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.

Isolation and Adaptation

Biologists agree that the huge diversity of species seen on earth have evolved from a common ancestor through the process of natural selection.  However the details of how this occurs have been hotly debated.  How do populations of organisms adapt to their local environment?  How are new species formed?  One theory called gradualism (which was the mechanism initially proposed by Darwin) suggests that adaptive changes occur in incremental steps.  So for example, if a bird had a short beak which was well adapted to environment A and some individuals from that population suddenly found themselves in environment B (lets just say that it is and island and there no subsequent contact between the two populations) and environment B favors individuals with long beaks then there will be natural variation of beak length within the population and those individuals with longer beaks will survive better and produce more offspring.  Over many generations mean beak length will become longer and longer until eventually population B have longer beaks than population A.  Such differences between populations could be modulated by relatively small changes in a large number of genes.  One question is whether adaptive changes can also occur on a more rapid timescale.  Examination of the fossil record indicates that in many cases a species changes very little over time and speciation events are relatively rapid (on a geological time scale) leading to an idea called punctuated equilibrium.  One question is whether traits instead of shifting gradually through intermediate stages can be changed directly from one state to another.  Using the previous example immediately after the birds from environment A colonized environment B the vast majority of birds would have relatively short beaks but a few individuals would have substantially longer beaks creating a bimodal distribution of beak size.  The longer beaked individuals would survive better and produce more offspring and over time the majority of individuals in population B would have long beaks.  In this case beak length would likely be controlled by a small number genes such that changing the regulation of one or a few genes would result in large changes in the size of the beak. 

Stickleback fish have provided a great natural experiment for studying evolutionary mechanisms.  The marine form of sticklebacks have invaded many North American lakes in separate colonization events.  Once a stickleback population becomes established in the lake environment several adaptions occur including a loss of armor plates and reduction of the pelvis.  Comparing marine and freshwater populations has allowed evolutionary biologists to observe how these fish adapt to their new environment and in some cases discover the underlying genetic mechanisms.

Tomorrow: What do sticklebacks have to tell us about the mechanisms of adaptive change?

Threespine stickleback (Gasterosteus aculeatus)