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.
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!