Charles Darwin and the Foundations of Evolutionary Theory

Darwin’s Scientific Contribution and the Voyage of the Beagle

Charles Darwin was born in England on February 12, 1809, and although he was not a star student—preferring nature watching and hunting to academic study—he eventually proposed what has been called the single best idea anyone has ever had. In $1831$, he was invited to join the HMS Beagle on a five-year voyage. The primary mission of the voyage, commanded by the captain, was to map coastlines and harbors, while Darwin was brought along to keep the captain company. During this time, the scientific community was undergoing a revolution; geologists suggested Earth was ancient and changing, and biologists suggested life changed through evolution, though no one had offered a scientific description of how that process occurred.

Darwin developed a theory of biological evolution that offered a scientific explanation for the unity and diversity of life. He proposed that modern organisms evolved through descent from common ancestors. In science, a theory is defined not as a hunch or guess, but as a well-established, scientific explanation of events in the natural world that can be used to make testable predictions. Darwin’s work confirmed that the living world is constantly changing. This theory remains the organizing principle for biological and biomedical sciences, helping researchers understand the emergence of drug-resistant bacteria, new influenza strains, and the potential impact of human-driven species extinction.

Patterns of Biological Diversity Observed by Darwin

Darwin focused on three distinct patterns of biodiversity during his travels: species vary globally, species vary locally, and species vary over time. Regarding global variation, Darwin noticed that different, yet ecologically similar, species inhabited separated, but ecologically similar, habitats around the globe. For example, in the grasslands of South America, he found flightless birds called rheas, which look and act like ostriches. However, rheas live only in South America, while ostriches live only in Africa. In Australia, he found a third large flightless bird, the emu. He also noted that European grassland species, such as rabbits, were absent from similar grasslands in Australia and South America, which instead housed kangaroos and other unique animals.

Local variation was evidenced by the fact that different, yet related, species often occupied different habitats within a local area. In South America, one species of rhea thrived in Argentina's grasslands, while a smaller species was adapted to the colder scrublands to the south. The Galápagos Islands, located approximately $1000\,\text{km}$ off the Pacific coast of South America, provided the most striking evidence. Despite being close to one another, the islands were ecologically different. The giant land tortoises on the islands exhibited traits that varied by environment. The Isabela Island tortoise has a dome-shaped shell and a short neck, suited for vegetation located close to the ground. In contrast, the Hood Island tortoise has a saddle-backed shell with a high opening and a long neck, allowing it to reach sparse, high-reaching vegetation. Darwin also collected mockingbirds and small brown birds with different beak shapes, which he later discovered were distinct species peculiar to the islands.

Variation over time was observed through the collection of fossils, which are the preserved remains or traces of ancient organisms. Darwin noticed that the fossil record included many extinct animals that were similar to, but different from, living species. A primary example was the glyptodont, an extinct giant armored animal. Glyptodonts resembled giant versions of modern armadillos, which lived in the same geographic area. Darwin questioned why the glyptodonts had disappeared and why modern armadillos resembled them, leading him to consider whether they shared a common ancestor.

Geological Influences: Hutton and Lyell

Darwin was profoundly influenced by geologists James Hutton and Charles Lyell, who concluded that Earth is extremely old and that the processes that changed Earth in the past are the same as those operating in the present. In $1785$, Hutton presented hypotheses on how geological processes like molten lava forming rock and sediment being squeezed into layers shaped the Earth. He proposed that forces beneath the surface push rock layers upward to build mountains, which are then worn down by rain, wind, and heat. Because these processes occur so slowly, Hutton argued that Earth must be much older than a few thousand years, introducing the concept of deep time.

Charles Lyell’s work, Principles of Geology, published its first volume in $1830$. Lyell argued for uniformitarianism, the idea that geological processes seen in action today—such as volcanoes releasing lava or rivers carving canyons—must be the same ones that shaped Earth millions of years ago. During his voyage, Darwin witnessed a volcano erupt in Chile and an earthquake that lifted a stretch of rocky shoreline more than $3\,\text{m}$ out of the sea. Seeing marine fossils in mountains thousands of feet above sea level further convinced Darwin that Lyell was correct. Darwin reasoned that if Earth could change over time, life could also change.

Evolutionary and Population Hypotheses: Lamarck and Malthus

Jean-Baptiste Lamarck proposed two of the first evolutionary hypotheses in $1809$. He suggested that organisms have an inborn urge to become more complex and perfect, changing their bodies by selectively using or not using various parts. He believed these acquired traits could be passed to offspring, a principle known as the inheritance of acquired characteristics. For instance, if a bird stretched its legs to stay above water, its legs would grow longer, and its offspring would inherit those longer legs. While today we know these hypotheses are unsupported—evolution does not move toward a predetermined "better" state and acquired traits are not inherited—Lamarck was important for being among the first to argue that species are not fixed and are adapted to their environments.

In $1798$, economist Thomas Malthus observed that humans were being born faster than they were dying, leading to overcrowding. He argued that if the human population grew unchecked, there would not be enough food or living space, and that forces like war, famine, and disease work against this growth. Darwin realized this logic applied even more strongly to other organisms. For example, a maple tree produces thousands of seeds and an oyster produces millions of eggs. If all descendants survived, they would eventually overrun the Earth. This led Darwin to the concept of the struggle for existence, where many individuals die and only a few survive to reproduce, a phenomenon known as differential reproductive success.

Artificial Selection and the Raw Material for Evolution

Darwin studied the practices of plant and animal breeders, who chose individual organisms with desirable inherited variations for breeding. For example, farmers would only breed cows that produced the most milk or trees that produced the largest fruit. Darwin called this process artificial selection, where nature provides the variations and humans select those they find useful. Before Darwin, scientists often viewed variations among individuals as unimportant defects. Darwin realized that inherited variation was crucial because it provided the raw material for a natural mechanism that could drive evolution. He tested these ideas himself by breeding fancy pigeon varieties to see how traits could be manipulated over generations.

Questions & Discussion

Analyzing Data: Darwin's Voyage

1. Identify three different biomes that Darwin visited on his voyage. (Using the biome map from Chapter 3, common examples include tropical rainforests in Brazil, grasslands in South America and Australia, and desert/scrublands).
2. Find an example of when Darwin visited the same biome on two different continents. (Darwin visited grasslands in both South America and Africa/Australia).
3. Which biome did you identify as the same on two continents? Are similar types of animals found on both continents? (In grasslands, Darwin found rheas in South America and ostriches in Africa; they were similar in being large and flightless but were different species).
4. How was a round-the-world voyage useful to Darwin for developing his theory of evolution? (It allowed him to see that species are often determined by their environments and geographic isolation, rather than being the same everywhere).
5. How does evidence from many places around the world, instead of only a single habitat or biome, help strengthen Darwin's theory of evolution? (Global evidence showed that similar environments don't always have the same species, suggesting that species evolve from local ancestors rather than being created for a specific climate).

Quick Lab: Variation in Peppers

1. Obtain a bell pepper, slice it open, and count the seeds. (Caution: Direct sharp edges away from self). Compare data with others using different colored peppers.
2. Calculate the average (mean) number of seeds in the class's peppers. By how much does the number of seeds in each pepper differ from the average? (This demonstrates mathematical variation within a species).
3. Think about variations Darwin observed. What questions might he have asked about the pepper data? (Darwin might ask if the number of seeds is an inherited trait, if peppers with more seeds have a higher chance of passing on their traits, and how environmental factors influence the number of seeds produced).