Evolution & Speciation

Learning Objective: A4.1.1 - Evolution as change in the heritable characteristics of a population

What is Evolution?

• “Heritable” means the traits are genetically passed from parents to offspring.

• Evolution only occurs in populations, not in individuals.

• Changes must accumulate across generations; one-time changes do not qualify as evolution.

Distinguishing Evolution from Acquired Traits

• Heritable characteristics are encoded in DNA and passed on via reproduction.

• Acquired characteristics (e.g., muscle gain from weightlifting, song learning in birds):

  • Are gained during an individual’s lifetime

  • Do not alter DNA

  • Are not passed on to offspring

Key Concept: Evolution requires changes to the genetic code, not just physical traits or learned behaviors.

Lamarckism vs Darwinian Evolution

Jean-Baptiste Lamarck (1809)

Theory of Use and Disuse: Organs used frequently become stronger; unused ones weaken.

Inheritance of Acquired Traits: Traits developed during a lifetime (e.g., strong arms from blacksmithing) are inherited by offspring.

• Environment causes new needs, leading to structural changes passed to the next generation.

• Example Lamarck gave:

  • Giraffes stretched their necks → longer necks passed on

Why Lamarckism is incorrect:

• Experiments show acquired traits are not inherited.

• No known mechanism exists for passing environmentally caused bodily changes to DNA.

Charles Darwin (1859)

Developed the theory of evolution by natural selection.

Darwin published On the Origin of Species in 1859. His theory was based on genetic variation, selection pressure, and inheritance.

Key concept: individuals with advantageous traits are more likely to survive and reproduce, passing those traits on.

Applied example with Giraffes: how to frame it correctly

• Incorrect (Lamarck): stretching → longer neck within lifetime → inherited.

• Correct (Darwin): offspring vary in neck length due to alleles → longer-neck individuals have survival advantage where tall trees are the main food source → they reproduce and pass on allele for longer necks → allele frequencies shift → population neck length increases over generations.

What Is a Scientific Theory vs a Scientific Fact?

🔍 Scientific Theories

• A scientific theory is a well-substantiated explanation of an aspect of the natural world.

• It is based on a large body of evidence, repeated testing, and observation.

• Theories explain the “why” and “how” of natural phenomena.

• A theory can never be proven in the absolute sense, but it can be strongly supported.

🔍 Scientific Facts

• A fact is an observable, measurable event or piece of data.

• It describes what is happening, but not why it happens.

• Facts can be used as evidence to support or challenge a theory.

The fact: Populations of organisms change over time.

The theory: Natural selection is the mechanism that explains how and why those changes occur.

Scientific Theories and Paradigm Shifts

• A paradigm is the dominant set of beliefs, values, and methods accepted by the scientific community.

• A paradigm shift occurs when new evidence or ideas challenge and replace an existing framework.

Paradigm Shifts in Evolutionary Biology

• Before Darwin: The dominant paradigm was Lamarckism—inheritance of acquired traits.

• After Darwin: The paradigm shifted to natural selection as the mechanism of evolution.

• Modern synthesis (20th century): Combined Darwin’s theory with Mendelian genetics → formed today’s evolutionary paradigm.