Sex evolved to help future generations fight infection, scientists show

December 20, 2016, University of Stirling
The scientists examined 6,000 Daphnia, waterflea, as part of the study. Credit: University of Stirling

Why does sex exist when organisms that clone themselves use less time and energy, and do not need a mate to produce offspring? Researchers at the University of Stirling aiming to answer this age-old question have discovered that sex can help the next generation resist infection.

Populations that clone themselves are entirely female and do not need sex to reproduce. As sex requires males, and males do not produce offspring themselves, an entirely clonal population should always reproduce faster than a sexual one.

Yet while some animal and can reproduce without sex, such as , starfish and bananas, sex is still the dominant mode of reproduction in the natural world.

Scientists know that sex allows genes to mix, allowing populations to quickly evolve and adapt to changing environments, including rapidly evolving parasites.

However, for sex to beat cloning as a reproduction strategy, there must be large-scale benefits that make a difference to the next generation. The theory has been difficulty to test as most organisms are either wholly sexual or clonal so cannot be compared easily.

A team of experts from the University of Stirling have taken an innovative approach to test the costs and benefits of sex. Using an organism that can reproduce both ways, the waterflea, researchers found sexually produced offspring were more than twice as resistant to infectious disease as their clonal sisters.

Dr Stuart Auld of the Faculty of Natural Sciences, said: "One of the oldest questions in evolutionary biology is, why does sex exist when it uses up so much time and energy?

"Sex explains the presence of the peacock's tail, the stag's antlers and the male bird of paradise's elaborate dance. But if a female of any of these species produced offspring on her own, without sex, her offspring should come to dominate, while the other females watch the redundant males fighting and dancing. So, why are we not surrounded by clonal organisms?

"By comparing clonal and sexual daughters from the same mothers, we found sexually produced offspring get less sick than offspring that were produced clonally. The ever-present need to evade disease can explain why sex persists in the in spite of the costs."

The waterfleas and their parasites were collected from the wild. Sexual and clonally produced daughters were harvested from the wild waterfleas and these were exposed to the parasites under controlled laboratory conditions.

The study is published in Proceedings of the Royal Society B.

Explore further: If you could clone yourself, would you still have sex?

More information: Sex as a strategy against rapidly evolving parasites, Proceedings of the Royal Society B, rspb.royalsocietypublishing.or … .1098/rspb.2016.2226

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tj10
2.1 / 5 (7) Dec 21, 2016
"Sex evolved to help future generations fight infection, scientists show"

No it didn't. Nothing ever evolved for a purpose. Scientists always speak of evolution as if it had a mind and purpose in where it was going. Such and such evolved for this and that. No. Shame on phys.org for not writing in a scientifically accurate way!

And the claim is that they "showed" this. No, they didn't. What they showed was that sexually produced offspring of water fleas were more than twice as resistant to infectious disease as their clonal sisters. That is what they showed.

This would be just as true if sex was a product of design. So let's be accurate here. All they showed was one benefit that sex confers on offspring, but that doesn't mean that is why it evolved, or even THAT it did/could evolve. The evolution of sex is simply assumed and then a possible reason for it is given, but it doesn't show that sex evolved or why. The logic here is severely lacking!
markgam
2.3 / 5 (3) Dec 21, 2016
"Nothing ever evolved for a purpose" is true, but it's just a way of saying that organisms that use sexual reproduction are more successful at adaptation to their environment vs asexual reproduction. Asexual reproduction was the only method of reproduction on this planet for about 2 billion years. Sex has only been around ~ 1.2 billion years. I guess the designer was hung over for a while after the launch party.
johne2123
2.3 / 5 (3) Dec 21, 2016
Why does sex exist when it uses up so much time and energy? Because it's fun! I figured that out without a double blind study. Where's my grant money?
Corevoice
2 / 5 (4) Dec 21, 2016
This in NO WAY explains a supposed spontaneous mutative advent of opposing, fully functional, complimentary reproductive systems. Neither does it explain that female DNA could theoretically be formed from male DNA but that male DNA could never be formed from female DNA for lack of a y chromosome.
rrrander
5 / 5 (1) Dec 21, 2016
Heterosexual sex may have evolved to prevent disease. Other kinds of sex kind of backfired in that regard.
docdave88
1.8 / 5 (5) Dec 21, 2016
Interesting but it still does not get to the first instance.

I watched, with fascination, when Carl Sagan did Cosmos. I remember him saying, and this is pretty close to a direct quote, "then one day, quite by accident, a hydrophyilc and a hydrophobic molecule joined together and the first cell wall began."

Okay, so far so good.

And in the primordial soup constantly bombarded by lightning I can even conceive of life beginning.

But sex?

Seriously?

Let's consider what would be involved.

In the gazillions of gallons of soup, bombarded by lightning and unfiltered cosmic rays and mutations were ongoign all of the time. Okay so far.

