Ch. 19 Descent With Modification and Natural Selection: Foundations of Biological Evolution

Chapter 19: Descent With Modification

Introduction to Biological Evolution

Descent with modification, also known as biological evolution, is a central concept in biology that explains how organisms change over generations. This process accounts for both the unity and diversity of life on Earth.

• Descent with modification: All living things are descended from ancestral species, with changes accumulating over time.

• Pattern: Observable facts about the results of evolution (e.g., similarities among species).

• Process: Mechanisms that produce evolutionary change (e.g., natural selection, mutation, sexual reproduction).

Evolution is supported by a vast amount of scientific evidence from multiple disciplines.

Adaptations and Diversity

Unity and Diversity of Life

Organisms are remarkably suited for life in their environments, sharing many characteristics while also displaying a rich diversity.

• Adaptations: Traits that improve an organism's ability to survive and reproduce in its environment.

• Unity: Shared characteristics among different organisms (e.g., genetic code, cellular structure).

• Diversity: The wide variety of forms and functions among living things.

Charles Darwin and the Voyage of the Beagle

Darwin's Observations and Influences

Charles Darwin (1809–1882) was a naturalist who traveled on the HMS Beagle, observing plants and animals in diverse environments. His observations led him to propose the theory of evolution by natural selection.

• Species in similar environments often resembled each other more than those in different environments.

• Fossils of extinct species resembled living local species.

• Darwin was influenced by earlier thinkers:

  • Jean-Baptiste Lamarck (1809): Proposed inheritance of acquired characteristics.

  • Charles Lyell (1830): Suggested geological processes are gradual and OLD EARTH.

  • Thomas Malthus (1798): Argued that population growth outpaces resources, leading to competition.

  • Alfred Russel Wallace (1858): Independently developed a hypothesis for natural selection.

Descent With Modification

Accumulation of Changes Over Time

Descendants of ancestral species lived in various habitats and accumulated modifications over millions of years, leading to the diversity of life observed today.

• Different modifications gradually accumulated in populations.

• Evolutionary relationships can be depicted as branching diagrams (phylogenies).

Mechanisms of Evolution: Natural Selection

Natural Selection Explained

Natural selection is the process by which organisms with traits better suited to their environment tend to survive and reproduce more successfully, passing those traits to future generations.

• Adaptation: Traits that increase survival and reproductive success.

• Inheritance: Adaptations are due to traits that are inherited from parents.

• Reproduction: Better-adapted individuals produce more offspring.

Artificial Selection

Humans can select for desirable traits in organisms, such as crops and livestock, demonstrating the principles of selection.

• Example: Breeding "fancy pigeons" for specific traits.

Darwin's Reasoning: The Logic of Natural Selection

Key Observations and Inferences

• Observation 1: There is variation in heritable traits in a population.

• Observation 2: More offspring are produced than can survive, and they compete for resources.

• Inference 1: Individuals with advantageous traits will leave more offspring than those without such traits.

• Inference 2: Favorable traits accumulate in the population over generations.

Important: Natural selection acts on populations, not individuals. Populations evolve, not individuals.

Evidence for Evolution

Scientific Support

Evolution is supported by extensive scientific evidence, including observations of natural selection in action.

• Examples include changes in beak length in soapberry bugs and adaptation in wild mice.

Case Study: Natural Selection in Wild Mice

Natural selection can be observed in wild populations, such as mice with different fur colors surviving better in matching environments.

• Individuals with traits that match their environment are more likely to survive and reproduce.

• Population adapts over time as favorable traits become more common.

Case Study: Soapberry Bugs

Changes in food sources can lead to evolutionary changes in populations, such as beak length in soapberry bugs adapting to new fruit types.

• Beak length changed over 30 years in response to fruit size.

• Deeper, longer beaks for larger fruit; shallower, shorter beaks for smaller fruit.

Drug Resistant Bacteria

Evolution in Action: Antibiotic Resistance

Certain strains of bacteria have evolved resistance to antibiotics, demonstrating natural selection in response to environmental changes.

• Using antibiotics changes the environment, selecting for bacteria that can survive and reproduce.

• Resistant bacteria become more common as susceptible bacteria are killed off.

Factor	Effect on Bacteria
Antibiotic use	Kills susceptible bacteria, allows resistant bacteria to survive
Mutation	Can produce resistance traits
Reproduction	Resistant bacteria reproduce, increasing their frequency

Summary Table: Key Concepts in Evolution

Concept	Definition	Example
Descent with modification	Species change over time, accumulating differences from ancestors	Darwin's finches
Natural selection	Process by which favorable traits become more common in a population	Drug resistant bacteria
Adaptation	Trait that increases survival and reproduction	Camouflage in wild mice

Key Equations and Terms

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

• Population: A group of individuals of the same species living in the same area.

• Mutation: A change in DNA that can introduce new traits.

Equation for Hardy-Weinberg Equilibrium:

$p^2 + 2pq + q^2 = 1$

Where p and q are the frequencies of two alleles in a population.

Additional info: The Hardy-Weinberg equation is included for context, as it is a foundational concept in population genetics and evolution.