Evo­lu­tion from com­pe­ti­tion

A study on lizards shows evo­lu­tion­ary change can oc­cur swiftly when two closely re­lated species com­pete

Down to Earth - - CONTENTS - APRA­JITA SINGH

Fight for habi­tat and food can lead to evo­lu­tion­ary changes in an­i­mal species, shows a

new study on lizards

IN­VA­SIVE species are con­sid­ered one of the most press­ing threats to wildlife. Dis­placed to a new habi­tat, of­ten as a re­sult of hu­man ac­tions, th­ese species spread ag­gres­sively in their new home, com­pet­ing with na­tive species for re­sources and al­ter­ing the ecosys­tem. How­ever, the sit­u­a­tion for na­tive species might not be as hope­less as thought, at least when it comes to com­pe­ti­tion be­tween closely re­lated species, as a small lizard na­tive to the US has shown.

About 15 cm in length, Ano­lis car­o­li­nen­sis, also called green anole, can typ­i­cally be found across the south­east­ern re­gion of the coun­try, an area that has re­cently been in­vaded by the Cuban brown anole, Ano­lis sagrei. One ef­fect of this in­va­sion has been com­pe­ti­tion over habi­tat and food be­tween the two. Both species live on tree perches, and the in­va­sion of brown anoles has forced green anoles, which nor­mally oc­cupy space from ground to tree crown, to shift to higher perches. The lizard seems to be at a dis­ad­van­tage here, as the higher perches are smooth and nar­row, and harder to grip than the lower branches.

How­ever, a study by re­searchers led by Yoel E Stu­art from the Univer­sity of Texas at Austin, US, shows that the lizards can evolve to adapt to their new habi­tat in a mat­ter of years.The study was pub­lished on Oc­to­ber 24 in Sci­ence.

The ex­per­i­ment was con­ducted on spoil-dredge is­lands in

Mos­quito La­goon, Florida, US. Dredge is­lands are hu­man-made struc­tures, cre­ated from dredge dur­ing the dig­ging of a water­way in the 1950s.Th­ese is­lands have vary­ing de­grees of veg­e­ta­tion and have been nat­u­rally colonised by A car­o­li­nen­sis. The team in­tro­duced A sagrei spec­i­men to three of six such is­lands in May 1995.

It was pre­dicted that the com­pe­ti­tion pre­sented by the in­vad­ing species would force green anoles to move to higher perches,which proved to be true. In Au­gust 1995, three months after brown anoles were in­tro­duced, an in­crease in the perch­ing height of green anoles was ob­served. On un­in­vaded is­lands, or ones that had not been colonised by brown anoles,no such change was seen.

A sec­ond pre­dic­tion said that the move would drive the evo­lu­tion of the toepads and lamellae of the lizard to bet­ter adapt it to higher branches. Lamellae cover the bot­tom of the toepads, help­ing them gain trac­tion. A greater num­ber of lamellae and a larger toepad would help the lizards in ma­noeu­vring at higher branches.

In 2010,re­searchers stud­ied the toepads and lamellae of brown anoles and found the pre­dic­tion to be true. The lizards were found to pos­sess larger toepads and more lamellae on is­lands where the two species were in com­pe­ti­tion for tree space than the ones where the brown anole was ab­sent. Most sur­pris­ingly, this change took only 20 gen­er­a­tions of lizards to oc­cur,a time pe­riod of about 15 years be­tween the in­tro­duc­tion of the in­va­sive species to the is­lands and this study.

“We did pre­dict that we would see a change, but the de­gree and quick­ness with which they evolved was sur­pris­ing,” said Stu­art. “To put this shift in per­spec­tive, if hu­man height were evolv­ing as fast as th­ese lizards’ toes, the height of an av­er­age Amer­i­can man would in­crease from about 5 feet 9 inches (175 cm) to­day to about 6 feet 4 inches (193 cm) within 20 gen­er­a­tions... Although hu­mans live longer than lizards, this rate of change would still be rapid in evo­lu­tion­ary terms.”

Be­fore ar­riv­ing at con­clu­sions, the team had to be sure that the evo­lu­tion had ac­tu­ally oc­curred due to the pres­ence of a closely re­lated in­va­sive species,and not due to some other process.To test this,they set up a gar­den ex­per­i­ment where eggs from both in­vaded and un­in­vaded is­lands were col­lected,and the hatch­lings raised in iden­ti­cal en­vi­ron­ments. The fact that the char­ac­ter­is­tics of th­ese lizards re­mained as they would have been on their re­spec­tive is­lands sug­gested that the changes were, in­deed, ge­netic. Other fac­tors that could have caused the changes in the lizards, such as a re­sponse to en­vi­ron­men­tal changes or the abil­ity of the brown anole to only colonise is­lands in­hab­ited by big-toed lizards, were also sys­tem­at­i­cally dis­counted.

Evo­lu­tion as a re­sult of in­ter­ac­tion be­tween two closely re­lated species takes place to re­duce com­pe­ti­tion be­tween them for food and habi­tat

Evo­lu­tion as a re­sult of in­ter­ac­tion be­tween two closely re­lated species is termed “character dis­place­ment”. Sim­ply put, it is an evo­lu­tion in the traits or char­ac­ter­is­tics of th­ese species to re­duce com­pe­ti­tion be­tween them, as has been ob­served in this study. Well-doc­u­mented cases of character dis­place­ment are rare, even though the phe­nom­e­non is con­sid­ered to be wide­spread (see ‘Evolv­ing to sur­vive’).

Martin Whit­ing, as­so­ciate pro­fes­sor at the Mac­quarie Univer­sity, Syd­ney, who heads his own lizard lab where he stud­ies their be­hav­iour, ecol­ogy and evo­lu­tion, ex­plains the im­por­tance of the study: “Character dis­place­ment, a common term in eco­log­i­cal lit­er­a­ture, has not pre­vi­ously been tested in real time. The au­thors use an old work-horse, the Cuban brown anole, to test if its in­tro­duc­tion in south-east US will elicit evo­lu­tion­ary change from the sole res­i­dent, the green anole.” Whit­ing was not part of this study.

“The au­thors were able to dis­count a se­ries of com­pet­ing hy­pothe­ses that might ex­plain their re­sults by the im­pres­sively com­pre­hen­sive and mul­ti­fac­eted ap­proach they took. This study will likely fea­ture in fu­ture stu­dent text­books as a ro­bust ex­am­ple of how character dis­place­ment through neg­a­tive in­ter­ac­tions, such as com­pe­ti­tion, can drive di­ver­gence be­tween closely re­lated species,par­tic­u­larly in a mod­ern world where hu­mans in­creas­ingly al­ter the en­vi­ron­ment and shift species distri­bu­tions.”

The study is of great rel­e­vance to our un­der­stand­ing of in­va­sive species and how they af­fect na­tive ecol­ogy.Na­tive species may be able to sur­vive the on­slaught of in­vaders by evolv­ing in re­sponse to the com­pe­ti­tion pre­sented by them, although the ex­tent to which this can oc­cur be­tween un­re­lated species is un­known. Stu­art says, “I think we most of­ten think of character dis­place­ment hap­pen­ing be­tween closely re­lated species be­cause we con­sider that closely re­lated species are the most likely to over­lap in niche space. How­ever, I think that any two species that com­pete for the same set of limited re­sources could ex­pe­ri­ence character dis­place­ment, even if they are not closely re­lated.”

Green anoles (left) un­der­went rapid evo­lu­tion due to com­pe­ti­tion for food and habi­tat from Cuban brown anoles (right)

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