Natural Selection Image Map


Ecological change and its consequence may need to be viewed with the perspective of centuries rather than years. But sometimes we have enough evidence to suspect what is taking place before our very eyes. We may see, for example, when one population is in the process of replacing another.

If fire destroys a forest, the barren land will experience a succession of replacement species -- first small trees and brush plants, then certain small mammals, then large trees, and finally more mammalian species. This succession will NOT occur, however, if the land washes away from rain.

This doesnít mean that an ecologist decides to camp out in a grassland for a few decades.  Usually an area will be photographed every few years.  When viewed in sequence, such pictures provide an accurate account of how populations grow and dwindle over time in a particular area. In your own experience, you may have seen the succession that takes place when a forest or brush is cleared from land and that land is turned into pasture. Can you name some species that might thrive in the forest or brush that could not exist in a pasture environment? 

When one population succeeds another, individuals in those populations must have the right genes -- genes that allow them to thrive in that environment. This leads us to the question of how to we know about gene frequencies.

Molecular Clocks

Although fossils can be very helpful in studying ecology, we canít always trace the fossil record forward to modern organisms.  By studying genetic material, which also changes over time, we can be certain that any gene present in an organism today is a descendant from a gene in the past.  So, if we can determine a rate of change for genetic information, we can determine how populations change over time. 

How can we figure out the time frame of genetic change?  We simply look for any events in the past that might have caused a population to split, and then, new species to occur.  Recently, geneticists have used the information that the Isthmus of Panama rose above the ocean surface about 5 million years ago.  This rise separated many populations of fish in the area.  By comparing the DNA of similar fish species on each side of this isthmus, scientists are able to determine both how populations change over time and how long it takes.



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