Evolution, Natural Selection, and Speciation: The Origins of Biodiversity

Evolution and Biodiversity

Introduction to Biodiversity and Evolution

Biodiversity refers to the variety of life forms found on Earth. Understanding its origins requires studying evolution, the process by which genetic changes occur in populations over time, leading to the diversity of species.

• Evolution: Genetic changes in populations of organisms through time that lead to differences among them.

• Population: A group of individuals of the same species living in the same area.

• Gene: A unit of DNA that provides a specific function in an organism.

• Allele: A version of a gene (e.g., flower color gene has alleles for purple or white).

• Status of Evolution: Evolution is considered a scientific theory, meaning it is a well-tested and reliable explanation.

Theses of Evolution from The Origin of Species

Core Principles

Charles Darwin's work established foundational principles for understanding evolution:

1. There is heritable variation in the traits of living things.

2. All species have descended, with modification, from common ancestors.

3. Species are modified (changed) through natural selection.

Heritable Variation

Individuals in populations differ from one another due to genetic variation. This variation is essential for evolution to occur.

• Example: Variation in shell patterns and colors among snails in a population.

Descent with Modification

Species change over generations, inheriting traits from ancestors but also acquiring new modifications.

• Example: Fossil records showing gradual changes in fish pelvic bones over time.

Natural Selection

Definitions and Concepts

Natural selection is a key mechanism of evolution, driving adaptation and change in populations.

• Natural Selection: Differences in survival and/or reproduction among individuals in a population that differ in one or more traits.

• Mechanism of Evolution: Natural selection is one way that evolution happens; other mechanisms exist but are not covered here.

• Adaptation: The adjustment of species to their environment through natural selection.

Classic Example: The Peppered Moth

Environmental changes can shift which traits are favorable:

• White peppered moths were abundant when trees were covered with light-colored lichens.

• After air pollution killed lichens, black moths became more abundant due to better camouflage against soot-blackened trunks.

Key Points About Evolution Through Natural Selection

• Individuals do not evolve. Populations do. Favorable traits become more common in the population over time.

• No directed purpose or goal. Natural selection simply favors traits that improve survival and reproduction.

• Favorable alleles arise by chance mutation, not on demand. Genetic variation is essential for natural selection to operate.

Human Activity and Natural Selection: Antibiotic Resistance

Antibiotic Resistance

Human use of antibiotics has led to the evolution of resistant bacteria, a clear example of natural selection in action.

• Antibiotics: Chemicals effective against bacteria, used to treat and prevent illness in humans and animals, and to sterilize environments.

• Selection Process: Bacteria with resistance alleles survive antibiotic treatment and reproduce, increasing the frequency of resistance in the population.

Stage	Description
Before Selection	Bacteria with varying resistance levels; antibiotics kill those with low resistance.
After Selection	Resistant bacteria survive and reproduce by simple division.
Final Population	Population is more resistant to antibiotics than the original one.

Mechanisms of Resistance Spread

• Conjugation: DNA is transferred directly from one cell to another, spreading resistance alleles.

• Simple Division: Bacteria reproduce rapidly, allowing resistant alleles to proliferate.

• Short Generation Times: New mutations and alleles arise quickly.

Solutions to Antibiotic Resistance

• Minimize use of antibiotics when unnecessary.

• Complete prescribed antibiotic courses.

• Avoid using human antibiotics in agriculture.

• Use multiple antibiotics for serious infections.

• Reserve certain antibiotics as drugs of last resort.

• Develop new antibiotics targeting bacteria in novel ways.

Speciation: The Formation of New Species

Definition of Species

A species consists of one or more populations of individuals that can interbreed and are reproductively isolated from other groups.

• Interbreeding: The ability to produce fertile offspring is key to defining species.

• Reproductive Isolation: Prevents gene flow between populations, leading to speciation.

Reproductive Isolating Mechanisms

• Isolation in Time: Different breeding or flowering periods.

• Use of Different Pollinators: Species may rely on different organisms for pollination.

• Behavioral Differences: Distinct mating rituals.

• Isolation of Gametes: Gametes may not be compatible.

• Hybrid Inviability and Infertility: Hybrids may not survive or reproduce.

Steps to Speciation

1. Isolation of populations (often geographic).

2. Evolution within populations.

3. Genetic differences arise, especially in genes controlling reproduction.

4. Reproductive isolating mechanisms develop.

5. Two distinct species are formed.

Additional info: Speciation is a gradual process, often requiring many generations and environmental changes to complete. Mechanisms such as genetic drift, mutation, and selection all contribute to the divergence of populations.