1. 

Crossing over of chromosomes during meiosis

2. 

Random assortment of maternal and paternal chromosomes during the production of egg and sperm

3. 

Random combination of egg and sperm at fertilization.

For further information on recombination see the Palomar College Web site.

For over 25 years Robert Vrijenhoek has studied unique populations of minnows in the small hillside pools of the Sonoran desert in Mexico. Here he found two different species of minnows living side-by-side, one an asexual reproducer and the other a sexual reproducer. Vrijenhoek has been trying to understand which conditions might favor the sexual minnows and which favor the asexual ones. To find out what he discovered about the value of sexual reproduction, view the Evolution Show Five video segment "Red Queen."