There are two sources of genetic variation: DNA mutation, and sexual reproduction. Sexual reproduction generates unique combinations of genes at three points in the sexual life cycle: during independent assortment and crossing over in meiosis I, and through random fertilization.

Consider a cell with three pairs of chromosomes. If the maternal chromosomes remained together and the paternal chromosomes remained together during meiosis I, there would only be two types of gametes. Since chromosomes assort independently, maternal and paternal chromosomes can combine in 2 to the 3rd power, or 8, ways to create gametes. Humans have 23 pairs of chromosomes, so each person can produce 2 to the 23rd power, or 8.4 million, different kinds of gametes! random fertilization means that any of a man’s 8.4 million possible types of sperm can unite with any of a woman’s 8.4 million possible types of eggs. So there are 8.4 million times 8.4 million, or 70 trillion different possible types of gametes that can be produced by a couple! Remember, that when we add the variation that crossing over contributes, we will have even MORE possible combinations of different gametes. That’s why brothers and sisters can be so different. Crossing over creates unique chromosomes composed of maternal and paternal parts. In humans, about 2 or 3 crossover events occur per chromosome, leading to an indefinite number of novel genetic combinations.

If genetic variation is the key to adaptation and evolution, how do genetically identical populations of asexually-reproducing organisms survive?

There are pros and cons to both types of reproduction. In stable environments, it makes sense to preserve the genetic makeup of successful individuals that are already adapted to the environment, so asexual reproduction is favored. In changing environments, sexual reproduction is advantageous, since new genetic combinations increase the probability that some individuals will have advantageous variations. Some species alternate between sexual and asexual reproduction throughout their life cycle depending on environmental conditions. The fungus Sordaria fimicola, which we’ll be observing in the demonstration, reproduces asexually when conditions are stable. However, if there is any sort of environmental stress, such as lack of nutrients, Sordaria will reproduce sexually to increase genetic diversity.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education