The version of evolution proposed by Charles Darwin focused on slow, gradual changes that only gradually built up the kinds of differences that separated species. However, this does not exclude the possibility of sudden and dramatic changes. In fact, some differences suggest that it may be difficult to understand what a transitional state is and may require a major leap.
The new study focuses on one major transition: from egg-laying to live birth in a range of closely related snail species. By sequencing the genomes of multiple snails, researchers identified changes in DNA associated with egg-laying. Despite its dramatic nature, a large number of genes were found to be associated with this change.
give up eggs
The snail in question is here a genus called retorina, mainly distributed around the North Atlantic Ocean. Many of these species lay eggs, but many transition to live birth. In these species, an organ that in other species coats the eggs with a protein-rich jelly acts as an incubator, allowing the eggs to develop until the young snails can crawl out of the parent’s shell. This is considered an advantage for animals that must lay eggs in environments that are unfavorable for survival.
The egg-laying species is so similar to its relatives that it was sometimes thought that they were simply variants of the egg-laying species. All of this suggests that childbirth evolved relatively recently, and gives us a good opportunity to understand the genetic changes that made it possible.
So a large international research team analyzed the genomes of more than 100 snails during spawning and birth. The resulting data was used to analyze things such as how closely different species are related and what genetic changes are associated with childbirth.
This result suggests that there are two distinct clusters of species that reproduce through live birth. In other words, there is a branch full of egg-laying species on the evolutionary tree of these snail species that separates the two groups that produce living snails. This structure is usually taken to indicate that life evolved twice, once in each of its two clusters.
But that doesn’t seem to be the case, as I’ll explain why later.
Rich variety
Separately, the researchers looked for regions of the genome associated with childbirth. And they found a lot of them – 88 in total. These 88 regions were identified in both clusters of spawned species, and their DNA sequences were very similar. This suggests that these regions have a single origin and are maintained in both of these lineages.
One possibility to explain this is that populations of animals born alive reverted to egg-laying at some point in their evolution. Alternatively, crossbreeding of spawning animals with live-born offspring can spread these mutations within the spawning population, eventually allowing live-birth again when there is enough variation in individual animals to create separate live-born offspring. It is possible that an offspring lineage was produced.
There is little genetic variation in the 88 regions identified as underlying birth causes, and certain genetic variations in each region are so advantageous that they spread throughout the population and overwhelm all other DNA versions. It has been suggested that it was replaced by But they found some distinct mutations that are rare outside egg-laying populations. This is enough for researchers to estimate the age at which these pieces of DNA underwent evolutionary selection.
The answer depends on which of the 88 segments you’re looking at, but it ranges from about 10,000 years ago to 100,000 years ago. This range suggests that the genetic regions that enable childbearing have been brought together gradually over many years, just as the traditional view of evolution suggests.
The researchers acknowledge that at least some of these regions likely evolved after live birth was already the norm, and simply made internal incubation more efficient. And there is no way to know how many (or which) mutants need to be present before birth is possible. But researchers now have an extensive list of genes to investigate to understand things better.
Science, 2024. DOI: 10.1126/science.adi2982 (About DOI).