Explaining Large-Scale Diversity within Midas Cichlid Fishes

A new study pub­lished in Evol­u­tion Let­ters cap­it­al­ises on a nat­ur­al exper­i­ment to shed light on when and how spe­cies diver­si­fy. Luke Turn­er reports:

Spe­ci­ation occurs when a new spe­cies is formed from a pre-exist­ing one, and can take place due to a vari­ety of eco­lo­gic­al factors. This diver­si­fic­a­tion is not equal among all groups of organ­isms, with some lin­eages tend­ing to diver­si­fy much more rap­idly than oth­ers. Eco­lo­gic­al oppor­tun­ity, which can res­ult from the appear­ance of a new niche, extinc­tion of dom­in­ant lin­eages or the evol­u­tion of advant­age­ous char­ac­ter­ist­ics, is thought to be a key influ­en­cer of diver­si­fic­a­tion rate. While eco­lo­gic­al oppor­tun­it­ies can explain diversity between dif­fer­ent clades, oth­er factors become rel­ev­ant when examin­ing the dif­fer­ences in diver­si­fic­a­tion rates among closely related lin­eages. For example, the tim­ing of col­on­isa­tion of a new hab­it­at affects how suc­cess­ful a spe­cies is in radi­at­ing, while sec­ond­ary waves of col­on­isa­tion also influ­ence the rate of diversification.

The vari­ation in diver­si­fic­a­tion among dif­fer­ent organ­isms is strik­ing, and cer­tain groups account for much more diversity than oth­ers. This is evid­ent with­in the ver­teb­rates, with just under half of around 60,000 spe­cies alive today being tele­ost fishes. Even more amaz­ing is the overrep­res­ent­a­tion of cich­lid fish spe­cies, as 1 in every 25 ver­teb­rate spe­cies in the world is a cich­lid fish. This astound­ing diversity makes the group an ideal case study when ask­ing ques­tions about how and why diver­si­fic­a­tion occurs with­in species.

Brand new research from Kautt et al. (2018), pub­lished in Evol­u­tion Let­ters, invest­ig­ates the factors that con­trib­ute to this diversity amongst the cich­lid fishes. The research focussed on a group of fish known as Midas cich­lids, which live in Nicara­guan vol­can­ic crater lakes and con­sist of 11 closely related lin­eages that have evolved rel­at­ively rap­idly. All of these Midas cich­lids ori­gin­ated from the same source pop­u­la­tion but have transitioned to the crater lakes and diver­si­fied into new spe­cies in less than 2000 generations.

After col­on­ising the vol­can­ic crater lakes, all of the Midas cich­lids exhib­ited changes in body mor­pho­logy, with a gen­er­al trend towards more slender body shapes. This change is thought to give the fish an advant­age due to the great depth of crater lakes. A more slender body is bet­ter suited to free swim­ming rather than man­oeuv­ring, which was more import­ant in the vast but shal­low lakes from which they ori­gin­ated. The degree of mor­pho­lo­gic­al diver­gence was found to be related to the size and shape of the crater lake itself, as these new envir­on­ments are smal­ler and more remote than the great lakes which they transitioned from. The pres­ence of smal­ler zones closer to the shore in crater lakes makes them more dis­sim­il­ar to their ori­gin­al hab­it­ats, and more dis­sim­il­ar crater lakes con­tain Midas cich­lids with great­er mor­pho­lo­gic­al diversity.

When invest­ig­at­ing the speed of this change in body shape, it was found that mor­pho­lo­gic­al diver­gence occurs rap­idly after the ini­tial col­on­isa­tion of a crater lake, but then slows down with time. This is thought to be due to the fish eco­lo­gic­ally diver­ging from their source pop­u­la­tion and rap­idly adapt­ing to the avail­able niche in the crater lake. It also fits with the the­or­et­ic­al pre­dic­tion that adapt­ive radi­ations are char­ac­ter­ised by short bursts of rap­id diver­si­fic­a­tion, instead of con­stant rates of change over time.

Anoth­er inter­est­ing dis­cov­ery was made when the research­ers looked at the crater lakes which con­tained more than one spe­cies of Midas cich­lid. Two crater lakes are inhab­ited by mul­tiple spe­cies, which are the product of small-scale adapt­ive radi­ation with­in the over­all diver­si­fic­a­tion of the Midas cich­lids from the source pop­u­la­tion. While these lakes provide a home for mul­tiple spe­cies, oth­ers con­tain only one Midas cich­lid spe­cies, so this begs the ques­tion; why did fur­ther diver­si­fic­a­tion occur in some lakes but not oth­ers? The answer to this lies with the depth of a par­tic­u­lar crater lake, as deep­er lakes are able to sus­tain more fish and there­fore allow for a great­er amount of mor­pho­lo­gic­al diversity. This vari­ation between hab­it­ats demon­strates that eco­lo­gic­al oppor­tun­ity can explain the imbal­ance in diver­si­fic­a­tion rates between dif­fer­ent lakes.

Spe­ci­ation can often be affected by hybrid­iz­a­tion (indi­vidu­als breed­ing with anoth­er spe­cies) and genet­ic intro­gres­sion (genes from one spe­cies mov­ing into the gene pool of anoth­er), as genet­ic exchange between closely related indi­vidu­als can be an import­ant factor in facil­it­at­ing diver­si­fic­a­tion. One instance of inter­breed­ing was iden­ti­fied between two closely loc­ated crater lakes, with gene flow occur­ring between them and lead­ing to the two pop­u­la­tions being very sim­il­ar in mor­pho­logy. In addi­tion, the research­ers found evid­ence that there had been sec­ond­ary con­tact between the source pop­u­la­tion and all of the crater lakes, indic­at­ing that a sec­ond­ary wave of col­on­isa­tion took place from the source to the crater lakes. This res­ul­ted from a rise in water levels as recently as 2000 years ago.

This research provides excit­ing insights into the factors that determ­ine dif­fer­ences in diver­si­fic­a­tion rates between closely related spe­cies. It shows that body shape diversity in the Midas cich­lids is largely con­trolled by eco­lo­gic­al oppor­tun­ity, under­lin­ing the power­ful role that nat­ur­al selec­tion plays in shap­ing biod­iversity. Although the Midas cich­lid fishes ori­gin­ated from the same source pop­u­la­tion, indi­vidu­als adap­ted to their new hab­it­at by devel­op­ing more slender bod­ies, allow­ing them to move more freely. Addi­tion­ally, diversity with­in a single crater lake was influ­enced by its depth, as deep­er lakes allowed fur­ther spe­ci­ation and could sus­tain mul­tiple Midas cich­lid species.

Luke Turn­er is a MSc Sci­ence Com­mu­nic­a­tion stu­dent at the Uni­ver­sity of Shef­field. The ori­gin­al study is freely avail­able to read and down­load from Evol­u­tion Let­ters here.