Tag Archives: Evolution

Why do we have sex?

It occurred to me a while ago that although I have posted a few articles about the sex lives of various animal species, I have never actually gone right back to basics and discussed why we even have sex in the first place. This may be an odd question to anyone who doesn’t have a background in biology but it’s an important one. Of course, animals have sex in order to reproduce, thus passing on their genetic material to the next generation. Without sex, we would eventually die out, right? Well… Perhaps not.

Sex is not essential

Many organisms, including some bacteria, can reproduce without sex (asexually). Instead of having the typical boy-meets-girl scenario we are so used to in our species and in the animal kingdom generally, many bacteria species simply divide themselves in two et voila – offspring. Simple and effective. Although this type of sexless reproduction is very rare in the animal kingdom, there is one rather well-documented case of asexual reproduction in a species of whiptail lizard. These lizard species consist entirely of females.

Yup. No males. BUT LAUREN, HOW DO THEY HAZ TEH BABIES?! I hear you ask? Well, firstly…one female will mount another, which may seem a bit odd and pointless, but is thought to trigger ovulation (the release of eggs) in the mounted female. But the part where sperm meets egg is just skipped, and her eggs simply divide and grow into embryos. The result of this asexual reproduction is that all the offspring the mother produces will be identical clones of her, carrying exactly the same genetic material. So sex is not always necessary for reproduction.

The costs of sex

Not only is sex just not essential for reproduction, it is also a bit of a pain in the ass of evolutionary theory. In species that rely on sexual reproduction (the majority of animal species), finding a member of the opposite sex who is willing to mate with you is not as easy as it may seem. Males of many species often compete to win over the females, and this is really costly in energy terms, whether it’s from the physical act of competition (which is often violent and can be to the death) or the energy that goes into developing the traits that ladies just seem to find irresistible.

The number of offspring a male peacock produces is significantly correlated with the number of eyes on his tail. So although a big energy investment, it may be worth it in the end.

There are costs for the female, too. The whiptail lizards described above produce babies each time that are exact clones of them: They share 100% of their genetic material. This is a massive WIN in Darwinian terms – you’ve passed on as much of your genetic material as you possibly can. But humans and virtually all other animals produce children that share only 50% of their genes. Yet sex has prevailed. Why?

(Not so) Sexy benefits

Sex does have its benefits, though. Y’all knew that already. Because we all know that we have sex in order to fight off parasites and diseases, right?! YEP. One of the most convincing theories of why we have sex is all to do with parasites and disease (how romantic).

If you are an asexually reproducing species and are only making clones of yourself, there is no genetic diversity. You are all the same, and so if a fatal parasite enters your environment and happens to be very good at attacking you, your whole family and thus genepool will soon disappear (EVOLUTIONARY FAIL). Sexual reproduction is essentially the mingling of genetic material, and this mingling results in much more diverse offspring than those produced asexually. When a male and a female of a given species reproduce, their offspring is not a simple blend of their two sets of genetic material. Each pair of chromosomes wraps around each other, swapping genes as cells divide into egg or sperm. The result of this is a completely unique individual (aww, see, your parent’s were telling the truth) who will be more likely to avoid infection of a parasite or disease that has adapted to infect a particular genetically common host.

The more diverse the individual members of a species, the easier it would be for them to adapt to changes in the environment such as the introduction of a disease or parasite. Of course, parasites and disease can also strike sexually reproducing organisms, but the effect is not as devastating as those seen in asexually reproducing populations.

Some scientists are unconvinced that this theory is enough to explain why sex persists, but it certainly is an interesting point to think about. In the mean time here is an almost-relevant video about parasites because IT IS COOL and just think – by having sex, we are perhaps protecting ourselves and our children from the same fate as this snail. YAY FOR SEX!

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Evolution giveth, and evolution taketh away.

So this article was brought to my attention by the super-cool Tom Houslay who tweeted it earlier this evening. The article is very short so do go and read it if you can. If you can’t be bothered, FINE, I will rip it apart summarise it later in this post.

The article is written by an organisation called the Institute for Creation Research who are a… Well I’ll just take a quote from their own website which describes exactly who they are.

For over four decades, the Institute for Creation Research has equipped believers with evidence of the Bible’s accuracy and authority through scientific research, educational programs, and media presentations, all conducted within a thoroughly biblical framework.

The article in question is criticising a paper published in the open access journal BMC Evolutionary Biology which investigates traits such as loss of sight and pigment in Mexican blind cavefish, and the advantages this may carry for this species. I’m not going to go into the paper in any detail, because that’s not what this post is about. This is about clearing up blatant misunderstandings about evolution. I’ll go through the ICR paper paragraph by paragraph, pointing out what (I think) is wrong with their arguments.

Evolution maintains that as more time passes, living things evolve to acquire better and more useful traits. As such, shouldn’t the loss of a useful trait, such as eyesight, be regarded as the opposite of evolution? Not so, say recent news reports on blind fish.

Uh, ok, first of all, evolutionary theory doesn’t maintain that things acquire “better” and “more useful traits”. Evolution is defined as any genetic change that is inherited over generations. This needn’t be particularly useful, and can involve the loss of traits, not just the invention of brand spanking new ones. Eyesight may well be deemed “useful” from our anthropomorphic view, but we are here talking about fish living in a cave in almost complete darkness. Not such a useful energy investment now, is it?

A New York University news release attributed the fishes’ loss of sight to “convergent evolution,” which makes little sense if evolution, as neo-Darwinists describe it, is supposed to generate new features and functions.

Convergent evolution is the process by which unrelated species evolve similar traits because they have had to adapt to similar environments or pressures. The most common example of this is the wing. Birds and bats both have wings. However, birds and bats are not closely related animals. Their last common ancestor did not have wings, therefore they both evolved wings independently.

For lots more examples of convergent evolution, there is a whole Wikipedia page of them (Hurrah!).

In this study, the authors note that the blind cavefish live alongside and are able to breed with a very genetically similar species of fish who are not blind. Yet, despite this gene flow and interbreeding with non-blind fish, the blind phenotype is still maintained in the cave populations. This leads the authors to conclude that there must be advantages to being a blind (as opposed to a seeing) cave fish. It is perhaps because eyes are costly things to maintain in terms of energy investment, and so it may be the case that Mexican cavefish with poor or no eyesight are able to invest that extra energy in something that is more useful to them in their environment.

So to repeat, evolution is not “supposed to generate new features and functions”.

But obtaining the fish sight system required an input of a massive quantity and quality of information. And making the fish blind merely required the loss of some of that information. How could attributing these opposite processes to “evolution” be scientifically accurate?

I don’t know what to say to this, I don’t really understand what they mean. “Making the fish blind” does not require the loss of information, it requires the reduction of energy cost associated with maintaining an essentially useless trait (DARK CAVE, remember, VERY DARK CAVE) which should be favoured by natural selection.

The study of blind cavefish can undoubtedly contribute valuable insight into the genetics of trait variations and the fishes’ potential to adapt and survive in varied environments.

Well, yeah, that’s kind of… That’s kind of adaptation, you know, natural selection, evolution…? Maybe they’re getting it…

But because evolution is supposed to make new traits or develop new and useful genetic information, mere losses and variations should not be called evolution.


Bradic, M. et al. 2012. Gene flow and population structure in the Mexican blind cavefish complex (Astyanax mexicanus). BMC Evolutionary Biology. 12: 9.

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