Is the rate of evolution faster in rainforest confined Amazonian birds living on richer soils?

Abstract

The Amazonian tropics in South America is home to a large diversity of organisms. This diversity has inspired numerous hypotheses that try to explain why the region is so species rich. Such hypotheses include the influence of productivity gradients – including those determined by the availability of water or ambient temperature - on speciation potential. In particular, the effective evolutionary time hypothesis implies that because the tropics will see higher metabolic rates and faster growth rates with shorter generation times, these effects should result in higher rates of evolution that might also promote faster rates of speciation. Within the Amazon rainforest, the Guiana Shield is less diverse and has nutrient poor soils compared to the nutrient rich soils and high species diversity of the remainder of Amazonia. This research project considers whether an edaphic gradient across the Amazon basin of soil nutrient status - and attendant variation in forest productivity – has any effect on the rate of evolution for resident birds. This involved comparing rates of microevolution for forest birds living on the nutrient poor soils of the Guiana Shield against rates for close relatives living on richer soils elsewhere in Amazonia. This has been done using the NADH dehydrogenase subunit 2 gene (ND2) for twenty one paired species contrasts. Fourteen out of twenty one of these contrasts displayed faster micro-evolutionary rates for species that occupied the nutrient rich soils of Amazonia compared to sister species occupying the nutrient poor soils of the Guiana Shield, producing a statistically significant result in favour of faster evolutionary rate for species living on richer soils. Such results indicate that the tempo of microevolution among birds may be responding to edaphic effects potentially due to an extension of the climatic energy effects suggested in the effective evolutionary time hypothesis. Prior to this study, no studies have attempted to test the relationship between soil quality and evolutionary rate within Amazonia or indeed in any other locality. It is therefore important to consider the implications of the findings of this research project, since it is now apparent that there may be other environmental factors apart from climate that may influence the rate of evolution. It also has implications for the conservation of the species that occupy the Guiana Shield since with reduced rates of microevolution they may be more vulnerable to human induced change.