A quarry site in Nevada carries the evolutionary history of a population of stickleback fish that resided there when it was a freshwater lake. In a short time span in evolutionary terms—about 10,000 years—the fish population can be seen to dramatically reduce the size of their pelvic spines. This particular fossil record is remarkably complete with nearly year-by-year detail which includes documentation of intermediate forms.
Fossil Record of Stickleback Evolution Background
By studying the fossils from different layers of the fossilized lakebed, one can follow the evolution of the local stickleback population over tens of thousands of years, since each layer corresponds to a specific year.
Sticklebacks from this quarry can be divided into three categories.
1) Complete pelvis: those with a complete pelvis and pelvic spines,
2) Intermediate pelvis: those with the pelvis reduced to two smaller bones and no pelvic spines,
3) Reduced pelvis: those with the pelvis reduced to one small bone and no pelvic spines.
The graph in the animation shows that initially, the lake was populated with sticklebacks with reduced pelvis. Reduced pelvis sticklebacks are not unusual in fresh-water lakes. About 10,000 years into the graph, the population changes rapidly to a stickleback form with a complete pelvis with spines. This was probably an "invasion" of the lake by the complete pelvis sticklebacks, perhaps due to the lake being temporarily joined to another habitat in which complete pelvis sticklebacks were abundant. However, after the initial invasion occurs, the reduced pelvis form re-evolves. This re-evolution happens relatively quickly--over the course of around 10,000 years--and is accompanied by the presence of sticklebacks with an intermediate form of pelvis.
From Lecture Three of the 2005 Holiday Lectures Series "Evolution: Constant Change and Common Threads"
Fossil Record of Stickleback Evolution Teaching Tips
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The 2005 Holiday Lectures Series "Evolution: Constant Change and Common Threads"
Fossil Record of Stickleback Evolution Overview Credits
Director: Dennis Liu, Ph.D.
Scientific Direction: David M. Kingsley, Ph. D.
Scientific Content: Satoshi Amagai, Ph.D.
Animator: Blake Porch