Macroevolution - The Origin of Species

Part 2


The Mechanism of Speciation

How do new species arrise? What are the mechanisms or conditions responsible for speciation?

Allopatry - speciation by physical isolation

When a species is split into 2 or more distinct populations by some geographical barrier speciation may proceed. In fact this is probably the most common cause of speciation.

Plate techtonics, desertification, volcanic activity, and other geologic processes conspire to produce veritable "islands" of flora and fauna isolated from one another.

"Island" in this context need only be an area difficult for most living things to imigrate to and from such as mountains or spring feed osasis scattered about within a sea of sand (a desert).

Sympatry - speciation within a population occupying the same habitat.

Sympatric speciation usually involves reproductive isolation between plants. It occurs through two main processes,
  1. hybridization and
  2. polyploidy

Hybridization. Plants are much more tollerant of interbreeding between similar but separate species. The new hybrids may have difficulty crossbreeding with either parent and so maintain their isolation living within the same boundaries.

Polyploidy . We now know that if the chromosomes of an organism do not segregate properly, an event called nondisjunction, the resulting progeny can have a chromosome number twice that of their parents. (chromosomes replicate in preparation for cell division but fail to undergo cytokinesis.) This mutant condition is called polyploidy.

An autopolyploid results when the individual has more than two chromosome sets all originating from its own species. If after mitosis cells fail to undergo cytokinesis the new tissue will be tetraploid (4n). Then all that is needed is for the new tissue to produce flowers which can self pollinate. Fertilization will produce seeds with the same tetraploid chromosome number.

Allopolyploids result when two different species interbreed and combine their chromosomes. If pollination and fertilization between two different plants take place the chromosome number in the resulting cells will be as if it is haploid since none of the chromosomes are homologous.

This usually ends the story because the plant, if it survives, will be sterile. Occasionally however a cell in the new hybrid (haploid) plant replicates its chromosomes but fails to undergo cytokinesis. Again the nondisjunction results in a doubling of the chromosome number, but more importantly each chromosome now has its homologous twin.

After meiosis the new gametes can self fertilize to produce a new diploid generation.