What is organic evolution ? Highlight the mechanisms governing organic evolution.

Defining Organic Evolution

Organic evolution is not a directed process with a pre-determined goal. It’s a branching, opportunistic process driven by several interacting mechanisms. It’s important to distinguish between microevolution (changes within a species) and macroevolution (evolution above the species level, leading to the formation of new species and higher taxa).

Mechanisms Governing Organic Evolution

1. Natural Selection

Natural selection, proposed by Darwin and Wallace, is the cornerstone of evolutionary theory. It operates on the principle that individuals with traits better suited to their environment are more likely to survive and reproduce, passing on those advantageous traits to their offspring. This leads to a gradual shift in the genetic composition of the population.

• Example: The peppered moth (*Biston betularia*) in England. During the Industrial Revolution, darker moths became more common as they were better camouflaged against soot-covered trees, avoiding predation by birds.

2. Genetic Drift

Genetic drift refers to random fluctuations in allele frequencies within a population, particularly pronounced in small populations. It’s a non-adaptive mechanism, meaning it doesn’t necessarily lead to adaptations that improve survival. Two key types of genetic drift are:

• Bottleneck Effect: A drastic reduction in population size due to a random event (e.g., natural disaster) leading to a loss of genetic diversity.
• Founder Effect: A small group of individuals colonizes a new area, carrying only a subset of the original population’s genetic diversity.

Example: The Northern elephant seal population experienced a severe bottleneck in the 19th century due to hunting, resulting in significantly reduced genetic variation.

3. Gene Flow (Migration)

Gene flow is the transfer of genetic material from one population to another. This can occur through the movement of individuals or gametes (e.g., pollen). Gene flow tends to reduce genetic differences between populations.

• Example: The migration of birds between different regions can introduce new alleles into local populations, increasing genetic diversity.

4. Mutation

Mutation is the ultimate source of all new genetic variation. It’s a change in the DNA sequence, which can be spontaneous or induced by environmental factors. Mutations can be beneficial, harmful, or neutral.

• Types of Mutations: Point mutations, frameshift mutations, chromosomal mutations.

While most mutations are neutral or harmful, occasionally a mutation arises that provides a selective advantage.

5. Recombination (Genetic Recombination)

Recombination occurs during sexual reproduction, specifically during meiosis. It involves the exchange of genetic material between homologous chromosomes, creating new combinations of alleles. This process significantly increases genetic variation within a population.

• Significance: Recombination doesn’t create new alleles, but it shuffles existing ones, providing the raw material for natural selection to act upon.

6. Non-Random Mating

Unlike random mating, non-random mating occurs when individuals choose mates based on specific traits. This can alter allele frequencies and lead to increased homozygosity.

• Example: Assortative mating (individuals with similar phenotypes mate more frequently) and inbreeding (mating between closely related individuals).

Evolutionary Processes at Different Scales

Process	Scale	Effect
Natural Selection	Micro & Macro	Adaptive changes, speciation
Genetic Drift	Micro	Random changes in allele frequencies, loss of diversity
Gene Flow	Micro	Homogenization of populations, introduction of new alleles
Mutation	Micro	Source of new genetic variation