Live birth is not a simple characteristic that could be produced by switching a single gene on or off. Retaining embryos inside the mother until they are sufficiently developed to survive requires coordinated changes in gestation length, uterine physiology, eggshell formation, gas exchange, nutrient transfer, hormonal control and the maternal immune response. To creationists, this is just the sort of complex biological system that must supposedly have appeared complete and fully functional in a single act of creation.
But that is not what the genomes of living animals show.
In a study recently published in Current Biology, an international team led by Dr Hongxin Xie of the University of Glasgow has reconstructed how live birth evolved in the Eurasian common lizard, Zootoca vivipara. The transition did not depend upon one miraculous genetic leap, or even upon one or two mutations of exceptionally large effect. It arose through natural selection acting upon numerous genetic changes distributed across the genome and accumulated over an extended period.
The common lizard offers an unusually revealing natural experiment because it includes both egg-laying and live-bearing lineages that diverged relatively recently. By comparing whole-genome sequences from lizards sampled across the species’ Eurasian range, the researchers could identify regions that resisted being homogenised by continuing gene flow between the reproductive forms. Many of these regions also carried evidence of selection associated with viviparity.
Especially significant was the finding that much of the difference lay not in protein-coding genes themselves, but in regulatory DNA controlling when, where and how strongly genes are expressed. Genes associated with these selected regions were more active in the uterus of pregnant live-bearing females. Evolution had therefore not needed to invent an entirely new collection of proteins from nothing. It had modified the genetic control system governing existing genes, gradually recruiting and coordinating biological processes already present in the ancestral egg-laying animal.
This is precisely how evolutionary theory explains the origin of apparent biological novelty. Small inherited changes need not individually produce a complete new reproductive system. Each may alter one component—perhaps the timing of eggshell deposition, the duration of egg retention or the activity of uterine tissue—and natural selection can preserve changes that improve reproductive success. Further changes can then accumulate upon that foundation until the eventual result differs substantially from the ancestral condition.
Continued gene flow makes the result still more instructive. Egg-laying and live-bearing lineages had not remained completely isolated: they continued to exchange genes. Nevertheless, selection preserved the combinations of regulatory variants that contributed to successful live birth. The relevant parts of the genome consequently became partial barriers to gene flow, while other regions could still pass between the lineages. This is evolution operating upon populations and genomic regions, not species transforming instantaneously in the caricature creationists are taught to attack.
The study also complements the recently reported evidence from the 236-million-year-old cynodont Chiniquodon theotonicus. That fossil study tentatively suggested that live birth might have originated surprisingly early in the mammalian stem lineage, but its conclusion was necessarily inferred from bone histology and estimated birth-to-adult body-mass ratios. The common-lizard research asks a different question. Rather than trying to establish how ancient viviparity might be, it uses living descendants to reveal a plausible genetic route by which such a complicated reproductive strategy can evolve.
Together, the two studies illuminate different parts of the same evolutionary story: fossils can provide evidence of when a major innovation may already have existed, while comparative genomics can reveal how natural selection assembled one. Neither requires a sudden creation event, an infusion of supernatural “information”, or a fully formed reproductive system appearing in a single generation. What the evidence reveals instead is the cumulative power of mutation, gene regulation and natural selection operating over time.
How Live Birth Can Evolve Gradually. Viviparity, or live birth, is sometimes presented as an all-or-nothing characteristic: an animal must either lay an egg or possess a complete system for retaining and nourishing embryos inside its body. In reality, living reptiles exhibit a continuum between laying eggs containing comparatively undeveloped embryos and giving birth to fully developed young.The paper in Current Biology was accompanied by a University of Glasgow news release:
A transition from egg laying to live birth can therefore proceed through a series of functional intermediate stages:
- Internal fertilisation already exists. Egg-laying lizards fertilise their eggs internally, so the embryos already begin their development inside the female reproductive tract. Evolution does not have to create this essential first step.
- Eggs are retained for longer. Females that keep their eggs inside for slightly longer lay embryos at a more advanced stage. Retention can protect embryos from predators, desiccation and fluctuating external temperatures, while allowing the mother to regulate their temperature by moving between warmer and cooler locations.
