Biology 1 - Lesson 22: Principles of Evolution and Natural Selection

Evolutionary biology provides the framework for understanding how life diversifies and adapts over time. Charles Darwin's concept of natural selection—supported by a wealth of modern genetic and ecological evidence—remains the cornerstone of evolutionary theory. This lesson explores the historical context, fundamental premises, and key lines of evidence for evolution, as well as the concept of evolutionary fitness and adaptation.

Foundations of Evolutionary Thought

Prior to Darwin, scientists recognized that species could change over generations, but lacked a unifying mechanism. Darwin (and independently Alfred Russel Wallace) proposed that variation among individuals, coupled with competition for limited resources, drives evolution through natural selection. Key influences included:

• Geology: Charles Lyell’s uniformitarianism suggested Earth is ancient, allowing gradual processes to shape life.
• Population Studies: Thomas Malthus’s essay on populations and resource limitations sparked Darwin’s idea of competition.
• Biogeography: Observations of finches and tortoises on the Galápagos Islands hinted at divergence from common ancestors.

Mermaid Diagram: Simplified Steps in Natural Selection

In natural selection, heritable variations that enhance survival or reproduction become more common in the population:

flowchart TB A[Genetic Variation in a Population] --> B[Competition for Resources] B --> C[Some Traits Confer Advantage] C --> D[Increased Survival/Reproduction] D --> E[Advantageous Alleles Passed On] E --> F[Population Gradually Changes]

A mermaid flowchart illustrating the core steps of natural selection, starting from heritable variation to population-wide shifts in traits.

Evidence for Evolution

Multiple lines of evidence support evolutionary change:

Key Evidence Types

Line of Evidence	Examples
Fossil Record	Progression of transitional forms (e.g., dinosaur-bird connections, whale ancestors)
Comparative Anatomy	Homologous structures (forelimbs in mammals), vestigial organs (human appendix, whale pelvic bones)
Biogeography	Endemic species on islands, convergent evolution in separate habitats
Embryology	Similar developmental pathways (e.g., pharyngeal arches in vertebrate embryos)
Molecular Biology	DNA/protein sequence similarities, universal genetic code

D3-Based Scatter Plot: Trait Variation and Fitness

Below is a hypothetical scatter plot showing a quantitative trait (e.g., beak depth) versus relative fitness in a bird population:

Hypothetical scatter plot showing a quantitative trait (beak depth) vs. relative fitness in birds. Traits associated with higher fitness can increase in the population over generations.

Evolutionary Fitness and Adaptation

Fitness in an evolutionary context measures an organism’s reproductive success relative to others in the population. Traits that enhance survival or mating success (under given environmental conditions) may be termed adaptive. Over generations, adaptive traits become more common, guiding the direction of evolution. However, environments change, so what’s adaptive in one context may be neutral or detrimental in another.

Conclusions

Darwin’s insights into variation, heredity, and differential reproductive success established the foundation for evolutionary biology. Contemporary evidence, from molecular homologies to direct observations of adaptation, consistently underscores natural selection as a critical mechanism by which populations evolve. By recognizing how traits arise, persist, or vanish under selection pressures, we better understand the dynamic interplay of organisms and environments shaping the biodiversity on Earth.