Natural Selection | Explanation, Types, & Its Process

Key Concepts

• Natural selection and its process

• Evolution and genetics

• Sources of variation

• Single-gene traits and polygenic traits

• Natural selection on single-gene traits

• Natural selection on polygenic traits

• Types of natural selection

Introduction:

Natural selection is one of the most important mechanisms of evolutionary change. It is the process responsible for the evolution of adaptive features in various species. Natural selection is a force that causes groups of organisms to change over time, leading to evolution. 

Natural selection means that organisms with traits that are best suited to their environment are more likely to survive and reproduce. These favorable traits are passed onto the next generation. 

Explanation:

Charles Darwin described the process of natural selection in his book “On the Origin of Species” in 1859. His thoughts on evolution explained how the many varied species on earth could be descended from a single ancestral species. 

Darwin’s theory of evolution encompasses the following fundamental ideas: 

1. Species change over time. 

2. All organisms are descended by the process of branching from common ancestors.  

3. Evolution is a slow process that takes a long time to complete. 

4. Natural selection is the mechanism of evolution. 

There are four principles of natural selection, which are as follows: 

1. Overproduction of offspring: All organisms produce more offspring so that more can survive to adulthood and reproduce.  

2. Inherited variation and adaptation: Most characteristics that an organism has are inherited from parent to offspring. Offspring vary in traits like color and size, etc. the favorable variations, i.e., adaptations that an organism acquires gets selected by nature and are inherited and passed on to the next generations. 

3. Struggle for existence: organisms struggle to survive due to environmental factors like predators, food supply, disease, and climate affect the size of a population. All these factors act against the survival of an organism and reduce its chances of successful reproduction. 

4. Natural selection: Individuals with favorable variations survive better and reproduce in a particular environment and are thus selected by nature. Whereas those with unfavorable variations perish. As a result, organisms with favorable variations will have more offspring and pass on these favorable traits compared to individuals without those features. 

Evolution and genetics: 

Darwin was unaware of how heredity worked when he developed his theory of evolution. He had no idea about the source of variation in a population. Mendel’s work was already published while Darwin was alive but no one paid attention to it until the 1900s.  

When scientists combined Mendel’s work and Darwin’s theory, they could understand how traits were inherited. Today, we can describe evolution in terms of genetics. 

Let’s recall some terms of genetics: 

• Genotype: Most organisms have two sets of genes – one set from each parent. These genes come in different forms, called alleles. The set of alleles found in an organism is called its genotype. An organism’s phenotype is determined by its genotype and environment. 
• Phenotype: An organism’s appearance and other characteristics or traits is called its phenotype. Natural selection works on the phenotypes and not directly on genes.  

• Gene pool: A gene pool made up of all the genes, including all the different alleles for each gene in a population at any given time. In a gene pool, some alleles are common. Others are rare. 

• Allele frequency: It is the ratio of the number of times an allele appears in a gene pool to the total number of alleles for that gene. The diagram below shows the allele frequencies for fur color in a mice population. In the given population, the frequency of the dominant allele  
  B (black) is 40 %. The frequency of the recessive allele b (brown) is 60 %. 

Sources of variation: 

We know that the members of a population differ from each other. Genetics explains the source of these variations. 

1. Mutations: A mutation is a genetic change. Most mutations do not have a major effect on fitness. Some mutations may lower fitness while others increase fitness. 

A mutation must be passed from one generation to the next in order for evolution to occur. The only mutations that can be passed on are those that are carried in germ cells. 

2. Sexual Reproduction: When sexual reproduction occurs the genes from two parents combine in new ways. This process produces millions of different gene combinations.  

Crossing-over also creates genetic variation. Crossing-over happens during meiosis when chromosome pairs trade pieces of DNA. Sexual reproduction creates new genotypes. But it does not change the frequency of alleles in the whole population. 

3. Lateral Gene Transfer: A few single-celled organisms have the ability to pass genes from one individual to another. The passing of genes to an organism that is not an offspring is called lateral gene transfer. 

Single-gene and polygenic traits: 

Genes control phenotypes in various ways. Sometimes a single gene controls a trait. In other cases, several genes work together to control a trait. 

