Saturday, 23 May 2015

The problem with sex.....

This takes me back..........

Most folk would say that the problem with sex is that they are not getting enough. But this post is not about sex, the act, rather it is about sex, the concept. From a biological perspective, sex is troublesome. Why bother finding a mate when you can reproduce asexually. In fact many organisms engage in asexual reproduction. Bacteria, plants, and even higher order animals can produce genetically identical copies of themselves without taking off 'their clothes'. Sexual reproduction is universal however, and even creatures which reproduce asexually often have a sexual option, if required.

The fact that sex has evolved under the ruthless and relentless mechanism of natural selection testifies to its biological usefulness. Evolution is not profligate or wasteful, therefore complex and energy expensive sexual reproduction must have earned its biological keep.

There are many advantages in reproducing asexually. Firstly, you produce an identical copy yourself. This is usually a good thing because the 'parent' organism will already be well adapted to its environment. Energy and resources are not spent on finding a suitable mate. The process is always faster than sexual reproduction and therefore organisms can rapidly populate their environment or colonise other favourable environments. Furthermore, all individuals have potential to reproduce. In sexual reproduction only half the population (females) can produce young. Also, the contribution to offspring in sexually reproducing organisms is one sided. The female carries and nourishes the young during in vitro development and in many species provide sole care for the youngsters until they can fend for themselves. In this context, the male's sole contribution is fertilisation. Once accomplished he is free to impregnate other females. Once a female is fertilised her reproductive option is closed. In this regard the male of the species can be seen as a parasite, exploiting females for his own reproductive advantage. The advantages of being an asexual female seem overwhelming. So why bother with sex?

The obvious difference between sexual and asexual strategies is that sexual reproduction provides genetic diversity. The melding of the genome of two genetically different individuals makes for endless genetic diversity. With the exception of identical twins, every organism produced is genetically unique. This diversity confers no particular advantage in a stable environment. The converse is true. Many of the sexually produced organisms are actually 'less fit' and will be removed from the gene pool by negative selection. Genetic diversity becomes a boon in circumstances of change, instability and catastrophe. By chance, the genetic configuration of some organisms will be best adapted to the new circumstances and therefore become successful. It matters nought to nature that most organisms can't adapt and therefore die. The few survivors will be left to pass on their better adapted genes to their offspring and populate the new environment.    

Another advantage of sexual over asexual reproduction centres around parasites. All species are infested with parasitic organisms. Parasites are creatures which hitch a free ride on or within their host. At best they do no harm and at worst they kill the host. There is an evolutionary war involved here. The host adapts by producing better defence mechanisms to prevent  parasitic infestation, in their turn, the parasite adapts to overcome the host's defence strategies. The key to the host maintaining the edge in this constant war is genetic diversity. Asexuality does not provide the genetic diversity to change and hence the parasite ultimately, wins.

All this brings me neatly to my own story. Many years ago I was involved in botanical research in an UK University and spent three years working on Dandelions (no shit). Dandelions are fascinating plants because the vast majority of sub-species set seed without recourse to fertilisation. Therefore, they reproduce asexually and the seeds from a particular plant will be genetically identical to each other and the parent plant. The Sussex Downs where I collected my samples were covered in Dandelions. An asexual strategy seemed to work for this ecosystem. Not surprisingly, the Sussex Downs are a very stable and predictable environment. On occasion, I needed Dandelions which reproduced sexually and the only place I could obtain these particular sub-species was next to coastal sand dunes. Asexual sub-species were completely absent in this environment. The sand dune system is subject to constant change and even catastrophe in the event of a storm. The sexual Dandelions thrived here because their genetic diversity allowed them to adapt to a precarious habitat. The sexual varieties were completely absent from the Sussex Downs. Here, stability reigns and the faster reproducing asexual Dandelions have the edge over the slower reproducing sexual plants.

Some organisms are just greedy and employ both strategies, as required. Bacteria are notoriously asexual and have a doubling time measured in minutes. This strategy enables bacteria to rapidly exploit any suitable environment. However, when things get tough, the bacteria get it on and engage in a form of primitive sexual activity which results in the exchange of genetic information. Aphids can also reproduce asexually and sexually when it suits. This bi-strategy gives them the best of both worlds. Higher organisms such as mammals don't seem to be able to choose an asexual mode of reproduction. Perhaps their complex biology and body plan prevents this.

So, hopefully, I've made a convincing argument for the utility of sex in biological systems. Pure asexual organisms have the potential to rule the world, but only when the going is good. When change comes along however, and if you wait long enough change will undoubtedly occur, it makes good sense to indulge in sex.            

Sex in a nutshell


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