Genetic bottlenecks and the Founder effect: lessons learnt from the Woolly Mammoth

By Megan Widdows

Dur­ing peri­ods of cli­mate change, spe­cies have three options: adapt and evolve to sur­vive in the chan­ging envir­on­ment, move to anoth­er area with bet­ter con­di­tions or, fail­ing this, die out and become extinct. While adapt­a­tion is the most desir­able option, it is sel­dom pos­sible when envir­on­ment­al change hap­pens very rap­idly, such as over the span of a single life­time. This is because evol­u­tion is a painstak­ingly slow pro­cess that takes place over the course of many generations.

One of the most extreme peri­ods of cli­mate change came at the end of the Pleis­to­cene peri­od, our most recent Ice Age, around 12,000 years ago. This marked the start of the Holo­cene peri­od, char­ac­ter­ised by sig­ni­fic­ant tem­per­at­ure increases and the melt­ing of thou­sands of kilo­metres of ice, caus­ing sea levels to rise considerably. 

The rap­idly rising tem­per­at­ures at the start of the Holo­cene era left the envir­on­ment unsuit­able for many spe­cies. The coin­cid­ing rising sea levels cut off many areas, cre­at­ing islands, sim­ul­tan­eously pre­vent­ing large scale migra­tion. The res­ult was a mass extinc­tion event and the loss of most spe­cies that were adap­ted to the gla­cial period. 

One not­able sur­viv­or was the Woolly Mam­moth. Although the woolly mam­moth was able to sur­vive the envir­on­ment­al changes, life would nev­er quite be the same. Small groups of mam­moths ended up isol­ated in small insu­lar pop­u­la­tions on the islands that were cre­ated when the sea levels rose. The most sig­ni­fic­ant pop­u­la­tion was on Wran­gel Island, off the coast of North East Siber­ia. This pop­u­la­tion sur­vived, in isol­a­tion, for anoth­er 6000 years. 

Recently, sci­ent­ists at the Uni­ver­sity of Stock­holm com­pared DNA isol­ated from the Holo­cene Wran­gel woolly mam­moth pop­u­la­tion with that of woolly mam­moths of the late Pleis­to­cene peri­od, in order to to cla­ri­fy how evol­u­tion is influ­enced by two pro­cesses: geo­graph­ic­al isol­a­tion and genet­ic bot­tle­necks. 

A genet­ic bot­tle­neck occurs when there is a quick and dra­mat­ic reduc­tion in the size of a pop­u­la­tion, in this case caused by the loss of many woolly mam­moths dur­ing the mass extinc­tion event at the end of the Pleis­to­cene peri­od. What’s left is a small, ran­dom assort­ment of sur­viv­ors. Over time, the pop­u­la­tion size can recov­er but has lower genet­ic diversity as all indi­vidu­als have des­cen­ded from a small num­ber of founders. 

Sim­il­arly, founder effects occur when a small group of indi­vidu­als are per­man­ently sep­ar­ated from the rest of the pop­u­la­tion. With­in the smal­ler group, there is far less genet­ic diversity that the pop­u­la­tion as a whole. Since any res­ult­ing des­cend­ants can only inher­it genes that were present in the founders, they will have lim­ited genet­ic diversity. 

The study found that in the Pleis­to­cene peri­od, before any geo­graph­ic­al isol­a­tion or extinc­tion events, the woolly mam­moth pop­u­la­tion was highly genet­ic­ally diverse. This means there were many dif­fer­ent alleles, or forms, of dif­fer­ent genes, cre­at­ing lots of vari­ation with­in the population. 

Imme­di­ately after the isol­a­tion to Wran­gel Island, the genet­ic diversity was reduced to single hap­lo­type, which is a group of genes that are inher­ited togeth­er from a single par­ent. This find­ing sug­gests that the pop­u­la­tion was most likely foun­ded by a very small num­ber of females, likely just a single mat­ri­arch­al herd. In this case, the woolly mam­moths exper­i­enced an extreme pop­u­la­tion bot­tle­neck and founder effect when they were trapped on Wran­gel Island. 

They also found that the muta­tion rate was much high­er in the Wran­gel Island pop­u­la­tion than in nor­mal pop­u­la­tions, which led to an increase in the num­ber of dele­ter­i­ous muta­tions. These are muta­tions that have a neg­at­ive effect on an organ­ism. In small pop­u­la­tions, dele­ter­i­ous muta­tions are not removed as effi­ciently as they are in large pop­u­la­tion, and can instead become fixed. This may be par­tic­u­larly likely for the woolly mam­moths of Wran­gel Island – it is unlikely that that their pop­u­la­tion size ever came close to the max­im­um capa­city of the island, mean­ing there was less com­pet­i­tion for sur­viv­al and all indi­vidu­als were able to repro­duce and pass on dele­ter­i­ous muta­tions to the next generation. 

After 6000 years, all of the Wran­gel Island mam­moths differed from oth­er mam­moths by three dis­tinct muta­tions. The most evol­u­tion­ar­ily import­ant was a non-syn­onym­ous muta­tion in the crit­ic­ally import­ant ATP syn­thase enzyme. A non-syn­onym­ous muta­tion is a change in DNA that alters the struc­ture of a pro­tein, res­ult­ing in a bio­lo­gic­al change in the organ­ism. ATP syn­thase is an enzyme that pro­duces ATP, a major energy stor­age molecule. In humans, this same muta­tion res­ults in dis­ease, and sim­il­ar neg­at­ive out­comes are likely to have occurred in the woolly mammoths.

Des­pite the small pop­u­la­tion size of woolly mam­moths on Wran­gel Island, the sci­ent­ists believe that fix­a­tion of this harm­ful muta­tion is actu­ally a con­sequence of the severe bot­tle­neck and founder effect when they were first segreg­ated from the main pop­u­la­tion. They believe that muta­tions arose at the time of the bot­tle­neck, explain­ing why all des­cend­ants of the found­ing woolly mam­moths have inher­ited these changes. 

The find­ings of this study have improved our under­stand­ing of the effects of genet­ic bot­tle­necks and lim­ited pop­u­la­tion sizes, not only in the past but, in spe­cies approach­ing extinc­tion today. It high­lights the power a single muta­tion, arising at an evol­u­tion­ar­ily vul­ner­able moment, can have on the future of a species. 

This is espe­cially top­ic­al as we face anoth­er cli­mate change and extinc­tion crisis. Envir­on­ments are chan­ging rap­idly and more than 160,000 spe­cies are lis­ted as endangered by the Inter­na­tion­al Uni­on for Con­ser­va­tion of Nature. Many con­ser­va­tion­ists are work­ing tire­lessly to pro­tect their hab­it­ats and increase the num­bers of these spe­cies. This study helps us to under­stand the long term genet­ic impact that bot­tle­necks may have on spe­cies brought back from near-extinction. 

The study men­tioned in this art­icle can be found here. A gloss­ary of terms can be found below.

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