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Archive for September, 2014

Applying Metabolic Scaling Laws to Predicting Extinction Risk

By Claire Asher, on 25 September 2014

The Earth is warming. That much were are now certain of. A major challenge for scientists hoping to ameliorate the effect of this on biodiversity is to predict how temperature increases will affect populations. Predicting the responses of species living in complex ecosystems and heterogenous environments is a difficult task, but one starting point is to begin understanding how temperature increases affect small, laboratory populations. These populations can be easily controlled, and it is hoped that the lessons learned from laboratory populations can then begin to be generalised and applied to real populations. Recent research from GEE academics attempted to evaluate the predictive power of a simple metabolic model on the extinction risk of single-celled organisms in the lab. Their results indicate that simple scaling rules for temperature, metabolic rate and body size can be extremely useful in predicting the extinction of populations, at least in laboratory conditions.

Current estimates suggest that over the next 100 years we can expect a global temperature rise of between 1.1°C and 6.4°C. This change will not be uniformly distributed across different regions however, with some areas expected to experience warming at twice the global average rate. Temperature is known to be a crucial component in some of the most basic characteristics of life – metabolism, body size, birth, growth and mortality rates. These characteristics have been shown to scale with temperature in an easily predictable way, formalised in the Arrhenius equation. This equation yields a roughly 3/4 scaling rule, so that as temperature increases, metabolism increases around 75% as fast. This relationship appears to hold true for a variety of taxa with different life histories and positions in the food chain. Models based upon this rule can be designed that are very simple, which makes it easy for scientists to collect the data needed to plug into the model. But are they accurate in predicting extinction?

Recent research conducted by GEE and ZSL academics Dr Ben Collen and Prof. Tim Blackburn, in collaboration with the University of Sheffield and The University of Zurich, investigated the predictive power of simple metabolic models on extinction risk in a single-celled protist Loxocephallus. They first collected data on the population and extinction dynamics of a population held at constant temperature. This data was fed into a model based on scaling laws for metabolic rates and temperature, which in turn attempted to predict extinction risk under different temperature changes. The researchers tested how real protists responded to temperature changes – for 70 days they monitored populations of the protist Loxocephallus under either decreases or increases in temperature. Populations began at 20°C and increased to 26°C or decreased to 14°C at different rates (0.5°C, 0.75°C, 1.5°C or 3°C each week). Most populations eventually went extinct, but these extinctions happened sooner in hotter environments, and mean temperature showed a strong correlation with the date at which the population went extinct. Extinction tended to happen sooner in populations subjected to more rapid warming.

None of this is particularly surprising, but what the researchers found when they ran their models was that, even with relatively minimal data to start out with (population dynamics under constant ‘normal’ conditions), and using only simple scaling laws to predict extinction, their model was able to accurately predict when populations would go extinct under different warming or cooling conditions, with an accuracy of 84%. One important factor was the specifics of the temperature changes that were input into the model – using average temperature across the experiment rather than actual temperature changes produced much less accurate results.

This research is a first step in creating models that may help us predict the future extinction dynamics of wild populations subjected to unevenly distributed climatic warming over coming decades. It is a long way from a simple model of a laboratory population to a model that can accurately predict the future of complex assemblages of wild animals that are also subject to predation, disease and a healthy dose of luck. But the fact that these models can work for simple systems in laboratory conditions is a great first start – if they didn’t work for these populations, we could be fairly sure they wouldn’t generalise to natural populations. This shows that simple phenomenological models based on basic metabolic theory can be useful to understand how climate change will effect populations.

Original Article:

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This research was made possible by funding from the Natural Environment Research Council (NERC).

