Types of Natural Selection

Natural Selection is how organisms better adapted to their environment tend to survive and reproduce, passing on favorable traits to the next generation. Over time, this process leads to evolutionary changes within populations. Natural Selection is one of the key mechanisms of evolution, alongside genetic drift, gene flow, and mutation. There are several types of natural Selection, each of which affects the genetic diversity of a population differently.

1.0Types of Natural Selection

Stabilizing Selection 

Stabilizing Selection favors the intermediate or average phenotypes in a population, reducing variation without changing the average value of a trait. It works against both extremes (i.e., individuals with extreme phenotypic values) and maintains the status quo.

Example

• Birth weight in human infants is a classic example of stabilizing Selection. 
• Very small or very large babies have higher mortality rates, whereas infants of average weight are more likely to survive.

Mechanism

• Individuals with traits near the mean are more likely to survive and reproduce.
• Reduces variation by selecting against extreme traits.

Effects on Population

• Genetic diversity decreases.
• Population becomes more uniform over time.

Illustration

• If a trait's distribution looks like a bell curve, stabilizing Selection makes the peak steeper, eliminating the tails of the distribution.

Directional Selection

Directional Selection occurs when one extreme phenotype is favored over other phenotypes, causing a population trait distribution shift over time.

Example

• Directional Selection occurs when one extreme phenotype is favored over other phenotypes, causing a population trait distribution shift over time.

Mechanism

• Selects for individuals with traits on one end of the phenotypic spectrum.
• Alters the mean value of the trait over time.

Effects on Population

• Genetic variation may remain unchanged initially but decrease over time if the selection pressure remains constant.
• The population's mean phenotype shifts in the direction of the favored extreme.

Illustration

• If the original population's trait distribution is normal, directional Selection shifts the entire curve toward one extreme.

Disruptive Selection (Diversifying Selection)

Disruptive Selection favors individuals at both extremes of the phenotypic range, leading to a bimodal distribution. It is the opposite of stabilizing Selection, promoting phenotypic diversity.

Example

• In a population of birds, individuals with very large or very small beaks may have an advantage if the available food consists of either very hard seeds (favouring large beaks) or very soft seeds (favouring small beaks), with no intermediate food types.

Mechanism

• Selects against individuals with average traits.
• Both extremes are favored, which can lead to speciation under certain conditions.

Effects on Population

• Increases phenotypic variation.
• This may form two or more distinct groups within a population, potentially leading to speciation.

Illustration

• The normal curve of trait distribution becomes two peaks as the individuals with intermediate traits are selected.

Balancing Selection

Balancing Selection maintains genetic diversity in a population by favoring multiple alleles, often through mechanisms like heterozygote advantage or frequency-dependent Selection.

Example

• Sickle cell anemia in humans is an example of balancing Selection. 
• Individuals heterozygous for the sickle cell allele (AS genotype) are more resistant to malaria, while homozygous individuals (AA or SS) either have normal blood cells or severe anemia. 
• This maintains the presence of the sickle cell allele in populations where malaria is prevalent.

Mechanism

• Heterozygote advantage: heterozygous individuals have a fitness advantage over either homozygote.
• Frequency-dependent Selection: the fitness of a phenotype depends on its frequency in the population (e.g., rare phenotypes may have an advantage simply because they are rare).

Effects on Population

• Maintains genetic variation.
• Prevents the fixation of any one allele.

Sexual Selection

Sexual Selection is a natural Selection in which certain traits increase an individual's chances of mating and reproducing, even if they don't necessarily improve survival. This can lead to exaggerated traits, especially in males.

Example

• The large, colorful tails of male peacocks result from sexual Selection. 
• Although the tail may make the peacock more visible to predators, it is attractive to female peacocks, thus increasing the male's chances of mating.

Mechanism

• Intersexual Selection: members of one sex (usually females) choose mates based on certain traits (e.g., bright feathers, elaborate songs).
• Intrasexual Selection: individuals of the same sex (usually males) compete for access to mates (e.g., antler size in deer).

Effects on Population

• This can lead to the development of secondary sexual characteristics.
• Traits favored by sexual Selection may be costly in terms of survival but provide reproductive advantages.

Artificial Selection

Artificial Selection is a directional Selection in which humans actively select for desired traits in organisms, such as crops, livestock, or pets. Unlike natural Selection, artificial Selection is driven by human preferences rather than environmental factors.

Example

• The domestication of dogs is an example of artificial Selection. 
• Over generations, humans have selected dogs with specific temperaments, sizes, and physical characteristics.

Mechanism

• Humans selectively breed individuals with desired traits, increasing the frequency of those traits in future generations.

Effects on Population

• Traits that would otherwise not be favored by natural Selection (e.g., exaggerated physical features in dog breeds) become more common.
• Genetic diversity is often reduced as specific traits are favored over others.