Evolution by Natural Selection: Foundations, Evidence, and Mechanisms

Chapter 22: Evolution by Natural Selection

Introduction to Evolution by Natural Selection

Evolution by natural selection, independently formulated by Charles Darwin and Alfred Russel Wallace, is a foundational theory in biology that explains how populations adapt to diverse environments. This theory reveals five key attributes of life, most notably that populations of organisms evolve over time. Darwin's publication, On the Origin of Species (1859), provided a comprehensive explanation supported by extensive evidence. Scientific theories, including evolution, consist of two main components:

• Pattern: Observations about the natural world (e.g., species change over time).

• Process: The mechanism that produces the observed pattern (e.g., natural selection).

The Rise of Evolutionary Thought

Historical Perspectives on the Diversity of Life

The theory of evolution by natural selection was revolutionary, replacing the long-held idea of special creation, which dominated Western thought for over 2000 years. Early models of life's diversity included:

• Plato's Typological Thinking: Each organism is an unchanging example of a perfect essence or type, created by a divine being.

• Aristotle's Scale of Nature: Species are fixed and organized into a linear sequence (the "great chain of being") based on increasing complexity, with humans at the top.

• Lamarck's Theory of Evolution: Proposed that simple organisms arise by spontaneous generation and evolve into more complex forms through inheritance of acquired characteristics (e.g., giraffes stretching their necks).

• Darwin and Wallace's Population Thinking: Emphasized variation among individuals in populations and proposed that individuals with certain traits produce more offspring, leading to evolutionary change.

The Pattern of Evolution: Have Species Changed, and Are They Related?

Descent with Modification

Darwin described evolution as "descent with modification," meaning that modern species are modified descendants of ancestral species. The pattern component of the theory predicts:

• Species change through time.

• Species are related by common ancestry.

Evidence for Evolution

• Fossil Record: Fossils are traces of organisms from the past, organized into the fossil record, which documents changes in life over time.

• Geologic Time: Sedimentary rocks form in layers, with younger layers on top. The geologic time scale, based on fossil positions, divides Earth's history into eons, eras, periods, and epochs. Radiometric dating shows Earth is about 4.6 billion years old, with life appearing 3.4–3.8 billion years ago.

• Extinction: Many fossils represent extinct species, indicating that species are dynamic and the array of life has changed over time. Over 99% of all species that have ever lived are now extinct.

• Transitional Features: Fossil species often show traits intermediate between ancestral and derived species, supporting the prediction that transitional forms should exist.

• Vestigial Traits: Reduced or nonfunctional structures similar to functional organs in related species (e.g., human tailbone, whale hip bones) provide evidence for change through time.

• Contemporary Evolution: Evolutionary changes can be observed in living populations, such as antibiotic resistance in bacteria and changes in finch beak shape.

• Biogeography: Similar species are often found in the same geographic area, as seen in Darwin's Galápagos finches, which share a common ancestor.

• Homology: Similarity among species due to shared ancestry, observed at three levels:

  • Genetic Homology: Similar DNA, RNA, or amino acid sequences (e.g., genetic code shared by all life).

  • Developmental Homology: Similar embryonic structures (e.g., pharyngeal pouches in vertebrate embryos).

  • Structural Homology: Similar adult morphology (e.g., vertebrate limb bones).

• Speciation: The formation of new species from preexisting species has been observed in contemporary populations (e.g., killer whales).

Internal Consistency of Evolutionary Evidence

Multiple independent data sets (fossil record, phylogenies, genetic data, vestigial traits) consistently support the theory of evolution by natural selection, making it a robust scientific explanation for the diversity of life.

The Process of Evolution: How Does Natural Selection Work?

Darwin’s Inspiration and Artificial Selection

Darwin was inspired by artificial selection in pigeon breeding and the work of Thomas Malthus, who described the "struggle for existence" due to limited resources. Darwin combined these ideas with observations of variation in natural populations to formulate the process of natural selection.

Darwin’s Four Postulates

1. Variation exists among individuals in a population.

2. Some trait differences are heritable.

3. Survival and reproductive success are highly variable.

4. Individuals with certain traits survive and reproduce more than others (not a random sample).

Modern synthesis condenses these into: Heritable variation leads to differential reproductive success.

Key Definitions

• Evolutionary Fitness: The ability of an individual to produce surviving, fertile offspring relative to others in the population.

• Adaptation: A heritable trait that increases fitness in a particular environment.

• Selection: Differential reproduction as a result of heritable variation.

Evolution in Action: Measuring Natural Selection in Populations Today

Case Study 1: Antibiotic Resistance in Mycobacterium tuberculosis

M. tuberculosis causes tuberculosis (TB). The rise of antibiotic-resistant strains is a major public health concern. Resistance often results from mutations (e.g., in the rpoB gene) that prevent antibiotics from binding to bacterial enzymes. This case demonstrates all four of Darwin’s postulates and shows that natural selection acts on individuals, but only populations evolve as allele frequencies change.

Case Study 2: Beak Size and Shape in Galápagos Finches

Peter and Rosemary Grant’s long-term research on medium ground finches showed that beak form and body size are heritable. During a drought, birds with deeper beaks survived better, leading to an increase in average beak depth in the population. Subsequent environmental changes favored smaller beaks, illustrating ongoing evolution in response to environmental variation. Beak traits are polygenic, with genes such as ALX1 and HMGA2 influencing shape and size.

Debunking Common Misconceptions about Natural Selection and Evolution

• Natural Selection Does Not Change Individuals: Only populations evolve; individuals do not change their traits during their lifetime.

• Not Lamarckian Inheritance: Natural selection sorts existing variants; it does not cause individuals to acquire new traits in response to the environment.

• Individuals Do Not Adapt: Acclimatization (phenotypic change) is not adaptation (genetic change in populations).

• Not Goal Directed: Evolution is not purposeful; mutations do not arise to solve problems.

• Not Progressive: Evolution does not necessarily produce more complex or "better" organisms.

• No Higher or Lower Organisms: All organisms are equally adapted to their environments.

• Traits Are Not Always Adaptive: Vestigial traits and silent mutations may have no effect on fitness.

• Fitness Trade-Offs: Adaptations often involve compromises (e.g., egg size vs. number, growth rate vs. lifespan).

• Genetic, Historical, and Environmental Constraints: Evolution is limited by genetic variation, historical legacy, and changing environments.

• Natural Selection Is Not the Only Process of Evolution: Other processes include genetic drift, gene flow, and mutation.