AboutZoos, Since 2008


How to be win­ner in the game of evolution

pub­lished 15 Jan­u­ary 2017 | mod­i­fied 15 Jan­u­ary 2017

A new study by Uni­ver­sity of Ari­zona biol­o­gists helps explain why dif­fer­ent groups of ani­mals dif­fer dra­mat­i­cally in their num­ber of species, and how this is related to dif­fer­ences in their body forms and ways of life. The study reveals that the most suc­cess­ful phyla have species that live on land, have a skele­ton and are parasites.

For mil­len­nia, humans have mar­velled at the seem­ingly bound­less vari­ety and diver­sity of ani­mals inhab­it­ing the Earth. So far, biol­o­gists have described and cat­a­logued about 1.5 mil­lion ani­mal species, a num­ber that many think might be eclipsed by the num­ber of species still await­ing discovery.

All ani­mal species are divided among roughly 30 phyla, but these phyla dif­fer dra­mat­i­cally in how many species they con­tain, from a sin­gle species to more than 1.2 mil­lion in the case of insects and their kin. Ani­mals have incred­i­ble vari­a­tion in their body shapes and ways of life, includ­ing the plant­like, immo­bile marine sponges that lack heads, eyes, limbs and com­plex organs, par­a­sitic worms that live inside other organ­isms (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).

simplified tree of lifeA sim­pli­fied evo­lu­tion­ary tree of six rep­re­sen­ta­tive ani­mal phyla, illus­trat­ing dif­fer­ences in body form, habi­tat, and species num­bers among them.
(Image credit: T. Jezkova/​Shutterstock/​Aaron Ambos/​J. Wiens)

Amid this daz­zling array of life forms, one ques­tion has remained as elu­sive as it is obvi­ous: Why is it that some groups on the evo­lu­tion­ary tree of ani­mals have branched into a dizzy­ing thicket of species while oth­ers split into a mere hand­ful and called it a day?

From the begin­nings of their dis­ci­pline, biol­o­gists have tried to find and under­stand the pat­terns under­ly­ing species diver­sity. In other words, what is the recipe that allows a phy­lum to diver­sify into many species, or, in the words of evo­lu­tion­ary biol­o­gists, to be “suc­cess­ful”? 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 the new study, Tereza Jezkova and John Wiens, both in the Uni­ver­sity of Arizona’s Depart­ment of Ecol­ogy and Evo­lu­tion­ary Biol­ogy, 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 how quickly species in each phy­lum mul­ti­plied over time (diver­si­fi­ca­tion). The results are pub­lished online on 3 Jan­u­ary in the jour­nal Amer­i­can Naturalist.

Jezkova and Wiens 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. Other traits, includ­ing those that might seem more dra­matic, had sur­pris­ingly lit­tle impact on diver­si­fi­ca­tion and species num­bers: Evo­lu­tion­ary accom­plish­ments such as hav­ing a head, limbs and com­plex organ sys­tems for cir­cu­la­tion and diges­tion don’t seem to be pri­mary acces­sories in the evo­lu­tion­ary “dress for success.”

Par­a­sitism isn’t cor­re­lated with any of the other traits, so it seems to have a strong effect on its own
John Wiens, Depart­ment of Ecol­ogy and Evo­lu­tion­ary Biol­ogy, Uni­ver­sity of Ari­zona, USA »

Wiens explained that when a host species splits into two species, it takes its par­a­site population(s) with it. “You can have a num­ber of par­a­site species liv­ing inside the same host,” he said. “For exam­ple, there could be 10 species of nema­todes in one host species, and if that host species splits into two, there are 20 species of nema­todes. So that really mul­ti­plies the diversity.”

The researchers used a sta­tis­ti­cal method called mul­ti­ple regres­sion analy­sis to tease out whether a trait such as par­a­sitic lifestyle is a likely dri­ver of species 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. And from that we picked six traits that each had a strong effect on their own. We then fed those six traits into a mul­ti­ple regres­sion model. And then we asked, ‘What com­bi­na­tion of traits explains the most vari­a­tion with­out includ­ing any unnec­es­sary vari­ables?’ — and from that we could reduce it down to three key variables.”

The authors point out that the analy­sis does not make any assump­tions about the fos­sil record, which is not a true reflec­tion of past bio­di­ver­sity, as it does not reveal most soft-​bodied ani­mals or traits like a par­a­sitic lifestyle. “We wanted to know what explains the pat­tern of diver­sity in the species we see today,” Wiens said. “Who are the win­ners, and who are the losers?”

Marine bio­di­ver­sity is in jeop­ardy from human activ­i­ties such as acid­i­fi­ca­tion from car­bon emis­sions, pos­ing an exis­ten­tial threat to many marine ani­mals, Wiens said.

Many unique prod­ucts of ani­mal evo­lu­tion live only in the oceans and could eas­ily be lost, so groups that have sur­vived for hun­dreds of mil­lions of years could dis­ap­pear in our life­time, which is ter­ri­ble,” he said. “Many of the ani­mals’ phyla that are losers in terms of present-​day species num­bers tend to be in the ocean, and because of human activ­ity, they may go com­pletely extinct.”

The study also sug­gests that man-​made extinc­tion may wage a heavy toll on Earth’s bio­di­ver­sity because of the effect of sec­ondary extinc­tions, Wiens explained.

When a species goes extinct, all its asso­ci­ated species that live in it or on it are likely to go extinct as well,” he said.

(Source: Uni­ver­sity of Ari­zona news release, 13.01.2017)

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