//
you're reading...

Biology

Little fish, new pond – watching stickleback evolution in real-time

Source

Lescak, E.A.; Bassham, S.L.; Catchen, J.; Gelmond, O.; Sherbick, M.L.; von Hippel, F.A.; and W.A. Cresko. “Evolution of stickleback in 50 years on earthquake-uplifted islands”. 2015. PNAS 112:52.  doi: 10.1073

But isn’t evolution a gradual, slow process?

When most people think about evolution, they see it as an extremely slow, gradual process that occurs over almost unthinkably vast timescales.  Darwin certainly believed that evolution progressed slowly.  While it’s true that evolutionary change requires a span of generations, for many reasons, it is actually possible to watch evolution occur in real-time, within a single human lifespan, and even a single researcher’s career.

How on earth is this possible?  Well, for one thing, generation times vary widely across different species.  A human generation is not the same length as a microbe’s generation.  E.coli has a generation time of 17 minutes under the proper conditions, meaning that (if I did my math correctly) you could potentially see 10,000 generations of E.coli in about 120 days.  That’s a lot of time for evolution to occur, though it doesn’t seem like a very long time to us long-lived creatures.  Even in organisms with longer generation times, it may be possible to watch evolution progress.

Under certain conditions, such as the unusually strong selective pressures associated with colonizing a new habitat, scientists now believe that rapid evolution is possible.  Lescak and her colleagues demonstrated such rapid evolution in threespine stickleback, a small northern-dwelling fish that has long been considered an exemplar evolutionary model system.

2.2 1

Figure 1: The extent of damage done by the 1964 earthquake, which was the largest ever recorded in North America

 

Uplifted islands from a tragic seismic event

            On March 27th, 1964, a devastating earthquake struck the southern coast of Alaska (Figure 1).  This catastrophic event uplifted islands in the Prince William Sound and the Gulf of Alaska, creating new ponds out of formerly marine habitat (Figure 2).  Oceanic threespine stickleback were either trapped in these new ponds as they formed or migrated into them soon after the earthquake.  Because they were then physically isolated from other populations of stickleback, they were able to evolve independently from those other populations.  Knowing the precise date of the isolation event, researchers were able to compare these new populations to older, more established populations of stickleback in the area.  This tragic event provided us with a unique opportunity to directly observe the earliest stages of population divergence, allowing us to ask how long divergence actually takes.

2.2 2

Figure 2: Layout of the study populations. Dark grey shaded areas of the islands existed before the 1964 earthquake, lighter shading indicates uplifted areas. Green squares indicate oceanic habitat, green circles indicate freshwater habitat with oceanic forms, blue circles indicate freshwater habitat with freshwater forms, and circles that are half green and half blue indicate freshwater populations that contain a mix of forms.

 

How do we study divergence?

These researchers collected samples from 21 oceanic and freshwater populations on three different islands (Figure 2).  The freshwater populations included some older (pre-1964) populations as well as many populations that were found in the new, uplifted pond habitat.  They took genetic samples and created a library of 130,000 differences to the DNA sequence, and assigned individuals into groups based on who shared which genetic differences.  They also looked at the anatomical differences across these individuals using a method called geometric morphometrics.  Geometric morphometrics is simply a quantitative way of studying shape. They used these data to determine whether oceanic and freshwater stickleback differed anatomically and genetically.

2.2 3

Figure 3: Variation in anatomy between oceanic (green) and freshwater (blue) populations of stickleback. Even excluding the most important shape variable (lateral plates), these researchers were able to distinguish quite easily between these two groups of populations, showing how well differentiated they actually are! You can think of PC1 as the most important axis of shape variation and PC2 as the second most important. These are actually composite axes that include several different variables of shape change as a way of distilling shape change down to the most dramatic changes.

 

Did these young populations diverge?

            The genetic analyses supported the idea that populations from the new ponds were derived independently from oceanic ancestors.  There was no evidence that these populations were colonized by pre-existing freshwater populations.  This result is incredibly important to establish off the bat, because had the researchers found that the new populations were derived from existing freshwater populations, any morphological differences between the oceanic stickleback and the new freshwater populations would likely be due to past evolution and not new, rapid evolution.  Because the researchers have evidence to support an independent derivation of these populations from the oceanic populations, they can truly say that any genetic and morphological differences they found between the oceanic and the new freshwater populations are due to rapid evolution that occurred in the fifty years since the earthquake.

And indeed, they did find differences between these types of populations, both genetically and morphologically (Figure 3).  When considering their anatomy, the young freshwater populations differ from their oceanic ancestors in similar ways as their older counterparts.   In fact, much of the variation in shape change between these groups can be explained by the loss of the lateral plates in freshwater populations, a common feature of stickleback evolutionary changes for freshwater habitats (Figure 4).  It is very clear from this study that these young freshwater populations have differentiated in a significant way from their oceanic counterparts in just fifty years.  This result suggests that most stickleback evolution occurs in the first few decades following colonization.  While this pattern may be limited to stickleback, it may also be a characteristic of evolution in general – we’ll need to study more systems at the earliest stages of their divergence in order to know either way (and this can be incredibly difficult to do!).

2.2 4

Figure 4: Stereotypical oceanic (top) vs freshwater (bottom) stickleback forms. Notice the red bony plates along the side of the oceanic stickleback that are conspicuously missing in the freshwater fish – these are the lateral plates. NB: these are not pictures taken from the current study; rather they represent the generic transition between these two habitat types.

 

Questions?  Comments?  Please sound off below!  I’d love to hear from you :)

Dina Navon
I am a doctoral candidate in the Organismic and Evolutionary Biology program at the University of Massachusetts Amherst. I’m interested in how an individual’s genes and the environment in which it grows come together to determine its physical traits. I study a group of closely related freshwater fish called cichlids which live in the African rift lakes like Victoria, Malawi, and Tanganyika.

Discussion

No comments yet.

Talk to us!

oceanbites photostream

Subscribe to oceanbites

@oceanbites on Twitter