Those spineless sticklebacks

dorsal and pelvic (held close to body) spines on stickleback

 

 

 

 

 

[More summaries from Barton’s Evolution]

 

Sticklebacks are sardine-sized fish with three spines on their backs and two on their bellies at least for marine species.  Over a period of 15,000 years, the fish wound up in inland seas at the end of the last ice age, and these turned into freshwater lakes.  Curiously, these freshwater sticklebacks lost their stickles to varying degrees.  Functionally, it seems that stickles made the fish just too big for their normal predators to swallow, and, if they did, the stickles hurt the soft mouth parts of the predators.  In freshwater, however, the main predators are dragonfly larvae, and the stickles actually help the larvae grab the slippery fish better.  Such large-scale evolution in morphology is rapid for just 15,000 years.  What changed in the fish genetically? 

 

 

When they crossed marine stickled fish with freshwater nonstickle fish, the F1 generation all have stickles.  When they crossed the F1 generation fish with each other, they got a 3:1 ratio of stickle:nonstickle.  So, of course, this points to a single gene with homozygous dominant in the marine fish.  In mice, there are three genes which control hindlimb, but not forelimb, development, which would be orthologous to pelvic development in the sticklebacks.  Through microsatellite mapping, they had identified the position of the gene responsible for the stickles in the crossing experiment above.  An ortholog to one of the three mouse genes, Pix1, was found to map to the same chromosome area.  So they found the single gene whose change in alleles drastically changed the morphology of the sticklebacks in such a short bit of evolutionary time.  Furthermore, the allele difference is not at the level of the amino acid sequence, but a difference in the promoters:  the dominant allele expresses in all the appropriate places in the body, but the recessive allele expresses everywhere except the pelvis.

 

 

There are several elements which make this story work well within an evolutionary paradigm.  A simple genetic change, a big morphological change upon which natural selection acts, and a shift in morphology between populations.  But consider also the human gene Ectodysplasin.  People with defective alleles have loss or reduction in teeth hair, sweat glands and eyebrows.  The ortholog to this gene in the sticklebacks controls the amount of body armor the fish have.  This also fits so well into the evolutionary paradigm:  fish and humans use the same basic gene to create very different, but somehow related, body parts.  As the body plan changes radically from fish to mammals and finally specifically into primates, the gene function adapts and gets used in new ways.

 

 

I think that it is the accumulation of these sorts of stories that makes the strongest case for evolution. 

 

 

Photo from:

http://fish.dnr.cornell.edu/nyfish/Gasterosteidae/threespine_stickleback.jpg