A big evolutionary change linked to many small differences

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By journalsofus.com

Image of a dark gray-black snail shell.
Enlarge / An example of a Littorina species, the common periwinkle.

The version of evolution proposed by Charles Darwin focused on slow, incremental changes that only gradually develop into the kind of differences that separate species. But that doesn’t rule out the possibility of sudden and dramatic changes. In fact, some differences make it difficult to understand what a transition state would look like, suggesting that a major leap may be necessary.

A new study looks at an important transition: the shift from egg-laying to live birth in a set of related snail species. By sequencing the genomes of multiple snails, the researchers identified DNA changes 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 in a genre called litorine, which are largely distributed throughout the North Atlantic. Many of these species lay eggs, but some of them have been born alive. In these species, an organ that coats the eggs with a protein-rich gelatin in other species acts as an incubator, allowing the eggs to develop until young snails can hatch from their parents’ shells. This is thought to be an advantage for animals that would otherwise have to lay eggs in environments that are not favorable for their survival.

Egg-laying species are so similar to their relatives that they were sometimes thought to be simply a variant of an egg-laying species. All of which suggests that live births have evolved relatively recently, giving us a good opportunity to understand the genetic changes that enabled it.

Thus, a large international team of researchers sequenced the genomes of more than 100 individual snails, both those that lay eggs and those that are born alive. The resulting data was used to analyze things like how closely related different species are and what genetic changes are associated with live births.

The results suggest that there are two separate groups of species that reproduce through live births. Put another way, in an evolutionary tree of these snail species, there is a branch full of egg-laying species that separates two groups that give rise to live snails. Typically, this structure is considered an indication that the live births evolved twice, once for each of the two groups.

But that doesn’t seem to be the case here, for reasons we’ll see later.

Many variations

Furthermore, the researchers looked for regions of the genome associated with live birth. And they found many of them: 88 in total. These 88 regions were identified in both groups of live-born species, and the DNA sequences within them were very similar. This suggests that these regions had a single origin and were maintained in both lineages.

One possibility to explain this is that a population of animals born alive again laid eggs at some point in their evolution. Alternatively, hybridization between egg layers and live births could have allowed these variations to spread within an egg-laying population and ultimately re-allowed live births when enough of them were present in individual animals, producing a separate lineage of live births.

The 88 regions identified as underlying the live births have very little genetic diversity, suggesting that a specific genetic variant in each region is so advantageous that it spread through the population, displacing all other versions of the stretch of DNA. However, they have detected some distinct variations that are rare outside egg-laying populations, enough to allow researchers to estimate the age at which these DNA fragments became evolutionarily selected.

The answer varies depending on which of the 88 segments you are looking at, but ranges from 10,000 to 100,000 years ago. That range suggests that the genetic regions that allow live births came together gradually over many years, exactly as the traditional view of evolution suggests.

The researchers acknowledge that it is likely that at least some of these regions evolved after live births were already the norm and simply improved the efficiency of internal incubation. And there is no way to know how many (or which) variants must be present before live births are possible. However, researchers now have an extensive list of genes to analyze to understand things better.

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

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