IN GENERAL, THE COMMON LIZARD GIVES BIRTH TO LIVE YOUNG, EXCEPT IN TWO POPULATIONS. ABI WARNER/SHUTTERSTOCK
Despite what many of us were once taught in school, not all reptiles lay eggs. In fact, one of the most widespread lizards in Europe give birth to live young. Now, it seems these common lizards (Zootoca vivipara) have done something once thought impossible – they have in effect "reversed" evolution to re-evolve egg laying.
It's already been observed that two separate sub-populations of the common lizard still lay eggs (despite their name meaning live-bearing in both Latin and Greek). With one found along the border of France and Spain and the other in the Alps, it was assumed that they simply represented relic populations of the egg-laying ancestors of the common lizard.
To test this, researchers carried out genetic analysis of over 70 lizards collected throughout their range in Europe, in order to flesh out a detailed evolutionary tree. Their results are published on bioRxiv.
It turns out that the tree is a little more complicated than they originally thought. The population of egg-laying lizards still knocking about in the Alps did indeed turn out to be a remnant group of the original egg-laying reptiles. But the lizards laying eggs in Spain were found to have re-evolved this ability, meaning that evolution basically went backwards.
This is interesting since in 1893, Belgian palaeontologist Louis Dollo presented a principle that in effect said that evolution is unidirectional, and so once an organism looses a complex trait (such as egg laying), it is not able to re-evolve it, even if it was to once again find itself living in the same environment. This is known as Dollo’s law, however the new analysis of the common lizard adds to mounting evidence that this is not strictly true.
Not only that, but evolutionarily speaking, the common lizard only evolved giving birth to live young relatively recently, within the last 2 million years or so. This means that the small renegade bunch in Spain must have regained egg laying even more recently, suggesting that perhaps they somehow retained the ability and that it was simply dormant within their DNA, before then being switched back on again.
This is not the first time that reptiles, or even lizards for that matter, have re-evolved egg laying. One such example is that of the Erycinae snakes – while most of these species are ovoviparous, at least three are known to lay eggs. It is thought that these three species likely re-evolved the ability after 60 million years, implying that they genuinely lost the genetic code for it, unlike the common lizards. Yet evolution still somehow managed to go backwards.