Natural Selection and Evolution

While natural selection is a mechanism of evolution but that doesn't mean that natural selection will always lead to evolution. For example, if you are studying a population and notice that the smaller organisms in that population are able to hide from predators more easily, you would expect that natural selection would lead to the population evolving to be smaller over time. However, as you continue to study the population over many generations, you notice that the offspring grow into adults of varying sizes regardless of parent size. This is because size isn't heritability for this particular population. It won't matter if you study the population for 4 generations or 50, the offspring will continue to range from small to large in size with natural selection leading to the larger organisms being consumed by predators. The only way for natural selection to lead to evolution is if the trait that nature is selecting for is able to be inherited from parent to offspring. 

There are many ways that selection can happen in the absence of natural selection, the first being artificial selection. Humans have been influencing selection for hundreds if not thousands of years. A great example of this is dog breeds which humans have created by selecting for different traits such as ear length, height, and face shape. In the case of bulldogs, artificial selection has lead to the breed being unable to mate or give birth naturally without human intervention not to mention a flat face that makes breathing difficult. So I think it's safe to assume that natural selection would not allow for bulldogs to exist. Another example would be natural disasters. Fires and floods lead to the deaths of organisms at random with no influence from natural selection. If you have a population of brown and black rabbits living near a river that floods and more of the black rabbits die than brown rabbits, the population will now produce more brown offspring, not because of natural selection but because more of the black rabbits were closer to the river. The same principle would apply to a herd of deer caught in a wildfire. In fact, this situation could lead to a selection that is against what natural selection would select for. Say, for example, the smaller deer were able to escape between the burning trees while the larger deer were unable to escape due to their large size. This would lead to the herd consisting mostly of smaller deer and without the genes for larger deer that would usually be selected for. 

Image credit: https://www.montananaturalist.org/blog-post/how-do-wildfires-impact-deer-elk/

Comments

  1. E GrahamFebruary 20, 2021 at 9:24 AM

    I like your example about how selection won't always lead to evolution. It is crucial to remember that heredity is essential for evolution to occur from natural selection

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  2. John Coffin (TA)February 22, 2021 at 7:44 AM

    Great job! Heritability (not hereditary, as you said, but I think that may have been a typo) is key for evolution to occur by natural selection. One thing to keep in mind is that natural disasters would be considered a component of natural selection. Some organisms that are better able to withstand natural disasters (ex: plants with fire-resistant seeds) will survive better and pass on those traits (assuming they are heritable) to their progeny, which will continue to have increased fitness. This process IS natural selection. While many natural disasters are human caused, they are ultimately selective forces from nature.

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