Parthenogenesis eliminates need for males
For some animals, sex roles are not an issue. For example, the common earthworm is hermaphroditic, having both male and female sex organs. Each worm is capable of producing offspring through a form of sexual reproduction. Amoeba and many other single-celled organisms reproduce by budding, which doesn’t involve sex at all. And viruses are incapable of reproduction except through the auspices of the infected host’s cellular machinery.
Reproduction is essential for life and many different forms of life have found alternative ways of reproducing without sex.
But it is a general working assumption in biology that for vertebrates (animals with backbones), sexual reproduction through the combination of male and female gametes is the normal way of things.
For one subset of vertebrate, though, it seems the male is unnecessary. The whiptail lizard – a resident of the southwestern United States and Central America – has adapted the ability to reproduce in a population made up entirely of females. Actually, there are 45 recognized species of whiptail lizards with only 15 of these species employing parthenogenesis – which is where females produce only diploid eggs that do not require fertilization.
These lizards are defined as engaging in obligatory parthenogenesis. That means they do not have an alternative mechanism. They have long ago given up the dichotomy of a male and female of the species.
Parthenogenesis requires some significant genetic changes for the lizard species but it has completely eliminated the need for a male sex.
The babies hatch and grow into adult female lizards that are capable of bearing the next generation. In effect, it is natural cloning.
The exact origin of these lizard species is somewhat tangled, but it appears that they normally arise from the mating of a male from a bisexual species with a female from a parthenogenic species or from the hybridization of two bisexual species. Normally, such inter-species breeding generates infertile offspring.
Such is the case, for example, of the mule which is the product from breeding a horse with a donkey.
For some of these interspecies couplings in whiptail lizards, the offspring remain fertile but have lost the need for males. They have become a new parthenogenic species.
Scientists have performed hormonal tests on the suspected ancestral species of some of the parthenogenic varieties and discovered that some males are sexually responsive to the hormone progesterone. Progesterone is normally considered a female hormone.
It is found in a number of species, including humans, and it is responsible for the maturation of eggs, among other things.
The working hypothesis is the mating between one of the progesterone sensitive males and a normal female results in a new generation which no longer required males for reproduction as the female has the necessary hormones to bring an egg to maturation.
While an all female population is rare enough, scientists have also discovered that these lizards still perform the instinctual mating rituals observable in their ancestors. That is, they take turns pretending to be the male and performing mating behaviour but without any sex act. They display themselves and will bite one another but it is all a show.
Further, females alternate in the laying of eggs.
The result is that twice as many offspring are produced compared to a bisexual population of the same size. After all, there are twice as many lizards generating babies. This has resulted in the parthenogenic species being very opportunistic and taking advantage of new territories.
If it was just one group of lizards which engaged in parthenogenesis, they would be an interesting oddity.
However, many different organisms reproduce in exactly the same way including many plants, some nematodes, water fleas, scorpions, aphids, bees and wasps. There are other vertebrate including fish, amphibians, reptiles, and a few species of birds which have been shown to engage in parthenogenesis. And this type of reproduction has been artificially induced in fish and amphibians.
Indeed, in the evolution of life on this planet, cloning may have been a dominate method of reproduction. Unfortunately, it is also genetically limiting and with the evolution of sexual reproduction, life became much more rapidly diverse.
All of this, of course, reminds me of the scene in Jurassic Park where the scientists discover that the dinosaurs are breeding in an “all female” environment. Yes, life will find a way to reproduce. I am just thankful that there is only one species of humans so it is unlikely that men will become unnecessary in the near future.