Sunday, March 29, 2009
Sudden change in the sticklebacks
On pages 56-57, Carroll discusses the evolution of the stickleback fish. The original ocean form of the fish, he explains, usually has a "continuous row of more than 30 armor plate running from head to tail" and this long armor plate may not be as good for maneuvering in water in contrast to shorter armor plates. He latter explains that in a short span of 11 years (1990-2001), the long armored sticklebacks in Loberg Lake, Alaska, are declining at a rapid rate while the other form, ones with shorter armor plates, are rising in surprising numbers. My question is, why would it be advantageous to have short armor plates in lake water because I would assume that there are more dangerous predators in the ocean, thus the sticklebacks would be expected to have short armor plates in the ocean and long armor plates in lakes. Furthermore, explain how such a rapid evolution is possible and the possibility that this observation isn't really an evolution, but rather a raise in one species and a decline in another.
Subscribe to:
Post Comments (Atom)
This comment has been removed by the author.
ReplyDeleteAll things being equal, it is more of a selective advantage to have a shorter armor plates, as it would allow for greater manueverability, as Carroll states. However, the fact that there are a greater number of predators in the ocean means that the fish must have more protection from potential attackers. The absence of these predators most likely allowed the stickleback fish to be rid of their need to maintain the cost of having larger armor plates (which not only consists of the aforementioned decrease in manueverability, but also a greater energy cost) and evolve to eventually develop smaller ones. Also, the rapid evolution of the stickleback fish may have been caused by their isolation from other species, which was caused by the receding of the glaciers, as noted by Carroll. Because there wouldn't be any threat to their well-being, there would be no need to have long armor plates.
ReplyDeleteAccording to David Kingsley at the Howard Hughes Medical Institute, the evolution of the fish is controlled by a gene called Eda, a gene that is also involved in the production of a signal protein in humans called ectodermal displasin. This particular protein signals the cells to create hair, teeth, and sweat. They were able to link these three characteristics by examining the absence of these in some men in India. The condition, called ectodermal dysplasia, is a disorder that is passed through the generations. As Kinglsey stated, "it turns out that armor plate patterns in the fish are controlled by the same gene that creates this clinical disease in humans....When these fish evolve this low-armored state they are using the same genetic mechanism. It's happening over and over again. It makes them more fit in all these different locations." After comparing the behavior of the protein in marine and freshwater fish, it became evident that the Eda gene was indeed causing the phenotypical change, as in marine fish, the gene was relatively silent, and was activated only when examining freshwater fish.
In addition to Eda, another gene that has been identified as controlling the size/length of the armor is Pitx1, which, according to Carroll, is "the major gene responsible for the reduction of the pelvic skeleton." (206)
http://www.hhmi.org/news/kingsley4.html
The eleven year time period attributed to the change in sticklebacks from long plates to short plates falls under a category of evolution known as micro-evolution. Micro-evolution is a relative term. This means that it can be defined as anything as minimal as a short change in DNA sequence between generations or anything as large (when compared to the sequence) as a phenotypic change. The unifying trait of all microevolutions is the fact that species do not change after these evolutions have come about. After all, didn’t the Gasterosteus aculeatus (stickleback) remain a Gasterosteus aculeatus after its change of armor plates? These fast acting, small yet useful microevolutions are made possible by an environmental change which makes the benefits of the mutation so strong that it catches on very quickly. In a study done on the repeated evolution of armor loss in Alaskan three-spined sticklebacks by the University of California-Berkley, it was mentioned that “importantly, novel environments may also have immediate disruptive effects on developmental processes that can expose novel genetic variants, some of which may have large effects on evolving phenotypes”. This is basically saying that if the environmental change is great enough, species will adapt in relatively short periods of time it will ensure that their population survives. The article gives the stickle back evolution time credit to the fact that the populations of Gasterosteus aculeatus were relatively isolated from other populations. This would give the evolved species a greater chance to spread and multiply, and thus become the prominent form of the fish in that area (1).
ReplyDelete1. William A. Cresko, Angel Amores, Catherine Wilson, Joy Murphy, Mark Currey, Patrick Phillips, Michael A. Bell. Charles B. Kimmel, and John H. Postlethwait. “Parallel genetic basis for repeated evolution of armor loss in Alaskan threespine stickleback populations”. December 19, 2003. PNAS 2004 101:6050-6055.