Why did single celled asexual beings become sexual?

Answer 1

DrHue nailed it (great answer).

Basically asexual reproduction is great for simple organisms that reproduce like gangbusters. As every new individual is basically a clone of its single parent, any environmental change (such as a new disease or toxin) that wipes out one individual could very well wipe out all of them. The only way they can adapt is through chance mutations ... and therefore they have to reproduce *ferociously* fast to increase the odds of producing the right mutations to offset environment changes ... or to move into new environments.

However, as an intermediate form of genetic exchange ... bacteria discovered something called 'bacterial conjugation', where tons of bacteria in close contact will actually exchange a little genetic material with each other. This both produces new genetic combinations, and gives each bacterium the chance of gettting something new and beneficial from its neighbors.

Another intermediate development was going from haploid to diploid organisms. In other words, in normal asexual replication (mitosis), a bacteria first duplicates a copy of its entire DNA, and then splits into two new individuals. However, if this is interrupted, then a single individual ends up with a double copy of its own DNA (it is what we call 'diploid'). This accident can be advantageous, because those bacteria now have a redundant copy, which allows it to fix its own DNA, and allows for more complex bacteria (as the DNA can get much bigger as it is now more robust to replication errors).

This leads naturally to the development known as 'meiosis'. This is where a diploid individual splits into two haploid individuals.

And finally, full sexual reproduction is when two haploid cells produced by meiosis, recombine to produce a new diploid cell. If the two haploid cells are from two different parents, we have sexual reproduction.

The advantages are that that this *exponentially* increases the generation of entirely new genetic combinations. *Every single individual* is the chance for an entirely new genetic combination that has never existed before ... different from either of its parents.

This not only makes the species *much* more adaptable to changes in environments, and to expand into brand new environments, but it is *really* fast at creating new species. This is why it has been really good at generating lots of new species that have this feature (sexual reproduction).

Answer 2

Exchange of genetic information is done by almost every species; in single-celled organisms, the process is called conjugation; in multi-cell, it is mostly sexual. Even though sexuality is hugely expensive, the rapid distribution of possibly favorable mutations is such a huge advantage to evolution that just about every species does it.
Postscript: As usual, secretsauce is right on the money, and alludes to an important point: with sexual reproduction, no two organisms (except for identical twins) have exactly the same genetic structure. This further encourages evolution via genetic variation.

Answer 3

It provides an advantage in adaptation or evolution.

Consider this, asexual reproduction allows an organism to replicate itself, but is limited to the DNA, genetic code or heritage it has received from it's single parent. There is a low chance of mutation, in other words sometimes changes will occur, but not very often. This type of organism is slow to adapt to changes in its environment.

If the organism (plant or animal) receives DNA from 2 separate sources then during the separation or recombining of the DNA double helix, a greater chance exists for mutations. Some mutations are good and some are bad. The ones that are good would enable the organism to better adapt to changes in the environment, giving it an survival/procreation advantage and would then be passed allow to more descendants. Eventually the mutated version would be more prevalent than the pre-mutated version.

Hope this helps, it comes down to this, a larger pool of genes is better then a smaller pool. That is why the world is dominated by organisms, plants and animals, that sexually reproduce.