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© 2013 Elsevier Inc. All rights reserved. Research into the sex ratio (proportion of individuals that are male) has been one of the most quantitatively successful areas of evolutionary biology. Relatively simple theory is able to explain why many animal species produce approximately equal numbers of males and females, why certain species have an excess of males or females, and why individuals of some species facultatively shift their offspring sex ratio in response to environmental conditions.

Original publication





Book title

Brenner's Encyclopedia of Genetics: Second Edition

Publication Date



412 - 413