Biodiversity does hit a ceiling + Genetic ‘memory’ of ancestral environments

The evolution of biodiversity: ever-increasing or did it hit a ceiling?

EXCERPT: Preserving biodiversity is one of the key debates of our time [...] Increasing knowledge on historical patterns of biodiversity could ... help us understand animals’ ability to adapt ... The traditional view is that species have increased in diversity continuously over the past 200 million years, particularly in the last 100 million, leading to more diversity now than ever before. But some recent studies suggest biodiversity has tended to stay largely the same, with only occasional surges.

‘Our findings strongly contradict past studies that suggested unbounded diversity increases at local and regional scales over the last 100 million years,’ said a fresh study on terrestrial species. It found no evidence of a rise in diversity in the past 66 million years, following a brief two- to three-fold increase over a couple of million years after the mass extinction of the dinosaurs at the end of the Cretaceous Period and as mammals began to thrive.

The story was similar for a study on marine species, with the researchers finding little change in ocean biodiversity over the past 200 million years. ‘We find that ecosystems are relatively stable in terms of how much diversity they have over tens to hundreds of millions of years,’ said Professor Richard Butler, a palaeobiologist [...] who worked on both studies as part of the TERRA project. ‘Rather than seeing exponential increases, it’s more like a short-term quite dramatic increase and then a relatively flat line.’

The findings imply that diversity is capped at a certain limit and that species numbers are ultimately limited by factors such as the availability of resources like water and space, says Prof. Butler. Various researchers have been moving towards this view for some time, he says, but new approaches like those adopted in the TERRA project have helped bolster the evidence... (MORE - details)

Genetic ‘memory’ of ancestral environments helps organisms readapt

EXCERPT: Organisms carry long-term “memories” of their ancestral homelands that help them adapt to environmental change, according to a new study that [...] provides new insights into how creatures adapt to changing environments, a topic that’s especially relevant today in the context of rapid climate change, which is creating challenges for plants and animals worldwide.

[...] In a set of experiments by University of Michigan biologists and their Chinese colleagues, researchers hatched and reared hundreds of chickens on the Tibetan Plateau, at an elevation of nearly 11,000 feet, and at an adjacent lowland site in China’s Sichuan Province. Some of the eggs from lowland chickens were hatched on the plateau, and some high-altitude eggs were hatched at a site 2,200 feet above sea level.

The goal was to assess the relative contributions of two types of phenotypic change—meaning changes to an organism’s observable physical characteristics or traits—to the process of environmental adaptation. “Plastic” phenotypic changes involve altered gene activity but no rewriting of the genetic code in DNA molecules, while mutations cause altered gene activity by modifying the sequence of letters in the code itself.

Evolutionary biologists have debated the relative roles of plastic and mutation-induced changes in adaptation, and whether the former serve as stepping stones to the latter. In the chicken study, researchers were specifically interested in how organisms readapt when reintroduced to ancestral environments. They found that plastic changes play a more prominent role when organisms return to an ancestral home than when they adapt to new environments.

[...] The findings were published May 22 in the journal Science Advances. To study the relative roles of plastic and DNA-sequence changes, the researchers looked at gene-expression differences between lowland and Tibetan chickens in five tissue types: brain, liver, lung, heart and muscle. To do that, they analyzed RNA transcriptomes from cells in those tissues.

The genome is made of DNA that contains the instructions needed to build an organism. For those instructions to be carried out, DNA must be read and transcribed into messenger RNA molecules. By analyzing the entire collection of RNA sequences in a cell, known as the transcriptome, researchers can determine when and where genes are turned on and off. Gene-expression studies provide snapshots of actively expressed genes under various conditions.

Changes in gene activity alter an organism’s phenotype, which includes its morphology, behavior and physiology. The term phenotypic plasticity refers to environmentally induced phenotypic changes that do not involve genetic mutations. In the chicken study, the researchers found that while many mutation-induced phenotypic changes were necessary when the animals first adapted to the Tibetan Plateau, plastic changes largely transformed the transcriptomes to the preferred state when Tibetan chickens were brought back to the lowland.

A similar result was seen with egg “hatchability,” the fraction of fertilized chicken eggs that hatched in the study. When lowland eggs were incubated on the unfamiliar Tibetan Plateau, hatchability was significantly lower than that of Tibetan chicken eggs. But when Tibetan eggs were incubated in the lowland—an environment familiar from the distant past—there was no significant difference in hatchability between the two groups. The egg result suggests that adaptive mutational changes are needed when an organism is brought to an unfamiliar environment for the first time, while plastic changes will do the trick when those same creatures return to an ancestral home.

Jianzhi Zhang’s team also analyzed transcriptomes from previous studies of guppies and E. coli bacteria and found comparable results... (MORE)

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