Bio 1112- Lecture 2

Darwinian Evolution

Introduction

Darwinian evolution is a foundational concept in biology, explaining the diversity of life through mechanisms of change over time. This study guide covers the history of evolutionary thought, Darwin's concept of 'descent with modification,' and the overwhelming evidence supporting evolutionary theory.

History of Evolutionary Thought

Early Ideas and Classification

• Scala Naturae: Aristotle (384–322 BCE) proposed that species were fixed and arranged them on a scale of increasing complexity, known as the scala naturae.

• Classification Systems: Carolus Linnaeus (1707–1778) developed a nested classification system and the binomial nomenclature (e.g., Homo sapiens).

Geological Influences

• James Hutton: Proposed gradual formation of Earth's features (e.g., valleys by rivers).

• Charles Lyell: Principles of Geology emphasized slow, continuous processes shaping Earth's surface.

Fossils and Change Over Time

• Fossils: Remains or traces of organisms found in sedimentary rock layers (strata).

• Paleontology: Developed by Georges Cuvier, who observed that older strata contain fossils less similar to current organisms.

Lamarck’s Hypothesis

• Use and Disuse: Body parts used extensively become stronger; unused parts deteriorate.

• Inheritance of Acquired Characteristics: Modifications acquired in one's lifetime can be passed to offspring.

• Note: This mechanism is not supported by experimental evidence.

Darwin’s "Descent with Modification"

Key Thinkers

• Charles Darwin (1809–1882): Proposed natural selection as the driving force of evolutionary change. Published On the Origin of Species (1859).

• Alfred Russel Wallace (1823–1913): Independently developed a similar theory of natural selection.

Tree Thinking

• Darwin viewed life’s history as a tree, with branches representing groups of organisms and forks representing common ancestors.

• Extinct groups are represented by unlabeled branches.

Modern Evolutionary Trees

• Large morphological gaps between related groups are explained by branching and extinction events.

• Example: Living elephant species split from a recent common ancestor; extinction of older species explains differences from nearest living relatives.

Mechanisms of Evolution

Natural Selection

• Definition: Individuals with certain inherited traits tend to survive and reproduce at higher rates because of those traits.

• Key Features:

  • Heritable traits increase survival and reproduction.

  • Frequency of favorable adaptations increases in a population.

  • Environmental changes may drive adaptation and speciation.

Artificial Selection

• Humans selectively breed organisms, changing the genetic structure of populations (e.g., domestic animals, crop varieties).

Examples

• Wild mustard selectively bred to produce cabbage, broccoli, kale, etc.

Darwin’s Observations and Inferences

Observation #1

• Members of a population vary in their inherited traits.

• Inference #1: Individuals with traits that increase survival and reproduction tend to produce more offspring.

Observation #2

• All species can produce more offspring than the environment can support; many fail to survive and reproduce.

• Inference #2: Unequal survival and reproduction leads to accumulation of favorable traits over generations.

Evidence Supporting Evolutionary Theory

1. Direct Observations of Evolutionary Change

• Natural selection observed in response to introduced species and drug-resistant bacteria.

• Example: Soapberry bugs in Florida evolved longer or shorter beaks depending on fruit size; similar patterns found in other regions.

2. Homology

• Anatomical Homology: Similar structures in related species due to common ancestry (e.g., vertebrate limbs).

• Comparative Embryology: Embryos show homologies not visible in adults (e.g., post-anal tail, pharyngeal arches).

• Vestigial Structures: Remnants of features that served a function in ancestors (e.g., snake pelvis and leg bones).

• Molecular Homology: Shared genetic code and genes among diverse organisms; evidence of common ancestry.

3. The Fossil Record

• Provides evidence of extinction, origin of new groups, and changes within groups over time.

• Example: Fossil record supports cetaceans as close relatives of even-toed ungulates; documents transitions from land to sea.

4. Biogeography

• Study of geographic distribution of species; supports evolution through patterns influenced by continental drift.

• Example: Freshwater fish in South America and Australia share ancestors from the time continents split from Pangea.

Convergent Evolution

• Evolution of similar (analogous) features in distantly related groups due to adaptation to similar environments, not common ancestry.

Summary Table: Types of Evidence for Evolution

Type of Evidence	Description	Example
Direct Observation	Evolutionary change documented in real time	Soapberry bug beak length
Anatomical Homology	Similar structures due to common ancestry	Vertebrate limbs
Molecular Homology	Shared genetic code and genes	Hemoglobin gene similarities
Fossil Record	Extinction, origin, and transitions of species	Land-to-sea transition in cetaceans
Biogeography	Geographic distribution of species	Galaxiidae fish in South America and Australia
Convergent Evolution	Analogous traits in unrelated groups	Sugar glider and flying squirrel

Key Terms

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

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

• Homology: Similarity due to shared ancestry.

• Analogous Traits: Similar features evolved independently in different lineages.

• Artificial Selection: Human-driven selective breeding.

• Biogeography: Study of species distribution across geographic areas.

Equations and Models

• Population Genetics: The change in allele frequency in a population can be modeled by the Hardy-Weinberg equation: $p^2 + 2pq + q^2 = 1$ where $p$ and $q$ are the frequencies of two alleles.

Conclusion

Darwinian evolution integrates diverse areas of biological study and is supported by extensive evidence from direct observation, homology, the fossil record, and biogeography. Ongoing research continues to expand our understanding of evolutionary processes.