- The eggshell becomes thinner. As more development takes place inside the mother, the embryo needs less protection from the external environment. A thinner shell also permits more efficient exchange of oxygen, carbon dioxide and water between maternal and embryonic tissues.
- Uterine support increases. Changes in blood supply, tissue structure, secretions and the transport of water and dissolved substances can improve conditions for embryos retained inside the reproductive tract. In some lineages, maternal tissues may eventually supplement the nutrients initially supplied in the egg yolk.
- Hormonal and immune systems are adjusted. Regulatory changes can extend gestation, coordinate uterine activity and prevent maternal immune responses from damaging the developing embryos.
- Young are born at an advanced stage. Once eggs are retained for almost the whole of embryonic development, further reduction of the shell leaves the young enclosed only by thin membranes that rupture during or soon after birth. The distinction between laying an egg and giving birth has then become extremely small.
This is not necessarily a rigid sequence followed identically by every lineage. Different components can evolve in different orders, and living reptiles display several intermediate reproductive arrangements. The important point is that every stage can be functional and potentially advantageous; there is no point at which a complete system must appear in a single generation.
The role of regulatory DNA
Much of this transition need not require the invention of new protein-coding genes. Egg-laying ancestors already possess genes involved in hormone signalling, eggshell production, blood-vessel growth, immune regulation, tissue remodelling and embryonic development. What must change is how those genes are used during pregnancy.
Regulatory DNA provides those instructions. It includes sequences such as promoters and enhancers that help determine:
- when a gene is switched on or off;
- where in the body it becomes active;
- how strongly it is expressed; and
- how long its activity continues.
A mutation in regulatory DNA might therefore cause an eggshell gene to become less active, a blood-vessel gene to become more active in uterine tissue or a hormone-controlled process to continue for longer. Each change can be small, heritable and subject to natural selection.
Accumulated across many generations and many regions of the genome, such regulatory changes can gradually reorganise existing biological machinery into a substantially different reproductive system. Complexity is produced cumulatively—not by a miraculous leap, but by descent with modification.
Scientists uncover the evolutionary process behind live birth in lizards
Scientists studying one of nature’s greatest reproductive innovations have discovered how some lizards evolved from laying eggs to giving birth to live young.
Scientists studying one of nature’s greatest reproductive innovations have discovered how some lizards evolved from laying eggs to giving birth to live young.
Montage of European common lizard, Zootoca viviparaImage credit:Meredith Kimble.
The study, led by researchers at the University of Glasgow and published in Current Biology, reconstructs how live birth evolved in the Eurasian common lizard. Researchers found that the reproductive change occurred, not through one large evolutionary leap as some scientists have previously assumed, but through the gradual accumulation of many small genetic changes over time.
The findings have implications for our wider understanding of reproductive evolution, and reveal never before seen information on how small, incremental changes at a genetic level can lead to big biological transformations.
The transition from laying eggs to giving birth to live young is one of the most significant innovations in animal evolution, and has happened multiple times in most vertebrate lineages, including in fish, amphibians, reptiles and mammals. However, because most transitions to live birth occurred tens or even hundreds of millions of years ago (as in mammals), until now scientists have struggled to understand the evolutionary process that first gave rise to this complex reproductive strategy.
Although lizards and humans reproduce in different ways, the structures involved in carrying and nourishing developing young have shared evolutionary origins. The egg sac around a baby in a live-bearing lizard is evolutionarily comparable to the membranes and placenta that surround and support a developing human baby during pregnancy.
The common lizard provides a rare opportunity to study this process because it contains both egg-laying and live-bearing lineages that diverged relatively recently in evolutionary time.
In the new study, the researchers analysed whole genome sequences from lizards sampled from across the species' range. They then used these data to reconstruct how those genetic differences accumulated during evolution and gave rise to live birth.
The analyses showed that rather than arising through one or two major genetic changes, the transition resulted from natural selection acting on many regions across the genome over a long evolutionary timescale. The changes occurred primarily in regulatory DNA that controls when and where genes are switched on during pregnancy. Consistent with this regulatory mechanism, genes in these regions were more highly expressed in the uterus of pregnant live-bearing lizards, suggesting that changes in gene regulation played a central role in the evolution of live birth.
