Monday, 10 August 2015

Revising Evolution Theory - The Scientific Way

Liolaemus nigriceps
Re-thinking 'adaptive radiation,' one of biology's most important concepts | ScienceDaily

When you see a headline like "Re-thinking 'adaptive radiation,' one of biology's most important concepts" it's bound to raise the eyebrows of the scientifically-minded. What's this? One of the fundamentals of evolution theory could be wrong? Shock horror!

It only takes a minor change of emphasis and the removal of any doubt by merely changing 'could be' to 'is' and you have the makings of another creationist article by Ken Ham or Ray Comfort and another merry tinkle of cash pouring in to help 'spread the good news' that evolution has been overthrown once more and 'evolutionists' all exposed as liars.

But what do we find when we look behind the headlines? What we find is an article about a science paper which merely suggests a slight revision of a poorly-defined term - 'adaptive radiation'. The idea of adaptive radiation is indeed fundamental, and remains unassailed by this paper. What is suggested is that the assumption that adaptive radiation happens early and relatively rapidly when a species comes up against a whole range of new evolutionary opportunities in the form of lots of vacant evolutionary niches waiting to be occupied. A rapid burst of adaptive evolution then occurs in which several new species evolve to fit the available niches. When the niches are full, this radiation ceases, or at least slows down considerably.

This is the kind of thing we see in the fossil record such as in the Burgess shales when multicellularity led to the so-called 'Cambrian Explosion'. We also see it in the evolution of flowering plants, in the evolution of hymenopterans (wasps, bees and ants), the evolution of birds, etc.

So what's the quibble? What do we need to re-think as a result of this paper?

Phylogenetic relationships within the Liolaemus radiation showing variation in body size (snout-vent length obtained by averaging male and female SVLs) across species (black bars, in mm). Clade colours indicate the eight main groups (or subgenera) within the genus *
A team from the University of Lincoln, UK, examined the evolution of the Liolaemus lizards, one of the most diverse and species-rich vertebrate lines. They showed that rather than a single, early radiation, as the Andes Mountains uplifted, creating a series of new niches, there were several equivalent waves of adaptive radiation leading to the many different related species of lizard. What the team propose is that the definition of 'adaptive radiation' be revised to drop any reference to an early burst but should include the situation where there are several pulses.

They chose these lizards because they have adapted to a wide range of environments from high to low altitude and to extremes of temperature and humidity and comprise some 240 different species


Adaptive radiation theory posits that ecological opportunity promotes rapid proliferation of phylogenetic and ecological diversity. Given that adaptive radiation proceeds via occupation of available niche space in newly accessed ecological zones, theory predicts that: (i) evolutionary diversification follows an ‘early-burst’ process, i.e., it accelerates early in the history of a clade (when available niche space facilitates speciation), and subsequently slows down as niche space becomes saturated by new species; and (ii) phylogenetic branching is accompanied by diversification of ecologically relevant phenotypic traits among newly evolving species. Here, we employ macroevolutionary phylogenetic model-selection analyses to address these two predictions about evolutionary diversification using one of the most exceptionally species-rich and ecologically diverse lineages of living vertebrates, the South American lizard genus Liolaemus.

Our phylogenetic analyses lend support to a density-dependent lineage diversification model. However, the lineage through-time diversification curve does not provide strong support for an early burst. In contrast, the evolution of phenotypic (body size) relative disparity is high, significantly different from a Brownian model during approximately the last 5 million years of Liolaemus evolution. Model-fitting analyses also reject the ‘early-burst’ model of phenotypic evolution, and instead favour stabilizing selection (Ornstein-Uhlenbeck, with three peaks identified) as the best model for body size diversification. Finally, diversification rates tend to increase with smaller body size.

Liolaemus have diversified under a density-dependent process with slightly pronounced apparent episodic pulses of lineage accumulation, which are compatible with the expected episodic ecological opportunity created by gradual uplifts of the Andes over the last ~25My. We argue that ecological opportunity can be strong and a crucial driver of adaptive radiations in continents, but may emerge less frequently (compared to islands) when major events (e.g., climatic, geographic) significantly modify environments. In contrast, body size diversification conforms to an Ornstein-Uhlenbeck model with multiple trait optima. Despite this asymmetric diversification between both lineages and phenotype, links are expected to exist between the two processes, as shown by our trait-dependent analyses of diversification. We finally suggest that the definition of adaptive radiation should not be conditioned by the existence of early-bursts of diversification, and should instead be generalized to lineages in which species and ecological diversity have evolved from a single ancestor.*

* © 2015 Pincheira-Donoso et al. Published under the terms of Creative Commons license (CC BY 4.0)

I must admit, I find the difference rather like splitting hairs but the scientific method is often about slight revisions and refinements rather than major changes or the overthrow of established principles, though they do happen, maybe once every few tens of years, but I wouldn't be at all surprised to see a creationist trumpeting yet another 'devastating' refutation of the Theory of Evolution by Natural Selection, the long-awaited death of 'Darwinism' and the triumph of Jesus. Praise the Lord and send money! In creationist circles with it's simplistic black and white, magical thinking, if an established scientific principle can be shown to be even slightly inaccurate or incomplete, this destroys the entire thing and their 'God did it by magic!' notion wins by default.

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