Chapter 17: Darwin’s Theory of Evolution Study Guide
Darwin’s Epic Journey
Charles Darwin and the HMS Beagle: * Born in England in 1809. * In 1831, Darwin began a five-year voyage on the HMS Beagle. * The voyage occurred during a period of significant scientific shift. Geologists were suggesting an ancient, changing Earth, and biologists suggested life changed through evolution.
Definition of Evolution: A process of biological change over time by which modern organisms have descended from ancient organisms. Darwin’s theory offered a scientific explanation for both the unity and the diversity of life by proposing how organisms evolved through descent from common ancestors.
Patterns of Biodiversity: Darwin focused on three specific patterns of diversity observed during his voyage: * Species Vary Globally: Darwin observed that different, yet ecologically similar, species inhabited separate, but ecologically similar, habitats around the globe. * Examples: Flightless, ground-dwelling birds like the Rheas in South America, Ostriches in Africa, and Emus in Australia. * Ecological Context: He noticed European rabbits and other grassland species were absent from South American and Australian grasslands, which instead hosted species like kangaroos found nowhere else. * Species Vary Locally: Different, yet related, species often occupied different habitats within a local area. * Examples: In South America, one rhea species lived in grasslands while a smaller species lived in colder scrubland. In the Galápagos Islands, the shape of a tortoise's shell could reveal which specific island it inhabited due to different ecological conditions. * Species Vary Over Time: Darwin collected fossils, which are the preserved remains or traces of ancient organisms. * Observation: The fossil record included extinct animals that were similar to, yet different from, living species. * Example: The extinct glyptodont, a massive armored animal, strongly resembled the modern armadillo.
Putting the Puzzle Together: * Darwin realized Galápagos mockingbirds were three separate species found nowhere else. * Small brown birds were identified as species of finches unique to the islands but resembling South American species. * This led Darwin to wonder if species were not fixed and could change through natural processes.
Ideas That Influenced Darwin
An Ancient, Changing Earth: * James Hutton: Proposed that geological forces beneath Earth's surface push rock layers upward, tilting or twisting them and building mountain ranges. He concluded Earth is much older than a few thousand years, introducing the concept of Deep Time—the idea that Earth's history is so long it is difficult for humans to imagine. * Charles Lyell: Developed the theory of Uniformitarianism, which states that the laws of nature are constant and geological processes observable today (like volcanoes and erosion) must be the same processes that shaped Earth millions of years ago. * Darwin's Connection: During the Beagle voyage, Darwin witnessed an earthquake in South America that lifted rocky shorelines more than out of the sea. He also found marine fossils thousands of feet above sea level, proving that geological processes could change the landscape over vast periods.
Lamarck’s Evolutionary Hypotheses: * Jean-Baptiste Lamarck: One of the first to suggest species evolve. He proposed two main ideas: 1. Selective Use and Disuse: Organisms could change during their lifetimes by using or not using body parts (e.g., a bird's legs getting longer by wading in deep water). 2. Inheritance of Acquired Characteristics: Traits altered during an individual's life (acquired characteristics) could be passed to offspring. * Scientific Accuracy: Today, Lamarck’s hypotheses are known to be incorrect because organisms do not have an inborn drive for perfection, and acquired traits (like muscle mass from exercise) do not change the DNA in germ cells (sperm/egg) and cannot be inherited. However, Lamarck is credited for recognizing the link between organisms and their environments.
Population Growth: * Thomas Malthus: An economist who observed that humans were being born faster than they were dying. He argued that unchecked population growth leads to overcrowding and a lack of resources, limited only by war, famine, and disease. * Darwin’s Application: Darwin realized most organisms produce far more offspring than humans (e.g., an oyster producing millions of eggs), but most die before reproducing. This led to the concept of differential reproductive success.
Artificial Selection: * Definition: The selective breeding of plants and animals by humans to promote desirable traits. * Process: Nature provides the inherited variations, and humans select those they find useful (e.g., cows that produce more milk or plants with larger fruit). * Darwin's Insight: Darwin recognized that natural variation was not a defect but the essential material for evolution.
