Darwin, Wallace, and the Voyage Evidence
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You should understand the basic concept of species and be familiar with the pre-Darwinian view that species were fixed and unchanging, as covered in the previous lecture.
Why this matters
The voyage of the Beagle provided the empirical foundation that transformed biology from a descriptive catalogue into a dynamic science of change.
Before 1831, naturalists collected specimens but lacked a unifying mechanism to explain their distribution. Charles Darwin’s five-year journey aboard HMS Beagle allowed him to observe geological strata, fossil records, and geographic patterns of life that contradicted the prevailing idea of independent creation. By correlating the physical history of the Earth with the biological history of its inhabitants, Darwin and Alfred Russel Wallace independently arrived at the theory of natural selection. This lecture examines the specific evidence from the voyage—particularly the Galápagos finches and the Andes—that forced a paradigm shift from fixity to descent with modification, establishing the framework for modern evolutionary biology.
The Voyage as a Natural Laboratory
Imagine standing on the shores of the Galápagos Islands in 1835. You are not just looking at birds; you are looking at a living experiment in adaptation. Darwin did not at first grasp the significance of the finches he collected; he did not even label them by island. The mockingbirds were the clearer clue at the time: different islands had different forms. Only later, after the ornithologist John Gould examined the specimens, did the pattern become clear: closely related finch species differ in beak shape, and beak shape matches diet. In plain language, natural selection is the process where individuals with traits better suited to their environment survive and reproduce more successfully than others. Over time, these advantageous traits become more common in the population. The voyage provided the "raw material" for this theory: the observation that species are not static entities but are linked by common ancestry and modified by their surroundings. The key insight was that the Earth is old enough for these gradual changes to accumulate, and that life is interconnected through a branching tree of descent rather than isolated creations. This shift in perspective—from seeing life as a collection of separate, fixed types to seeing it as a continuous, changing process—was one of the great intellectual revolutions in the history of science.
Geological Time and Fossil Evidence
One of the most critical pieces of evidence Darwin gathered was the discovery of fossils in the Andes mountains. High in the Andes he found fossil marine shells, and near Uspallata a stand of fossil trees that had once grown at sea level. In Chile in 1835 he saw a great earthquake raise the coastline by a metre or more. Together these showed that land that is now high mountains had once been sea floor and had risen in many small steps. This supported the concept of geological change over vast periods of time. Darwin realised that the Earth’s surface was not static but had undergone significant physical changes, providing the necessary time for biological evolution to occur. The presence of extinct species in the fossil record, closely related to living species, suggested a continuity of life forms across time. This evidence of "descent with modification" was crucial because it showed that species could change and that extinction was a natural part of the process. The fossil record provided a historical narrative of life, showing that the past was not identical to the present, but was connected to it through a series of gradual transformations. This historical perspective was essential for understanding how current biodiversity came to be.
Biogeography and the Galápagos Finches
The distribution of species across the globe provided further evidence for evolution. Darwin noted that species on different islands were more similar to each other than to species on the mainland, despite living in different environments. For example, the finches on the Galápagos Islands were more similar to each other than to finches on the South American mainland. This pattern suggested a common ancestor that had colonised the islands and then diversified into different species to exploit different ecological niches. The variation in beak size and shape among the finches was directly related to their diet: some had large, strong beaks for cracking hard seeds, while others had slender beaks for probing flowers for nectar. This adaptation to specific food sources demonstrated how natural selection could drive the divergence of a single ancestral population into multiple species. The geographic isolation of the islands prevented gene flow between populations, allowing each group to evolve independently. This biogeographic evidence was powerful because it showed that the distribution of life was not random but was shaped by historical processes and environmental pressures. It provided a clear example of how geographic barriers and ecological differences could lead to speciation, a key mechanism in the theory of evolution.
The Mechanism of Natural Selection
While the evidence from fossils and biogeography suggested that species change over time, it did not explain how this change occurs. Darwin and Wallace independently proposed the mechanism of natural selection to fill this gap. The theory is based on three key observations: variation, inheritance, and differential survival. First, individuals within a population vary in their traits. Second, many of these traits are heritable, meaning they can be passed from parents to offspring. Third, more offspring are produced than can survive due to limited resources, leading to a struggle for existence. Individuals with traits that give them a survival or reproductive advantage are more likely to pass those traits on to the next generation. Over time, this leads to an accumulation of advantageous traits in the population, resulting in evolutionary change. This process is not directed or goal-oriented; it is a blind, statistical process driven by environmental pressures. The result is that populations become better adapted to their environments over time. This mechanism provided a scientific explanation for the diversity of life, replacing the need for supernatural intervention. It showed that evolution is a natural process, governed by the same physical and biological laws as other phenomena. The theory of natural selection thus became the central organising principle of biology, unifying the study of life under a single, coherent framework.
The concept
Natural selection
Differential survival and reproduction of heritable variation.
A common misconception is that natural selection acts on individuals to make them "better" or more complex. In reality, selection acts on populations, and the outcome is simply a better fit to the current environment, which may involve simplification or loss of traits if they are no longer advantageous. Evolution has no goal or direction; it is a response to immediate selective pressures. C
Worked example
Problem. A population of 1,000 finches has two beak size variants: small (S) and large (L). Initially, 600 have small beaks and 400 have large beaks. During a drought, the survival rate for small-beaked finches is 0.5, and for large-beaked finches is 0.8. Calculate the frequency of the large-beaked type in the next generation, assuming offspring have the same beak type as their parents (true breeding) and the population returns to 1,000 individuals after reproduction.
- Calculate the number of survivors for each phenotype: Small-beaked survivors = \( 600 \times 0.5 = 300 \). Large-beaked survivors = \( 400 \times 0.8 = 320 \).
- Determine the total number of survivors: \( 300 + 320 = 620 \) individuals.
- Calculate the frequency of the large-beaked phenotype in the survivor pool: \( f(L) = \frac{320}{620} \approx 0.516 \).
- Assume the population grows back to 1,000 individuals with the same phenotypic ratio. The number of large-beaked finches in the next generation is \( 1000 \times 0.516 = 516 \).
- The frequency of the large-beaked type rises from 0.400 to about 0.516 in a single generation. (With a real single-gene trait, allele and phenotype frequencies differ, because heterozygotes carry both alleles.)
Units check. The result is a dimensionless frequency, which is consistent with the definition of allele frequency.
Three quick tests
- What key observation from the Galápagos Islands supported the idea of common ancestry?
Answer
The similarity of finch species across different islands, despite differences in beak size and shape, suggested they descended from a common ancestor that diversified in response to local conditions. - How did the fossil evidence from the Andes challenge the idea of local creation?
Answer
The presence of freshwater animal fossils at high altitudes indicated that the land had undergone geological changes, supporting the idea that species migrated and adapted over time rather than being created in their current locations. - What are the three key components of the mechanism of natural selection?
Answer
Variation in traits, heritability of those traits, and differential survival and reproduction due to environmental pressures.
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Every lecture and every concept in this unit, with prerequisites and threads.
Lecture notes checked line by line by the editor; every number in the worked example was recomputed.
Concepts cited: T-017 · Threads: evolution