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Natural Selection & Adaptation: Definition, Theory & Examples

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  • 0:06 Studying the Evolution…
  • 0:25 Natural Selection
  • 2:18 Adaptation
  • 3:00 Evolution in Practice
  • 5:18 Lesson Summary
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Lesson Transcript
Instructor: Greg Chin
How does natural selection help shape the amazing types of animals we witness around us? In this lesson, we'll explore adaptations and what they can tell us about a species' past evolution.

Studying the Evolution of Traits

Whew. Counting all those hamsters was hard work, but I think we have a lot of interesting data to start a project. One interesting research project might be to try to determine how a flying hamster could have evolved. But we probably should do a little research in the library first before we try to develop a hypothesis.

Natural Selection

We've seen that natural selection plays an important role in the evolution of a population of organisms. For instance, in our example of a volcano erupting on a remote island, natural selection favored the flying hamsters with the largest, strongest wings. Those hamsters were able to escape the island and continue to pass on their genetic information to their offspring in a new location. We can say that those hamsters were more fit than the ones that could not make it to another piece of land. However, this is not fitness in the common sense.

When we speak of evolutionary fitness, we're really talking about a measurement of the ability of a trait to increase or decrease the relative contribution of offspring by an individual to the next generation. That's a fancy way of saying that a phenotype that improves the fitness of an individual will improve its viability or reproductive success relative to other individuals in the population. For instance, the large, strong wings of the island flying hamsters conveyed a higher fitness level to those hamsters compared to the rest of the population after the volcano erupted. Note that before the volcanic eruption, those big wings didn't allow those hamsters to contribute a higher proportion of progeny to the next generation by surviving better or reproducing more successfully. The trait neither improved nor detracted from the fitness of the individual. Only after the volcanic eruption was the trait of having stronger wings beneficial, because those hamsters could escape the devastation.

Recall that this is one of the major principles of Darwin's theory of evolution. Variation within a population provides the genetic variability for improvements to the population.

Adaptation

That variation provides the basis for the population to adapt to environmental changes, such as the volcanic eruption on the island. We can say that the larger, stronger wings are adaptations that better equip the hamsters to deal with an unstable habitat. The ability to breathe fire may be another example of an adaptation. We could hypothesize that maybe a mutation in a gene first allowed a hamster to breathe smoke to confuse predators while it made its escape. Through a series of additional mutations, that trait could have evolved into the more direct form of fire-based protection. Now, let's consider how hamsters might have evolved the ability to fly.

Evolution in Practice

Let's assume that a single mutation didn't just suddenly enable the ability to fly. What if an ancient hamster developed an extra limb? With the other four legs in place for walking, this limb could have been free to evolve into a wing-like structure. As the new limb became more and more wing-like, the benefit to the individual's fitness increased. Maybe an early wing-like structure increased the fitness of the hamster by bestowing the ability to parachute, or glide away from danger. Slight increases in fitness over time could slowly select for the winged hamster we are studying today. If we look at the wing more closely, our hypothesis is supported by the fact that the wing bones appear to be very similar to the other hamster limb bones.

Human arms and bird wings are homologous structures
Evolution example

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