Monday, 25 August 2014

Unintelligently Designed Hummingbirds

Blue-tailed-Emerald, Chlorostilbon mellisugus. Photo: Bjørn Christian Tørrissen
Evolution of sweet taste perception in hummingbirds by transformation of the ancestral umami receptor

Hummingbirds are unique amongst birds in living off nectar and being able to detect sweetness. Typically, hummingbirds only feed from flowers with nectar containing more than 10% sugars. The way this evolved illustrates a couple of basic principle of diversification by evolution and the lack of a directing intelligence or plan behind it. Something else for creationists to misrepresent, lie about or ignore but never, under any circumstances, face up to.

The sense of taste and smell are intimately linked and, so far as detection of different odours, pheromones and basic tastes like bitterness are concerned, involve a group of genes which are highly flexible across species being lost or gained according the species-specific ecology. New receptors typically arise by gene duplication and mutation with new capabilities which convey an advantage quickly being incorporated into the species genepool.

Ruby-throated hummingbird, Archilochus colubris.
However, two basic taste detection systems - savoury (or "umami") and sweet - do not follow this pattern and in fact are highly conserved across species with very little variance in the coding DNA even between distantly related species. In vertebrates, this system involves three G protein-coupled receptors known as T1Rs and designated T1R1, T1R2 and T1R3 respectively. Detection of sweetness involves T1R1 and T1R2; detection of umami involves T1R1 and T1R3. Some mammals such as cats, which are obligate carnivores, have lost T1R2 so are unable to detect sweetness. Giant pandas, which live almost exclusively on bamboo, have lost T1R1 and so can detect neither sweetness or umami.

All known birds, including hummingbirds, lack T1R2 so should be unable, using the 'normal' detection process, to detect sweetness and yet the ability to detect sweet nectar would appear to be essential to hummingbirds and something that would be expected to set them apart from other birds.

Purple-throated Carib, Eulampis jugularis. Photo: Charles J. Sharpe.
An additional 'problem' for evolution was that loss of T1R2 seems to have occurred in the pre-avian dinosaurs since it is present in at least one alligator - a sister group to birds - so we can be sure neither hummingbirds nor their last common ancestor with swifts - the family hummingbirds evolved from - had the ability to make it. Swifts are known to not be able to detect sweetness. Clearly, this ability evolved anew in the ancestral hummingbird and was probably the evolutionary breakthrough which led to the explosive radiation of new species and to the evolution of new plant-bird coevolutionary complexes similar to the various plant-insect coevolutionary complexes.

This paper by Baldwin et al, published in Science a few days ago, suggests a mechanism by which this happened:

Abstract
Sensory systems define an animal's capacity for perception and can evolve to promote survival in new environmental niches. We have uncovered a noncanonical mechanism for sweet taste perception that evolved in hummingbirds since their divergence from insectivorous swifts, their closest relatives. We observed the widespread absence in birds of an essential subunit (T1R2) of the only known vertebrate sweet receptor, raising questions about how specialized nectar feeders such as hummingbirds sense sugars. Receptor expression studies revealed that the ancestral umami receptor (the T1R1-T1R3 heterodimer) was repurposed in hummingbirds to function as a carbohydrate receptor. Furthermore, the molecular recognition properties of T1R1-T1R3 guided taste behavior in captive and wild hummingbirds. We propose that changing taste receptor function enabled hummingbirds to perceive and use nectar, facilitating the massive radiation of hummingbird species.


Incidentally, I wrote about the discovery of a fossil believed to be the first example of a nectivorous bird only last May, in Closing The Gaps - Early Bird Shows Evolution. This fossil was found to have pollen grains and, possibly significantly if it was transitional between swifts and hummingbirds, the remains of insects in its stomach.

Kunstformen der Natur, plate 99: Trochilidae, Ernst Haeckel 1904
(Drawn from specimens. Body postures are fanciful)
So, what we have here is a lovely example of exaptation. The ability to detect umami, essential for insectivorous swifts, became less important as an ancestor to the hummingbirds, in exploiting the ready supply of insects feeding on nectar, and ingesting energy-rich nectar in the process, discovered a whole new source of energy and so opened up a whole new range of niches to move into. Although hummingbirds still rely on insects and spiders in their diet to supply amino acids, minerals and vitamins, as a much less important structure, a mutation which switched the T1R1-T1R3 system over from detecting umami to detecting sweet in a new way would have been a considerable advantage.

We also see an explosive radiation of new species and a burst of evolution as this allowed a species with this new ability to exploit new niches that it opened up, each occupying a unique niche in the form of a new mutually beneficial relationship with flowering plants as their main pollinator species. As these relationships became more and more specialised, the selection pressure to evolve barriers to interbreeding would have increased, so enhancing the tendency for speciation. Barriers to interbreeding between closely related species of birds almost always include changes in plumage which forms part of mating rituals, so we see a proliferation of plumage, just as we would expect.

But what we don't see in any of this is an intelligent plan. Why would an intelligent creator do away with the ability to detect sweetness in an early dinosaurian ancestor of birds and then re-invent it millions of years later instead or retaining it or using the same method for hummingbirds that it had supposedly designed for mammals and even the remote ancestors of birds? Did it not know it was going to design hummingbirds to feed on nectar?

Instead, what we see is exactly what we would expect to see as the product of a mindless, purposeless, unintelligent process lacking any foresight or ability to plan ahead. In other words, we see no evidence of design or intelligence but lots of evidence for Darwinian evolution by natural selection.

By the way, the standard pre-primed reflexive "that's microevolution, not macroevolution" argument can't be used here because we are discussing the evolution not even of varieties or subspecies, or even a new species; we are discussing the evolution of a whole family of birds and over 100 genuses which forms a major branch of the avian order. And we are discussing how this came about as the result of a relatively small change in the genome of a swift.

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1 comment :

  1. @Rosa: Maybe a bit off topic, but anyhow I want to draw your attention to this article: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3433997/ . That article is maybe not so much about unintelligent design, but instead a blow to young-earth creationists. Here are two quotes from the article:

    1) The oldest evidence for an association between humans and honeybees is a cave painting in Spain depicting honey hunting, dated at around 7000 years ago (Crane 1999).

    2) Based on comparisons of mtDNA loci, the A. mellifera lineage split from other extant honeybees at least 6 million years ago, and the subspecies began diverging around 1 million years ago. These estimates are based on a commonly used rate of mtDNA sequence divergence in insects of 2% per million years (Brower 1994).

    BTW: Here's is another article summarizing the same news: http://phys.org/news/2014-08-evolutionary-history-honeybees-revealed-genomics.html . Three quotes from that article:

    1) Another unexpected result was that honeybees seem to be derived from an ancient lineage of cavity-nesting bees that arrived from Asia around 300,000 years ago and rapidly spread across Europe and Africa. This stands in contrast to previous research that suggests that honeybees originate from Africa.

    2) "The evolutionary tree we constructed from genome sequences does not support an origin in Africa, this gives us new insight into how honeybees spread and became adapted to habitats across the world", says Matthew Webster.

    3) Hidden in the patterns of genome variation are signals that indicate large cyclical fluctuations in population size that mirror historical patterns of glaciation. This indicates that climate change has strongly impacted honeybee populations historically.

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