Biogeography is the study of how species and ecosystems are distributed across geographic areas and through geological time. This field examines the patterns of life on Earth, revealing how and why organisms are found where they are. Evolution refers to the changes in heritable characteristics within biological populations over successive generations. Biogeography offers compelling observations that support the concept of evolution by showing distinct patterns in where organisms live.
The Foundation: Biogeographical Principles
Biogeography uncovers patterns in species distribution. One such pattern is endemism, where a species is found exclusively in a particular defined geographic area and nowhere else. These unique distributions suggest species evolved in isolation or within specific environmental confines. Another observed pattern is disjunct distributions, where populations of a single species or a closely related group are found in widely separated geographical regions. These non-random distributions indicate historical and evolutionary forces have shaped life’s presence across the planet.
Global Distribution of Life: A Story of Common Ancestry
Worldwide patterns of species distribution provide evidence for evolution. Consider the distribution of marsupials, which are primarily found in Australia and South America, with a few species in North America. This pattern points to a shared ancient lineage before continental separation. After the ancient supercontinent Gondwana broke apart, these marsupials diversified independently on the now-isolated landmasses. Large, flightless birds like ostriches in Africa, emus in Australia, and rheas in South America also share a common flightless ancestor, evolving into distinct species on different continents, explained by common descent and evolutionary divergence.
Island Biogeography: Evolution in Isolation
Islands provide clear examples of evolutionary processes, as their isolation limits gene flow, leading to unique evolutionary pathways and endemism. When a single ancestral species colonizes an island, it can diversify into many distinct forms adapted to different ecological niches, a process known as adaptive radiation. Darwin’s finches on the Galápagos Islands illustrate this, where an ancestral finch diversified into species with unique beak shapes adapted to different food sources. Hawaiian honeycreepers, found only in Hawaii, also show adaptive radiation, evolving from a single ancestral species into many distinct forms. Unique environmental pressures on islands, coupled with limited gene flow, accelerate evolutionary change, resulting in unique species.
Continental Drift: Shaping Life’s Journey
Continental drift has profoundly influenced the distribution of species over vast timescales, as the movement of Earth’s tectonic plates has caused continents to shift and separate, directly impacting the dispersal and isolation of populations. The break-up of the supercontinent Pangea led to the physical separation of ancestral populations. This geographical isolation resulted in allopatric speciation, where new species arise from an ancestral population due to reproductive isolation. The distribution of ancient plant fossils, such as Glossopteris, found across modern-day India, Australia, Antarctica, South Africa, and South America, provides evidence of this past supercontinent. The distribution of the Nothofagus (southern beech) tree genus across South America, Australia, New Zealand, and New Guinea also reflects ancient southern landmass connections, with both distributions understood by considering Earth’s geological history and subsequent evolutionary processes.