Natural Selection - Crash Course Biology #14

CrashCourse
30 Apr 201212:44
EducationalLearning
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TLDRThe video explains the concept of natural selection, first proposed by Charles Darwin. It outlines the four key principles: populations have variations, variations can be inherited, populations produce more offspring than available resources can support leading to competition, and traits that increase fitness lead to better chances of survival and reproduction. It then describes different modes of selection like directional, stabilizing, disruptive, and sexual selection. Examples are given such as peppered moths, giraffe necks, peacock tails. The video also covers how artificial selection has shaped domesticated plants and animals over generations through selective breeding.

Takeaways
  • 😀 Natural selection is the process by which certain inherited traits make organisms more fit and multiply, changing the genetic makeup of populations.
  • 🧬 Variation, heritability, overpopulation, and differential survival/reproduction drive natural selection.
  • 🦋 Peppered moths and Darwin's finches exemplify natural selection through camouflage and specialized adaptations.
  • 👪 Darwin studied his own inbred family to observe the effects of crossbreeding.
  • 🔀 There are different modes of selection: directional, stabilizing, disruptive, and sexual.
  • 🦉 Traits favored by natural selection enhance survival and reproductive success.
  • 🐎 Sexual selection promotes traits that attract mates or deter rivals.
  • 🐕 Artificial selection is human-driven selective breeding of plants and animals.
  • 😯 Over time, divergent selective pressures can lead to new species.
  • 🧬 Ultimately, natural selection alters the genetic composition of populations.
Q & A

  • What observation by Darwin about populations led to his idea of the 'struggle for existence'?

    -Darwin observed that populations often have far more offspring than resources like food and water can support. This competition for limited resources is what Darwin called the 'struggle for existence'.

  • What are the four basic principles of natural selection based on Darwin's observations?

    -The four principles are: 1) Different members of a population have individual variations 2) Many variations are heritable 3) Populations often have more offspring than resources can support 4) Heritable traits affecting fitness lead to variations in survival and reproduction.

  • How did Darwin's marriage to his cousin Emma Wedgwood demonstrate problems caused by inbreeding?

    -Darwin and Emma had 10 children, 3 of whom died young likely due to inherited weaknesses caused by inbreeding. Additionally, Darwin noted that his surviving children were 'not very robust' - 3 were unable to have children, probably another effect of inbreeding.

  • What is the difference between an organism's phenotype and genotype?

    -An organism's physical form and characteristics are its phenotype. An organism's genetic makeup of alleles is its genotype. Natural selection causes changes in genotype frequencies over generations.

  • What are some examples of the different modes of selection?

    -Directional selection favors extremes of a trait over time e.g. light to dark moth coloration. Stabilizing selection favors intermediate traits e.g. human birth weight. Disruptive selection favors extremes while selecting against common traits e.g. parasite susceptibility vs. fecundity in Daphnia.

  • How does sexual selection differ from natural selection?

    -Sexual selection involves competition for mates rather than resources. It selects for traits seen as attractive to the opposite sex or that give an advantage against same-sex rivals.

  • What was significant about the beak variations Darwin observed in Galapagos Island finches?

    -The finches had beaks adapted to the specific food sources on each island. This demonstrated superior inherited traits leading to increased fitness and ability to survive and reproduce.

  • How does artificial selection by humans relate to natural selection?

    -Artificial selection encourages some heritable traits over others, like plant and animal breeding. It demonstrates the same principles of selection pressures and fitness over time as natural selection.

  • What does it take for natural selection to occur in a population?

    -There must be 1) heritable variations, some of which 2) increase competitiveness and fitness, leading to 3) those variations becoming more frequent in the population over generations.

  • How can one species turn into another species over time?

    -When selection pressures become so strong populations become very genetically different, eventually they cannot interbreed successfully - this forms a new species.

Outlines
00:00
🦋 Moths and Natural Selection

Paragraph 1 introduces the concept of natural selection using the example of light and dark colored peppered moths in 19th century England. It describes how soot from coal factories made tree bark darker, camouflaging dark moths and making light moths more visible to predators. Over time, the dark moth population increased from 2% to 95% as they were better adapted to survive and reproduce.

05:03
🧬 Principles of Natural Selection

Paragraph 2 further explains the core principles behind natural selection - variation in traits, heritability of traits, overproduction of offspring and ensuing competition, and differential reproductive success favoring advantageous traits. It emphasizes that natural selection drives evolution by changing the genetic makeup of populations.

10:06
😷 Modes and Examples of Selection

Paragraph 3 describes different modes of selection - directional, stabilizing, disruptive - with examples of each. It also covers sexual selection for attractive traits or fighting ability. The paragraph ends noting that humans drive artificial selection through selective breeding of plants and animals.

Mindmap
Keywords
💡Natural selection
Natural selection is one of the main mechanisms of evolution. It is the process by which organisms with traits that make them better adapted to their environment tend to survive and reproduce more, leading to changes in the genetic makeup of a population over generations. The video explains how Darwin observed finches with different beak shapes adapted to the food sources on different Galapagos Islands as an example of natural selection.
💡Adaptation
An adaptation is a trait or characteristic that helps an organism survive and reproduce in its environment. Adaptations are shaped by natural selection over many generations. The video gives examples of finch beaks adapted for eating different foods as adaptations that arose through natural selection.
💡Variation
Variation refers to the genetic and phenotypic differences between individuals in a population. Variation provides the raw material for natural selection. The video explains that natural selection requires a population to have heritable variations, some of which lead to higher fitness.
💡Fitness
Fitness is a measure of an organism's ability to survive and reproduce in its environment. Individuals with favorable heritable traits tend to have higher fitness. The video discusses Darwin's observation that finches with beak adaptations were more 'fit' for their environment.
💡Species
A species is a group of organisms that can interbreed and produce fertile offspring. The video explains that over time, natural selection can lead to enough changes that populations become separate species unable to interbreed, like Darwin's finches.
💡Population
A population is a group of organisms of the same species living in a defined area. Populations evolve through shifts in genetic makeup over generations, not individuals. The video emphasizes that natural selection leads to changes across populations.
💡Inheritance
Inheritance is the process by which genetic information is passed from parents to offspring. The video states that natural selection requires heritable trait variations that can be inherited by future generations.
💡Directional selection
Directional selection favors extreme versions of a trait, shifting the distribution in one direction. The video gives the example of peppered moth coloration shifting from light to dark due to directional selection.
💡Stabilizing selection
Stabilizing selection favors intermediate traits and acts against extremes. The video uses human birth weight as an example, with very low and high weights selected against.
💡Disruptive selection
Disruptive selection favors extremes at both ends of a trait distribution. The video describes an example in a Daphnia population selected for susceptibility to a parasite but also increased reproduction.
Highlights

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Transcripts

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