Biogeography, the study of how species are distributed across Earth, offers compelling support for the theory of evolution. It reveals that the patterns of life on our planet are not random, but rather reflect historical processes of change and adaptation. Understanding where organisms live provides insights into the mechanisms driving evolutionary diversification.
The Study of Life’s Distribution
Biogeography explores the geographical distribution of life forms, both in the present and throughout geological time. This field examines patterns in where species occur, investigating the underlying reasons for these distributions. It integrates knowledge from biology, ecology, geography, and geology to explain how species ranges are shaped and how organisms interact with their environments.
Geographic Barriers and New Species
Physical barriers, such as vast oceans, towering mountain ranges, or expansive deserts, play a significant role in evolution by isolating populations. When populations are separated, gene flow between them ceases, leading to independent evolutionary paths. Different environmental pressures in these isolated areas cause distinct adaptations, eventually resulting in the formation of new species, a process known as speciation.
Islands are particularly illustrative examples, often hosting unique species found nowhere else, a condition called endemism. The Galapagos Islands, for instance, are renowned for their endemic species, such as Darwin’s finches, which diversified from a single ancestral species to fill various ecological niches. Isolation on islands can accelerate evolutionary changes due to unique selective pressures and reduced competition.
Evidence from Shared Ancestry
The distribution of living species and fossils provides strong evidence for common ancestry and historical geological events, especially continental drift. Disjunct distributions, where related species are found in widely separated areas, are often explained by the past arrangement and movement of Earth’s continents. Identical fossils and geological similarities on now-distant landmasses support their past connection.
A prime example is the ancient seed fern Glossopteris, whose fossils are found across South America, Africa, India, Australia, and Antarctica. The distribution of this plant was key evidence supporting the theory that these landmasses were once part of the supercontinent Gondwana. Similarly, marsupial mammals, predominantly in Australia and South America, reflect their spread across Gondwana before the continents drifted apart.
Similar Adaptations in Different Places
Convergent evolution illustrates how unrelated species develop similar traits when adapting to comparable environmental conditions or ecological roles. This process shows environmental pressures can guide evolution towards similar solutions, even in organisms with distinct evolutionary histories. The resulting similarities are due to adaptation, not shared ancestry.
A classic illustration involves aquatic animals like sharks, dolphins, and the extinct ichthyosaurs, all of which evolved streamlined body shapes and fins for efficient movement through water. On land, cacti in the Americas and euphorbias in Africa, though distantly related, independently developed succulent stems, spines, and water-storing capabilities to thrive in arid desert environments. These parallel adaptations highlight the powerful influence of environmental selection on the forms and functions of life.