Biogeographical Evidence
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You should understand the basic mechanisms of natural selection and be familiar with the distinction between homologous and analogous structures from the previous lecture.
Why this matters
The geographic distribution of life provides a spatial map of evolutionary history.
While molecular data offers a microscopic view of ancestry, biogeography provides a macroscopic, geographic perspective. It demonstrates that the distribution of species is not random but is structured by historical barriers, such as continental drift and oceanic isolation. This evidence strongly supports the theory of common descent, as it shows that related species are found in geographically contiguous areas, reflecting their shared evolutionary lineage and the physical constraints of their environments.
The Geographic Pattern of Life
Imagine the Earth as a giant puzzle where the pieces are continents. If life were created independently in every location, we would expect to see a random mix of organisms everywhere. However, we observe a striking pattern: similar environments on different continents often host different species, while similar species are found in geographically close regions. This is the core of biogeographical evidence. Consider the case of marsupials. In Australia, we find kangaroos, koalas, and wombats. In South America, we find opossums and shrew opossums. In North America, we find only the opossum. This distribution is not random. It reflects the historical movement of these animals across land bridges and their subsequent isolation. When continents were connected, these animals could migrate. When they separated, they evolved independently in their new environments. This pattern suggests that all these marsupials share a common ancestor that lived when the continents were joined. Island faunas show the same logic, as the next section describes. These patterns are consistent with the theory of evolution by natural selection. They show that life is not static but changes over time and space. The distribution of species is a record of their evolutionary history, shaped by geographic barriers and environmental pressures. This evidence complements other lines of evidence, such as homology and molecular data, to build a comprehensive picture of common descent.
Continental Drift and Vicariance
One of the most powerful pieces of biogeographical evidence is the distribution of species across continents. Alfred Wegener proposed the theory of continental drift in the early 20th century, suggesting that continents were once joined in a supercontinent called Pangaea and have since drifted apart. This idea was initially met with skepticism but was later supported by evidence from plate tectonics. The distribution of certain species supports this theory. For example, the fossil record shows that the seed fern Glossopteris are found in South America, Africa, India, Australia, and Antarctica. These continents are now separated by vast oceans, and its large seeds could not have crossed them. However, if these continents were once joined, the plants could have spread across the landmass. As the continents drifted apart, the populations became isolated and evolved independently. This process is known as vicariance. It occurs when a geographic barrier, such as an ocean or a mountain range, splits a population into two or more isolated groups. Over time, these groups evolve into distinct species. The result is a pattern of related species found on different continents, reflecting their shared ancestry and the historical separation of their habitats. Vicariance is a key mechanism in biogeography. It explains why related species are found in geographically contiguous areas. It also provides a testable prediction: if two continents were once joined, we should find related species on both continents. This prediction has been confirmed in many cases, such as the distribution of marsupials and the fossil record of Glossopteris. The concept of vicariance is important because it shows that the distribution of life is not random but is shaped by historical events. It provides a framework for understanding the patterns of biodiversity we observe today. By studying the distribution of species, we can reconstruct the history of life on Earth and gain insights into the processes that drive evolution.
Island Biogeography
Islands are natural laboratories for studying evolution. They are isolated from the mainland, which limits the movement of organisms and creates unique environments. This isolation allows for the rapid diversification of species, a process known as adaptive radiation. The Galápagos finches are a classic example of adaptive radiation. Each Galápagos island has its own mix of finch species, all closely related to one another and most similar to seed-eating birds of the South American mainland. This suggests that a single ancestral population colonised the islands and then diversified into different species to exploit different food sources. The geographic isolation of the islands allowed this diversification to occur. Another example is the honeycreepers of the Hawaiian Islands. These birds have diversified into a wide variety of species, each adapted to a different ecological niche. The geographic isolation of the islands has allowed for the evolution of unique species that are found nowhere else in the world. Island biogeography also provides evidence for the role of chance in evolution. The species that colonise an island are often the result of random events, such as a bird being blown off course by a storm. This randomness can lead to the evolution of unique species that are not found on the mainland. The study of island biogeography has led to the development of the Theory of Island Biogeography, which predicts that the number of species on an island is determined by the balance between immigration and extinction. This theory has been tested in many cases and has provided valuable insights into the processes that drive biodiversity. Island biogeography is an important area of study because it provides a clear example of how evolution works. It shows that species can diversify rapidly in response to new environments and that geographic isolation can lead to the evolution of unique species.
Disjunct Distributions
Disjunct distributions occur when related species are found in geographically distant locations, separated by unsuitable habitat. This pattern can be explained by a combination of vicariance and long-distance dispersal. One example of a disjunct distribution is the protea family (Proteaceae): proteas grow in southern Africa, and their relatives the banksias and grevilleas in Australia, despite the vast ocean separating these two continents. This distribution can be explained by the fact that these two regions were once part of the supercontinent Gondwana. As Gondwana broke up, the family's lineages became isolated and evolved independently. Another example is the distribution of the genus Nothofagus, commonly known as southern beech. This genus lives in South America, New Zealand, Australia, New Guinea and New Caledonia, and is known as fossils from Antarctica. This distribution is consistent with the theory of continental drift, as these continents were once joined in Gondwana. Disjunct distributions can also be explained by long-distance dispersal. For example, some seeds can be carried by wind or water over long distances. This can lead to the colonization of new areas and the evolution of new species. The study of disjunct distributions provides valuable insights into the history of life on Earth. It shows that the distribution of species is not random but is shaped by historical events and the ability of organisms to disperse.
The concept
The evidence for common descent
Homology, fossils and molecules point to shared ancestry.
A common misconception is that biogeographical evidence alone is sufficient to prove common descent. While it is a powerful line of evidence, it must be considered in conjunction with other lines of evidence, such as homology, fossils, and molecular data. Biogeography shows that the distribution of life is structured by historical barriers, but it does not directly show the mechanisms of evolution. It is the combination of all these lines of evidence that provides a comprehensive picture of common descent. C
Worked example
Problem. A storm blows 10,000 small birds out to sea. Each bird independently has a probability of 0.01 of reaching an island 100 km away. What is the expected number that arrive, and what is the probability that none arrive?
- Each bird is a trial with success probability \(p = 0.01\); there are \(N = 10\,000\) trials.
- Expected arrivals: \(E = Np = 10\,000 \times 0.01 = 100\) birds.
- Probability that a given bird fails: \(1 - p = 0.99\).
- Probability that all fail: \(0.99^{10\,000} = e^{10\,000 \ln 0.99} = e^{-100.5} \approx 2 \times 10^{-44}\), effectively zero.
- Interpret: rare events become near-certain when there are many trials. Over thousands of years and many storms, even a weak chance of dispersal makes colonisation of remote islands likely, which is why islands are stocked mainly with good dispersers such as birds, bats, insects and wind-borne plants.
Units check. \(E\) is a count of birds; a probability is a pure number between 0 and 1.
Three quick tests
- What is vicariance?
Answer
Vicariance is the process by which a geographic barrier splits a population into two or more isolated groups, leading to the evolution of distinct species. - Give an example of adaptive radiation.
Answer
The Galápagos finches are a classic example of adaptive radiation, where a single ancestral population diversified into different species to exploit different food sources. - How does continental drift support the theory of evolution?
Answer
Continental drift explains the distribution of related species across different continents, reflecting their shared ancestry and the historical separation of their habitats.
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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-020 · Threads: evolution