Creationism in Crisis - How A Marine Snail Transitioned From Egg-Laying To Live Births.

Adult snails adapted to different habitats.
The larger snail is adapted for defense against crab attacks, while the small snail is adapted to live in areas with strong wave exposure.

© Sophie Webster

ISTA | The Snail or the Egg?

Live-bearing has allowed Littorina snails to occupy and adapt to a diverse range of habitats. This has led to the evolution of many ‘ecotypes’ that vary in size, shape, and behavior.

© Fredrik Pleijel

When did eggs evolve? Eggs, in the context of reproduction, have been around for a very long time in the evolutionary history of life. The evolution of eggs is a complex process that occurred over millions of years, and different types of eggs have evolved in different lineages of organisms.

The earliest evidence of eggs comes from fossils of small, shelled eggs that date back to the Early Cambrian period, around 540 million years ago. These eggs are believed to belong to a group of extinct marine animals. However, it's important to note that these early eggs were likely quite different from the eggs we are familiar with today.

The evolution of eggs in the context of vertebrates, such as birds, reptiles, and mammals, is a more recent development. Amniotic eggs, which have a protective shell and membranes that allow them to be laid on land rather than in water, evolved in the lineage leading to reptiles. This adaptation was crucial for the colonization of terrestrial environments.

Birds, a group of reptiles, are known for their hard-shelled eggs, and their evolutionary history is marked by the development of various features related to flight and parental care. Mammals, on the other hand, have live births or lay eggs with soft shells (monotremes like the platypus and echidna).

In summary, the evolution of eggs is a complex process that spans millions of years and involves various lineages of organisms, with different types of eggs evolving at different times in the history of life.

An example of the scientific ignorance that underpins creationism is in the boringly repetitive "Gotcha!" question from creationist fools, "Which came first; the chicken of the egg?", as though they believe chickens had ancestors that never laid eggs and the chicken species started with a single individual, which either hatched from an egg or was given birth to.

Of course, creationists believe chickens were created out of dirt by magic without any ancestors, so in their childlike, magical view of things their question makes some sort of sense.

In reality of course, ancestors of chickens laid eggs and egg-laying originated hundreds of millions of years ago at about the time multicellular organisms evolved.

All placental mammals have had live births ever since this evolved in Mammalia about 140 million years ago, but live birth has evolved independently in other classes multiple times. It is frequently found in the Reptilia, for example, and at least one insect, the tsetse fly.

A marine snail, Littorina saxatilis, is known to have transitioned from egg-laying to live young-bearing in the last 10,000 years - a mere blink in the evolutionary time scale. An international team of researchers, led by Dr. Sean Stankowski of the Austrian Institute of Science & Technology (ISTA) have now worked out the 50 genetic changes involved in this , which probably began about 100,000 years ago. Their research is explained in an ISTA news release:

The Snail or the Egg?

How marine snails switched from laying eggs to giving birth

Animals reproduce in one of two distinct ways: egg-laying or live birth. By studying an evolutionarily recent transition from egg-laying to live-bearing in a marine snail, collaborative research by the Institute of Science and Technology Austria (ISTA), the University of Sheffield, and the University of Gothenburg has shed new light on the genetic changes that allow organisms to make the switch. The results were published in Science.

The egg did come first. Egg-laying arose deep in evolutionary time, long before animals even made their way onto land. Throughout evolution, there have been many independent transitions to live-bearing across the animal kingdom, including insects, fish, reptiles, and mammals. Yet, these examples have taught us very little about the number of genetic changes it takes to go from eggs to live offspring.

Now, an international team of researchers led by ISTA postdoc Sean Stankowski has used a humble marine snail to reveal the genetic changes that underpin the transition to live-bearing. The main advantage of investigating this phenomenon in marine snails: Live-bearing evolved within the past 10,000 years in these organisms—a blink of an eye in evolutionary terms. Thus, these marine snails could provide a unique chance to reveal the genetic basis of live-bearing. “Almost all mammals give live birth, and this function has accompanied their evolution for around 140 million years. Yet, in this study, we can investigate how live-bearing evolved completely independently, and much more recently, in marine snails,” says Stankowski. The team’s central finding: the switch to live-bearing is caused by around 50 genetic changes that are scattered around the snail genome.

One species, over a hundred names

The seaside marine snail Littorina saxatilis is the most misidentified creature in the world, The Guardian reported in 2015. Over centuries, scientists have described it as a new species or subspecies more than a hundred times, despite it being commonly found throughout the North Atlantic shores. This whole confusion must have stemmed from this species’ many shell variations and habitats. On top of this, L. saxatilis has a unique reproductive mode: it has evolved live-bearing while related marine snails that share its habitat lay eggs. “Scientists have mainly studied shell variation within L. saxatilis rather than what differentiates the species from its egg-laying relatives. The reality is that this snail species is the odd one out when it comes to its breeding strategy,” says Stankowski.