Evolution by Natural Selection
Publication of Theory: Darwin delayed publishing for 20 years until Alfred Russel Wallace published an almost identical essay in 1858. Darwin then published On the Origin of Species in 1859.
The Struggle for Existence: Members of a population must compete for a finite supply of food, living space, and other necessities.
Variation and Adaptation: * Adaptation: Any heritable characteristic that increases an organism’s ability to survive and reproduce. These can be structural (body parts), physiological, or behavioral.
Survival of the Fittest: * Fitness: Describes how well an organism can survive and reproduce in its environment. * Concept: Individuals with high fitness are well-suited to their environment; those with low fitness die or leave few offspring. Evolutionarily, "survival" means passing adaptations to the next generation.
Natural Selection: The process where individuals with variations most suited to their environment survive and leave more offspring. It occurs when: 1. More individuals are born than can survive (Struggle for existence). 2. Natural heritable variation affects survival/reproduction (Variation and adaptation). 3. Fitness varies among individuals (Differential reproductive success).
Common Ancestry: * Descent with Modification: Living species descended, with changes, from common ancestors over many generations. * Principle of Common Descent: All species—living and extinct—are united by descent from ancient common ancestors.
Evidence of Evolution
Biogeography: The study of where organisms live now and where they and their ancestors lived in the past. * Closely Related but Different: Island species (like Galápagos finches) evolve differently based on their specific island environments. * Distantly Related but Similar: Similar habitats (like grasslands) lead to similar adaptations in unrelated species (like rheas and emus).
The Age of Earth and Fossils: * Earth's Age: Radioactive dating indicates Earth is approximately years old ( years). * Fossil Discoveries: Modern fossils document intermediate forms showing how modern species (like whales or birds) evolved from extinct ancestors.
Comparing Anatomy and Development: * Homologous Structures: Structures shared by related species and inherited from a common ancestor (e.g., the limb bones of vertebrates). Purpose may vary, but the basic structure remains the same. * Vestigial Structures: Inherited structures that have lost most or all of their original function (e.g., the hipbone of a dolphin or tiny limb remnants in legless lizards). * Analogous Structures: Body parts that share a common function but not a common structure/origin (e.g., the wing of a bee vs. the wing of a bird). * Embryology: Similar patterns of early development in vertebrates suggest descent from a common ancestor.
Genetics and Molecular Biology: * Common Genetic Code: All living things use DNA and RNA to carry information. This code is nearly identical in bacteria, fungi, plants, and animals. * Molecular Homology: Homologous genes, such as Hox genes (which determine the development of body parts and limb growth), are found in almost all multicellular animals.
Testing Natural Selection: The Grants
The Study: Peter and Rosemary Grant have spent over 40 years studying finches on the island of Daphne Major in the Galápagos.
Key Findings: * They documented that beak size and shape are heritable traits. * During a severe drought, seed abundance decreased and seed hardness increased. Birds with larger beaks were better able to crack the hard seeds and survived at higher rates. * Result: The average beak size of the population increased in just a few years, providing direct observation of natural selection in the wild.
Questions & Discussion
Q: What do birds use their beaks for, and why are there differences? * A: Birds use beaks for eating; differences are dictated by the specific food sources available in their environments.
Q: If you exercise and build big muscles, will your children inherit them? * A: No; building muscles is an acquired trait and does not change the DNA in germ cells, so it cannot be passed to offspring.
Q: Why is survival not enough to describe biological fitness? * A: Biological fitness requires both survival and the ability to reproduce and pass on adaptations to the next generation.
Q: What was Darwin’s key contribution to science? * A: He developed a comprehensive theory of evolution that provided a scientific explanation for the unity and diversity of life through the process of natural selection.
Q: What happens after population growth exceeds available resources (Point A)? * A: There results a struggle for existence where not all individuals can survive, leading to differential reproductive success.