The Origin of Meiosis and How Sexual Reproduction Prevailed.

We’re studying now one of my favorite topics, evolution. The other day, we went after school to watch with our teacher the documentary film “Charles Darwin and the Tree of life”, presented by David Attenborough. I really enjoyed the documentary, but after watching it a question arose in my mind:

Supposedly, a characteristic doesn’t just appear. Is a step by step process, where each of these steps are a random mutation which presents an advantage for the individual which will increase the probability that the individual will survive and will pass it’s genes to it’s offspring. But what if this steps didn’t have any advantage to the individual at first and the process would not be beneficial until it was completed? 

The process I was thinking of was meiosis. If you think about it, what would be the advantage of an early meiosis without any outcome? It could not just appear like a complete process, and, In case it was an advantage on it’s first steps, why can we not find any organism which present this intermediate state of primal meiosis? 

For this reasons I decided to do some research on this topic.

The Origin of Meiosis

The Origin of Meiosis is still Unknown, but there are two main theories which try to explain where it evolved from:

—} Meiosis Evolved from Mitosis: 

In this theory, supposedly the first prokaryotic cells evolved Mitosis first. Then, when Mitosis was settled, Meiosis and sexual reproduction appeared.

The main support for this theory is that both Mitosis and Meiosis are very similar in terms of steps and they use almost the same molecular machinery (For example, The use of Spindle Fibers generated by the centrioles which separate the chromosomes during Anaphase). 

But this still doesn’t explain how variation processes appeared, like Chiasmata and crossing over.

—} Meiosis Evolved from prokaryotic sex:

Prokaryotic sex is a complex process which in simple terms consists in one bacteria releasing (Usually a plasmid of) DNA to the surroundings which is then taken up by another Bacteria and binds to it’s own DNA. This the simplest and most ancient way of achieving variation inside a single “specie”.

Support for this theory is that the earliest eukaryotic cells carry the genes for early meiosis, but are not expressed. This happens for example in Giardia Intestinalis.

However neither of this theories show in a clear way what was the origin of Meiosis.

How is Sexual reproduction an advantage? How did it prevailed?

An obvious advantage of sexual reproduction is that it leads to variation. However, this is a long term process, and Evolution doesn’t work like that. Evolution is based on short term advantages, advantages which increase the probability that the mutated individual would pass it’s genes to the offspring. Long term processes which don’t help the individual to survive in it’s first stages are much less likely to pass to the next generation, because they aren’t an advantage for the individual even though it takes energy for carrying it out.

This shows how improbable it was for Sexual reproduction to appear. The first Sexual reproductive individuals would be in competition with self replicating/cloning organisms and they would have to survive until sexual reproduction was actually an advantage.

With the next diagram I want to show how Sexual reproduction is a long term advantage over self replicating. 

During the second generation of sexual reproductive individuals, There would be both weak individuals and Strong individuals which reproduce this way, and they would have to compete against self replicating “normal” individuals. During this first competition, weak sexual reproductive individuals would die against normal self-replicating individuals, lowering drastically the amount of sexual reproductive organisms. However, Strong sexual reproductive individuals would survive, and normal self-replicating individuals would die against them. 

Therefore, over time, strong sexual reproductive individuals would be the only ones to prevail showing how variation is a long term advantage over cloning and self replicating. The survival of the fittest.

This is one of the blog posts that have taken me more time to do, but it can’t express how I feel about this topic and how amazingly interesting it can be.