For creationists, who typically know little or nothing about evolution and so have no understanding that it is a process that takes place in a population, not an event that happens to individuals, sexual reproduction is a fertile source of disinformation and misunderstanding with which to attack the Theory of Evolution.
For instance, they frequently argue that the probability of the 'first man' and 'first woman' evolving together is so highly unlikely that their myth of the spontaneous creation out of dirt of a man and the cloning of a woman by magic is a far more rational alternative explanation. The science is attacked because when they try to force fit their childish myth of every species having a single ancestral male and female couple into the science of evolution, it doesn't fit well. The possibility of the myth being wrong must never be contemplated, because that would risk having to change their mind.
However, science works by addressing questions, not by throwing stones at infantile straw-man parodies of alternative ideas and declaring victory by default.
For science, sexual reproduction presents evolutionary biology with a different long-standing puzzle. An asexual organism can pass on its entire genome without finding a mate, competing for one or investing energy in courtship. In species with separate sexes, there is also the familiar “twofold cost of males”: only females produce offspring, whereas every member of an asexual population can potentially reproduce.
All else being equal, an asexual lineage should therefore increase more quickly than a sexual one, and indeed there are a few examples of where this is true - a single female vine weevil can quickly populate a window box or plant pot with devastating consequences for the owner as I have found to my cost; a single marbled crayfish can rapidly spread through a new river system, out-competing any native species.
Yet sexual reproduction is widespread among eukaryotes and has persisted for hundreds of millions of years. This does not imply that evolution requires foresight or that sex was introduced by a designer for some future purpose. It means that, under many circumstances, the descendants of organisms that exchange and recombine genetic material must acquire advantages sufficient to offset the immediate costs.
One important advantage is that sex reshuffles the genome. In an asexual lineage, genes are inherited together as a largely indivisible package. A beneficial mutation can therefore carry neighbouring neutral or mildly harmful mutations with it as it spreads through a population—a process known as genetic hitchhiking. Sexual reproduction and recombination can break up these associations, separating advantageous mutations from some of the damaging evolutionary baggage accompanying them.
A team led by Shreyas V. Pai and Parris T. Humphrey in Michael Desai’s laboratory at Harvard University has now demonstrated this process through experimental evolution. In a study published in Science, the researchers propagated populations of the budding yeast Saccharomyces cerevisiae for 960 generations. Some populations remained strictly asexual, while others underwent a single round of sexual reproduction and recombination at regular intervals.
All the populations began from the same ancestral strain and evolved under the same favourable laboratory conditions. Nevertheless, the populations allowed occasional sex increased their fitness in that environment by about 8 per cent, compared with 5.7 per cent among the asexual populations. Evolution was not merely inferred from similarities between living species or reconstructed from fossils; changes in relative reproductive success were measured as they accumulated under controlled conditions.
The difference became still more revealing when the evolved populations were tested under altered conditions, including higher temperature, increased salinity, reduced phosphate availability and changed acidity. The sexual populations generally retained higher fitness, enjoying advantages of between 2 and 5.6 per cent over the asexual populations. The strictly clonal lineages had adapted to their original environment, but in doing so had accumulated hitchhiking mutations that made them more specialised and less successful elsewhere. Recombination had enabled the sexual lineages to discard more of that baggage and evolve as better generalists.
The experiment does not claim to identify the single reason sexual reproduction originally evolved, nor need there be one universal reason why it persists. Parasites, changing environments, DNA repair and the combination of beneficial mutations may all contribute under different circumstances. What this research supplies is direct evidence for another important advantage: sex can prevent local adaptation from becoming excessive specialisation, leaving a population better equipped to survive when its surroundings change.
Why Is Sexual Reproduction an Evolutionary Puzzle? Sexual reproduction is widespread among eukaryotes, yet it carries substantial costs compared with reproduction by cloning. An asexual organism can reproduce without finding a mate and can pass an almost complete copy of its genome to every offspring. Sexual reproduction usually transmits only half of each parent’s nuclear genome and expends time and energy on producing gametes, attracting mates and competing for them.
In species with separate males and females, there is also what evolutionary biologists call the twofold cost of males. An asexual female producing only reproducing daughters could, in principle, found a lineage that grows twice as quickly as a sexual female producing equal numbers of sons and daughters, because the sons do not themselves bear offspring. This particular cost does not apply to yeast in the same way, but it illustrates the broader problem: sexual reproduction must provide compensating advantages if natural selection is to maintain it.
Sex can also break up combinations of genes that are already working successfully. Consequently, evolutionary biologists do not assume that sex must be advantageous merely because it is common. They investigate the circumstances under which recombination produces benefits large enough to outweigh its immediate costs.
