Evolution: Patterns, Processes, and Evidence

Lineages, Traits, and Evolutionary Patterns

Understanding the basic terminology and concepts is essential for studying evolutionary biology. This section introduces key terms and the foundational ideas of evolutionary change.

• Lineage: A group of organisms connected over many generations, representing a continuous line of descent.

• Trait: A characteristic or feature of an organism, such as eye color, beak shape, or metabolic ability.

• Convergent Evolution: The independent evolution of similar traits in different lineages, often due to similar environmental pressures. For example, wings in bats and birds evolved separately.

• Homology: Traits shared by two or more species because they were inherited from a common ancestor. For example, the forelimbs of humans and whales are homologous structures.

• Descent with Modification and Divergence: The process by which species descend from common ancestors, accumulate changes (modifications), and diverge into different lineages over time.

Example: The forelimb bones in humans, cats, whales, and bats are structurally similar due to shared ancestry, but have diverged in function.

Evolutionary Processes

Evolution is driven by several processes, some of which are random and others non-random.

• Mutation: Random changes in DNA that introduce new genetic variation.

• Genetic Drift: Random changes in allele frequencies, especially in small populations.

• Natural Selection: Non-random process where individuals with advantageous traits survive and reproduce more successfully.

Random Processes: Mutation and genetic drift are random; natural selection is not.

Population Growth and Limiting Factors

Population size is influenced by resource availability and environmental factors.

• Unlimited Resources: Populations with infinite resources grow exponentially.

• Limiting Factors: Environmental constraints (e.g., food, space, predation) limit population growth, leading to logistic growth curves.

Example: If a species' trait is not adaptable to its environment, survival and reproduction may decrease, limiting population growth.

Populations Evolve, Not Individuals

Evolution is a change in the genetic composition of a population over generations. Individuals do not evolve; rather, populations do as allele frequencies shift over time.

Natural Selection: Requirements and Mechanisms

For a trait to evolve by natural selection, it must meet three criteria:

• Variation: Individuals differ in the trait.

• Differential Survival and Reproduction: Some variants confer higher fitness, leading to greater reproductive success.

• Heritability: The trait must be passed from parents to offspring.

If any of these properties are absent, evolution by natural selection cannot occur.

Example: If a trait is not heritable, even if it affects survival, it will not be passed to the next generation.

Natural Selection vs. Evolution

• Natural Selection without Evolution: If trait differences do not affect the genetic composition of the population, natural selection occurs but evolution does not.

• Evolution without Natural Selection: Genetic drift or mutation can change allele frequencies without selective pressure.

Example: Pesticide resistance in insects can evolve through natural selection if resistance is heritable; random changes in small populations can also cause evolution without selection.

Historical Perspectives and Evidence for Evolution

Pre-Darwinian Views and Geological Change

In the 1800s, the prevailing view in Europe was that species were individually created ("special creation"). This changed with new scientific discoveries.

• Charles Lyell's Hypothesis: Proposed that Earth's features result from slow, continuous processes (uniformitarianism), implying an ancient Earth.

• Darwin's Observations: Earthquakes, raised shorelines, and geological formations provided evidence for gradual change over long periods.

Biogeography and the Principle of Succession

• Biogeographical Patterns: Darwin observed that species in different locations were similar but adapted to their environments, supporting evolution.

• Principle of Succession: Living species in a region often resemble fossils from the same region, indicating descent with modification.

Artificial Selection and Natural Selection

• Artificial Selection: Humans breed organisms for desired traits, demonstrating that selection can cause species to change.

• Natural Selection: The environment, rather than humans, determines which traits are favored.

Example: Breeding dogs for specific traits is artificial selection; finch beak variation in the Galapagos is natural selection.

Dispersal and Adaptive Radiation

• Dispersal: Organisms colonize new habitats by moving to new locations (e.g., birds, plants, spiders, lizards).

• Adaptive Radiation: A single ancestral species gives rise to many species adapted to different environments, as seen in Galapagos finches.

Darwin's Main Ideas

• Descent with Modification: Species change over generations and share common ancestors.

• Natural Selection: Inherited traits that enhance survival and reproduction become more common over time.

Microevolution, Macroevolution, and the Fossil Record

Microevolution vs. Macroevolution

• Microevolution: Small-scale changes in gene frequencies within a population over a few generations (e.g., mutation, gene flow, genetic drift, natural selection).

• Macroevolution: Large-scale evolutionary changes that occur over long periods, leading to the origin of new species and major changes in traits.

Stratigraphy and Fossil Formation

• Principle of Stratigraphy: The study of rock layers (strata) and their age relationships. Older fossils are found in lower layers; newer fossils are closer to the surface.

• Sedimentary Rocks: Formed from the accumulation of sediments, often containing fossils that provide a record of past life.

Extinction, Transitional Forms, and Evidence for Evolution

• Extinction: Demonstrates that species can disappear over time.

• Transitional Fossils: Show features of both ancestral and derived groups, illustrating evolutionary change (e.g., Tiktaalik, Archaeopteryx).

Tiktaalik: A Transitional Tetrapod

• Hypothesis: Tiktaalik was predicted to be a transitional form between fish and tetrapods, found in 375-million-year-old rocks.

• Tetrapod Features: Neck, strong ribs, limb bones similar to arms/legs.

• Fish Features: Scales, fins, fish-like body shape.

Tetrapods: Definition and Examples

• Tetrapods: Vertebrate animals with four limbs. Major groups include amphibians, mammals, birds, and reptiles.

• Example: Frogs (amphibians), humans (mammals), eagles (birds), lizards (reptiles).

Archaeopteryx: Transitional Form Between Reptiles and Birds

• Bird Features: Feathers.

• Reptile Features: Teeth, long tail.

Vestigial Structures

• Definition: Body parts inherited from ancestors that have lost most or all of their original function.

• Examples in Humans: Tailbone, appendix, goosebumps, ear-moving muscles, wisdom teeth.

• Significance: Evidence for evolution, showing descent from ancestors with different lifestyles.

Types of Homology

• Molecular Homology: Similarities in DNA or proteins indicate shared ancestry.

• Structural Homology: Similar body structures inherited from a common ancestor, even if used differently (e.g., vertebrate forelimbs).

• Developmental Homology: Similarities in embryonic development suggest evolutionary relationships.

Example: The humerus bone in the forelimbs of humans and whales is homologous.

Recent Examples of Evolution

• Pesticide Resistance: Insects have evolved resistance to pesticides within the past 50 years due to natural selection.

Research on Non-Human Organisms

• Reason: Humans share many genes and biological processes with other organisms due to common ancestry. Studying model organisms helps understand human biology and disease.