Evidence of evolution from biogeography

The argument

Biogeography — the study of where species live and why — provides striking confirmation of evolution, because the global distribution of life follows the pattern that descent with modification predicts rather than one of mere suitability of habitat. If species had simply appeared wherever conditions suited them, similar environments around the world would host similar creatures; instead, what we find is that the inhabitants of a region are most closely related to other species in that same region, even across very different habitats, reflecting their shared ancestry and where their ancestors happened to live and diversify. Islands offer the clearest demonstration. Remote oceanic islands such as the Galápagos and Hawaii are home to unique species found nowhere else, and crucially these endemic species most resemble those of the nearest mainland, as descent from colonising ancestors predicts. Darwin's Galápagos finches — a cluster of related species adapted to different food sources, all clearly derived from a common South American ancestor — are a textbook case of a single colonising lineage radiating into many forms. Such islands also conspicuously lack native land mammals and amphibians, which could not cross the ocean to reach them, an absence that makes sense by descent and dispersal but not by independent creation in suitable habitats. The distribution of related groups across continents tells the same story, tracing the movement and splitting of landmasses over geological time — marsupials concentrated in Australia and the Americas, for instance. The map of life is a map of evolutionary history and geography, not of habitat alone. Because biogeography reveals exactly the distribution that common descent and dispersal predict, this argument holds, the theory of evolution by natural selection is correct.

Premises

Counter-arguments

A careful critic grants that biogeography strongly evidences common descent and dispersal, but notes it is largely neutral between natural selection and other evolutionary mechanisms — drift, extinction and continental drift shape distributions too — so it confirms that lineages descend and disperse rather than pinpointing selection as the driver. The argument's title claims support for "the theory of evolution by natural selection," which biogeography underdetermines. Critics also observe that resemblance among a region's species is complicated by convergent evolution, which produces similar forms in similar habitats on separate continents, and by human-introduced species that scramble natural ranges. The pattern is powerful evidence for descent and dispersal, but a looser fit to the specific selective mechanism than the argument implies.

Rejecting the premises

[Rejecting P1] Regional resemblance supports shared ancestry, but convergent evolution also yields similar forms in similar habitats on different continents, so resemblance alone is not decisive for the mechanism. [Rejecting P3] The distribution of related groups across continents evidences dispersal and common descent directly, but is largely silent on whether natural selection rather than drift or other processes drove the changes.

Further reading

https://www.nap.edu/read/6024/chapter/4