Proposed evolutionary advantages of sex
- Combining beneficial mutations — the Fisher–Muller hypothesis: In separate asexual lineages, different advantageous mutations must compete with one another. Sexual recombination can bring them together in the same descendants, allowing adaptation to proceed more rapidly.
- Slowing the accumulation of harmful mutations — Muller’s ratchet: In a finite asexual population, the least mutation-burdened genetic class can occasionally disappear by chance. Without recombination, it cannot easily be reconstructed, so mildly harmful mutations may accumulate progressively. Sex can recreate offspring carrying fewer of them.
- Keeping pace with parasites — the Red Queen hypothesis: Parasites and pathogens continually evolve to exploit common host genotypes. By producing genetically varied offspring, sexual reproduction can make it harder for parasites adapted to one generation of hosts to infect the next.
- Preparing populations for environmental change: Recombination generates new combinations of existing genetic variants. When temperature, food supply, competitors or other conditions change, a varied population is more likely to contain individuals suited to the new circumstances.
- Purging hitchhiking genetic load: A beneficial mutation can carry linked neutral or mildly harmful mutations through a population with it. Recombination can separate the beneficial mutation from these genetic hitchhikers, allowing selection to retain the advantage while discarding some of the associated cost.
The Harvard yeast experiment provides direct evidence for this final mechanism. Asexual populations became well adapted to their constant laboratory environment but accumulated linked mutations that reduced their performance elsewhere. Occasional sexual recombination separated more of those harmful hitchhikers from the useful adaptations. The sexual populations therefore gained greater fitness in their original environment while remaining less specialised and better able to tolerate altered conditions.
These explanations are not mutually exclusive, and no single advantage need account for sexual reproduction in every organism. Its benefits will vary with population size, mutation rate, environmental instability, parasite pressure and the structure of the genome. The persistence of sex may therefore reflect several evolutionary advantages acting separately or together under different conditions.
Glossary
Asexual reproduction: Reproduction without the fusion of gametes, usually producing offspring genetically very similar to their parent.
Beneficial mutation: A genetic change that increases reproductive success in a particular environment.
Deleterious mutation: A genetic change that reduces survival or reproductive success.
Fitness: An organism’s relative success in surviving and contributing descendants to later generations; it does not simply mean physical strength or health.
Genetic hitchhiking: The spread of a neutral or harmful genetic variant because it is linked to a beneficial mutation favoured by natural selection.
Genetic linkage: The tendency of genetic variants located close together on the same chromosome to be inherited together.
Genetic load: The reduction in a population’s average fitness caused by deleterious genetic variants.
Pleiotropy: The situation in which one gene or mutation affects more than one characteristic, sometimes beneficially in one respect but harmfully in another.
Recombination: The reshuffling of genetic material during sexual reproduction, creating new combinations of parental genetic variants.
Generalist: An organism or lineage capable of performing reasonably well under a range of environmental conditions.
Specialist: An organism or lineage closely adapted to a restricted set of conditions, often at the cost of poorer performance elsewhere.
Once again, a biological feature that creationists may portray as the product of deliberate planning acquires an explanation from ordinary, observable processes. Mutation supplies variation, selection favours variants that reproduce more successfully, and recombination changes which mutations are inherited together. No organism has to anticipate a future environmental change, and no external intelligence has to rearrange its genes. Populations undergoing occasional sex simply leave more adaptable descendants—and evolution does the rest.
The paper in Science was accompanied by a this report from the Harvard Gazette:
The why behind sex
Research details answer to a question that’s more complex than it seems
Why is there sex — or, more precisely, sexual reproduction? If you’re an evolutionary biologist, the answer isn’t obvious.
Sexual reproduction is biologically costly, requiring not only competition for mates, but also the creation of entire populations in which only half the individuals can give birth. That reality has led scientists to wonder why it arose in the first place. After all, in asexual reproduction — as occurs when bacteria divide — not only can you skip the wooing and its costs, but every individual is a potential mother.
New research from a group of Harvard biologists dives into the benefits that come with sexual reproduction, particularly in changing environments.
The work was conducted in the lab of Fisher Professor of the Natural Sciences Michael Desai and led by Ph.D. student Shreyas Pai and Parris Humphrey, then a postdoctoral fellow. The team used a strain of yeast as a model organism. The yeast, commonly used in bread baking and beer brewing, has the scientific advantage of being able to reproduce either asexually or sexually.
Another advantage is that yeast produce new generations in only 90 minutes, taking the guesswork out of experiments on evolution, which is generally understood to promote slow change over long stretches of time. With a new generation every hour and half, Desai’s team was able to observe changes after 960 generations in just four months, something that would take hundreds to thousands of years in large mammals.
The research, published in the journal Science in April and supported by the National Science Foundation, the National Institutes of Health, and the National Institute of General Medical Sciences, began with a dozen identical populations of yeast growing in ideal conditions. All initially reproduced asexually, but every 100 generations, designated lines were induced to reproduce sexually for a single generation.
