Thursday, February 5, 2009

Red Queens or Court Jesters: How Species Evolve

A review in this month's science by Michael Benton discusses two prominent models of evolution.Science article  The abstract and some snippets of the article are below:

Evolution may be dominated by biotic factors, as in the Red Queen model, or abiotic factors, as in the Court Jester model, or a mixture of both. The two models appear to operate predominantly over different geographic and temporal scales: Competition, predation, and other biotic factors shape ecosystems locally and over short time spans, but extrinsic factors such as climate and oceanographic and tectonic events shape larger-scale patterns regionally and globally, and through thousands and millions of years. Paleobiological studies suggest that species diversity is driven largely by abiotic factors such as climate, landscape, or food supply, and comparative phylogenetic approaches offer new insights into clade dynamics. 

According to Benton, abiotic factors play a more prominent role when the geographic and temporal scale is large:

Much of the divergence between the Red Queen and Court Jester world views may depend on scale (2) (Fig. 1): Biotic interactions drive much of the local-scale success or failure of individuals, populations, and species (Red Queen), but perhaps these processes are overwhelmed by substantial tectonic and climatic processes at time scales above 105 years (Court Jester). It is important not to export organism-level processes to regional or global scales, and it is likely that evolution operates in a pluralistic way (3).

Figure 1 Fig. 1. Operation of Red Queen (biotic causation) and Court Jester (abiotic causation) models at different geographic and temporal scales (A). The Red Queen may prevail at organismic and species level on short time scales, whereas the Court Jester holds his own on larger scales. The stippled green shape shows an area where Red Queen effects might be identified erroneously, but these are likely the result of spatial averaging of regional responses to climate change and other complex physical perturbations that may be in opposite directions, and so cancel each other, suggesting no controlling effect of the physical environment on evolution. Physical-environmental disruptions may elicit biotic responses along the red line separating Red Queen and Court Jester outcomes (B). The usage here is the microevolutionary Red Queen, as opposed to the macroevolutionary Red Queen that posits constant extinction risk, a view that has been largely rejected (31). Illustration based on (2). [View Larger Version of this Image (69K GIF file)]

Large-Scale Controls on Species Diversity

... Biotic factors, such as body size, diet, colonizing ability or ecological specialization, appear to have little effect on the diversity of modern organisms, although abundance and...life-history characteristics (short gestation period, large litter size, and short interbirth intervals) sometimes correlate with high species richness (16).

 

Geographic and tectonic history has generated patterns of species diversity through time. The slow dance of the continents as Pangaea broke up during the past 200 My has affected modern distribution patterns. Unique terrestrial faunas and floras, notably those of Australia and South America, arose because those continents were islands for much of the past 100 My. Further, major geologic events such as the formation of the Isthmus of Panama have permitted the dispersal of terrestrial organisms and have split the distributions of marine organisms. A classic example of vicariance is the fundamental division of placental mammals into three clades, Edentata in South America, Afrotheria in Africa, and Boreoeutheria in the northern hemisphere, presumably triggered by the split of those continents 100 Ma (17). Other splits in species trees may relate to dispersal events, or there may be no geographic component at all. 

Species richness through time may correlate with energy. The species richness–energy relationship (18) posits correlations with evapo-transpiration, temperature, or productivity, and studies of terrestrial and marine ecosystems have shown that these factors may explain as much as 90% of current diversity, although relationships between species diversity and productivity change with spatial scale (19). Over long time spans, there are strong correlations between plankton morphology and diversity and water temperature: Cooling sea temperatures through the past 70 My, and consequent increasing ocean stratification, drove a major radiation of Foraminifera, associated with increasing body size (20). More widely, there is close tracking between temperature and biodiversity on the global scale for both marine and terrestrial organisms (21), where generic and familial richness were relatively low during warm "greenhouse" phases of Earth history, coinciding with relatively high origination and extinction rates. 

A much-studied manifestation of energy and temperature gradients is the latitudinal diversity gradient (LDG), namely the greater diversity of life in the tropics than in temperate or polar regions, both on land and in the sea. There are two explanations (22): (i) the time and area hypothesis, that the tropical belt is older and larger than temperate and polar zones, and so tropical clades have had longer to speciate, or (ii) the diversification rate hypothesis, that there are higher rates of speciation and lower rates of extinction in the tropics than elsewhere. There is geological and paleontological evidence for a mixture of both hypotheses (23, 24). 

Species diversity may increase by the occupation of new ecospace. The number of occupied guilds, that is, broad ecological groupings of organisms with shared habits, has increased in several steps through time...(25). Further, marine animals have shown several step increases in tiering, the ability to occupy and exploit different levels in the habitat: At times, burrowers have burrowed deeper, and reef-builders have built taller and more complex reefs. Analogous, if even more dramatic, expansions of ecospace have occurred on land, with numerous stepwise additions of new habitats, from the water-margin plants and arthropods of the early Paleozoic to the forests and upland habitats of the later Paleozoic when land animals first burrowed, climbed, and flew, through the introduction of herbivory, giant size, endothermy, and intelligence among vertebrates, and the great blossoming of flowering plants (with associated vast expansions in diversity of plant-eating and social insects and modern vertebrates)...(26). 

The other mode of species increase globally or regionally is by niche subdivision, or increasing specialization. This is hard to document because of the number of other factors that vary between ecosystems through time. However, mean species number in communities (alpha diversity) has increased through time in both marine (15, 25) and terrestrial (10) systems, even though niche subdivision may be less important than occupation of new ecospace in increasing biodiversity. Further, morphological complexity may be quantified, and a comparative study of crustaceans shows, for example, that complexity has increased many times in parallel in separate lineages (27).

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