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Adaptation and Evolution of Populations

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Adaptation and Natural Selection
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Lesson Objectives

  • Define adaptaion.
  • Explain natural selection.
  • Understand the difference between inherited and acquired traints.


Why would an organism match its background? Wouldn't it be better to stand out?

An organism that blends with its background is more likely to avoid predators. If it survives, it is more likely to have offspring. Those offspring are more likely to blend into their backgrounds.

Adaptation

The characteristics of an organism that help it to survive in a given environment are called adaptations . Adaptations are traits that an organism inherits from its parents. Within a population of organisms are genes coding for a certain number of traits. For example, a human population may have genes for eyes that are blue, green, hazel, or brown, but as far as we know, not purple or lime green.

Adaptations develop when certain variations or differences in a population help some members survive better than others ( Figure 3.1 ). The variation may already exist within the population, but often the variation comes from a mutation , or a random change in an organism’s genes. Some mutations are harmful and the organism dies; in that case, the variation will not remain in the population. Many mutations are neutral and remain in the population. If the environment changes, the mutation may be beneficial and it may help the organism adapt to the environment. The organisms that survive pass this favorable trait on to their offspring.

Natural Selection

The mechanism for adaptation is natural selection . Traits become more or less common in a population depending on whether they are beneficial or harmful. An example of adaptation by natural selection can be found in the deer mouse, species Peromyscus maniculatus . In Nebraska this mouse is typically brown, but after glaciers carried lighter sand over the darker soil in the Sand Hills, predators could more easily spot the dark mice. Natural selection favored the light mice, and over time, the population became light colored.

Figure 3.1 An explanation of how adaptations develop.

This story is covered in more detail here: http://news.harvard.edu/gazette/story/2009/08/mice-living-in-sand-hills-quickly-evolved-lighter-coloration/ .

Natural selection means that the inherited traits of a population change over time. Inherited traits are features that are passed from one generation to the next. For example, your eye color is an inherited trait. You inherited your eye color from your parents. Inherited traits are different from acquired traits , or traits that organisms develop over a lifetime, such as strong muscles from working out.

Natural selection explains how organisms in a population develop traits that allow them to survive and reproduce. Natural selection means that traits that offer an advantage will most likely be passed on to offspring. Take the giant tortoises on the Galápagos Islands as an example. If a short-necked tortoise lives on an island with fruit located at a high level, will the short-necked tortoise survive? No, it will not, because it will not be able to reach the food it needs to survive. If all of the short necked tortoises die, and the long-necked tortoises survive, then, over time, only the long-necked trait will be passed down to offspring. All of the tortoises with long-necks will be "naturally selected" to survive.

Every plant and animal depends on its traits to survive. Survival may include getting food, building homes, and attracting mates. Traits that allow a plant, animal, or other organism to survive and reproduce in its environment are called adaptations .

Natural selection occurs when:

  1. There is some variation in the inherited traits of organisms within a species.
  2. Some of these traits will give individuals an advantage over others in surviving and reproducing.
  3. These individuals will be likely to have more offspring.

Summary

  • A population has genetic variations, possibly due to mutations. Favorable variations may allow an organism to be better adapted to its environment and survive to reproduce.
  • Beneficial traits are favored in a population so that they may become better represented.
  • Natural selection is the process by which organisms with traits that better enable them to adapt to their environment will tend to survive and reproduce in greater numbers.
  • Natural selection occurs when there is some variation in the inherited traits, some of these traits will give individuals an advantage over others, and the individuals with certain traits will be more likely to have more offspring.

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