How within a population provides the genetic variability

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.

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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

In fact, if we compare the limb bones of the hamster to a human arm, those bones are similar as well.

Any structures which are shared by two or more different organisms and are inherited from a common ancestral structure are called homologous. Homologous structures develop from a common ancestral origin but may have diverged in appearance and function over time. For instance, different selective pressures altered the appearance and primary function of a human arm and a bird wing, but both limbs possess similar bones, as depicted by the colors in this diagram.Contrast this homologous relationship to the relationship between the wing of a bird and the wing of an insect. Both structures allow the organism to fly, so you might think they are homologous as well. However, if you consider the anatomy of the structures, an insect wing is clearly different than that of a bird wing. For instance, the bird wing possesses bones while the insect wing doesn’t.

Traits or structures that are similar in appearance or function but that evolved separately are called analogous.The ability of insects and birds to fly is an example of convergent evolution, because selective pressure resulted in the independent evolution of flight in each of these organisms.

Lesson Summary

In summary, fitness is a measurement of the ability of a trait to increase or decrease the relative contribution of offspring by an individual to the next generation.

If a trait increases the fitness of an individual, that organism will contribute a higher proportion of the offspring in the next generation relative to individuals without that trait. An adaptation is a trait that enhances the survival or reproductive success of an organism. A homologous structure is a structure shared between different organisms which evolved from a common ancestor. An analogous trait is a trait that may be similar in appearance or purpose but which evolved independently in the two organisms in question. Convergent evolution occurs when selective pressure results in the independent evolution of similar traits in two or more different organisms.

Lesson Objectives

After watching this lesson, you should be able to:

  • Define fitness, adaptation and convergent evolution
  • Explain the difference between homologous and analogous traits
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