Natural selection in populations (article) | Khan Academy
May 9, We quantified phenotypic variation in longevity, locomotor behavior, .. the relationship of polymorphism within species to divergence between. Natural selection need not result in 75% of the variation in plate number between Directional selection is characterized by a linear relationship between fitness and. Jun 9, Phenotypic variation in a population of organisms can contribute to natural selection and evolution. Find out what A lot more is known about the relationship between genes and phenotypes. Let's take the example of hair.
A wolf may have a genotype that calls for a long coat of fur, but if she gets infested with ticks and rubs her fur off, the phenotype will be an individual with bare spots. The Process of Natural Selection Natural selection is the mechanism by which phenotypes and genotypes change for a population over many generations.
In other words, it describes the underpinnings of evolution. The central precept of natural selection is that variation exists in a population and that because certain individuals with certain traits are better at staying alive, they have a better chance of surviving and are more efficient at passing on their genes.
Over the course of generations, natural selection can cause directional change in a trait, favor the extremes in a trait, or reduce variation in the population. Natural Selection Operates on Phenotype The traits that an individual expresses, its phenotype, are what will give that individual an advantage or disadvantage in the struggle to survive and reproduce.
Histogram showing height in inches of male high school seniors in a sample group. The histogram is roughly bell-shaped, with just a few individuals at the tails 60 inches and 77 inches and many individuals in the middle, around 69 inches.
Image modified from " Continuous variation: Quantitative traits ," by J. Kimball CC BY 3. We can see if natural selection is acting on a polygenic trait by watching how the distribution of phenotypes in the population changes over time.
How natural selection can shift phenotype distributions There are three basic ways that natural selection can influence distribution of phenotypes for polygenic traits in a population. To illustrate these forms of selection, let's use an imaginary beetle population, in which beetle color is controlled by many genes and varies in a spectrum from light to dark green. In stabilizing selection, intermediate phenotypes are more fit than extreme ones.
Measuring Natural Selection on Genotypes and Phenotypes in the Wild
For example, medium-green beetles might be the best camouflaged, and thus survive best, on a forest floor covered by medium-green plants. Stabilizing selection tends to narrow the curve.Genetic variation, gene flow, and new species
One extreme phenotype is more fit than all the other phenotypes. For example, if the beetle population moves into a new environment with dark soil and vegetation, the dark green beetles might be better hidden and survive better than medium or light beetles.
Directional selection shifts the curve towards the favorable phenotype. Height or hair color are examples. A natural range of variability may exist in those traits among all the members of a species.
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Imagine, for example, a butterfly species that has a distribution of tongue length, say, from 12 millimeters to about 30 millimeters. If there is a change to a predominance of long, tubular flowers in their environment, then butterflies with longer tongues will have an easier time getting food.
Those butterflies could be healthier than others and more successful at breeding, or they may be more likely to survive long enough to reproduce.
Phenotype and Environment As in the butterfly example, natural selection occurs when an organism's physical characteristics make it either more or less suited to thrive in an environment. The physical characteristics are called the phenotype; therefore, natural selection works directly on phenotype.
The phenotype of an organism is determined by both environmental influences and genotype.