• Single Gene Traits: A trait that is controlled by only one gene is called a single-gene trait. Single-gene traits may have only two or three phenotypes. For example, in a type of snail, a single gene controls stripes on the shell. The gene has two alleles, one code for striped shells and one for plain shells. There are only two possible phenotypes, i.e., with or without stripes. 

• Polygenic Traits: The trait that is controlled by two or more genes is a polygenic trait. Polygenic traits can have several possible genotypes and phenotypes.  

An example of a polygenic trait in humans is height. Individuals can be very tall, very short, or any height in between. A bell curve is typical for polygenic traits. 

How natural selection works: 

The word fitness describes how healthy an individual is. But in terms of evolution, fitness has a different meaning. Fitness describes individuals that have traits that help them survive and reproduce. Their traits are favored by natural selection. Individuals with high fitness have more offspring and pass on more of their genes. The alleles that produce these traits are becoming more common in the population. This is the process of evolution. 

Natural selection acts differently on single-gene traits and polygenic traits. 

Natural selection on single-gene traits: 

In single-gene traits, natural selection can bring changes in allele frequencies. When natural selection favors one trait over another, the allele for the favored trait becomes more common over time. 

For example, consider the population of brown lizards. A single gene that controls color mutates to form two new alleles: red and black. What happens to lizards with these new alleles?  

Red lizards are easier to see. They often get eaten before they can reproduce. As a result, the red allele will probably vanish.  

Black lizards can warm up in the sun rapidly than brown lizards. The warmer black lizards can escape from predators more quickly than brown lizards. Therefore, the frequency of the black allele is likely to increase. Over time, black lizards will become more common. 

Natural selection on polygenic traits: 

More than one gene controls polygenic traits and these traits produce a wide range of phenotypes. This range can often be shown by a bell curve.  

Natural selection can act on polygenic traits in one of three ways:  

1. Directional selection 

2. Stabilizing selection 

3. Disruptive selection 

1. Directional selection: 

Sometimes natural selection favors organisms at one end of the bell curve. When individuals at one end of the curve have higher fitness than the others, directional selection takes place.  

Consider a group of birds living on an island. There are many large seeds with thick shells on the island, but there are only a few small seeds. As a result, birds with larger beaks would be able to eat more food than those with smaller beaks. Larger beaks would allow birds to live longer and produce more offspring. The average size of beaks would increase with time. 

Natural selection would move in one direction—toward larger beaks. 

2. Stabilizing selection: 

Sometimes natural selection favors the average individual. When individuals present near the center of the bell curve have the highest fitness, stabilizing selection takes place. This type of selection shifts the ends of the curve closer to the middle. Human babies are affected by stabilizing selection.  

For example, babies that are very small are usually less healthy and less likely to survive. Whereas babies that are very large may have trouble being born. In this case, the fitness of the extremes is lower than that of the average. 

3. Disruptive selection:  

Sometimes the traits that are most extreme are most likely to survive and reproduce. While the average types have a harder time surviving and reproducing. This situation is called disruptive selection. Disruptive selection can eventually result in two distinct phenotypes. These phenotypes are shown by a curve with two peaks.  

Assume that a bird population lives on an island where medium-size seeds are rare. The majority of seeds are either large or small. Birds with very small or very large beaks would have a better chance of surviving. Due to this, the population might split into two groups: one with smaller beaks and one with larger beaks. 

Summary

• Natural selection means that organisms with traits that are best suited to their environment are more likely to survive and reproduce. These favorable traits are passed on to the next generation.

• Charles Darwin described the process of natural selection in his book “On the Origin of Species’ in 1859.

• Genetic variation arises from mutation, sexual reproduction, and lateral gene transfer.

• A trait that is controlled by only one gene is called a single-gene trait. Single-gene traits may have only two or three phenotypes.

• The trait that is controlled by two or more genes is a polygenic trait. Polygenic traits can have several possible genotypes and phenotypes.

• Natural selection can bring changes in allele frequencies in single-gene traits. When natural selection favors one trait over another, the allele for the favored trait becomes more common over time.

• Natural selection can act on polygenic traits in one of three ways:

1. Directional selection: favors organisms at one end of the bell curve.

2. Stabilizing selection: favors individuals present near the center of the bell curve.

3. Disruptive selection: favors traits that are most extreme.