Why Natural Selection Acts on Phenotype, Not Genotype

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  • 0:02 Descent With Modification
  • 1:08 An Editing Process
  • 2:09 Environmental Interactions
  • 4:36 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Evolution works through the mechanism of natural selection. This mechanism influences the genetic make-up of populations, acting on the population's physical traits. In this lesson, we'll explore examples of why this is.

Descent With Modification

Each organism is well-suited for its environment. For example, polar bears do well in the Arctic, but would not enjoy a life in Miami. But even within the same environment we can find differences between species, such as beak size and shape in birds or coloration and patterns on snakes. Charles Darwin observed this variation as he traveled around the world on the Beagle. And it was these observations that eventually led him to his theory of evolution, the idea that all species on Earth are descendants of ancestors that were different from themselves.

You may have heard some controversy surrounding the theory of evolution, but because it is a scientific theory it is a thoroughly tested and overwhelmingly accepted explanation of natural phenomena. This means that it has been scientifically tested over and over, and has yet to be disproven. Other scientific theories that are on par with evolution include Einstein's theory of relativity, the Big Bang theory, and Newton's theory of gravity. See what I mean?

An Editing Process

Evolution is the explanation that all organisms are descendants of their different-looking ancestors, something that Darwin called 'descent with modification.' But natural selection is the selection process or mechanism for this inheritance. Basically, over millions of years, organisms spread throughout various habitats and accumulate a whole host of adaptations that allow them to thrive in certain environments.

I need to be very clear, though, that while natural selection occurs between the interactions of individual organisms and their environments, it only alters populations. That is, individuals themselves do not adapt to meet their environmental needs within their lifetimes. Instead, natural selection is like an editing process - it leads to the success or failure of individuals within the population. Those individuals that are successful pass on their genetic information to the next generation, while those that fail do not.

Environmental Interactions

The interactions between individuals and their environment is what determines whether their genetic information will be passed on or not. This is why natural selection acts on phenotypes instead of genotypes. A phenotype is an organism's physical traits, while a genotype is an organism's genetic makeup.

This may sound counter-intuitive since the genetic makeup does get passed on from generation to generation through reproduction. But the phenotype includes not only an organism's physical traits but also things like its metabolism and behavior, and the genotype may include information that doesn't affect the organism at all. This includes a wide variety of inheritable diseases and recessive traits that are often carried on genes, but with no physical expression.

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