By Claire Asher, on 14 June 2013
The tree of life is a vast tangle of branches and twigs representing around 9 million species. Understanding the patterns and processes by which these species originated is a fundamental topic in evolutionary biology. Previous studies have suggested that the generation of new species may not be uniform over time, often with an initial burst of speciation, slowing as the lineage expands to fill the available niches. However, recent research in the department of GEE suggests this may not always be the case, and a previous focus on island species may have skewed our perspective of species accumulation.
When a new lineage begins to diverge, adapting to exploit new habitats or lifestyles, this often leads to an adaptive radiation, with many new species appearing over a short space of time. This initial rush of new species eventually begins to slow, as all the available niches become saturated, and slowly speciation grinds to a halt. This is the traditional view of how lineages have diversified to produce the vast array of species we see today (as well as many who have long since gone extinct). However, much of the research in this area has focussed on species confined to islands and lakes, where different processes may influence the appearance of new species. Continental species have a larger area to expand into, and a wider array of climatic conditions to cope with, providing a different set of parameters which may influence speciation. Recent research in the department of Genetics, Evolution and Environment suggests the pattern of species generation in continental lineages may differ from that of islands.
In their recent paper in Systematic Biology, Day and colleagues determined species relationships for 81 species of squeaker catfish from Africa in order to investigate the patterns of species generation and diversification during their 35 million year evolution. Using genetic data along with fossil evidence, they constructed a squeaker catfish tree and estimated divergence dates and historical geographic distributions. This tree revealed an almost constant rate of species generation over time. Across 35 million years of evolution, through major climatic and environmental changes, the squeaker catfish have been churning out new species at approximately 1 every 8 million years, and look set to keep going for another 90 million years, or so.
So why don’t the continental, river-dwelling catfish follow the same rules developed through years of studying island and lake-dwelling species? Continents are, of course, generally much larger than islands, and rivers provide more opportunity for movement than lakes. Continents allow for more constant species generation, as there is more space and habitat diversity available for species to move into and exploit. The climatic changes in Africa over the last 30 million years may have also helped the catfish maintain constant production of new species – creating a variable environment which is much less likely to become saturated with species. The squeaker catfish have survived through major tectonic activity in the East African Rift valley, including volcanoes and earthquakes, as well as huge climatic changes that influenced sea level and temperature. By combining phylogenetic and biogeographical data, Day et al suggest these environmental variations may have played an important role in African catfish diversification.
The processes shaping species diversification are varied and complex, and differ markedly between geographically isolated lineages (island / lake ecosystems) and wider ranging ones (e.g. continental rivers). Continental groups may take longer to reach species saturation, with environmental fluctuations facilitating continued species generation at a relatively constant rate. Moving beyond ‘model organisms’ and well-studied systems has the potential to reveal new processes and patterns, and illuminate old ones.
This project was made possible by funding from the Natural Environment Research Council (NERC)