Chapter 22: Evolution by Natural Selection – Study Notes

Chapter 22: Evolution by Natural Selection

Introduction

Evolution by natural selection is a foundational theory in modern biology, explaining how species change over time and how they are related through common ancestry. This chapter explores the evidence for evolution, the mechanisms behind it, and the implications for understanding biodiversity.

Darwin & Wallace: Evolution by Natural Selection

Historical Context and Core Principles

• Darwin and Wallace independently proposed that species change through time, not in a linear or progressive pattern, but based on variation among individuals in populations.

• Population thinking replaced typological thinking, emphasizing the importance of individual variation within populations.

• Individuals with certain traits have higher reproductive success, leading to changes in populations over generations.

Core Principles:

• Common Ancestry: All organisms are connected by descent from ancestors.

• Natural Selection: Survival and reproduction depend on traits that affect fitness in a given environment.

Patterns and Evidence for Evolution

Descent with Modification

• Evolution is described as descent with modification: modern species are modified descendants of ancestral species.

• Two main predictions:

  1. Species change through time.

  2. Species are related by common ancestry.

Evidence 1: The Vastness of Geologic Time

• Fossils are traces of past organisms, forming the fossil record.

• Sedimentary rocks form in layers, with younger layers on top of older ones, allowing for relative dating of fossils.

• Geologic time scale divides Earth's history into eons, eras, periods, and epochs.

• Absolute dating uses methods like radiometric dating (e.g., C14 half-life = 5730 years) to assign ages to rocks and fossils.

Key Fact: Radiometric dating shows Earth is about 4.6 billion years old.

Evidence 2: Extinction

• Many fossils represent extinct species, showing that species are dynamic and change over time.

• Over 99% of all species that have ever lived are now extinct.

• Extinctions have occurred continuously throughout Earth's history.

Evidence 3: Transitional Features

• Fossil species often show transitional features—traits intermediate between ancestral and derived species.

• This supports the idea of gradual change and common ancestry.

• Law of succession: Fossil and living species in the same area are often similar, indicating descent with modification.

Example: Tiktaalik is a transitional fossil showing features between fish and tetrapods.

Evidence 4: Vestigial Traits

• Vestigial traits are reduced or nonfunctional structures similar to functioning organs in related species.

• Examples include tiny hip bones in whales, reduced wings in flightless birds, and eye sockets in blind cave fish.

Evidence 5: Species Are Changing Today

• Evolution can be observed in real time, such as the evolution of antibiotic resistance in bacteria and changes in beak shape in finches.

• Evolutionary changes can occur over days, weeks, months, or years—not just over millennia.

Evidence 6: Similar Species in the Same Place

• Darwin observed that species on the Galapagos Islands were similar to those on the mainland, suggesting common ancestry.

• DNA analysis places Galapagos finches on a phylogenetic tree, confirming their shared ancestry.

Evidence 7: Related Species Share Homologies

• Homology is similarity due to shared ancestry, recognized at three levels:

  1. Genetic Homology: Similarity in DNA, RNA, or amino acid sequences. The genetic code is nearly universal among organisms.

  2. Developmental Homology: Similarity in embryonic development, such as pharyngeal pouches in vertebrate embryos.

  3. Structural Homology: Similarity in adult morphology, such as the limb bones of vertebrates.

Mechanisms and Internal Consistency

Testing Evolutionary Theory

• Theory of evolution by natural selection is testable through observation and experimentation.

• Multiple independent data sets (fossils, DNA, morphology) provide internal consistency for evolutionary predictions.

• Example: The evolution of whales is supported by fossil transitions, DNA evidence, and vestigial structures.

Key Terms and Concepts

• Natural Selection: Differential survival and reproduction of individuals due to differences in phenotype.

• Fitness: The ability of an organism to survive and reproduce in its environment.

• Adaptation: A heritable trait that increases an organism's fitness.

• Speciation: The process by which one species splits into two or more descendant species.

Table: Types of Homology

Type of Homology	Description	Example
Genetic	Similarity in DNA, RNA, or protein sequences	Genetic code, gene sequences
Developmental	Similarity in embryonic development	Pharyngeal pouches in vertebrate embryos
Structural	Similarity in adult morphology	Limb bones in vertebrates

Summary

• Evolution by natural selection explains the diversity of life and is supported by multiple lines of evidence.

• Species change over time and are related by common ancestry.

• Evidence includes fossils, vestigial traits, homologies, and direct observations of evolutionary change.

Additional info: For further reading, see the provided links on natural selection in high-altitude Tibetan populations and the evolution of whales.