Simple Example of Macroevolution

(source)

We start with two species of Spartina cordgrass, one is a European cordgrass (Species A) the other is an American species of cordgrass (Species B). When they interbreed they produce a sterile hybrid cordgrass (Species C). Because Species C cannot produce seeds it is reduced to reproduction via vegetative propagation, a process in which “new” individual plants arise without seeds or spores from parent plants, and doesn’t spread very fast as a result. However, after a while there is a polyploidy event in which during reproduction the number of chromosomes is doubled inside a cell where basically the nucleus of a cell divides but the cell doesn’t. You can visualize this pretty easily by looking at a normal example of cell mitosis here, the orange thing in the middle is the cell nucleus where the cell’s chromosomes are stored which are basically the filing cabinets for DNA. Now imagine that middle part dividing and the rest of the cell remaining the same, basically what you’ve done is double the number of chromosomes. Ta-da, you now understand polyploidy.

In our hybrid cordgrass (Species C) the result is a new species that can now reproduce sexually (Species D), and that is better able to survive in its environment. As a result it spreads faster than either Species A, B, or C. At the same time it cannot reproduce with species C, seeing as the hybrid is sexually sterile, nor can it reproduce with species A or B due to the large difference in chromosomes.


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