Natural Selection Study Guide
Introduction
Charles Darwin initially theorized about the concept of natural selection in his book The Origin of Species following his landmark trip to the Galápagos Islands. Natural selection occurs when specific traits offer an advantage or disadvantage in survival or reproductive success. Advantageous traits are passed down to the next generation, while disadvantageous traits are phased out of the population. Due to this selection pressure, the alleles linked with those characteristics will alter in frequency over time.
What is Natural Selection?
Natural selection is the process by which organisms adapt and change within their environment. Dominant, recessive, and codominant qualities are selected for and passed down through this process. Advantageous traits increase an individual’s chances of surviving and reproducing, and thus can be passed down to help the next generation thrive.
Because natural selection operates on phenotypes rather than alleles, heterozygotes can maintain disadvantageous recessive genes with no detrimental consequences. The benefits of an allele might shift over time due to environmental changes. Previously innocuous genes can become advantageous or deleterious depending on the environmental pressures affecting the individual.
Natural Selection and the Peppered Moth
The study of the peppered moth (Biston betularia) in England during the Industrial Revolution in the 1800s is a famous example of natural selection. Before the Industrial Revolution, the peppered moth population was largely light in color, with black (pepper-like) speckles on the wings.
The “peppered” color was remarkably similar to the bark and lichens that grew on the trees in the area. This helped the moths blend in when resting on a tree, making it more difficult for moth-eating birds to discover and devour them.
Another phenotype appeared in the population from time to time. These moths were heterozygotes with a hyperactive dominant pigment allele, resulting in a solid black coloring. As you might expect, black moths were considerably easier to locate by birds, putting this genotype at a distinct disadvantage. However, when the Industrial Revolution gained traction, the situation reversed.
Large industries began blowing massive volumes of coal smoke into the air, covering the area with black soot, including lichens and trees. Suddenly, the light-colored moths were easier to identify for birds, while the black moths had the upper hand. The dark pigment allele’s frequency increased considerably, and the black moth phenotype accounted for 98 percent of all moths recorded by 1895.
Air pollution in England began to decline in the 1960s due to new environmental restrictions. As the soot levels fell and the trees returned to their old, lighter hue, it was the ideal time to see how the peppered moth population would react. A progressive increase in the frequency of the lighter-colored phenotype was found in many follow-up investigations. By 2003, the dark phenotype had dropped to less than 10% in most regions of England, with a maximum frequency of 50%.
Classification of Natural Selection
Depending on the environmental pressures exerted on the population, natural selection can be classified as directional, balancing, disruptive, or sexual. Let’s go over each of these categories below.
Directional Selection
Directional selection occurs when advantageous traits allow one portion of the population to survive and reproduce better than the other. Environmental forces that favor one phenotype over the other lead the frequency of the corresponding beneficial genes to progressively rise.
In the instance of peppered moths, the selection pressure moved three times: first, it preferred lighter pigment; secondly, as pollution increased, the pressure turned to favor darker pigment; and lastly, as pollution decreased, the pressure shifted back to favoring light-colored moths. Each of these shifts are an example of directional selection.
Balancing or Stabilizing Selection
Balancing selection occurs when the selective force prefers the intermediate phenotype over the extremes of a characteristic. Humans, for example, have an average birth weight that balances the requirement for children to be tiny enough to avoid problems during pregnancy and labor while still being large enough to maintain a healthy body temperature after birth.
Disruptive or Diversifying Selection
Disruptive selection is the opposite of balancing selection in which the extremes of a characteristic are favored over the intermediate. Disruptive selection can lead to a population developing into two different species if individuals with mid-range features are chosen against.
The wide variety of finch species discovered by Darwin on his trip to the Galápagos Islands provided strong evidence of disruptive selection leading to speciation. Seed-eating birds had either large beaks capable of swallowing large seeds or small beaks capable of extracting little seeds. On the islands, few plants produced medium-sized seeds.
Because few plants on the islands produced medium-sized seeds, birds with medium-sized beaks would struggle to eat both the large and small seeds more than their extreme counterparts. Darwin hypothesized that the population of mid-size beaks separated into two species throughout time due to this pressure.
Sexual Selection
Sexual selection focuses on reproductive success (the capacity to reproduce effectively and rear children) rather than survival. Sexual selection encourages characteristics that entice a partner, such as the colorful feathers of a peacock or the size of a stag’s antlers.
✅ Conclusion
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Mutation, gene flow, genetic drift, and natural selection are the four processes that drive evolution.
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Natural selection is the process by which organisms adapt and change within their environment.
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The study of the peppered moth (Biston betularia) in England during the Industrial Revolution in the 1800s is a famous example of natural selection.
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Depending on how the pressure is exerted on the population, natural selection can be classified as directional, stabilizing, disruptive, or sexual.
FAQs
1. What is natural selection?
Natural selection is a natural process in which organisms that are more suited to their environment survive and reproduce more than those not.
2. What are the 4 conditions required for natural selection to occur?
Reproduction, inheritance, variation in the fitness of organisms, and diversity in individual traits among population members. Natural selection occurs automatically if they are satisfied.
3. How does darwin define natural selection?
As an explanation for adaptation and speciation, Charles Darwin proposed his theory of evolution by natural selection. Natural selection, he stated, is the “principle by which any little variation (of a feature) is kept if it is helpful.”
4. What are the 4 types of natural selection?
- Stabilizing Selection
- Directional Selection
- Disruptive Selection
- Sexual Selection
5. What are Darwin’s 5 points of natural selection?
Natural selection is a basic mechanism that affects the evolution of populations of living organisms through time. It’s so simple that it can be broken down into five simple processes, abbreviated as VISTA: Variation, Inheritance, Selection, Time, and Adaptation.
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Sources
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Natural Selection https://flexbooks.ck12.org/cbook/ck-12-biology-flexbook-2.0/section/5.21/primary/lesson/natural-selection-bio/ Accessed on 7 Dec 2021
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Natural Selection https://en.wikipedia.org/wiki/Natural_selection Accessed on 7 Dec 2021
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Natural Selection https://www.britannica.com/science/natural-selection Accessed on 7 Dec 2021
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Natural Selection https://www.biologyonline.com/dictionary/natural-selection Accessed on 7 Dec 2021