Let’s start with a couple of basic definitions! There are lots of more specific and technical terms that I could go into but I won’t. Suffice to say, if you are interested in the biological processes at work, there is information out there on that. For my purposes, I think there are probably just two definitions we need to be familiar with:
Parthenogenesis is a type of asexual reproduction in which a female gamete or egg cell develops into an individual without fertilisation.
In asexual reproduction, one individual produces offspring that are genetically identical to itself.
It all gets very complicated at a cellular level and not being a biologist I wouldn’t want to even attempt to explain it but I wanted to make sure that it was clear that parthenogenesis and asexual reproduction are not always interchangeable.
The word parthenogenesis means virgin creation and these virgin births are known to occur in a range of species including many insects, snakes, lizards, Komodo dragons and even, rarely, in turkeys. In lab conditions, mammals have had virgin births but only when artificially helped by researchers.
To help us understand the realities of being a virgin mother, I wanted to look at a few examples, starting with a population of Bynoes gecko in Australia gave up entirely on sexual reproduction and are now an all-female species. This type of reproduction tends to occur in harsh climates – arid deserts and arctic areas – and are almost exclusively hybrids. The thinking is that the population of one species got split and evolved separately before coming back together. The two groups then reproduced sexually, creating sterile hybrids who over time evolved to reproduce without sex.
On a spectrum of reproduction, it’s possible that whiptail lizards would come next. There are as many as 50 types of whiptail lizard that reproduce without sex, but despite this, the exclusively female New Mexico whiptail still engage in ‘fake sex’ to be stimulated in order to reproduce. Whilst not essential, the lizards that are stimulated lay more eggs.
Then there are the animals that engage in both types of reproduction. Queen bees produce male drones by parthenogenesis but workers are made by sexually reproducing with drones. Boa constrictors normally reproduce sexually, but not always and komodo dragons, sharks, turkeys and swordfish also primarily reproduce sexually but parthenogenesis can account for up to 5% of babies. Note that it some species, females are able to store sperm for a considerable length of time after encountering a male and so genetic testing is required to confirm parthenogenesis.
The advantage of asexual reproduction is that a female can restart a population in the absence of a male – think about a female which has found itself on an island with no other creatures of the same species. It’s also easier for an initially small population to take over an area. Other benefits include not having to expend energy finding, attracting and keeping a mate and when babies are born, they are made up solely from mums genes.
Whilst an all female population sounds enticing, there are downsides… clones are unable to evolve and adapt genetically to change, there is an increase risk of being affected by parasites and disease. As the entire population are all vulnerable to the same external factors, one small factor can result in them all dying. As I noted above, mammals don’t naturally engage in parthenogenesis – we can’t reproduce without male and female dna… even just one missing gene can result in conditions such as angelmans syndrome and prada willi syndrome.
On the whole, species that reproduce without sex, don’t last that long:
“from time to time organisms evolve to give up sex, reproducing asexually instead. When this happens, any genetic differences between a parent and a child are, by definition, due to mutation only. At first asexual organisms often flourish. But their glory is fleeting. For reasons that remain mysterious, the loss of sex is almost always followed by swift extinction, apparently, without sex you are doomed.”
– Olivia Judson
Of course, there are always exceptions, and in this case, one of the exceptions is the bdelloid rotifer which is an all female species that has been cloning itself for about 85 million years (Judson). To overcome the problem of identical genetics, they get new genes by picking up DNA from the environment. This protects them from the main dilemmas facing most parthenegenic species.