"Long before it's in the papers"
January 27, 2015


Evolution is predictable, study suggests

July 21, 2013
Courtesy of UC Davis
and World Science staff

If you could hit the re­set but­ton on ev­o­lu­tion and start over, would bas­ic­ally the same spe­cies ap­pear? Yes, ac­cord­ing to a study of Car­ib­be­an liz­ards pub­lished July 19 in the jour­nal Sci­ence.

Pairs of Ano­lis spe­cies from dif­fer­ent Great­er An­til­le­an is­lands that have in­de­pend­ent­ly evolved match­ing mor­pholo­gies, or body forms. Anoles di­ver­si­fying on four is­lands re­peat­ed­ly col­o­nized the same adap­tive peaks on a shared ev­o­lu­tiona land­scape, re­sult­ing not just in con­ver­gence among a few spe­cies pairs, such as those shown he­re, but in the con­ver­gence of en­tire is­land anole fau­nas, re­search­ers said. From left to right, the top row de­picts gi­ant tree crown spe­cial­ists Ano­lis cu­vieri (Puerto Rico; pho­to by J. Losos) and A. gar­mani (Ja­maica); sec­ond row de­picts the twig spe­cial­ists A. gar­ri­doi (Cu­ba) and A. oc­cul­tus (Puerto Ri­co); third row de­picts trunk and ground spe­cial­ists A. cy­botes (His­pan­io­la; pho­to by B. Falk) and A. lin­eato­pus (Ja­maica); fourth row de­picts grass spe­cial­ists A. alu­mi­na (His­pan­io­la; pho­to by M. Lan­destoy) and A. alu­taceus (Cu­ba). Im­ages not oth­er­wise marked are by L. Mah­ler. (Cred­it: Luke Mah­ler, UC Da­vis)

Bi­ol­o­gists have long de­bat­ed ev­o­lu­tion’s pre­dictabil­ity over mil­lions of years, said Luke Mah­ler of the Uni­vers­ity of Cal­i­for­nia, Da­vis, one of the re­search­ers. For ex­am­ple, the late Ste­phen Jay Gould pre­dicted that if you “re­wound the tape” on ev­o­lu­tion and started over, you would get a to­tally new out­come, as small events could snow­ball in­to large con­se­quenc­es.

On the oth­er hand, there are a num­ber of ex­am­ples of spe­cies in si­m­i­lar habi­tats that evolve in­de­pend­ently in­to sim­i­lar-looking forms, such as the cich­lid fish­es of Af­ri­can lakes.

“It’s a big ques­tion in ev­o­lu­tion­ary bi­ol­o­gy, but very hard to test,” Mah­ler said.

Mah­ler found his test sub­jects in the Anole liz­ards that live on four neigh­bor­ing is­lands—Cuba, His­pan­io­la (Hai­ti and the Do­min­i­can Re­pub­lic), Ja­mai­ca and Puerto Rico. Anoles be­gan col­o­niz­ing these is­lands, all si­m­i­lar in cli­mate and ecol­o­gy, about 40 mil­lion years ago. Once there, they be­gan to mul­ti­ply, re­sult­ing in a di­vers­ity of spe­cies on each.

The re­search­ers stud­ied 100 of the 119 Anole liz­ard spe­cies from the is­lands, tak­ing mea­sure­ments of their bod­ies from wild and mu­se­um spec­i­mens and com­par­ing them across is­lands.

They found a strik­ing de­gree of con­ver­gence—on each is­land, ev­o­lu­tion had pro­duced a set of very sim­i­lar-looking liz­ards oc­cu­py­ing si­m­i­lar en­vi­ron­men­tal niches.

The “adap­tive radia­t­ions”—the way spe­cies branch out in­to new spe­cies—”match on all four is­lands,” Mah­ler said. “With few ex­cep­tions, each spe­cies on an is­land has a match on the oth­er is­lands.”

The four is­lands al­so show si­m­i­lar “adap­tive land­scapes,” the re­search­ers found. An adap­tive land­scape is a fun­da­men­tal con­cept in ev­o­lu­tion­ary bi­ol­o­gy but dif­fi­cult to show in prac­tice. Peaks on an adap­tive land­scape rep­re­sent var­i­ous com­bina­t­ions of fea­tures that will be fa­vored by ev­o­lu­tion, where­as val­leys are just the op­po­site. Spe­cies with si­m­i­lar habits will tend to clus­ter on the same peak.

For Anole liz­ards, their niche might be liv­ing on tree-trunks, or among twigs high in a tree, or down in the grasses on the ground. Each calls for dif­fer­ent adapta­t­ions, and cre­ates a dif­fer­ent adap­tive peak. “The cool part is that we now have a way of mod­el­ing the adap­tive land­scape that ex­plains this con­ver­gence,” Mah­ler said.

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If you could hit the reset button on evolution and start over, would basically the same species appear? Yes, according to a study of Caribbean lizards published July 19 in the journal Science. Biologists have long debated evolution’s predictability over millions of years, said Luke Mahler of the University of California, Davis, one of the researchers. For example, the late Stephen Jay Gould predicted that if you “rewound the tape” on evolution and started over, you would get a totally new outcome, as small events could snowball into large consequences. On the other hand, there are a number of examples of species in similar habitats that evolve independently into similar-looking forms, such as the cichlid fishes of African lakes. “It’s a big question in evolutionary biology, but very hard to test,” Mahler said. Mahler found his test subjects in the Anole lizards that live on four neighboring islands—Cuba, Hispaniola (the countries of Haiti and the Dominican Republic), Jamaica and Puerto Rico. Anoles began colonizing these islands, all similar in climate and ecology, about 40 million years ago. Once there, they began to multiply, resulting in a diversity of species on each. The researchers studied 100 of the 119 Anole lizard species from the islands, taking measurements of their bodies from wild and museum specimens and comparing them across islands. They found a striking degree of convergence—on each island, evolution had produced a set of very similar-looking lizards occupying similar environmental niches. The “adaptive radiations”—the way species branch out into new species—”match on all four islands,” Mahler said. “With few exceptions, each species on an island has a match on the other islands.” The four islands also show similar “adaptive landscapes,” the researchers found. An adaptive landscape is a fundamental concept in evolutionary biology but difficult to show in practice. Peaks on an adaptive landscape represent various combinations of features that will be favored by evolution, whereas valleys are just the opposite. Species with similar habits will tend to cluster on the same peak. For Anole lizards, their niche might be living on tree-trunks, or among twigs high in a tree, or down in the grasses on the ground. Each calls for different adaptations, and creates a different adaptive peak. “The cool part is that we now have a way of modeling the adaptive landscape that explains this convergence,” Mahler said.