Friday, 4 September 2015

How Rapid Evolution Works

Guppies. Ideal subject to study evolution.
Change in environment can lead to rapid evolution -- ScienceDaily

Many things in science are counter-intuitive, which is the basic reason we use control groups and other methods designed to remove bias, and also why we don't simply do science by thinking about things in the belief that we can work out the answers by intuition.

Human intuition is not the powerful tool that people who dispute science on the basis of personal incredulity, often backed by profound ignorance of the subject, like to imagine.

Take, for instance, this nice little example, admittedly only one study which may not be generally representative, which shows that genetic response to radical environmental change can be the opposite of what is intuitively expected at least initially.

Kimberly Hughes, Professor of Biological Science at Florida State University, and colleagues took wild guppies from a stream which had predatory cichlids and placed them in predator-free streams. Guppies are small fish which reproduce rapidly, producing about three generations a year, so any genetic responses to environmental change should be detectable within a few years.

What they found was perhaps surprising.

Genes can respond to environmental change in one of two ways:
  1. By changing their activity level - termed gene plasticity.
  2. By evolutionary change where variants conveying an advantage over and above other variants in the new environment, increase in the population. (Normal Darwinian evolution, genetic drift, etc.)

The team found that gene plasticity, as measured by changes in 135 genes sequenced from brain tissue, changed in ways which ran counter to what would intuitively be expected. The changes were in genes concerned with internal functions such as metabolism, immune responses and development.

However, these early changes were then reversed later by evolutionary changes, suggesting the plastic changes in the 'wrong' direction had caused the genes to evolve more rapidly than they otherwise would. This runs counter to the intuitive expectation that plastic and evolutionary change should both be in the same direction.

Phenotypic plasticity is the capacity for an individual genotype to produce different phenotypes in response to environmental variation. Most traits are plastic, but the degree to which plasticity is adaptive or non-adaptive depends on whether environmentally induced phenotypes are closer or further away from the local optimum. Existing theories make conflicting predictions about whether plasticity constrains or facilitates adaptive evolution. Debate persists because few empirical studies have tested the relationship between initial plasticity and subsequent adaptive evolution in natural populations. Here we show that the direction of plasticity in gene expression is generally opposite to the direction of adaptive evolution. We experimentally transplanted Trinidadian guppies (Poecilia reticulata) adapted to living with cichlid predators to cichlid-free streams, and tested for evolutionary divergence in brain gene expression patterns after three to four generations. We find 135 transcripts that evolved parallel changes in expression within the replicated introduction populations. These changes are in the same direction exhibited in a native cichlid-free population, suggesting rapid adaptive evolution. We find 89% of these transcripts exhibited non-adaptive plastic changes in expression when the source population was reared in the absence of predators, as they are in the opposite direction to the evolved changes. By contrast, the remaining transcripts exhibiting adaptive plasticity show reduced population divergence. Furthermore, the most plastic transcripts in the source population evolved reduced plasticity in the introduction populations, suggesting strong selection against non-adaptive plasticity. These results support models predicting that adaptive plasticity constrains evolution6, 7, 8, whereas non-adaptive plasticity potentiates evolution by increasing the strength of directional selection The role of non-adaptive plasticity in evolution has received relatively little attention; however, our results suggest that it may be an important mechanism that predicts evolutionary responses to new environments.*

Cameron K. Ghalambor, Kim L. Hoke, Emily W. Ruell, Eva K. Fischer, David N. Reznick, Kimberly A. Hughes.
Non-adaptive plasticity potentiates rapid adaptive evolution of gene expression in nature.
Nature, 2015; DOI: 10.1038/nature15256

*Copyright © 2015, Rights Managed by Nature Publishing Group. Reproduced under licence #3700911233397

Of course, this doesn't mean that there is some sort of pre-selection of genes to be evolved later, and some preparatory planning stage going on here - so creationists and ID proponents should calm down and not get over-excited. What it means is that drift in gene activity, if that drift runs counter to the environmental change, results in those randomly selected genes evolving more quickly. It can all be explained by science with no resort to magic.

But it does have implications for other situations where environmental change causes evolutionary change, such as the response of cancers to chemotherapy, the response of food plants to climate change and the response of pathogens to antibiotics, etc.

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