1 for Unit 7

Natural Selection Overview

• Natural selection is a key mechanism in evolution that operates by scrutinizing variations in organisms worldwide.

• Key Quotes:

  • Charles Darwin described natural selection as a process improving organic beings according to their conditions of life.

Charles Darwin

• Background: English naturalist, well-known for research conducted in the Galapagos Islands.

Darwin’s Research

• Focused on biogeography: the geographic distribution of species.

• Hypothesis: Organisms migrated from South America to the Galapagos Islands, where they diversified and evolved into new species.

• Special emphasis on the finches as a primary example of his research.

Darwin’s Conclusions: Theory of Natural Selection

• Key Points:

  • Variation exists within natural populations.

  • Many offspring are born than can survive (overproduction).

  • Leads to a struggle for existence due to competition for survival.

  • Characteristics that are beneficial tend to become more common (adaptations).

  • Over time, steady input of new variations leads to the emergence of new species.

Comparison: LaMarck vs. Darwin

• LaMarck's Theory:

  • Believed in adaptations via acquired traits during an organism's life (e.g., giraffe neck stretching passes to offspring).

• Darwin's Theory:

  • Emphasizes natural selection where the most fit (long-necked giraffes) survive and pass on traits.

Understanding Fitness

• Definition: Reproductive success; individuals with certain phenotypes produce more surviving offspring.

• Commonly referred to as “survival of the fittest.”

  • High fitness: ability to survive and reproduce.

  • Low fitness: inability to survive or reproduce.

True or False Statements

• Statement 1: "Natural selection gives organisms what they need."

  • Answer: False. Natural selection acts on existing variations.

• Statement 2: "Natural selection acts for the good of species."

  • Answer: False. It operates on individual traits without any intention to benefit the species.

• Statement 3: "Natural selection involves organisms trying to adapt."

  • Answer: False. It operates on random mutations and genetic variations.

• Statement 4: "Fittest means strongest or fastest."

  • Answer: False. "Fittest" refers to reproductive success, not overall health or speed.

Evolution and Descent with Modification

• Harvey proposed that evolution means change in genetic makeup over time, explaining descent with modification.

• Key Components:

  • Heritable traits change generationally due to natural selection.

Process of Natural Selection

• Definition: A mechanism in which certain traits increase survival and reproductive rates.

• Mechanics:

  • Natural selection acts on phenotypic variations.

  • Selective pressures from ever-changing environments can drive this process.

Key Observations by Darwin

1. Heritability of Traits: Characteristics are passed from parent to offspring.

2. Overproduction of Offspring: Leads to competition for resources and survival, influencing which traits become more prevalent in a population.

Examples of Natural Selection

• Antibiotic Resistance in Bacteria: Resistant individuals survive treatment and reproduce, leading to a populace that is increasingly resistant.

• Peppered Moths: The shift during the Industrial Revolution where darker moths had a survival advantage against soot-covered trees.

Selection Graphs

• Types of Natural Selection:

  • Disruptive Selection: Favors extreme phenotypes.

  • Stabilizing Selection: Favors intermediate phenotypes.

  • Directional Selection: Favors one extreme phenotype.

Artificial Selection vs. Natural Selection

• Artificial Selection: Selective breeding by humans for desirable traits in domesticated species (e.g., pets, livestock).

• Natural Selection: Operates without human intervention, acting on traits beneficial for survival and reproduction.

Selective Breeding / Artificial Selection

• Human-directed variation in species leads to targeted traits being passed down (example: breeding in dogs and livestock).

Practical Application Exercise

• Example of Natural Selection:

  • Scenario of pacas (rodents) losing body mass due to a recent fire depleting food, leading to thinner subsequent generations is an example of descent with modification due to environmental pressures.