Natural Selection Lesson in Biology - Definition, Stages, And Examples

Lesson Overview

• What Is Natural Selection in Biology?
• How Does Natural Selection Work?
• Stages of Natural Selection
• Natural Selection Examples
• What Is Sexual Selection in Natural Selection?
• Levels of Natural Selection

Life on Earth is remarkably diverse, shaped by evolution-the gradual change in heritable traits over generations. A key driver is natural selection. 

Within a population, individuals vary; some traits offer survival and reproductive advantages. These traits are more likely passed on, gradually shifting a population's characteristics. Natural selection thus drives adaptation and diversification.

What Is Natural Selection in Biology?

Natural selection is the differential survival and reproduction of individuals due to differences in their heritable traits. It's a process where organisms with traits better suited to their environment tend to survive and produce more offspring, thereby increasing the frequency of those advantageous traits in a population over generations. This leads to the adaptation of populations to their surroundings.

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How Does Natural Selection Work?

Natural selection, the driving force behind evolution, operates through a simple yet powerful mechanism. It favors individuals with advantageous traits, leading to their increased survival and reproduction, ultimately shaping the characteristics of a population over time.  

• It's a process: Natural selection isn't actively "trying" to achieve a specific outcome. It's a natural consequence of variation, heritability, and environmental pressures.
  
• Acts on existing variation: Selection doesn't create new traits. It works with the variations that already exist within a population, favoring those that are advantageous in the current environment.
  
• Depends on the environment: The "fitness" of a trait is context-dependent. A trait that's beneficial in one environment might be neutral or even harmful in another. Environments change, and so can the direction of selection.
  
• Leads to adaptation: Over time, natural selection can lead to adaptation, where populations become better suited to their environment. This is not a conscious process; it's a result of the differential survival and reproduction of individuals with advantageous traits.
  
• Survival of the fittest: "Fitness" in evolutionary terms refers to reproductive success, not just physical strength or dominance. The individuals that leave the most offspring are the "fittest," regardless of how strong or dominant they are.
  
• It's ongoing: Natural selection is not a one-time event. It's a continuous process that operates as long as there is variation, heritability, and environmental change. This means that populations are constantly evolving and adapting to their surroundings.

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Different Types of Natural Selection Quiz

Stages of Natural Selection

Natural selection happens through various stages, it begins with the help of variations and ends at adaptation. 

• Variation: This is the starting point. Individuals within a population are not identical; they exhibit variations in their traits. These variations can be in physical characteristics (size, color, shape), physiological processes (metabolism, disease resistance), or behavioral patterns. The ultimate source of this variation is genetic mutation, though other processes like genetic recombination during sexual reproduction also play a role. These variations arise randomly – they aren't caused by the environment. 
  
• Inheritance: For natural selection to act, the variations must be heritable. This means that the traits are passed down from parents to offspring through genes. Only heritable traits can be subject to selection. Traits that are acquired during an individual's lifetime, for example, muscle mass gained through exercise are not heritable and therefore not relevant to natural selection. 
  
• Selection: This is where the environment comes into play. Environmental pressures, also known as selective pressures, are factors that affect an organism's survival and reproduction. These can include:
  • Predation: Predators influence the survival of prey, favoring traits that enhance camouflage, speed, or defense mechanisms. 
  • Competition: Competition for resources like food, water, or mates can favor traits that make individuals more efficient at acquiring those resources. 
  • Environmental conditions: Factors like temperature, rainfall, or availability of sunlight can favor traits that allow organisms to better tolerate these conditions. 
    
• Differential Survival and Reproduction: This is the core of natural selection. Individuals with certain variations that make them better suited to the environment are more likely to survive and reproduce. They have a higher fitness. "Fitness," in the evolutionary sense, doesn't necessarily mean physical strength. It means the ability to survive and, crucially, reproduce successfully, passing on their genes to the next generation. Individuals with less advantageous traits are more likely to die or have fewer offspring. 
  
