Natural selection is usually the most powerful mechanism or process causing evolution to occur, however, it only selects among the existing variation already in a population.  It does not create new genetic varieties or new combinations of varietiesOne of the sources of those new combinations of genes is recombination during meiosis.  It is responsible for producing  genetic combinations not found in earlier generations.

Sperm and ova are radically different from somatic cells in the number of chromosomes that they contain.  Both male and female sex cells normally get only half of the pair of parent chromosomes (23 for humans).  Which half goes to any one sex cell is a matter of chance.

Net effect of the meiosis
process in terms of
chromosome numbers

  schematic drawing of the two division processes of meiosis and the net effect in terms of the number and kind of sex cells produced in men and women and the halving of the number of chromosomes

At conception, a single sperm and an ovum combine their chromosomes to produce a zygote with the normal full set of 46, but with a new combination of chromosomes distinct from either parent.

Sperm and ovum
combining their
chromosomes in
a new zygote

  drawing of sperm and ovum combining their chromosomes in a new zygote

New combinations of existing genes are produced at the beginning of meiosis when the ends of chromosomes break and reattach, usually on their homologous chromosome.  This crossing-over process results in an unlinking and recombination of parental genes.  In the example below, one end of each chromosome of this homologous pair is exchanged along with the genes that they contain.  The next generation inherits chromosomes with partially new sequences of alleles.

Crossing-over   drawing of crossing-over unlinking genes as homologous chromosomes separate in the formation of sex cells

The consequence of this recombination is the production of sperm and ova that can potentially add even greater diversity to a population's gene pool.  However, it does not result in new alleles.  Subsequently, recombination by itself does not cause evolution to occur.  Rather, it is a contributing mechanism that works with natural selection by creating combinations of genes that nature selects for or against.


  Previous Topic 

  Return to Menu  

  Practice Quiz  

  Next Topic  

Copyright 1997-2012 by Dennis O'Neil. All rights reserved.
illustration credits