The Importance of Size in the Evolution of Complexity in Ants

By Claire Asher, on 16 September 2014

Ants are amongst the most abundant and successful species on Earth. They live in complex, cooperative societies, construct elaborate homes and exhibit many of the hallmarks of our own society. Some ants farm crops, others tend livestock. Many species have a major impact on the ecosystems they live in, dispersing seeds, consuming huge quantities of plant matter and predating other insect species. One of the major reasons for their enormous success is thought to be the impressive division of labour they exhibit. Theory suggests that, during the evolution of ants, increases in colony size drove increases in the complexity of their division of labour. However, there have been few previous attempts to test the hypothesis. A recent paper by GEE’s Professor Kate Jones and Phd student Henry Ferguson-Gow tested this hypothesis across the Attine ants, a large neotropical group including the famous leaf-cutter ants.

Ants, along with other social insects such as some bees, wasps and termites, are eusocial. This means that reproduction in their societies is dominated by just one or a few queens, while most of the colony members never reproduce, but instead perform other important tasks such as foraging, nest construction and defence. This system initially puzzled evolutionary biologists, because it poses the question, “how do non-reproductive workers pass on their genes?”. More specifically, “how can genes evolve to generate different morphology and behaviour in workers if they never reproduce and pass those genes on?”. This question was resolved in the 1960s, when W.D. Hamilton proposed the concepts of inclusive fitness and kin selection. He pointed out that although members of the non-reproductive worker caste do not directly pass on their genes, they are helping to ensure the survival of their siblings. Closely related individuals, such as siblings, share a large percentage of their genetic information, so by helping relatives, you are indirectly passing on your genes. Inclusive fitness is a measure of the total reproductive success of an individual, including direct fitness (gained by producing your own offspring) and indirect fitness (gained by helping relatives to reproduce). Kin selection, a form of natural selection, can therefore favour genes that cause sterility in the worker caste through it’s positive effects on the reproductive success of relatives.

When eusociality first began to evolve, colonies were probably small and although the worker caste likely refrained from reproduction most of the time, they weren’t completely sterile. In small colonies, keeping your reproductive options open makes a lot of sense – if the queen dies you may have a good chance of taking over the colony and reproducing yourself. Through evolutionary time, however, colony size increased in some lineages, and it is thought this may have driven increasing specialisation and commitment of individuals to their queen and worker roles. As colony size increases, your chances for gaining any kind of direct fitness start to decrease very rapidly. As a worker it’s a much better bet to do what you can to maximise your indirect fitness benefits in large colonies, and this can be achieved by becoming increasingly specialised for your particular role. Increases in division of labour, for example, as individuals specialise more in particular tasks, may lead to increase colony efficiency and success. In turn, this may allow for the evolution of larger colonies, resulting in a positive feedback loop whereby increases in colony size lead to increases in division of labour which lead to increases in colony size, and so on. This force may have lead to the evolution of ant species with enormous colonies – over a million workers can be found in some leaf-cutter colonies!

GEE Researchers Professor Kate Jones and Henry Ferguson-Gow, along with colleagues at the University of East Anglia and the University of Bristol, produced a phylogenetic tree for the Attine Ants (a group containing over 250 species), and mapped social and environmental data onto this tree in order to test for the effects of colony size and environment on the evolution of more sophisticated division of labour. The Attini are a good group of ants to test this hypothesis in, as they show large variation in colony size and the extent of morphological divergence between the queen and worker caste.

They collected published data on social traits (colony size, worker size, queen size) and environmental conditions (daytime temperature, seasonality in temperature and precipitation) for over 600 observations of populations for 57 species of Attine ant, including every single Attine genus. Using supertree methods, they constructed a phylogeny for the attine ants, which enabled them to control for evolutionary relationships and to estimate the speed at which evolutionary changes occurred.

Colony size ranged from 16 to 6 million individuals, with the largest colonies exhibited by the fungus growing leaf-cutter ants Atta and Acromyrmex. The authors found that increases in colony size through evolution are strongly associated with increases in both worker size variation (representing division of labour within the worker caste) and queen worker dimorphism (representing reproductive division of labour). Colony size showed a positve correlation with variation in size within the worker caste, and a weaker, but positive correlation with queen-worker dimorphism. Environmental factors such as temperature, rainfall and seasonality did not have any effect on colony size, indicating that climate and other environmental variables have not been an important factor in driving the evolution of increased colony size.

