Evolution, Natural Selection, and the Origin of Biodiversity

Evolution and the Origin of Biodiversity

Introduction to Biodiversity and Evolution

Biodiversity refers to the variety of life forms on Earth. To understand its origins, it is essential to study evolution, the process by which populations of organisms change genetically over time, leading to the diversity of species observed today.

Key Definitions in Evolution

• 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., a flower color gene may have alleles for purple or white).

• Theory (in science): An explanation that has been extensively tested and can be relied upon with a high degree of confidence.

Theses of Evolution from The Origin of Species

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 heritable, meaning it can be passed from parents to offspring.

Descent with Modification

All species share common ancestors and have diverged over time through gradual changes. Fossil evidence, such as changes in bone structure, supports this concept.

Natural Selection

Natural selection is the process by which certain traits become more common in a population because they confer a survival or reproductive advantage.

• Definition: 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 occurs, resulting in adaptation to the environment.

Classic Example: The Peppered Moth

• White peppered moths were abundant in England when trees were covered with light-colored lichens, providing camouflage from predators.

• After industrial pollution killed lichens and darkened tree trunks, black moths (previously rare) became more common because they were better camouflaged and less likely to be eaten.

Discussion Questions

• Did individual white moths become black when the environment changed? (No, individuals do not change color; the frequency of black moths increased due to selection.)

• Would natural selection create black moths if none existed in the population? (No, natural selection can only act on existing variation.)

• Do individuals want to change their color? (No, changes occur by chance mutation, not by desire.)

Key Points About Evolution Through Natural Selection

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

• No directed purpose or goal. Natural selection is not goal-oriented; it simply favors traits that increase survival and reproduction in a given environment.

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

Natural Selection in Action: Herbicide and Antibiotic Resistance

Herbicide Resistance in Weeds

• When a population of weeds is exposed to an herbicide (e.g., Round-Up), most plants may die, but those with a resistant genotype survive and reproduce.

• If all plants have the same genotype and no mutations occur, evolution cannot happen.

Antibiotic Resistance in Bacteria

• Antibiotics are chemicals that kill bacteria. Overuse in medicine and agriculture selects for resistant bacteria.

• Bacteria can transfer resistance alleles between cells and species via conjugation (direct DNA transfer).

• Bacteria reproduce rapidly, so resistance alleles can spread quickly.

Stage	Description
Before selection	Population contains both low and high resistance bacteria.
After selection	Antibiotic kills low resistance bacteria; high resistance bacteria survive and reproduce.
Final population	Population is more resistant overall than the original.

Solutions to Antibiotic Resistance

• Minimize unnecessary use of antibiotics.

• 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 with novel mechanisms.

Speciation: The Formation of New Species

Definition of Species

• A species is one or more populations of individuals that can interbreed (produce fertile offspring) and are reproductively isolated from other groups.

• Reproductive isolation is key: if two populations evolve barriers to interbreeding, they become different species.

Reproductive Isolating Mechanisms

• Temporal isolation: Breeding at different times.

• Pollinator isolation: Use of different pollinators.

• Behavioral isolation: Differences in mating rituals.

• Gametic isolation: Gametes cannot fuse.

• Hybrid inviability/infertility: Hybrids do not survive or are sterile.

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. Populations become two different species.

Example: Geographic barriers such as rivers or mountains can separate populations, leading to independent evolution and eventual speciation.