How to be winner in the game of evolution13 January 2017

A simplified evolutionary tree of six representativeanimal phyla, illustrating differences in body form,habitat, and species numbers among them. Credit: T.Jezkova/Shutterstock/Aaron Ambos/J. Wiens

A new study by University of Arizona biologistshelps explain why different groups of animals differdramatically in their number of species, and howthis is related to differences in their body forms andways of life.

For millennia, humans have marveled at theseemingly boundless variety and diversity ofanimals inhabiting the Earth. So far, biologistshave described and catalogued about 1.5 millionanimal species, a number that many think might beeclipsed by the number of species still awaitingdiscovery.

All animal species are divided among roughly 30phyla, but these phyla differ dramatically in howmany species they contain, from a single speciesto more than 1.2 million in the case of insects andtheir kin. Animals have incredible variation in theirbody shapes and ways of life, including the plant-like, immobile marine sponges that lack heads,

eyes, limbs and complex organs, parasitic wormsthat live inside other organisms (e.g. nematodes,platyhelminths), and phyla with eyes, skeletons,limbs and complex organs that dominate the land interms of species numbers (arthropods) and bodysize (chordates).

Amidst this dazzling array of life forms, onequestion has remained as elusive as it is obvious:why is it that some groups on the evolutionary treeof animals have branched into a dizzying thicket ofspecies while others split into a mere handful andcalled it a day?

From the beginnings of their discipline, biologistshave tried to find and understand the patternsunderlying species diversity. In other words, what isthe recipe that allows a phylum to diversify intomany species, or, in the words of evolutionarybiologists, to be "successful?" A fundamental butunresolved problem is whether the basic biology ofthese phyla is related to their species numbers. Forexample, does having a head, limbs, and eyesallow some groups to be more successful and thushave greater species numbers?

In the new study, Tereza Jezkova and John Wiens,both in the University of Arizona's Department ofEcology and Evolutionary Biology, have helpedresolve this problem. They assembled a databaseof 18 traits, including traits related to anatomy,reproduction, and ecology. They then tested howeach trait was related to the number of species ineach phylum, and to how quickly species in eachphylum multiplied over time (diversification). Theresults are published in the journal AmericanNaturalist.

Jezkova and Wiens found that just three traitsexplained most variation in diversification andspecies numbers among phyla: the most successfulphyla have a skeleton (either internal or external),live on land (instead of in the ocean), and parasitizeother organisms. Other traits, including those thatmight seem more dramatic, had surprisingly littleimpact on diversification and species numbers:

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evolutionary accomplishments such as having ahead, limbs, and complex organ systems forcirculation and digestion don't seem to be primaryaccessories in the evolutionary "dress for success."

"Parasitism isn't correlated with any of the othertraits, so it seems to have a strong effect on itsown," said Wiens.

He explained that when a host species splits intotwo species, it takes its parasite population(s) withit.

This colorful chocolate chip sea star, along with seacucumbers and sea urchins belongs to the Echinoderms,the only phylum with a body plan of five-fold symmetry.Credit: Ethan Daniels/Shutterstock

"You can have a number of parasite species livinginside the same host," he said, "for example, therecould be ten species of nematodes in one hostspecies, and if that host species splits into two,there are 20 species of nematodes. So that reallymultiplies the diversity."

The researchers used a statistical method calledmultiple regression analysis to tease out whether atrait such as parasitic lifestyle is a likely driver ofspecies diversification.

"We tested all these unique traits individually,"Wiens explained, "for example, having a head,having eyes, where the species in a phylum tend to

live, whether they reproduce sexually or asexually,whether they undergo metamorphosis or not; andfrom that we picked six traits that each had a strongeffect on their own. We then fed those six traits intoa multiple regression model. And then we asked,'what combination of traits explains the mostvariation without including any unnecessaryvariables?'—and from that we could reduce it downto three key variables."

The authors point out that the analysis does notmake any assumptions about the fossil record,which is not a true reflection of past biodiversity asit does not reveal most soft-bodied animals or traitslike a parasitic lifestyle.

"We wanted to know what explains the pattern ofdiversity in the species we see today," said Wiens."Who are the winners, and who are the losers?"

Marine biodiversity is in jeopardy from humanactivities such as acidification from carbonemissions, posing an existential threat to manymarine animals, Wiens said.

"Many unique products of animal evolution live onlyin the oceans and could easily be lost, so groupsthat have survived for hundreds of millions of yearscould disappear in our lifetime, which is terrible.Many of the animals phyla that are losers in termsof present-day species numbers tend to be in theocean, and because of human activity, they may gocompletely extinct."

The study also suggests that man-made extinctionmay wage a heavy toll on Earth's biodiversity dueto the effect of secondary extinctions, Wiensexplained.

"When a species goes extinct, all its associatedspecies that live in it or on it, are likely to go extinctas well."

More information: Tereza Jezkova et al. WhatExplains Patterns of Diversification and Richnessamong Animal Phyla?, The American Naturalist(2017). DOI: 10.1086/690194

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Provided by University of ArizonaAPA citation: How to be winner in the game of evolution (2017, January 13) retrieved 26 December 2021from https://phys.org/news/2017-01-winner-game-evolution.html

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