Just a few explosive lineages account for most species on Earth

A sweeping analysis of the “tree of life” finds that most living species belong to a relatively small number of lineages that diversified unusually quickly.

Across plants, animals, and other kingdoms, most branches have not grown evenly through time. Instead, a few have exploded into dense thickets of species, while many others remain sparsely populated.

The British evolutionary biologist J.B.S. Haldane is said to have quipped that any divine being evidently had “an inordinate fondness for beetles.”

The joke contains a truth biologists have wrestled with for decades: some groups dominate the ledger of life, while others are comparatively species-poor.

More than 40 percent of extant insects are beetles, 60 percent of birds are passerines, and more than 85 percent of plants are flowering plants – an unevenness that begs an explanation.

Spanning Earth’s many species

To move beyond anecdotes, researchers compiled a vast cross-taxonomic dataset. They then asked whether the concentration of species in a few very large groups is a general feature of life on Earth.

The team analyzed the distribution of species richness and diversification rates across clades – lineages that share a common ancestor – spanning land plants, insects, vertebrates, all animals, and a synthesis “across life.”

For land plants, the tally included 10 phyla, 140 orders, and 678 families, together representing more than 300,000 species. For insects, 31 orders and 870 families covered over one million known species.

Vertebrates made up 12 classes encompassing more than 66,000 species, while animals overall accounted for 28 phyla and 1,710 families, totaling more than 1.5 million species.

A final roll-up across 17 kingdoms and 2,545 families included more than two million species. The team compared species counts, clade ages, and diversification rates for each clade.

Earth’s diversity comes from rare explosions

“Here we show for the first time that most living species do indeed belong to a limited number of rapid radiations,” said John Wiens, a professor at the University of Arizona. “That is, they form groups with many species which evolved in a relatively short period of time.”

“If we look among the kingdoms of life, among animal phyla, and among plant phyla, we find that more than 80 percent of known species belong to the minority of groups with exceptionally high rates of species diversification.”

The result stayed consistent no matter how researchers sliced the tree – by high-level phyla, mid-level orders, or family-level groupings. They found that most species cluster in a few branches with above-average diversification rates, while the majority of branches remain comparatively species-poor.

The spark behind biodiversity surges

So-called rapid radiations can follow the opening of new ecological opportunities or the evolution of key innovations.

When a small founding flock of grassquit birds reached the Galápagos roughly 2.5 million years ago, it diversified into Darwin’s finches.

When powered flight evolved in early bats about 50 million years ago, it unlocked a rich set of niches. Such episodes don’t just add species; they reset the balance of diversity across the tree.

Traits that tip the scales

“Our results imply that most of life’s diversity is explained by such relatively rapid radiations,” Wiens said. “We also suggest key traits that might explain these rapid radiations, based on our results and results of earlier studies.”

“These traits include multicellularity in plants, animals, and fungi across the kingdoms of life; the invasion of land and the adoption of a plant-based diet in arthropods among animal phyla; and the emergence of flowers and insect pollination in flowering plants among plant phyla.”

These innovations changed the rules of engagement with the environment, allowing lineages to exploit new resources, spread into new habitats, and diversify faster than their relatives.

The statistical signal is not unique to one group or one rank. It appears from the vast diversity of insects to the great branches of land plants and vertebrates.

In practical terms, most of the species we encounter – and most of the biodiversity we aim to conserve – sit within a minority of lineages. These lineages experienced unusually fast growth spurts through evolutionary time.

Bacteria remain a vast unknown

One major caveat is the kingdom Bacteria. Scientists have formally described only about 10,000 bacterial species, yet they estimate true bacterial diversity to be in the millions to trillions.

At the same time, bacteria originated roughly 3.5 billion years ago, which spreads speciation events across immense time and implies a low overall diversification rate.

The authors point out that if bacteria actually have far more species than we currently know, then bacteria would make up most of life on Earth – very different from what their study found. They emphasized that their results mainly apply to the species we’ve already identified.

Conserving Earth’s species diversity

Recognizing that most species are concentrated in a handful of fast-diversifying clades reframes both evolutionary theory and conservation practice. It sharpens questions about the ecological triggers and genetic innovations that spark radiations.

It also helps triage conservation priorities: protecting the engines of diversification may conserve not only many species today but also the capacity for future diversity.

Haldane’s quip about a deity’s “inordinate fondness for beetles” captured the pattern with wit. This analysis shows the pattern is not an exception but a rule written across much of life.

The study is published in the journal Frontiers in Ecology and Evolution.

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