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Big Questions in Ecology and Evolution$
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Thomas N. Sherratt and David M. Wilkinson

Print publication date: 2009

Print ISBN-13: 9780199548606

Published to Oxford Scholarship Online: November 2020

DOI: 10.1093/oso/9780199548606.001.0001

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PRINTED FROM OXFORD SCHOLARSHIP ONLINE (oxford.universitypressscholarship.com). (c) Copyright Oxford University Press, 2021. All Rights Reserved. An individual user may print out a PDF of a single chapter of a monograph in OSO for personal use. date: 17 October 2021

Why Sex?

Why Sex?

(p.25) 2 Why Sex?
Big Questions in Ecology and Evolution

Thomas N. Sherratt

David M. Wilkinson

Oxford University Press

There is considerable confusion about the meaning of sex. It can be taken to mean gender (male or female), a form of recreation (‘hot sex’), or a form of procreation (‘sexual reproduction’). To most biologists, sex is none of the above. Instead, one definition of sex would simply be a process that combines genetic material from more than one individual. By this definition, sex is not in itself reproduction. For one thing, reproduction is not a necessary consequence of sex (many bacteria can simply share DNA through hooking up via conjugation), and sex is not always needed for reproduction (dandelions, greenfly, and starfish, to name a few, can all produce viable offspring without it). In fact, the specific act of combining genetic material can be thought of as the precise opposite of reproduction since it typically involves the coming together of the genetic material of two cells (‘gametes’) to create one (‘zygote’), rather than the splitting of one cell into two. All that said, in eukaryotic species (like us, with chromosomes housed discretely within a nuclear membrane) sex is a precursor to reproduction. Indeed, in some species including humans, other mammals, and many insect species, sex is an essential step in the production of offspring. To understand what sex is, we must first cover some basic genetics. Let us begin by thinking about the process of combining two gametes to produce a zygote—a fertilized egg. Naturally, if you simply combine all of the genetic material present in the nucleus of one of your typical cells with that derived from some lucky mate, then any resultant zygote would contain double the number of chromosomes. For example, humans are diploid, and have 23 pairs of chromosomes (one chromosome from each pair derived from each parent). Therefore, if you simply combine chromosomes from the diploid cells of two potential human parents, then the resulting zygote would have 46 pairs of chromosomes, and if two such individuals mated then their offspring would have 92 pairs of chromosomes. Clearly such accumulation of genetic material would quickly get out of hand, and cells would rapidly become obese with chromosomes.

Keywords:   allele, beneficial mutation, chromosomes, dandelions, environmental variability, fecundity, gametes, haploids, isogamy

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