Development biology

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Origin of Sexual Reproduction

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1. Sexual reproduction is an adaptive feature which is common to almost all multicellular
organisms and various unicellular organisms.
2. Currently, the adaptive advantage of sexual reproduction is widely regarded as a major
unsolved problem in biology.
3. One prominent theory is that sex evolved as an efficient mechanism for producing
variation, and this had the advantage of enabling organisms to adapt to changing
environments.
4. Another prominent theory, is that a primary advantage of outcrossing sex is the masking of
the expression of deleterious mutations.
5. Sex does, however, come with a cost.

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6. In reproducing asexually, no time nor energy needs to be expended in choosing a mate and,
if the environment has not changed, then there may be little reason for variation, as the
organism may already be well-adapted.
7. However, very few environments have not changed over the millions of years that
reproduction has existed. Hence it is easy to imagine that being able to adapt to changing
environment imparts a benefit.
8. Sex also halves the amount of offspring a given population is able to produce.
9. Sex, however, has evolved as the most prolific means of species branching into the tree of
life.
10. Diversification into the phylogenetic tree happens much more rapidly via sexual
reproduction than it does by way of asexual reproduction.

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• Many protists reproduce sexually, as do the multicellular plants, animals, and fungi.
• In the eukaryotic fossil record, sexual reproduction first appeared by 1.2-2 billion years
ago in the Proterozoic Eon.
• All sexually reproducing eukaryotic organisms likely derive from a single-celled common
ancestor.
• It is probable that the evolution of sex was an integral part of the evolution of the first
eukaryotic cell.
• There are a few species which have secondarily lost this feature, such as Bdelloidea and
some parthenocarpic plants.

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Phanerozoic
Pre-Cambrian

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• Organisms need to replicate their genetic material in an efficient and reliable manner.
The necessity to repair genetic damage is one of the leading theories explaining the origin
of sexual reproduction.
• Diploid individuals can repair a damaged section of their DNA via homologous
recombination, since there are two copies of the gene in the cell and if one copy is
damaged, the other copy is unlikely to be damaged at the same site.
• A harmful mutation in a haploid individual, on the other hand, is more likely to become fixed
(i.e. permanent), since any DNA repair mechanism would have no source to recover the
original undamaged sequence from.
• The most primitive form of sex may have been one organism with damaged DNA
replicating an undamaged strand from a similar organism in order to repair itself.

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