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


From tiny to titanic in 24 million generations

Jan. 31, 2012
Courtesy of Monash University
and World Science staff

It takes 24 mil­lion genera­t­ions for a mouse-sized an­i­mal to evolve to an ele­phant’s size—but trav­el­ing along the re­verse ev­o­lu­tion­ary path goes more than ten times faster, new re­search sug­gests.

The stu­dy, pub­lished Jan. 30 in the jour­nal Pro­ceed­ings of the Na­t­ional Acad­e­my of Sci­ences, ex­amined in­creases and de­creases in mam­mal size that have occurred since the the di­no­saurs died out 65 mil­lion years ago.

A mouse-to-elephant size change would take at least 24 mil­lion gen­er­a­tions based on the max­i­mum speed of ev­o­lu­tion in the fos­sil rec­ord, ac­cord­ing to the work of Al­is­tair Ev­ans and co-authors. Be­com­ing smaller can hap­pen much faster than be­com­ing big­ger: the ev­o­lu­tion of pyg­my ele­phants took 10 times few­er gen­er­a­tions than the equiv­a­lent sheep-to-elephant size change. (Cred­it: Al­is­tair Ev­ans, Da­vid Jones, IMPPS)

“We con­cen­trat­ed on large-scale changes in body size. We can now show that it took at least 24 mil­lion gen­er­a­t­ions to make the pro­ver­bi­al mouse-to-ele­phant size change – a mas­sive change, but al­so a very long time,” said ev­o­lu­tion­ary bi­ol­o­gist Al­is­tair Ev­ans of Mo­n­ash Uni­vers­ity in Aus­tral­ia, who led the stu­dy. 

“A less dra­mat­ic change, such as rab­bit-sized to ele­phant-sized, takes 10 mil­lion genera­t­ions.”

The study looked at 28 dif­fer­ent mam­mal lin­eages, in­clud­ing ele­phants, pri­ma­tes and whales, from var­i­ous con­ti­nents and oceans. Size change was tracked in genera­t­ions rath­er than years to al­low mean­ing­ful com­par­i­son be­tween spe­cies with dif­fer­ing life spans.

Changes in whale size oc­curred at twice the rate of land mam­mals, said study co-author Er­ich Fitz­ger­ald, sen­ior cu­ra­tor of ver­te­brate pal­e­on­tol­ogy at Mu­se­um Vic­to­ria in Aus­tral­ia. “This is probably be­cause it’s eas­i­er to be big in the wa­ter – it helps sup­port your weight,” he ex­plained. He added that “the huge dif­fer­ence in rates for get­ting smaller and get­ting big­ger is really as­tound­ing – we cer­tainly nev­er ex­pected it could hap­pen so fast!”

Many min­ia­ture an­i­mals, such as the pyg­my mam­moth, dwarf hip­po and “hob­bit” people lived on is­lands, he went on, which helps ex­plain the size re­duc­tion. “When you do get smaller, you need less food and can re­pro­duce faster, which are real ad­van­tages on small is­lands.”

Ev­ans said the study was un­ique be­cause most pre­vi­ous work had fo­cused on microev­o­lu­tion, small changes that oc­cur with­in a spe­cies. “In­stead we con­cen­trat­ed on large-scale changes,” he not­ed. The re­search sheds light both on con­di­tions that let cer­tain mam­mals thrive and grow big­ger, and those that slow that in­crease and pos­sibly con­trib­ute to ex­tinc­tion, ac­cord­ing to the re­search­ers.

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It takes 24 million generations for a mouse-sized animal to evolve to an elephant’s size—but traveling along the reverse evolutionary path goes more than ten times faster, new research suggests. The study, published Jan. 30 in the journal Proceedings of the National Academy of Sciences, describes increases and decreases in mammal size following the extinction of the dinosaurs 65 million years ago. “We concentrated on large-scale changes in body size. We can now show that it took at least 24 million generations to make the proverbial mouse-to-elephant size change – a massive change, but also a very long time,” said evolutionary biologist Alistair Evans of Monash University in Australia, who led the study. “A less dramatic change, such as rabbit-sized to elephant-sized, takes 10 million generations.” The study looked at 28 different mammal lineages, including elephants, primates and whales, from various continents and oceans over the past 70 million years. Size change was tracked in generations rather than years to allow meaningful comparison between species with differing life spans. Changes in whale size occurred at twice the rate of land mammals, said study co-author Erich Fitzgerald, senior curator of vertebrate palaeontology at Museum Victoria in Australia. “This is probably because it’s easier to be big in the water – it helps support your weight,” he explained. He added that “the huge difference in rates for getting smaller and getting bigger is really astounding – we certainly never expected it could happen so fast!” Many miniature animals, such as the pygmy mammoth, dwarf hippo and ‘hobbit’ hominids lived on islands, he went on, helping to explain the size reduction. “When you do get smaller, you need less food and can reproduce faster, which are real advantages on small islands.” Evans said the study was unique because most previous work had focused on microevolution, small changes that occur within a species. “Instead we concentrated on large-scale changes in body size,” he noted. The research sheds light on conditions that let certain mammals thrive and grow bigger, and circumstances that slow that increase and possibly contribute to extinction, according to the researchers. do it