Natural Selection and Evolution: Key Concepts and Evidence

Natural Selection and Evolution

Introduction to Evolutionary Theory

Evolution by natural selection is a foundational concept in modern biology, explaining the diversity of life and the adaptation of organisms to their environments. This topic covers the historical development of evolutionary thought, the contributions of key scientists, and the evidence supporting evolution.

• Evolution: The change in the heritable characteristics of biological populations over successive generations.

• Natural Selection: The process by which organisms better adapted to their environment tend to survive and produce more offspring.

• Biodiversity: The variety and variability of life forms within a given ecosystem, region, or on the entire Earth.

Concept Map: Evolution in Biology

Evolution explains the increase or decrease of biodiversity, influences the production of genetic diversity, and leads to the development of phylogeny (the evolutionary history and relationships among species).

• Evolution creates and influences biodiversity.

• Genetic diversity is produced and altered through evolutionary processes.

• Phylogeny is constructed based on evolutionary relationships.

Historical Models of Evolution

Western Thinking: Models of Diversity Over Time

Three major models have shaped Western understanding of how life changes over time:

• Typological Thinking: Species are static and unchanging; variation is ignored.

• Lamarckian Evolution: Species change through time via inheritance of acquired traits. For example, giraffes stretch their necks and pass longer necks to offspring.

• Darwin and Wallace's Model: Species change through time and share common ancestry. Variation within populations is essential, and natural selection acts on this variation.

Darwin & Wallace: A Revolutionary Perspective

Darwin and Wallace overturned the idea that species are static. They replaced typological thinking with population thinking and proposed a testable mechanism for evolutionary change: natural selection.

• Species are dynamic and change over time.

• Variation within populations is critical for evolution.

• Mechanisms of change can be scientifically tested.

• Example: Observations from the Galapagos Islands contributed to Darwin's theory.

Pattern of Evolution

Descent with Modification and Common Ancestry

The pattern of evolution describes how species change over time and are related by common ancestry.

• Descent with Modification: Species change over time, producing new, modified species from ancestral species.

• Common Ancestry: All species are related through descent from a common ancestor.

Example: The evolutionary tree (phylogeny) shows the relationships among modern tetrapods and their common ancestor.

Evidence for Evolution

Types of Evidence

Multiple lines of evidence support the theory of evolution, including fossil records, vestigial traits, and homology.

Type of Evidence	Prediction 1: Species Change Through Time	Prediction 2: Species Are Related by Common Ancestry
Fossil Record	Layers of sedimentary rock contain fossils of extinct species.	Similar species are found in the same geographic areas (e.g., fossil species found in Galapagos Islands).
Transitional Forms	Fossils document gradual changes and intermediate forms.	Related, geographically close species share traits.
Vestigial Traits	Traits that are reduced or nonfunctional in some species but functional in others (e.g., human tailbone).	Shared vestigial structures indicate common ancestry.
Homology	Similar structures in different species due to shared ancestry (e.g., limb bones in vertebrates).	Homologous structures can be traced to a common ancestor.

Sedimentary Rocks and Geologic Time

Sedimentary rocks provide evidence for the vastness of geologic time, showing layers that contain fossils and document evolutionary changes over millions of years.

• Rock layers (strata) reveal the sequence of evolutionary events.

• Fossils found in different layers indicate changes in species over time.

Key Terms and Concepts

• Vestigial Traits: Reduced or nonfunctional traits inherited from ancestors.

• Homology: Similarity in traits due to shared ancestry.

• Phylogeny: The evolutionary history and relationships among species.

• Transitional Forms: Fossils or organisms that show intermediate states between ancestral and derived species.

Summary of Darwin's Four Postulates

Darwin's Four Postulates of Natural Selection

Darwin proposed four postulates that explain how natural selection leads to evolution:

1. Individuals in a population vary in their traits.

2. Some trait differences are heritable.

3. More offspring are produced than can survive; only some survive and reproduce.

4. Individuals with advantageous traits are more likely to survive and reproduce, passing those traits to the next generation.

Common Misconceptions

Misconceptions about Evolution and Natural Selection

• Evolution does not occur in individuals, but in populations over generations.

• Natural selection does not create new traits; it acts on existing variation.

• Evolution is not goal-directed; it is a response to environmental pressures.

Additional info: These notes expand on brief slide points to provide a comprehensive overview suitable for exam preparation in General Biology.