• Adaptation and Evolution: Over generations, the frequency of the advantageous traits increases in the population. This is because individuals with these traits are more likely to reproduce, and their offspring inherit those beneficial genes. The population gradually changes, becoming better adapted to its environment. This change in the genetic makeup of a population over time is evolution. Adaptation is the outcome of natural selection. It's the process by which populations evolve to become better suited to their environment. 
  
  

Natural Selection Examples

1. Darwin's Finches:

• The scenario: On the Galapagos Islands, Charles Darwin observed finches with different beak shapes. He realized these variations were adaptations to the different food sources available on each island.
• The selection: During droughts, birds with deeper, stronger beaks were better able to crack open tough seeds, surviving and reproducing more successfully.
• The outcome: Over time, the average beak depth of the finch population on those islands increased.
  

2. Peppered Moths:

• The scenario: In England, peppered moths existed in light and dark forms. During the Industrial Revolution, pollution darkened tree trunks.
• The selection: Darker moths were better camouflaged against the sooty trees, making them less likely to be eaten by birds.
• The outcome: The population shifted towards a higher proportion of dark moths. When pollution decreased, the trend reversed, and lighter moths became more common again.
  

3. Antibiotic Resistance:

• The scenario: Bacteria populations can evolve rapidly. When antibiotics are used, most bacteria are killed, but some may have random mutations that confer resistance.
• The selection: These resistant bacteria survive and reproduce, while susceptible bacteria are eliminated.
• The outcome: Over time, the proportion of antibiotic-resistant bacteria increases, making infections harder to treat. This is a major public health concern.
  

4. Giraffe Necks:

• The scenario: Giraffes feed on leaves high in trees.
• The selection: Giraffes with longer necks could reach higher leaves, giving them a competitive advantage in accessing food.
• The outcome: Over generations, the average neck length of giraffes increased.
  

5. Pesticide Resistance:

• The scenario: Similar to antibiotic resistance, insects can develop resistance to pesticides.
• The selection: Insects with genes that confer resistance survive pesticide applications and reproduce.
• The outcome: The population evolves to become more resistant to the pesticide, requiring stronger or different chemicals to control them.
  
  

What Is Sexual Selection in Natural Selection?

Sexual selection is a specific type of natural selection that acts on traits influencing an individual's ability to attract mates. It's driven by the advantage some individuals have over others in attracting or securing mates, rather than directly by survival. 

This can lead to the evolution of seemingly extravagant or even detrimental traits, like the peacock's tail, which makes the bird more vulnerable to predators but increases its chances of mating.

Sexual selection can operate through two main mechanisms:

• Intrasexual selection: This involves competition within a sex, usually males, for access to mates. Think of deer battling with their antlers or male birds singing to establish territory. The winners of these contests are more likely to mate.
  
• Intersexual selection: This involves mate choice, where individuals of one sex (usually females) choose mates based on certain traits. These traits can signal good genes, health, or resources. The peacock's tail, the bright plumage of some birds, and the elaborate songs of others are thought to be products of female choice.
  
  

Levels of Natural Selection

Natural selection does not operate solely at the level of the individual organism. It can act at multiple levels, though the individual is generally considered the primary unit of selection:

• Gene level: Richard Dawkins proposed the "selfish gene" concept, arguing that selection can act on individual genes, favoring those that are most successful at replicating themselves, even if it's at the expense of the organism.
  
• Individual level: This is the most commonly considered level. Selection acts on the variations among individuals, favoring those with traits that enhance their survival and reproduction.
  
• Group level: While controversial, some argue that selection can act on groups of individuals. This would favor groups with traits that enhance their survival and persistence, even if those traits are detrimental to some individuals within the group. This is a complex area of research.
  
• Species level: While not strictly natural selection in the same way, some patterns of extinction and diversification can be seen as a form of selection at the species level, where some species are more likely to persist and diversify than others.

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natural selection - cells - organelles