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4.3: Natural Selection

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    How is this deer mouse well adapted for life in the forest?

    Notice how its dark coloring would allow the deer mouse to easily hide from predators on the darkened forest floor. On the other hand, deer mice that live in the nearby Sand Hills are a lighter, sand-like color. What caused the deer mice to be so well adapted to their unique environments? Natural selection.

    Natural Selection

    The theory of evolution by 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 (Figure below).

    Drawing of attached or free human earlobes, which is an inherited trait
    Figure \(\PageIndex{1}\): Human earlobes may be attached or free. You inherited the particular shape of your earlobes from your parents. Inherited traits are influenced by genes, which are passed on to offspring and future generations. Things not influenced by genes are not passed on to your offspring. Natural selection only operates on traits like earlobe shape that have a genetic basis, not on traits that are acquired, like a summer tan.

    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; individuals with those traits have a better chance of surviving. Evolution occurs by natural selection.

    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. Organisms that are not well-adapted, for whatever reason, to their environment, will naturally have less of a chance of surviving and reproducing.

    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. Without this variation, natural selection would not be possible.
    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.

    Imagine how in the Arctic, dark fur makes a rabbit easy for foxes to spot and catch in the snow. Therefore, white fur is a beneficial trait that improves the chance that a rabbit will survive, reproduce, and pass the trait of white fur on to its offspring (Figure below). Through this process of natural selection, dark fur rabbits will become uncommon over time. Rabbits will adapt to have white fur. In essence, the selection of rabbits with white fur - the beneficial trait - is a natural process.

    A white Arctic hare in the snow
    Figure \(\PageIndex{2}\): The white fur of the Arctic hares may make it more difficult for fox and other predators to locate hares against the white snow.

    Why So Many Species?

    Scientists estimate that there are between 5 million and 30 million species on the planet. But why are there so many? Different species are well-adapted to live and survive in many different types of environments. As environments change over time, organisms must constantly adapt to those environments. Diversity of species increases the chance that at least some organisms adapt and survive any major changes in the environment. For example, if a natural disaster kills all of the large organisms on the planet, then the small organisms will continue to survive.

    Evolution Acts on the Phenotype

    Natural selection acts on the phenotype (the traits or characteristics) of an individual. On the other hand, natural selection does not act on the underlying genotype (the genetic makeup) of an individual. For many traits, the homozygous genotype, AA, for example, has the same phenotype as the heterozygous Aa genotype. If both an AA and Aa individual have the same phenotype, the environment cannot distinguish between them. So natural selection cannot select for a homozygous individual over a heterozygous individual. Even if the "aa" phenotype is lethal, the recessive a allele, will be maintained in the population through heterozygous Aa individuals. Furthermore, the mating of two heterozygous individuals can produce homozygous recessive (aa) individuals. However, natural selection can and does differentiate between dominant and recessive phenotypes.

    Carriers

    Since natural selection acts on the phenotype, if an allele causes death in a homozygous individual, aa, for example, it will not cause death in a heterozygous Aa individual. These heterozygous Aa individuals will then act as carriers of the a allele, meaning that the a allele could be passed down to offspring. People who are carriers do not express the recessive phenotype, as they have a dominant allele. This allele is said to be kept in the population's gene pool. The gene pool is the complete set of genes and alleles within a population.

    Summary

    • Evolution occurs by natural selection, 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. Evolution is due to differences in the survival and reproduction of individuals within a population.
    • 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.
    • Natural selection acts on the phenotype (the traits or characteristics) of an individual, not on the underlying genotype.
    • Carriers of a trait can show no symptoms of a recessive disease and, yet, still pass it on to their offspring.

    Review

    1. What's the difference between an acquired and inherited trait?
    2. Define natural selection.
    3. What is an adaptation?
    4. What is required for natural selection to take place?
    5. How many species are there on the planet?
    6. What's the difference between phenotype and genotype? Does natural selection act on the genotype or phenotype?
    7. Explain how a lethal recessive gene can stay in the gene pool.