MECHANISM OF EVOLUTION

 

MECHANISM OF EVOLUTION

Introduction

Evolution is the gradual change in the genetic composition of populations over successive generations. According to the Modern Synthetic Theory (Neo-Darwinism), evolution occurs through changes in the gene pool of a population. These changes arise from genetic variation, which is produced by mutation and genetic recombination, and are acted upon by natural selection. Other evolutionary forces such as gene flow, genetic drift, and reproductive isolation also contribute to evolution.

The mechanism of evolution therefore refers to the biological processes that generate, maintain, and modify genetic variation, ultimately leading to adaptation and the formation of new species.

Evolution: Modern Definition

Evolution is defined as:

A change in the allele frequencies of a population over successive generations.

Important Point

Evolution occurs at the population level, not at the individual level.

What Is the Mechanism of Evolution?

Evolution occurs through the interaction of several biological processes:

1. Variation
2. Mutation
3. Genetic recombination
4. Natural selection
5. Gene flow
6. Genetic drift
7. Reproductive isolation

Flow Chart of Evolution

Mutation + Recombination
↓
Genetic Variation
↓
Natural Selection Acts on Variation
↓
Differential Survival and Reproduction
↓
Change in Allele Frequency
↓
Adaptation
↓
Speciation
↓
Evolution

Genetic Variation

Definition

Variation refers to the differences in characteristics among individuals of the same species.

Example:

• Human height
• Eye colour
• Blood groups
• Skin colour
• Fingerprints

No two individuals are genetically identical except identical twins.

Importance of Variation

Variation is called the raw material of evolution because:

• It provides diversity within populations.
• It enables organisms to adapt to changing environments.
• It allows natural selection to act.
• It reduces the risk of extinction.
• It leads to speciation over long periods.

Without variation, evolution cannot occur.

Types of Variation

1. Genetic (Heritable) Variation

Produced by changes in genes or chromosomes.

Characteristics:

• Passed from parents to offspring.
• Permanent.
• Important for evolution.

Examples:

• Blood groups
• Eye colour
• Genetic disorders

2. Environmental (Non-heritable) Variation

Produced by environmental influences.

Examples:

• Body weight
• Skin tanning
• Muscle development
• Nutrition-related changes

Characteristics:

• Not inherited.
• Does not contribute directly to evolution.

Continuous and Discontinuous Variation

Continuous Variation	Discontinuous Variation
Controlled by many genes	Controlled by one or few genes
Shows gradual differences	Shows distinct categories
Quantitative	Qualitative
Example: Height	Example: ABO blood groups

 

Sources of Genetic Variation

Genetic variation is produced mainly by:

1. Mutation
2. Genetic recombination
3. Gene flow
4. Random fertilization

Mutation

Definition

Mutation is a sudden, random, heritable change in the genetic material (DNA or chromosomes).

The term "mutation" was introduced by Hugo de Vries while studying the evening primrose (Oenothera lamarckiana). Mutation is the ultimate source of new genetic variation.

CHARACTERISTICS OF MUTATIONS

• Random
• Sudden
• Heritable (if occurring in germ cells)
• Rare
• Usually neutral or harmful
• Occasionally beneficial

Beneficial mutations are important in evolution.

Types of Mutations

A. Gene Mutation (Point Mutation)

Occurs due to changes in the nucleotide sequence of a gene.

Examples:

• Substitution
• Insertion
• Deletion

Example

Sickle Cell Disease

A single nucleotide substitution in the β-globin gene changes glutamic acid to valine.

B. Chromosomal Mutation

Occurs due to structural changes in chromosomes.

Types

1. Deletion

Loss of chromosome segment.

2. Duplication

Repeated chromosome segment.

3. Inversion

Chromosome segment reverses orientation.

4. Translocation

Exchange of chromosome segments between non-homologous chromosomes.

C. Genome Mutation

Changes in chromosome number.

Examples:

• Aneuploidy
• Polyploidy

Polyploidy is common in plants and can rapidly produce new species.

Causes of Mutations

Spontaneous Mutations

Occur naturally during DNA replication.

Induced Mutations

Caused by mutagens.

Physical Mutagens

• X-rays
• Gamma rays
• UV radiation

Chemical Mutagens

• Nitrous acid
• Mustard gas
• Benzene derivatives

Biological Mutagens

• Certain viruses
• Transposable genetic elements

Significance of Mutation

Positive Effects

• Produces new alleles.
• Increases genetic diversity.
• Provides material for natural selection.
• Can produce advantageous adaptations.

Negative Effects

• Genetic diseases
• Reduced fitness
• Lethal mutations

Genetic Recombination

Definition

Genetic recombination is the formation of new combinations of genes during sexual reproduction.

Unlike mutation, recombination does not create new alleles, but rearranges existing alleles into new combinations.

Mechanisms of Recombination

1. Crossing Over

Occurs during Prophase I (Pachytene stage) of meiosis. Homologous chromosomes exchange corresponding segments.

Importance

Produces recombinant chromosomes.

2. Independent Assortment

Occurs during Metaphase I. Chromosomes align independently. Each gamete receives a different combination of maternal and paternal chromosomes.

