Aquatic vertebrates show larger, more frequent evolutionary size shifts
A comparison of 21,374 vertebrate species found a recurring habitat pattern in reconstructed changes of body size. The reconstructed shifts were greatest and most common in water, smallest and rarest among flying animals, and intermediate on land. The peer-reviewed statistical study links this pattern to environmental constraints while accounting for ancestry and other background differences.
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Water, land and flight leave different patterns
Aquatic vertebrates stood at one end of a repeated pattern in the evolution of body size: their inferred large changes were both bigger and more frequent. Flying lineages occupied the other end, with terrestrial animals between them. Mark Pagel, Ciara O’Donovan and Andrew Meade identified this pattern through a peer-reviewed comparison spanning 21,374 species, including mammals, birds, reptiles, amphibians and fishes. The analysis concerns size increases as well as decreases.[1]
Ancestral shifts are reconstructed statistically
The researchers combined present-day size measurements with evolutionary trees. A statistical model allowed gradual variation alongside rarer, larger changes and yielded 2,408 large shifts for the final size comparison. These are estimates along ancestral branches, rather than events witnessed directly. The comparisons accounted for branch duration, geological timing, shared ancestry and body-temperature regulation. Groups measured by length and those measured by mass were placed on a common scale.[1]
Bats resemble birds and aquatic mammals resemble fishes
Mammals that returned to water approached the size-shift pattern of fishes; bats approached birds. Independently compiled population data also placed aquatic variation above terrestrial and flying variation. The authors interpret the recurring ordering as evidence that habitats impose different constraints on size evolution. Some habitat transitions have few examples, and historical estimates depend on the trees and model assumptions. The habitat ordering applies to body-size estimates, with no equivalent rule established for other evolving traits.[1]