But for sex to develop requires the leap of faith that at some point two separate entities had to have mutually complementary mutations at the same time and within proximity to get together.

If you are going to make that leap of faith, why not deliberate design?
stevemoxon
2.3 / 5 (3) Dec 21, 2016
This is an old theory and only a subsidiary answer. Better adaptation is the flip side of the more important problem of dealing with accumulated gene replication error.
See the brand-new book, SEX DIFFERENCE EXPLAINED: From DNA to Society – Purging Gene Copy Errors, by Steve Moxon.
It's a 'bottom-up' from biology, cutting-edge holistic understanding of men/women: the first time anyone has properly attempted to put forward a truly integrated account of human sociality, utilising all the latest lines of evidence in theory re male hierarchy, female 'personal network', the very different in-grouping according to sex, and pair-bonding. It destroys the basis of feminism in revealing that there is no such thing as any sort of 'oppression' of women, who are privileged because of their sex.
Published as a monograph by the journal, New Male Studies, it's available on Amazon in book and Kindle formats, and is open-access at the journal's website.
ET3D
4 / 5 (4) Dec 21, 2016
rrrander, Heterosexual sex may have evolved to prevent disease, but germs adapted. There were a lot more STD's over history than homosexual specific ones.

docdave88, "why not deliberate design"? Because then you have to ask yourself why design something with so many downsides, instead of, for example, perfecting single sex reproduction. The logical conclusion is naturally that the Designer was sexually obsessed, which then begs the question that if sex was created to be enjoyable, why do so many religious people think of enjoying sex as a sin?
eric96
3 / 5 (2) Dec 21, 2016
The question still remains as to how and when sex was introduced, but part of the reason has been explained here. But fundamentally, the 3 reasons are: longevity, adaptability and sustainability. With clones, both the good and the bad aspects are emphasized; this is analogous to the simultaneous peak and fall of a species (genetic demise). With sex, "usually" the worst aspects are left extinct. Hence sex in the long run ensures species longevity, adaptability and long term sustainability.
eric96
3 / 5 (2) Dec 21, 2016

But there is something much more profound than this, and there is an answer to the old problem of who came first, the chicken or the egg?

What you need to understand was that the chicken was not always a chicken, the organism evolved to have all the qualities that make a chicken a chicken; this happened as fast as hundreds of years or as slow as hundreds of thousands or millions or years. In the knowledge that each generation had parents who had sex, its pretty clear that the unfinished chicken gave birth to the more finished chicken. So the answer to the question depends on how specific (think genetic makeup) you define a chicken, by layman's standard the chicken came before the egg. By scientific/genetic standard, the egg came before the chicken.

This is the truth, and everyone from a certain point of view is right.
Whydening Gyre
3 / 5 (2) Dec 21, 2016
ET3D@
rrrander, Heterosexual sex may have evolved to prevent disease, but germs adapted. There were a lot more {heterosexual} STD's over history than homosexual specific ones.

Excellent point.
Stevemoxon@
See the brand-new book, SEX DIFFERENCE EXPLAINED: From DNA to Society – Purging Gene Copy Errors, by Steve Moxon.

Ya made me laugh. Even if it was an applicable reference to the topic, your entry still is non-compliance with Comment Policy. But - good try...
Docdave88@
Interesting but it still does not get to the first instance.

Oh, to be the fly on the wall with that one, eh?
With all the gazillions of early cells, in the gazillions of gallons of primordial soup, with all the gazillion environment inputs causing gazillions of mutations, over a couple of billions of years - sex (as we know it today) as a gene transfer method, championed out as the most efficient.
So far...:-)
Bart_A
1 / 5 (3) Dec 22, 2016
tj10---I totally agree with your scientific reasoning. It is so un-scientific for "scientists" to slap the "evolved" label on some observation, that has nothing to do whatsoever to showing that evolution is real.

The readers who gave your comment a "1" rating should be ashamed of themselves.

Corevoice
1 / 5 (1) Dec 23, 2016

This in NO WAY explains a supposed spontaneous tandem mutative advent of opposing, fully functional, complimentary reproductive systems. Neither does it explain that female DNA could theoretically be formed using solely male DNA but that male DNA could never be formed using solely female DNA for lack of a y chromosome.
Whydening Gyre
5 / 5 (1) Dec 23, 2016
To the "design" minded -
The Universe, and everything IN it (including biological systems), only REacts. There is no PRE planning involved.
A cellular entity is faced with an environmental roadblock (which includes other "entities") and adapts. Fortuitous circumstance is the only thing standing between the 1st entity's adaptation and survival.
Life as we know it is a result of those fortuitous adaptations.
THAT is evolution.
SURFIN85
not rated yet Jan 03, 2017
"This in NO WAY explains a supposed spontaneous tandem mutative advent of opposing, fully functional, complimentary reproductive systems."

So prove Intelligent Design.

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