VARIATION

TOPIC 3: VARIATION

Lesson Objectives

• Explain variation in population

3.1: INTRODUCTION

Definition: Variation refers to differences between individuals of a plant or animal species, arising from genetic and environmental factors.

Sources of Genetic Variation:

• Mutation: Any change in the DNA sequence.

• Recombination: The process of shuffling genes during reproduction.

Importance: Significant genetic variation enhances the survival of species in changing environments as some individuals will retain advantageous traits to pass to their offspring.

Types of Variation

Major Types

• A - Discontinuous Variation: Sudden shifts between two traits with no intermediates.

• B - Continuous Variation: Displays a gradual transition from one extreme to another. Characterized by slight differences that can blend.

Comparison of Continuous and Discontinuous Variation

• Continuous Variation: No distinct categories for individuals. Gradual differences observed in phenotypic spectrum. Quantitative characters can be measured. Normal distribution curve observed.

• Discontinuous Variation: Clearly defined categories. No intermediate phenotypes. Qualitative characters without measure.

SOURCES OF VARIATION

Types of Variation Sources:

• Chromosome Mutation

• Gene Mutation: Includes insertion, deletion, substitution, inversion.

• Random Assortment: Chromosome combination reshuffling.

• Random Fertilization: Combination of any male and female gametes.

3.2: SELECTION

Lesson Objective:

• Explain natural selection

Types of Selection

• Natural Selection: The process by which individuals better adapted to their environment tend to survive and reproduce.

• Artificial Selection: Breeding of organisms by humans to produce desired traits.

Natural Selection Processes

• Favorable Genes: Selected; unfavorable genes eliminated.

• Adaptability: Organisms that adapt to environmental changes reproduce successfully, contributing to the gene pool.

Types of Natural Selection

• Stabilizing Selection: Intermediate phenotypes are favored; extremes are eliminated (e.g., human birth weight).

• Directional Selection: Favors one extreme phenotype; shifts distribution curve in one direction. Example: Industrial melanism in peppered moths where darker moths had survival advantages.

• Disruptive Selection: Favors two extreme phenotypes and acts against intermediates in fluctuating environments (e.g., Galapagos finches with different beak sizes).

3.3: SPECIATION

Definition: Speciation is the formation of new species through evolution.

Biological Species Concept:

A species is a group of populations that can interbreed to produce fertile offspring.

Modes of Speciation

• Allopatric Speciation: Occurs due to geographical separation, leading to reproductive isolation as populations evolve independently. For example, a river may split a species, preventing gene flow.

• Sympatric Speciation: New species arise from populations within the same geographical area. This can occur through processes like polyploidy in plants, where an increase in chromosome number results in reproductive isolation.

Factors in Speciation

• Isolation: Physical barriers or behavioral differences can isolate populations, preventing interbreeding.

• Genetic Drift: Random changes in allele frequencies, which can lead to significant differences in small populations.

• Hybridization: Breeding between closely related species, which may introduce new genetic combinations and can occasionally lead to the formation of new species.

• Adaptive Radiation: Rapid formation of new species originating from a common ancestor adapting to various environments, such as Darwin's finches in the Galapagos, where different beak shapes evolved to utilize different food sources.

Isolation and Reproductive Barriers

Types of Isolation

• Prezygotic Barriers: Prevent fertilization (e.g., temporal isolation where species mate at different times, habitat isolation where species occupy different habitats, behavioral isolation where differences in mating behavior prevent breeding, mechanical isolation where reproductive organs are incompatible, gametic isolation where sperm and egg cannot fuse).

• Postzygotic Barriers: Prevent viable offspring from developing (e.g., hybrid inviability where hybrids fail to develop properly, hybrid sterility where hybrids are sterile, hybrid breakdown where offspring are fertile but their descendants are inviable or sterile).

Genetic Drift

Definition: A random process where allele frequencies change over time, especially impactful in small populations.

Examples

• Founder Effect: Rare alleles become prevalent in a small founding population due to limited genetic diversity.

• Bottleneck Effect: A drastic reduction in population size leads to loss of genetic variability, making the population vulnerable to future changes and diseases.

Hybridization and Adaptive Radiation

• Hybridization: Breeding between closely related species can lead to viable offspring that may themselves establish a new species.

• Adaptive Radiation: A type of evolution in which organisms diversify rapidly into a wide variety of forms to adapt to different environments.

Key Examples

Darwin's finches with different beak shapes adapted to available food sources.

Conclusion

Understanding variation, selection, and speciation is crucial for grasping evolutionary biology and the dynamics of populations in nature.