Breed­ing be­tween close kin is thought to be fraught with ev­o­lu­tion­ary pit­falls: it tends to sad­dle off­spring with dan­ger­ous lev­els of ge­net­ic de­fects. 

But ev­o­lu­tion­ary the­o­ry pre­dicts that un­der some cir­cum­stances, in­breed­ing may have ben­e­fits that out­weigh the costs. Re­search­ers say have found ev­i­dence to back this idea. 

In some spe­cies, “close in­breed­ing may be ex­plained by rel­a­tives be­ing bet­ter par­ents,” Timo Thün­ken and col­leagues of the Uni­ver­si­ty of Bonn, Ger­ma­ny, wrote in the Feb. 6 is­sue of the re­search jour­nal Cur­rent Bi­ol­o­gy.

Pel­vi­ca­chro­mis tae­ni­a­tus. Na­tive to low­er Ni­ge­ria and Cam­e­roon in West Af­ri­ca, slight­ly over three inches long as an ad­ult, and va­r­i­a­ble in col­or and mark­ings, it is a har­dy and fast-re­pro­duc­ing fish.

Although they didn’t re­com­mend this parent­ing stra­te­gy for humans, the re­search­ers noted the be­ne­fi­cial ef­fect among an Af­ri­can cich­lid fish, Pel­vi­ca­chro­mis tae­ni­a­tus. These fish, in which both par­ents help care for young, pre­ferred un­fa­mil­iar, close kin rath­er as mates, the bi­ol­o­gists said.

Pa­ren­ting costs en­er­gy, and kin­ship tends to fa­vor co­op­er­a­tion. In­deed, ob­ser­va­tions of this fish showed that re­lat­ed par­ents “were more co­op­er­a­tive and in­vested more” in par­ent­ing, the sci­en­tists wrote. The rea­son, they added, is likely that a male mat­ing with his sis­ter en­sures that his young re­ceive ex­tra copies of his genes—from their mo­ther. That would pro­mote his own ev­o­lu­tion­ary suc­cess.

So in­breed­ing is­n’t uni­form­ly bad, Thün­ken and col­leagues wrote: ev­o­lu­tion­ar­ily speak­ing, “an in­di­vid­ual has to trade off the costs against the ben­e­fits” of it.

The main drawback is that in­breed­ing can br­ing to­geth­er cou­ples with the same gene de­fects. The mu­ta­tions are of­ten harm­less in them be­cause there are back-up cop­ies of the genes. But in­breed­ing par­ents can pool the mu­ta­tions in off­spring, over­whelm­ing the back­up sys­tems.

There was no ev­i­dence of this oc­cur­ring in the fish, Thünken and col­leagues wrote, for un­clear rea­sons. Per­haps good pa­ren­ting made up for bad genes, they spec­u­lat­ed. An­oth­er pos­si­bil­i­ty, they wrote, is a pos­si­ble self-cor­rect­ing mech­an­ism tied to in­breed­ing. The­o­rists be­lieve that re­peat­ed in­breed­ing may lead to die-offs of bad­ly mu­tat­ed in­di­vid­uals. That “purges” the pop­u­la­tion’s bad genes, stav­ing off ge­net­ic dis­in­te­gra­tion.