Losing the egg, one step at a time

An eye-opening moment was when Stankowski inferred the phylogenetic tree, or evolutionary “family tree”, of L. saxatilis and other related, egg-laying Littorina species, using whole-genome sequences. He showed that, although live-bearing is the only trait that distinguishes L. saxatilis from its egg-laying relatives, L. saxatilis did not seem to form a single evolutionary group. It was this mismatch between reproductive strategy and ancestry that ultimately allowed Stankowski and his collaborators to disentangle the genetic basis of live-bearing from other genetic changes throughout the snail genome. “We were able to identify 50 genomic regions that together seem to determine whether individuals lay eggs or give birth to live young,” says Stankowski. “We don’t know exactly what each region does, but we were able to link many of them to reproductive differences by comparing patterns of gene expression in egg-laying and live-bearing snails.” Overall, the results suggest that live-bearing evolved gradually through the accumulation of many mutations that arose over the last 100,000 years.

The costs and benefits of live-bearing

The research shows that the switch to live-bearing allowed snails to spread into new areas and habitats where egg-layers cannot survive and reproduce. But the precise benefits of live-bearing in these snails remain a mystery. “We don’t know for sure, but the transition from egg-laying to live-bearing may have arisen by natural selection favoring increased egg retention time, with the eggs eventually hatching inside the mother. We speculate that eggs might have been more susceptible to drying out, physical damage, and predators,” says Stankowski. In live-bearers, offspring are protected from the elements until they can fend for themselves, he adds. But by solving one problem, live-bearing would have certainly created others. “The extra investment in offspring would have almost certainly placed new demands on the snails’ anatomy, physiology, and immune system. It’s likely that many of the genomic regions we identified are involved in responding to these types of challenges”.

Marine snails on the seashore. Littorina snails are common on the rocky shores of Europe, the UK, and the East Coast of the USA.

© Daria Shipilina

Mapping the function of each gene

Although the work sheds new light on the transition from eggs to live offspring, many questions remain to be answered. “Most genetic innovations are in fact really old and tangled up on an evolutionary scale which makes it difficult to study their origin,” says Stankowski. “These snails have allowed us to do exactly that, but we have only begun to scratch the surface of what they can teach us about the origins of novelty.” As a next step, the researchers want to map the function of each mutation. “We aim to understand how each genetic change shaped the snails’ form and function on the way to live-bearing, step by step,” concludes Stankowski.

The team give more technical details in the abstract to their paper in Science

Editor’s summary

Traits are constantly changing in populations, but the emergence of a trait that provides a new function is a rarer occurrence. Two studies now examine how such traits arise. Stankowski et al. identified the multiple genome regions associated with the transition from egg-laying to live-bearing reproduction in a clade of snails, demonstrating a polygenic basis of live-bearing reproduction with alleles that accumulated in response to selection over time. Chomicki et al. found that the spontaneous occurrence of three variable traits created a novel mechanism for trapping insects in pitcher plants, which arose separately in two different species. Together, these studies provide insight into the emergence of complex, transformative traits (see the Perspective by Elmer). —BEL and CNS


Abstract

Key innovations are fundamental to biological diversification, but their genetic basis is poorly understood. A recent transition from egg-laying to live-bearing in marine snails (Littorina spp.) provides the opportunity to study the genetic architecture of an innovation that has evolved repeatedly across animals. Individuals do not cluster by reproductive mode in a genome-wide phylogeny, but local genealogical analysis revealed numerous small genomic regions where all live-bearers carry the same core haplotype. Candidate regions show evidence for live-bearer–specific positive selection and are enriched for genes that are differentially expressed between egg-laying and live-bearing reproductive systems. Ages of selective sweeps suggest that live-bearer–specific alleles accumulated over more than 200,000 generations. Our results suggest that new functions evolve through the recruitment of many alleles rather than in a single evolutionary step.

Sean Stankowski et al.
The genetic basis of a recent transition to live-bearing in marine snails.
Science 383, 114-119(2024). DOI:10.1126/science.adi2982

Copyright © 2024 The Authors
Published by American Association for the Advancement of Science.
Reprinted with kind permission under licence #5702991425725.

A number of things for creationists to ignore here:

1. The dependence of the scientists on the Theory of Evolution by Natural Selection to explain the facts, with no hint that it might not be fit for purpose.
2. The genetic evidence of transition from egg-laying, through prolonged egg retention to live-birth, each stage of which would have given an advantage in the snail's environment.
3. The process lasted about 100,000 years, i.e., ten times longer than Earth has existed, according to creationist superstition.
4. Does a different method of reproduction constitute 'macro'-evolution, or is this just ordinary evolution?
5. The stupidity and scientific ignorance of the 'chicken and egg' question.

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Thank you for sharing!

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