Stickleback Evolution

Sticklebacks are an excellent species for studying how populations adapt to their local environment through natural selection because nature has done an experiment for us.  The oldest (ancestral) population of sticklebacks is a marine form that spends most of its life in the sea and returns to freshwater to breed.  Just over 10,000 years ago much of North America was covered with ice and as this ice receded lakes were formed.  At that time different populations of marine sticklebacks found their way to and colonized freshwater lakes, these populations underwent rapid adaptation to the freshwater environment.  For example fish in freshwater lakes tended to lose the spines on their pelvis and the armor plates usually found in marine fish.  This resulted in fish from freshwater populations looking remarkably similar to each other even though each population is more closely related to the marine population than to each other.  This phenomenon of populations that live in similar environments developing similar morphological (body shape and structure) characteristics is called parallel evolution. 

Natural selection works because individuals that are better adapted to their habitat live longer and have more offspring than those that are poorly adapted.  However, in order for natural selection to act there must be variation in the natural population.  Where do the morphological characteristics seen in the freshwater populations come from?  The body plan of every organism is encoded in its DNA; genes are regions of the DNA that encode proteins.  Every aspect of an organism's anatomy, physiology and behavior is dependent on the structure of these proteins and where and when they are expressed.   Variation could result from differences in the DNA sequence of a gene itself which would alter the structure of the protein.  However changing the structure of a protein is likely to diminish or even inactivate its function.  Most proteins (and thus genes) act in many different parts of the body and at different times; changes that broadly disrupt protein function are likely to do more harm than good.  On the other hand changes in the regulatory regions of DNA control where, when and how much of a protein is expressed will result in smaller, more targeted changes.  Differences in these regulatory parts of the genome may have the specific types of effects that we see in stickleback populations and allow for rapid adaptation.  This is exactly what researchers in Dr. David Kingsley's lab at Stanford University have found.  They were able to locate specific genes that account for morphological differences between populations however in each case genetic differences were found not in the gene region that determines the structure of the protein but rather in regulatory regions that control when and where the gene is expressed.  This is an exciting result that shows that variation in a single region of DNA can have a big effect on the morphology of a particular body part without causing overall deleterious effects.  This type of genetic variation would allow for the rapid adaption to new environments seen in stickleback populations and could eventually lead to the creation of new species.