and may even be detrimental.

Sometimes there are fitness trade-offs, where one trait may negatively impact another.

In addition, mutations - which cause new variation and new traits - occur randomly and by chance.

Natural selection is not goal-driven and does not strive toward a particular phenotype. Fitness in each generation depends on its present environment, not the past environment, and not a future environment.

As evolution works by making traits that already exist more of less common, it cannot "create" new needed phenotypes. And due to this, more genetically diverse populations are more able to respond to disturbances as they are more likely to have something that can survive or withstand the change.

In addition, not every trait is an adaptation - or even has an impact on fitness. There are traits that can be passed on without having any impact on fitness, such as those that drift randomly across generations. There are also what are known as spandrels, which arise as a byproduct of another trait, not because they themselves increase fitness. They are named after architectural spandrels, which is the triangular shape above an arch that is required to make an arch.

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