Big evolutionary change is associated with lots of small differences

The version of evolution proposed by Charles Darwin focused on slow, incremental changes that only gradually lead to the kind of differences that separate species. But this does not rule out the possibility of sudden and dramatic changes. Indeed, some differences make it difficult to understand what a transitional state might look like, suggesting that a big leap may be required.

A new study looks at one major shift: the shift from egg-laying to live births in a group of related snail species. By sequencing the genomes of multiple snails, the researchers identified changes in DNA associated with egg laying. It turns out that a large number of genes are associated with the change despite its dramatic nature.

Give up eggs

The snails in question are present Sex called LitorinaWhich is widely distributed around the North Atlantic Ocean. Many of these species lay eggs, but a number of them become live births. In these species, the organ that covers the eggs with a protein-rich jelly in other species acts instead as an incubator, allowing the eggs to develop until the young snails can crawl out of their parents' shells. This is thought to be an advantage for animals that may have to lay their eggs in environments unsuitable for their survival.

The egg-laying species is so similar to its relatives that it has sometimes been thought that it is simply a variant of the egg-laying species. All of this suggests that live births have evolved relatively recently, giving us a good opportunity to understand the genetic changes that enabled them.

Therefore, a large international team of researchers sequenced the genomes of more than 100 individual snails, both laying eggs and giving birth live. The resulting data was used to analyze things like how closely related different species are, and what genetic changes are associated with live birth.

The results indicate that there are two separate groups of species that reproduce through live births. In other words, on the evolutionary tree of these types of snails, there is a branch full of egg-laying species that separates two groups that give birth to live snails. Typically, this structure is seen as an indication that live births evolved twice, once for each of the two groups.

But that does not appear to be the case here, for reasons we will discuss.

Lots of differences

Separately, the researchers looked for regions of the genome associated with live births. And they found a lot of them – 88 in total. These 88 regions were identified in both groups of organisms, and the DNA sequences within them were very similar. This suggests that these regions had a single origin and were preserved in both lineages.

One possibility to explain this is that a group of living animals returned to laying eggs at some point in their evolution. Alternatively, hybridization between egg layers and live births could have allowed these differences to spread within the egg-laying population, eventually re-enabling live births when enough of them were present in individual animals, producing a separate live birth line .

The 88 regions identified as live births contained very little genetic diversity, suggesting that a specific genetic variant in each region is so beneficial that it has swept through the population, crowding out all other versions of the stretch of DNA. However, they picked up some distinct differences that are rare outside of egg-laying populations, enough to allow researchers to estimate the age at which these parts of DNA underwent evolutionary selection.

The answer varies depending on which of the 88 fragments you look at, but ranges from about 10,000 to 100,000 years ago. This range suggests that the genetic regions that enable live births have been gradually assembled over many years – just as the traditional view of evolution suggests.

The researchers acknowledge that at least some of these areas likely evolved after live births had already become the norm, simply improving the efficiency of internal incubation. There is no way to know how many (or which) variables must be present before live births are possible. However, researchers now have an extensive list of genes to look at to understand things better.

Science, 2024. DOI: 10.1126/science.adi2982 (About digital IDs).