How important is gene flow?

There’s a handy art­icle avail­able from the American Journal of Botany that’s caught my eye: Is gene flow the most import­ant evol­u­tion­ary force in plants? by Norman C. Ellestrand. It opens with a strong statement.

Some sci­ent­ists con­sider the word “evol­u­tion” to be more or less equi­val­ent with “nat­ural selec­tion” or adapt­a­tion. They would, of course, be wrong.

DNA sequence on awall

DNA. Photo by John Goode / Flickr.

Ellestrand states that bio­lo­gical evol­u­tion is the change in allele fre­quen­cies in a pop­u­la­tion over time, and that this is due to four evol­u­tion­ary forces: muta­tion, selec­tion, drift, and gene flow. Gene flow is import­ant because even low levels of gene flow can have a large impact, coun­ter­act­ing the other evol­u­tion­ary forces.

So what is gene flow?

It’s the move­ment of alleles from one pop­u­la­tion to another. For example, Schulze et al. pub­lished a paper Searching for gene flow from cul­tiv­ated to wild straw­ber­ries in Central Europe. They were look­ing at cul­tiv­ated straw­ber­ries, which are octoploid and bred to be tasty and look­ing to see if genes humans had selec­ted were mov­ing into wild pop­u­la­tions of dip­loid straw­ber­ries. It’s an import­ant ques­tion because if genes are flow­ing then farms could be con­tam­in­at­ing local wild­life. They found it wasn’t hap­pen­ing, even though hybrid ber­ries were possible.

But not all plants are the same. Take car­rots for example. Rong et al. looked at gene flow in wild car­rot pop­u­la­tions (subscription-only access till Nov 2014). They found that gene flow could hap­pen at the scale of a kilo­metre or more, and that mow­ing road­side verges helped spread genes. It also hap­pens among alpine plants.

It’s led to a change of opin­ion about gene flow over the past thirty years. In a press release for the paper, Ellestrand said: “When I first star­ted doing plant patern­ity stud­ies in the 1980s, our lab assumed that gene flow was lim­ited. But we kept identi­fy­ing ‘impossible fath­ers’ that could not be assigned to our study pop­u­la­tion. Surely, these couldn’t be fath­ers from out­side of our wild radish populations—hundreds of meters away? But after exclud­ing all other pos­sib­il­it­ies, the improb­able turned out to be the answer. And the paradigm of lim­ited gene flow in plants began to crumble.”

Ellestrand now describes gene flow as “idio­syn­cratic, but often sig­ni­fic­ant.” Self-pollinating plants will not have the gene flow that out­cross­ing plants do. Wind-pollinated plants can have more gene flow than insect pol­lin­ated plants. It isn’t just inter­est­ing for its out sake, it has import­ant con­sequences for biology.

One is that gene flow can act as an ‘evol­u­tion­ary glue’, as Ellestrand calls it. By swap­ping alleles around the pop­u­la­tion between each other, there is an evol­u­tion­ary unit that it makes sense to call a spe­cies. Without gene flow you simply have a group of things that look sim­ilar at the moment because they’re under sim­ilar selec­tion pressures.

He also argues that pop­u­la­tions can evolve gene flow to become units, giv­ing the example of adap­tion to toxic metals among some plants in Britain as an example of how gene flow can isol­ate and form new populations.

There are also cur­rent con­cerns. Ellestrand raises the issue of gene flow of mater­ial from GMO crops to wild rel­at­ives. This is sim­ilar to the work done by Chen et al. on trans­genic rice. You don’t even need a par­ent plant to cause this kind of trouble. Ellestrand points to his own research on weeds that were des­cend­ants of inter– or intrataxon hybrids.

His con­clu­sion could seem at odds with the rest of the paper start­ing as it does:

Is gene flow the most import­ant evol­u­tion­ary force in plants?
That’s a silly question!

He argues that it’s a silly ques­tion, because you need many forces and look­ing at gene flow in isol­a­tion ignores the con­straints that gene flow flows through.

If it is a silly ques­tion, then it’s a silly ques­tion that’s worth ask­ing. In his press release he says: “This review paper tells the story of gene flow’s rise to respect among plant evol­u­tion­ary bio­lo­gists, a fact that hasn’t yet pen­et­rated bio­logy in gen­eral that is still mired in selection/adaptation-only think­ing.” To return to the top of the post, I would have been happy con­flat­ing evol­u­tion with nat­ural selec­tion. I think Ellestrand has made a very good argu­ment that I would be wrong doing that.


Ellstrand N.C. (2014). Is gene flow the most import­ant evol­u­tion­ary force in plants?, American Journal of Botany, DOI:

See also:

Buehler D., Graf R., Holderegger R. & Gugerli F. (2012). Contemporary gene flow and mat­ing sys­tem of Arabis alpina in a Central European alpine land­scape., Annals of Botany, DOI:

Chen L.J., Lee D.S., Song Z.P., Suh H.S. & Lu B.R. (2003). Gene flow from cul­tiv­ated rice (Oryza sativa) to its weedy and wild rel­at­ives., Annals of Botany, DOI:

Rong J., Xu S., Meirmans P.G. & Vrieling K. (2013). Dissimilarity of con­tem­por­ary and his­tor­ical gene flow in a wild car­rot (Daucus carota) meta­pop­u­la­tion under con­trast­ing levels of human dis­turb­ance: implic­a­tions for risk assess­ment and man­age­ment of trans­gene intro­gres­sion., Annals of Botany, DOI:

Schierenbeck K.A. & Ellestrand N.C. (2009). Hybridization and the evol­u­tion of invas­ive­ness in plants and other organ­isms, Biological Invasions, 11 (5) 1093–1105. DOI:

Schulze J., Stoll P., Widmer A. & Erhardt A. (2011). Searching for gene flow from cul­tiv­ated to wild straw­ber­ries in Central Europe., Annals of Botany, DOI:


DNA by John Goode / Flickr. This image licensed under a Creative Commons by licence.

Alun Salt. ORCID 0000-0002-1261-4283

When he's not the web developer for AoB Blog, Alun Salt researches something that could be mistaken for the archaeology of science. His current research is about whether there's such a thing as scientific heritage and if there is how would you recognise it?