Evolution and asexual reporduction

My 14-yr old son, a DEEP thinker, sprung this one on me on a hectic, activity-filled weekend. I was too busy trying to remember which of 4 kids had to be where and when, not to mention coordination pickups. I feel like I should KNOW this answer, but mental fatigue has prevented me from examining it closely. I KNOW the smart folks who hang out here can give me the best answer!

So, his question is: “How does evolution impact organisms that reproduce asexually? Since they do not meld 2 different sets of genetic material, do they evolve as Darwin theorizes, for mutations that are beneficial?”

I will feel dumb, probably, after reading the answers, but end of the school year is a time when moms are NOT at their intellectual best…so thanks!

Great question! The evolutionary biology specialists will be along soon to give more detailed answers, I’m sure, but in the meantime:

Yes, asexually-reproducing beings such as single-celled organisms evolve in response to changes in their environment. For a fascinating example, read about Richard Lenski’s long-term E coli evolution experiment.

Just because they aren’t mixing two different sets of genetic material when they reproduce doesn’t mean that mutations in their genetic material don’t sometimes occur. And when they do occur, the ones that make the organism more successful in its environment increase its chances of living long enough to reproduce.

Well, there are the bdelloid rotifers who phased out sexual reproduction about 40 million years ago. They cap their genes with whatever DNA they can get a hold of, and are sometimes endowed with same traits they gave the original organism. These are past directly onto the daughters (there are no male bdelloid).

Mutations occur through exposure to mutagens (like uv light) or random mistakes in copying the DNA. Those mutations can be neutral, harmful or beneficial depending on the environment and organism. Asexually reproducing organisms can also transfer genetic info horizontally (organism to organism, not through reproduction) by swapping parts of copies of their DNA. Have him look up bacterial transduction. Often drug resistance genes are transferred that way.

Whether an organism reproduces sexually or asexually has not real impact in the rate of evolution. Bacteria generally reproduce asexually, and they evolve faster than other organisms.

So long as mutations are occurring, it doesn’t matter much how those mutations are passed onto offspring. Those offspring with beneficial mutations will survive better and ultimately dominate the population. Those with detrimental mutations will die off. The exact same principles of “Darwinian evolution” apply and the work exactly the same way. All that sexual reproduction does is allow for beneficial mutations to mingle with unrelated lines. It doesn’t produce novel genetic material and it doesn’t alter the survival advantage/disadvantage of any genetic trait.
Ask your son to imagine what would happen if one individual in a population of asexually reproducing organism, say a lizard, developed a mutation that allowed it to run 50% faster. Obviously that lizard would produce more eggs than other lizards, and the lizards that hatched from those eggs would produce more eggs and so forth. Very shortly the slower lizards would starve to death because all their food was stolen by the fast lizards. The whole population will evolve into a population of fast lizards, exactly as it would if the lizards had been reproducing sexually.

The only difference is that in the case of the asexually reproducing lizards, the genes form every other lizard alive at the time of the original speed mutation will become extinct. The entire population will be descended from just one individual.

The reason why sexual reproduction has some evolutionary value is that it allows for the conservation of multiple gene lines for multiple survival traits.

For example, in an asexually reproducing population, if the climate gets hotter and drier, then an individual with mutations that allows it to tolerate heat better will produce more offspring. And an individual with mutations that allows it to tolerate dry better will also produce more offspring. But no individual in the new population will be better at tolerating both heat and dry. Instead we will have a mixed population of dry tolerating individuals and heat tolerating individuals. And if heat tolerance provides a bigger advantage than dry tolerance, then the heat tolerance genes may actually become extinct *despite *being advantageous. To produce an individual that tolerates both we need to wait until one of the heat tolerating individuals evolves dry tolerance or vice versa, which may never happen.

However if a sexually reproducing population is faced with a warming and drying climate, then individuals that tolerate heat will mate with organisms that tolerate dry, and the result will be an offspring that tolerates the new climate much better than either parent. In a very few generations the population will be composed solely of heat and dry tolerant organisms.

As you, and you son, can hopefully see, sexual reproduction doesn’t actually increase the pace of evolution of itself. Beneficial genes still need to be produced randomly and be passed on to subsequent generations. All that sexual reproduction does is allow beneficial genes from different organisms to combine in novel ways.

That is not always a benefit either, since it means that if a genetically perfect organism evolves in a sexually reproducing population, it will become extinct in a single generation when that perfect gene combination is blended with less well-adapted genes from the other parent.

As a result, the general rule is that asexual reproduction is an evolutionary advantage where conditions are stable because it allows a gradual but continuous progression towards an optimal form. Sexual reproduction is favoured in unpredictably changing environments because it allows for the rapid combination of multiple novel solutions.

There’s a reason why most asexually reproducing animals live in the water.

Asexual reproducers still reproduce and pass on their genes. If a bacterium picks up a useful mutation, then its offspring carrying that useful mutation are more likely to do well. Natural selection works just fine.

Sexual reproduction isn’t necessary for evolution, but it does speed up the process, in a way. It allows for more combinations of different genes to be tested in the environment more quickly than if each mutation had to happen sequentially. More variation = quicker evolution.

It’s also worth pointing out that many asexual organisms, like bacteria, also engage in some sort of horizontal gene transfer, where they pick up DNA from the environment or from other organisms in a sort of pseudo-sexual process.

Pedantic nitpick of an otherwise excellent post:

I think you must mean that it’s the dry tolerance genes which might become extinct, because the heat tolerance genes are more advantageous.


Organism evolved a long time before sexual reproduction was “invented”. Therefore sexual reproduction is clearly not necessary for evolution. However, sexual reproduction has its advantages: sexual reproduction make it easier to weed out deleterious mutations and combine beneficial mutations in different genes, which is especially important to maintain large and complex genomes. An organism with a diploid genome can have two different alleles of a gene which may be advantageous under different conditions: if allele A is optimal under conditions a and allele B under conditions b, diploid genomes may be more suitable to maintain both alleles in the population under fluctuating environmental conditions.

Of course, when you give him his answer, he will either:
a. have forgotten he asked the question
b. shrug and say “Thanks mom. I already googled it”
c. come back with a supplementary about hermaphrodites.

The other advantage to sexual reproduction is that sometimes, you can have two (or more) traits which are close to neutral by themselves, but advantageous in their combination. Sexual reproduction allows for the trying of many different combinations, combinations which may not have occurred before, even without mutation.