The findings provide one of the clearest reconstructions yet of how a complex evolutionary innovation emerges.
Live birth is one of the most remarkable evolutionary innovations among vertebrates, but because it evolved so long ago in most animals, we've had very little opportunity to understand how it actually arose. The common lizard provides a unique window into this transition. By reconstructing its evolutionary history using whole genomes, we were able to show how a complex trait can emerge through many small genetic changes that accumulated over time.
Perhaps most excitingly, we found that much of this evolution involved changes in gene regulation rather than changes to the genes themselves. Regulatory DNA acts like an instruction manual, determining when and where genes are switched on during pregnancy, providing a flexible way for evolution to build an entirely new reproductive system.
Dr Hongxin Xie, lead author.
School of Biodiversity, One Health, and Veterinary Medicine
College of Medical, Veterinary & Life Sciences
University of Glasgow
Glasgow, UK.
Despite continued gene flow between egg-laying and live-bearing lizard lineages during the mating process, the research team also found that any reproductive adaptations were maintained in the lizards who had them, helping to maintain the reproductive change became established in some species.This is one of the few systems in the world where we can still observe both the ancestral egg-laying form and the derived live-bearing form within the same species. That makes it possible to reconstruct an evolutionary transition that is usually hidden deep in the past.
Dr Hans Recknagel, co-author of the study
School of Biodiversity, One Health, and Veterinary Medicine
College of Medical, Veterinary & Life Sciences
University of Glasgow
Glasgow, UK.
One of the longstanding questions in evolutionary biology is how complex adaptations arise. Do they evolve through a few major genetic changes, or through the accumulation of many small ones? Our study provides a remarkable example of the latter. We show that one of the most complex reproductive innovations in vertebrates evolved through many small regulatory changes distributed across the genome, gradually assembling a completely new reproductive strategy. These findings not only explain how live birth evolved in lizards, but also provide a broader framework for understanding how complex evolutionary innovations arise across the tree of life.
Professor Kathryn Elmer, senior author of the study.
School of Biodiversity, One Health, and Veterinary Medicine
College of Medical, Veterinary & Life Sciences
University of Glasgow
Glasgow, UK.
The research was carried out by scientists from the University of Glasgow and international collaborators from China and France.
Publication:
The importance of this research lies not merely in showing that live birth evolved, but in revealing the genomic pattern left by that evolution. Selection acted upon numerous regions of the common-lizard genome over an extended period, particularly upon regulatory sequences controlling uterine gene activity. The result was not the sudden appearance of an entirely new reproductive apparatus, but the progressive modification and coordination of biological processes already present in an egg-laying ancestor.
Continuing gene flow between egg-laying and live-bearing lineages makes the evidence especially instructive. Although much of their DNA could pass between the reproductive forms, variants important to their different reproductive strategies were preserved by selection. The genome consequently records both their continuing relationship and the evolutionary divergence maintained between them—exactly the sort of complex, untidy history expected when populations evolve, interbreed and adapt.
The findings also expose the familiar creationist caricature in which evolution supposedly requires one animal suddenly to produce offspring with a fully formed new reproductive system. Evolutionary biology proposes no such event. Longer egg retention, thinner shells, altered uterine activity and changes in hormonal or immune regulation can each arise incrementally and function before every feature associated with full viviparity is present. Natural selection can preserve each useful modification, allowing later changes to build upon what already works.
Nor can this evidence be dismissed with the claim that mutations cannot produce “new genetic information”. Changes in regulatory DNA created heritable differences in when, where and how strongly genes were expressed. Those differences altered reproductive biology, were tested by natural selection and persisted despite genetic exchange with egg-laying populations. Calling that anything other than the generation and accumulation of biologically meaningful genetic information merely redefines “information” to exclude any example that contradicts creationist dogma.
Together with the tentative fossil evidence for ancient viviparity in the cynodont Chiniquodon, the study supplies two complementary views of reproductive evolution. One looks deep into the fossil record for evidence of when live birth may have existed; the other reads living genomes to reconstruct how such an innovation could arise. Neither reveals a discontinuity requiring supernatural intervention. Instead, both fit the same coherent scientific picture: complex reproductive systems have histories, and those histories were written by inheritance, variation and natural selection—not by magic.
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