3. Random Fertilization

Any sperm may fertilize any ovum. This creates enormous genetic diversity.

Importance of Recombination

• Produces genetic diversity.
• Generates new gene combinations.
• Increases adaptability.
• Enhances evolutionary potential.
• Maintains healthy populations.

Comparison: Mutation Vs Recombination

Mutation	Recombination
Produces new alleles	Rearranges existing alleles
Rare	Occurs in every meiosis
Random	Regular process
Can create new traits	Produces new combinations of existing traits
Ultimate source of variation	Major source of diversity among offspring

 

Natural Selection

Definition

Natural selection is the process by which individuals possessing favorable heritable traits survive and reproduce more successfully than others.

Proposed by Charles Darwin. Natural selection acts on phenotypes, but evolution occurs through changes in genotypes (allele frequencies).

Principles of Natural Selection

1. Overproduction

Organisms produce more offspring than can survive.

Examples:

• Fish produce thousands of eggs.
• Plants produce thousands of seeds.

2. Struggle for Existence

Because resources are limited, competition occurs.

Types

A. Intraspecific

Competition within the same species. Example: Two deer competing for mates.

B. Interspecific

Competition between different species. Example: Lions and hyenas competing for prey.

C. Environmental

Competition against environmental factors.

Examples:

• Drought
• Flood
• Disease
• Cold

3. Variation

Individuals differ genetically. Natural selection acts upon these differences.

4. Differential Survival

Individuals possessing advantageous characteristics survive better.

5. Differential Reproduction

Surviving individuals produce more offspring. Beneficial alleles increase in frequency.

Types of Natural Selection

1. Stabilizing Selection

Characteristics

• Favors average phenotype.
• Eliminates extremes.
• Maintains stability.

Example: Human birth weight. Very small and very large babies have lower survival.

2. Directional Selection

Characteristics

• Favors one extreme.
• Population shifts toward one side.

Example: Industrial melanism in the Peppered Moth.

3. Disruptive Selection

Characteristics

• Favors both extremes.
• Eliminates intermediate individuals.
• May lead to speciation.

Example: Seed-eating birds with either very large or very small beaks.

Natural Selection In Action

Industrial Melanism

Organism: Peppered moth (Biston betularia)

Before industrialization:

• Light-colored moths predominated due to camouflage on lichen-covered trees.

After industrialization:

• Soot darkened tree trunks.
• Dark-colored moths survived better.
• Their frequency increased.

This demonstrates directional selection.

Antibiotic Resistance

Some bacteria possess mutations that confer resistance. When antibiotics are used:

• Susceptible bacteria die.
• Resistant bacteria survive.
• Resistant bacteria reproduce.

Result: The population becomes resistant over time.

Pesticide Resistance

Repeated pesticide use selects resistant insects.

Examples:

• Mosquitoes resistant to insecticides.
• Crop pests resistant to pesticides.

Relationship Between Variation, Mutation, Recombination, And Natural Selection

Mechanism	Role in Evolution
Variation	Provides diversity
Mutation	Creates new alleles
Recombination	Produces new gene combinations
Natural Selection	Selects favorable variations
Evolution	Results from changes in allele frequencies

 

Adaptation

Adaptation is an inherited characteristic that increases an organism's ability to survive and reproduce in its environment.

Examples:

• Camouflage in insects
• Streamlined body of fish
• Thick fur in polar bears
• Long neck of giraffes (through selection of naturally occurring variation)

Adaptations arise gradually through natural selection acting on heritable variation.

Evolutionary Outcome

When advantageous alleles continue to accumulate: Population changes genetically.

Eventually:

• Reproductive isolation develops.
• New species arise.

This process is known as speciation.

Summary Flow Chart

Mutation
↓
New Alleles
↓
Recombination During Meiosis
↓
New Gene Combinations
↓
Genetic Variation in Population
↓
Natural Selection
↓
Survival of Better-Adapted Individuals
↓
Increased Frequency of Beneficial Alleles
↓
Adaptation
↓
Speciation
↓
Evolution

High-Yield Facts

·       Evolution is a change in allele frequencies in a population over generations.

·       Population—not the individual—is the unit of evolution.

·       Variation is the raw material for evolution.

·       Mutation is the ultimate source of new genetic variation.

·       Recombination creates new combinations of existing alleles but does not produce new alleles.

·       Crossing over occurs during Pachytene stage of Prophase I.

·       Independent assortment occurs during Metaphase I of meiosis.

·       Natural selection acts on phenotypes, whereas evolution is reflected in changes in genotypes (allele frequencies).

·       The three major patterns of natural selection are:

·       Stabilizing selection

·       Directional selection

·       Disruptive selection

·       Industrial melanism, antibiotic resistance, and pesticide resistance are classic examples of natural selection in action.

Summary

• Variation is essential for evolution; without heritable variation, natural selection cannot operate.
• Mutation introduces new alleles, whereas recombination reshuffles existing alleles.
• Natural selection is not random; it consistently favors individuals with advantageous heritable traits under specific environmental conditions.
• Evolution does not have a predetermined goal—it reflects changes in populations as they adapt to changing environments through the interaction of variation, mutation, recombination, and natural selection.