Chapter 22: Evolution by Natural Selection – Study Notes

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Chapter 22: Evolution by Natural Selection

Introduction to Evolution by Natural Selection

Evolution by natural selection is a foundational theory in biology, independently formulated by Charles Darwin and Alfred Russel Wallace. This theory explains how populations adapt to diverse environments and reveals that populations of organisms evolve over time.

Pattern: Observations about the natural world (e.g., species change over time).
Process: The mechanism that produces the observed pattern (natural selection).

Darwin's On the Origin of Species (1859) provided extensive evidence for this theory, which is now widely supported by scientific data.

22.1 The Rise of Evolutionary Thought

Historical Perspectives on Diversity

Typological Thinking (Plato): Each organism is a perfect, unchanging type created by a divine being.
Scale of Nature (Aristotle): Species are fixed and organized in a linear hierarchy from simple to complex, with humans near the top.
Lamarckian Evolution: Proposed that simple organisms arise by spontaneous generation and become more complex over time. Suggested that acquired traits (e.g., a giraffe's long neck) are inherited by offspring.
Darwin & Wallace: Proposed that evolution is based on variation within populations, not a linear progression. Individuals with advantageous traits leave more offspring.

Revolutionary Aspects: The theory overturned the idea of static species, replaced typological with population thinking, and provided a testable mechanism for evolutionary change.

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

Descent with Modification

Darwin described evolution as descent with modification: modern species arise from ancestral species through gradual changes over time. The theory predicts:

Species change through time.
Species are related by common ancestry.

Evidence for Change through Time

Fossils: Traces of organisms from the past. The fossil record documents the existence, diversity, extinction, and change of many life forms.

Evidence 1: The Vastness of Geologic Time

Sedimentary rocks form in layers; younger layers are deposited on top of older ones.
Geologic time scale: Divides Earth's history into eons, eras, periods, and epochs based on fossil positions.
Radiometric dating: Uses the steady decay of radioactive isotopes to assign absolute ages to rocks and fossils.

Key findings: Earth is about 4.6 billion years old; earliest life dates to 3.4–3.8 billion years ago.

Evidence 2: Extinction Changes the Species Present over Time

Many fossils are of extinct species, showing that species are dynamic and the array of life has changed.
Over 99% of all species that have ever lived are now extinct.

Evidence 3: Transitional Features Link Older and Younger Species

Law of succession: Fossil species are similar to living species in the same area.
Transitional features: Traits in fossils that are intermediate between ancestral and derived species (e.g., tetrapod limb evolution).

Evidence 4: Vestigial Traits

Vestigial trait: A reduced or nonfunctional structure similar to functioning organs in related species (e.g., human tailbone, whale hip bones).
Vestigial traits are evidence of evolutionary change and inconsistent with special creation.

Evidence 5: Species Can Be Observed Changing Today

Evolutionary changes have been documented in real time (e.g., antibiotic resistance in bacteria, beak shape in finches).
Evolution can occur over days, weeks, or months—not just millions of years.

22.3 The Process of Evolution by Natural Selection

Darwin’s Four Postulates

Variation exists among individuals in a population.
Some trait differences are heritable.
Survival and reproductive success are variable.
Individuals with certain traits produce more offspring (not a random sample).

Modern summary: Evolution by natural selection occurs when heritable variation leads to differential reproductive success.

Key Definitions

Fitness: The ability to produce surviving, fertile offspring relative to others.
Adaptation: A heritable trait that increases fitness in a particular environment.
Selection: Differential reproduction due to heritable variation.

22.4 Evolution in Action: Case Studies

Antibiotic Resistance in Mycobacterium tuberculosis

TB rates surged due to antibiotic-resistant strains.
Resistance traced to a point mutation in the rpoB gene, altering RNA polymerase and preventing antibiotic binding.
Natural selection favored resistant bacteria, increasing their frequency in the population.

Beak Size Evolution in Galapagos Finches

Grants’ research showed beak size and shape are heritable.
Droughts and environmental changes led to shifts in beak size due to selection for traits best suited to available food.
Beak depth is a polygenic trait influenced by genes such as ALX1 and HMGA2.

22.5 Common Misconceptions about Natural Selection and Evolution

Natural selection does not change individuals: Only populations evolve as allele frequencies shift.
Not Lamarckian inheritance: Individuals do not acquire new traits in response to need; selection sorts existing variation.
Acclimatization vs. adaptation: Acclimatization is a non-heritable change in an individual; adaptation involves genetic change in a population.
Evolution is not goal-directed or progressive: Mutations are random, and evolution does not necessarily produce more complex or 'better' organisms.
No higher or lower organisms: All species are equally adapted to their environments.
Not all traits are adaptive: Vestigial traits and silent mutations may not affect 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 existing genetic variation, evolutionary history, and changing environments.
Other evolutionary processes: Besides natural selection, evolution can occur via genetic drift, gene flow, and mutation.

Table: Three Levels of Homology

Level	Description	Example
Genetic Homology	Similarity in DNA, RNA, or amino acid sequences	Genetic code is nearly universal
Developmental Homology	Similarity in embryonic development	Pharyngeal pouches in vertebrate embryos
Structural Homology	Similarity in adult morphology	Limb bones in vertebrates

Table: Evidence for Evolution

Evidence	Description	Example
Fossil Record	Shows change through time	Transitional forms, extinct species
Homology	Similarity due to common ancestry	Genetic, developmental, structural
Direct Observation	Evolution in action	Antibiotic resistance, finch beak evolution

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