“What explains patterns of diversification and richness among animal phyla?”

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Tereza Jezkova and John J. Wiens

Three traits explain most of the variation in the diversity of animal phyla, representing >80% of all known species

New research helps reveal why some groups of animals are more successful than others

A simplified evolutionary tree of 6 representative animal phyla, illustrating differences in body form, habitat, and species numbers among them.
[Image credits: Shutterstock (top three), Aaron Ambos (snail), John Wiens (lion, grasshopper)]

A new study helps explain why different groups of animals have such different numbers of species, and how this is related to differences in their body forms and ways of life.

All animal species are divided among ~30 phyla, but these phyla differ dramatically in how many species they contain, from a single species to more than 1.2 million (insects and relatives). However, the explanation for the remarkable variation in biodiversity among animal phyla remains largely unknown. Animals also have incredible variation in their body shapes and ways of life. For example, animals include plant-like, immobile marine phyla that lack heads, eyes, limbs, and complex organs (sponges), parasitic worms that live inside other organisms (e.g. nematodes, platyhelminths), and phyla with eyes, skeletons, limbs, and complex organs that dominate the land in terms of species numbers (arthropods) and body size (chordates). A fundamental but unresolved problem is whether the basic biology of these phyla is related to their species numbers. For example, does having a head, limbs, and eyes allow some groups to be more successful and thus have greater species numbers?

In a new study, researchers from the University of Arizona have helped resolve this problem. They assembled a database of 18 traits, including traits related to anatomy, reproduction, and ecology. They then tested how each trait was related to the number of species in each phylum, and to how quickly species in each phylum multiplied over time (diversification). They found that just three traits explained most variation in diversification and species numbers among phyla: the most successful phyla have a skeleton (either internal or external), live on land (instead of in the ocean), and parasitize other organisms. They also found that many dramatic traits had surprisingly little impact on diversification and species numbers, such as having a head, limbs, and complex organ systems for circulation and digestion. Read the Article