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Why are there males and females? Algae helped explain this bizarre phenomenon

Posted December 7, 2017

Why are there two sexes? What determined that two kinds of gametes are needed to create a new life? These questions are really interesting for scientists, but at the same time cannot be answered in a simple way. Technically, two sexes are not needed males and females, so why do we have them? A scientist from the University of Adelaide employed algae to help explain this phenomenon.

Males and females evolved to have different sizes of gametes, which might be because species became bigger. Image credit: KDS444 via Wikimedia(CC BY-SA 3.0)

Males produce moving gametes that are small, while females produce bigger gametes that are non-motile. However, technically, these two types are not necessary for reproduction. Scientists have created a Disruptive Selection Theory, which seeks to explain how population evolves from having one size gametes to two different sizes of gametes. However, it is very difficult to explain the course of evolution and it all gets very complicated. Jack da Silva from the University of Adelaide picked up some green algae to explain how this works.

The theory says that organisms evolve to be larger and so gametes have to be larger as well, to provide enough nutrients. Larger at the same times means fewer in number. However, bigger number of gametes can help fertilization and so these functions are split. Rather than having two large gametes meet with low chances of fertilization, it is better to have one large gamete for necessary nutrients and a bunch of smaller motile ones to increase chances of fertilization. The Disruptive Selection Theory says that the size difference cannot be smaller than three times, because then gametes will evolve back into being the same size.

Algae is perfect for this research, because its gametes vary considerably between types, sometimes being the same and sometimes – different sizes. And they did support the theory. In cases where one gamete was larger, it was always bigger by a factor of three. Dr. Jack da Silva, author of the study, said: “This provides the first test that is specific to this theory – previous tests have been about predictions that are common to this and similar theories. To date there hasn’t been a lot of strong evidence in support of any of the competing theories, but here we have confirmed that the classic theory is probably on the right track”. This research can help understand evolution itself much better, but a lot remains to be answered.

While we know one gamete has to be considerably bigger than the other, we still don’t fully understand why it must be by 3 times. It would also be interesting to see the path human gametes went in the course of evolution, but it may be too difficult for now.

Source: University of Adelaide

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