The scientists first confirmed earlier work showing that in a constant, favorable environment, sexually reproducing populations gain more fitness than asexually reproducing populations. Over those nearly 1,000 generations, the fitness of sexually reproducing populations increased 8 percent over the ancestral population, compared with a 5.7 percent increase for asexual populations.
Next, the researchers tested evolution in different environments — one saltier, one hotter, another with decreased acidity, and one with lower phosphate. In these altered conditions, researchers found that sexually reproducing populations had fitness advantages of between 2 percent and 5.6 percent over asexual populations.
While those numbers may not seem large enough to matter, Desai said that over long periods of time, an advantage of even a fraction of a percentage point can allow a particular population to dominate.
Selective differences of fractions of a percent in large populations can drive total shifts in population composition. A several percent effect in a microbe is large. It might take 50 to 100 generations to take over.
Professor Michael M Desai, senior author.
Department of Organismic and Evolutionary Biology
Harvard University
Cambridge, MA, USA.
Desai said that the work also probed the mechanism by which sexual reproduction is advantageous in a shifting environment. Some genes are “pleiotropic,” meaning that they affect the expression of several traits, some of which can be adaptive, some harmful. Harmful traits that affect an individual’s ability to survive or reproduce are reduced or removed from the population naturally. But traits that are neutral or only mildly harmful can survive as what scientists call “hitchhiking load,” particularly if other traits of the same gene provide an adaptive advantage.
When the environment changes for the worse, as it did for the experimental yeast, some of those previously neutral or mildly disadvantageous mutations may threaten survival. That is where sexual reproduction gets its edge.
Asexual populations have no way to eliminate the threat because their offspring are clones and genetically identical. Sexually reproducing populations, on the other hand, produce offspring with a mix of their parents’ genes, some of which may not carry the harmful “hitchhikers.” Over time, that reduces or eliminates the harmful effects of those genes.
We showed that, without sex, it’s not just that the populations are accumulating deleterious baggage, they’re accumulating baggage that maybe is not so bad in the environment they’re evolving in, but would hurt you elsewhere. In contrast, when you have sex, you purge that hitchhiking pleiotropic cost and prevent yourself from specializing quite as much just to the environment that you’re in. That can be a big advantage when the environment shifts or fluctuates.
Professor Michael M Desai.
While Desai and colleagues worked with microbes, he said that the effects extend into more complex organisms where asexual reproduction, while rarer, is still represented. In the plant kingdom, for example, cuttings can grow into fully mature plants that are genetically identical to the parent. Among animals, certain species of lizard can undergo parthenogenesis, in which their eggs are self-fertilized, producing offspring that are clones of their parents.
We believe that this general mechanism by which sex speeds adaptation and purges hitchhiking deleterious load should be a general advantage of sex across all sorts of organisms. I think it’s great that we can learn so much about how evolution acts and how the ways in which we reproduce, copy, and re-sort our genomes makes sense all the way across these enormous differences in organisms’ complexity and scale.
Professor Michael M Desai.
Publication:
This experiment does not claim to provide a single explanation for the original evolution of sexual reproduction. What it does provide is direct evidence for one important reason sex can persist despite its immediate costs. Recombination allowed the yeast populations to adapt more rapidly, separated advantageous mutations from harmful hitchhikers and left the resulting lineages less narrowly specialised when conditions changed.
Nor was evolution merely assumed or reconstructed from events in the distant past. Genetically comparable populations were propagated under controlled conditions for 960 generations, after which their reproductive fitness was measured. The sexual and asexual populations accumulated different genetic combinations, adapted at different rates and performed differently when tested in unfamiliar environments. That is evolution—heritable change accompanied by differential reproductive success—observed and quantified in the laboratory.
The results also illustrate why biological complexity does not require foresight. No yeast cell anticipated that its descendants might encounter heat, salt, altered acidity or a shortage of phosphate. Mutations arose without regard to future needs; genetic linkage caused some harmful variants to hitchhike alongside useful ones; recombination reshuffled those variants; and natural selection preserved the combinations that reproduced most successfully. A population can consequently acquire greater adaptability even though no individual, process or supervising intelligence planned for the future.
Creationism contributes nothing explanatory to this picture. Declaring sexual reproduction to have been designed neither explains its substantial costs nor predicts why recombination should purge linked genetic baggage, accelerate adaptation and reduce overspecialisation. Evolutionary theory does all three and allows those predictions to be tested experimentally. Sexual reproduction persists not because it was installed with a purpose, but because lineages that reshuffled their genomes sometimes left more adaptable descendants than lineages that did not. Once again, the appearance of planning emerges naturally from mutation, inheritance, recombination and selection—with no planner required.
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