Friday, 31 May 2013

Rapid Evolution in Brazil

Functional Extinction of Birds Drives Rapid Evolutionary Changes in Seed Size

Yet another example of observed rapid evolution is published in this week's Science. This time it is rapid evolution in Brazilian palm trees due to human activity with no evidence whatsoever of any intelligence being involved in the process, nor of any being required.
Local extinctions have cascading effects on ecosystem functions, yet little is known about the potential for the rapid evolutionary change of species in human-modified scenarios. We show that the functional extinction of large-gape seed dispersers in the Brazilian Atlantic forest is associated with the consistent reduction of the seed size of a keystone palm species. Among 22 palm populations, areas deprived of large avian frugivores for several decades present smaller seeds than nondefaunated forests, with negative consequences for palm regeneration. Coalescence and phenotypic selection models indicate that seed size reduction most likely occurred within the past 100 years, associated with human-driven fragmentation. The fast-paced defaunation of large vertebrates is most likely causing unprecedented changes in the evolutionary trajectories and community composition of tropical forests.
Functional Extinction of Birds Drives Rapid Evolutionary Changes in Seed Size; Mauro Galetti, et al.
Science 31 May 2013: 340 (6136), 1086-1090. [DOI:10.1126/science.1233774]
This is a lovely example of how evolutionary change will occur without any new mutation arising simply because the environment has changed. I have shown before how it is not necessarily the information contained in the genome which needs to change but the meaning of that information as determined in the context of the environment in which it finds itself. See Evolution - The Meaning of Information and Rapid Human Evolution.

Over time, these Brazilian palms had evolved to have their seeds dispersed by a range of bird species by being eaten by them and excreted some distance away (neatly giving the seed a little fresh fertiliser to start in on its way in the process). Larger seeds obviously produce larger seedlings and so will have been favoured in areas where 'large-gape' birds were present because these birds could swallow large seeds whole, but, with other bird species present, which could disperse smaller seeds, there would have been little pressure on palms towards producing only large seeds; instead they produce a range of seed sizes.

But, in several areas, under human pressure in the last 100 years or so, many of these large-gape bird species have become locally extinct making it impossible for palms with larger seeds to get dispersed. Not surprisingly, in these areas, as this paper has demonstrated, palms have evolved to produce smaller seeds. With this human-induced environmental change, the relative frequency of alleles of genes favouring smaller seeds in these Brazilian palm trees has shifted - and that is all evolution is.

This reminds me of a similar though more drastic example of how humans can disturb a balanced ecosystem from the Indian Ocean island of Mauritius where several large frugivores (fruit eaters), including the Dodo and a species of giant tortoise have been exterminated by man, resulting in the near-extinction of several plants that depended on them. One such example was the relationship between the tambalacoque (Sideroxylon grandiflorum) or 'Dodo Tree' which was reported in 1973 as being near extinct with only 13 specimens having germinated since the Dodo went extinct 300 years earlier. This was an over-simplification and exaggerated the problem but never-the-less it serves to illustrate the point:
In 1973, the tambalacoque, also known as the "Dodo tree", was thought to be dying out on Mauritus, to which it is endemic. There were supposedly only 13 specimens left, all estimated to be about 300 years old. Stanley Temple hypothesised that it depended on the Dodo for its propagation, and that its seeds would germinate only after passing through the bird's digestive tract. He claimed that the tambalacoque was now nearly coextinct because of the disappearance of the Dodo. Temple overlooked reports from the 1940s that found that tambalacoque seeds germinated, albeit very rarely, without being abraded during digestion. Others have contested his hypothesis and suggested that the decline of the tree was exaggerated, or seeds were also distributed by other extinct animals such as Cylindraspis tortoises, fruit bats or the Broad-billed Parrot. According to Wendy Strahm and Anthony Cheke, two experts in the ecology of the Mascarene Islands, the tree, while rare, has germinated since the demise of the Dodo and numbers several hundred, not 13 as claimed by Temple, hence discrediting Temple's view as to the Dodo and the tree's sole survival relationship.

It has also been suggested that the Broad-billed Parrot may have depended on Dodos and Cylindraspis tortoises to eat palm fruits and excrete their seeds, which became food for the parrots. Anodorhynchus macaws depended on now-extinct South American megafauna in the same way, but now rely on domesticated cattle for this service.

Just another example of evolution in progress, driven as always by the environment selecting for fitness to survive in that environment and environmental change producing a change in allele frequency. If the Brazilian palms in question had not existed in the presence of a range of frugivorous birds but had been forced down an evolutionary path dictated by a single, or small number of, large-gaped species of birds, we would now be looking at impending local extinctions of these trees.

All in all, no evidence there of intelligent design, and all of it easily explained by Darwinian Evolution.

Functional Extinction of Birds Drives Rapid Evolutionary Changes in Seed Size
Mauro Galetti, Roger Guevara, Marina C. Côrtes, Rodrigo Fadini, Sandro Von Matter, Abraão B. Leite, Fábio Labecca, Thiago Ribeiro, Carolina S. Carvalho, Rosane G. Collevatti, Mathias M. Pires, Paulo R. Guimarães Jr., Pedro H. Brancalion, Milton C. Ribeiro, and Pedro Jordano
Science 31 May 2013: 340 (6136), 1086-1090. [DOI:10.1126/science.1233774]

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