This study finds strong support for the size-complexity hypothesis, which suggests that during the evolution of eusociality, increases in colony size both drove and were driven by increases in division of labour and in specialisation of the queen and worker castes to their respective roles. This pattern may have also occurred during other major transitions in evolution, such as the evolution of multicellularity, which shares many similarities with the evolution of eusociality (e.g closely related group members, division of labour). The relationship between group size and complexity may therefore have been a crucial force in the evolution of complex life, and in the major evolutionary innovations that have generated the diversity of life we see today.

Original Article:

() Science

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This research was made possible by funding from the Natural Environment Research Council (NERC).

Understanding Catfish Colonisation and Diversification in The Great African Lakes

By Claire Asher, on 5 September 2014

Why some regions or habitats contain vast, diverse communities of species, whilst others contain only relatively few species, continues to be the subject of scientific research attempting to understand the processes and conditions that allow and adaptive radiation. The Great African Lakes exist as freshwater ‘islands’, with spectacularly high levels of biodiversity and endemism. They are particularly famous for the hyperdiverse Cichlid fish, but they are also home to diverse assemblages of many other fish, such as catfish. Recent research in GEE investigated the evolution of Clatoteine catfish in Lake Tanganyika, to investigate the forces driving evolutionary radiations in the Great African Lakes. Their results suggest that evolutionary time is of key importance to catfish radiations, with recently colonised groups showing less diversity than long-standing species.

Lake Tangayika is the World’s second largest freshwater lake, covering 4 countries in the African Rift Valley (Tanzania, the Democratic Republic of the Congo, Burundi and Zambia). It is home to the highest diversity of lake-dwelling catfish on Earth, however the evolutionary history of these catfish is not fully understood. GEE academic Dr Julia Day and PhD Student Claire Peart, in collaboration with colleagues at the Natural History Museum, London and the South African Institute for Aquatic Biodiversity, investigated the evolutionary history of nocturnal Claroteine catfishes in Lake Tangayika. This group of catfish offers an excellent opportunity to investigate the influence of different factors in evolutionary diversification, as it includes multiple genera with varying range sizes and habitat types.

The Drivers of Diversification

Previous research has suggested a number of factors that are important in enabling adaptive radiations that can produce extremely high levels of biodiversity – deep lakes that experience lots of sunlight tend to favour evolutionary diversification. Diversification is also more common for species that have had a lot of evolutionary time in which to diverge and that experience high levels of sexual selection. Interestingly, although lake depth is important, the total size of the lake does not appear to be so important for diversification. A large geographical area to diversify into may influence the duration of adaptive radiations, however, with river-dwelling species showing more consistent species-production through time. This data suggests that adaptive radiations may be, to some extent, predictable, however much previous work has focussed on key model groups such as the Cichlid fish, and these hypotheses need to be generalised to other species and locations.

Molecular Phylogeny of Claroteine Catfish,
showing independent colonisation of
Chrysichthys brachynema

The authors sequenced nuclear and mitochondrial genes from 85 catfish covering 10 of the 15 species of Claroteine catfish, in order to construct an evolutionary tree for the sub-family. Estimates of the relationships between species and the evolutionary timescales of colonisation and divergence allowed the authors to distinguish between the possibilities of single or multiple colonisation events, and the processes driving diversification. The results indicated that most Claroteine catfish in Lake Tangayika originate from a single colonisation of the lake between 5 and 10 million years ago, followed by evolutionary radiations to produce the variety of species present today. One species, Chrysichthys brachynema was the exception to this rule, having independently colonised the lake around 1 – 2 million years ago. This species has not shown adaptive radiation since colonisation, probably because of the relatively short time it has been present in the lake. These results support previous work that has suggested that time is an important factor in producing highly diverse species assemblages.

Original Article:

() Molecular Phylogenetics and Evolution

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This research was made possible by funding from the Natural Environment Research Council (NERC), the National Council for Scientific and Technological Development (CNPQ), the National Geographic Society, and the Percy Sladen Memorial Trust Fund.