For more than 100 years biologists have wondered why sex exists, with the asexual alternative of simple cloning more closely aligned with the fast and efficient reproduction at the heart of Darwinian natural selection.
In a fascinating News & Views article for Nature – one the world’s leading scientific journals – Dr Matthew Goddard from the University of Lincoln, UK, sheds light on new research which helps us answer this age-old question.
Dr Goddard, Reader in the School of Life Sciences at the University of Lincoln, has led his own extensive research into the patterns and processes of populations and communities, including the evolution and maintenance of sexual reproduction, and reviews the newest findings in this subject area.
Writing in Nature, published on 24th February 2016, Dr Goddard explains: “Sex involves the shuffling (recombination) of chromosomes from different parents, followed by the separation of these newly-shuffled chromosomes into reproductive gametes, which then fuse through mating. As well as being more complicated than asexual reproduction, sex also risks breaking apart collections of genes that have proven effective. In animals, sex means that fewer offspring are produced as only females give birth, and mate finding and courtship impose further uncertainties. Given these disadvantages, it is not immediately clear why sexual reproduction is maintained.”
However, Dr Goddard’s own research has previously confirmed that sex between organisms allows natural selection to proceed more effectively because it increases genetic variation. Examining the reproduction of yeast populations, his experiments revealed that sex assists adaptation not just by more rapidly combining beneficial mutations, but also by unlinking positive mutations from negative ones – a process that doesn’t occur during asexual reproduction.
The new study published in Nature by McDonald et al. explores this concept in minute detail – focussing on the molecular mechanisms underpinning the advantage that sex has in speeding up adaptive evolution.
The research evolved sexual and asexual yeast populations in a simple laboratory environment, to which the sexual populations adapted more rapidly, before using powerful DNA sequencing approaches to dissect and track various mutations in the different groups.
The results showed that sex improves the efficiency with which selection acts on individual mutations. They also found that while groups of positive and negative mutations remained together in asexual populations, so that some negative mutations become common “by hitchhiking”, no mutation groups persisted in sexual populations and as a result negative mutations did not become common.
“These comprehensive experiments provide the long-awaited confirmation that sex accelerates adaptation by plucking mutational rubies from the rubbish,” Dr Goddard said. “Sex shuffles mutations among genomes, enabling natural selection to act on individual mutations more efficiently, eliminating negative changes while favouring positive ones. Selection is comparatively blinded in asexual populations, as the effects of individual mutations are consistently hidden in genomes.”
Dr Goddard’s article is available to read in full online: http://www.nature.com/nature/journal/vaop/ncurrent/full/nature17304.html.
To access the research paper by McDonald et al. visit: http://www.nature.com/nature/journal/vaop/ncurrent/full/nature17143.html.