Effective evolutionary time

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The hypothesis of effective evolutionary time [1] attempts to explain gradients, in particular latitudinal gradients, in species diversity. It was originally named "time hypothesis". [2] [3]

Contents

Background

Low (warm) latitudes contain significantly more species than high (cold) latitudes. This has been shown for many animal and plant groups, although exceptions exist (see latitudinal gradients in species diversity). An example of an exception is helminths of marine mammals, which have the greatest diversity in northern temperate seas, possibly because of small population densities of hosts in tropical seas that prevented the evolution of a rich helminth fauna, or because they originated in temperate seas and had more time for speciations there. It has become more and more apparent that species diversity is best correlated with environmental temperature and more generally environmental energy. These findings are the basis of the hypothesis of effective evolutionary time. Species have accumulated fastest in areas where temperatures are highest. Mutation rates and speed of selection due to faster physiological rates are highest, and generation times which also determine speed of selection, are smallest at high temperatures. This leads to a faster accumulation of species, which are absorbed into the abundantly available vacant niches, in the tropics. Vacant niches are available at all latitudes, and differences in the number of such niches can therefore not be the limiting factor for species richness. The hypothesis also incorporates a time factor: habitats with a long undisturbed evolutionary history will have greater diversity than habitats exposed to disturbances in evolutionary history.

The hypothesis of effective evolutionary time offers a causal explanation of diversity gradients, although it is recognized that many other factors can also contribute to and modulate them.

Historical aspects

Some aspects of the hypothesis are based on earlier studies. Bernhard Rensch, [4] for example, stated that evolutionary rates also depend on temperature: numbers of generation in poikilotherms, but sometimes also in homoiotherms (homoiothermic), are greater at higher temperatures and the effectiveness of selection is therefore greater. Ricklefs refers to this hypothesis as "hypothesis of evolutionary speed" or "higher speciation rates". [5] Genera of Foraminifera in the Cretaceous and families of Brachiopoda in the Permian have greater evolutionary rates at low than at high latitudes. [6] That mutation rates are greater at high temperatures has been known since the classical investigations of Nikolay Timofeev-Ressovsky et al. (1935), [7] although few later studies have been conducted. Also, these findings were not applied to evolutionary problems.

The hypothesis of effective evolutionary time differs from these earlier approaches as follows. It proposes that species diversity is a direct consequence of temperature-dependent processes and the time ecosystems have existed under more or less equal conditions. Since vacant niches into which new species can be absorbed are available at all latitudes, the consequence is accumulation of more species at low latitudes. [1] All earlier approaches remained without basis without the assumption of vacant niches, as there is no evidence that niches are generally narrower in the tropics, i.e., an accumulation of species cannot be explained by subdivision of previously utilized niches (see also Rapoport's rule). The hypothesis, in contrast to most other hypotheses attempting to explain latitudinal or other gradients in diversity, does not rely on the assumption that different latitudes or habitats generally have different "ceilings" for species numbers, which are higher in the tropics than in cold environments. Such different ceilings are thought to be, for example, determined by heterogeneity or area of the habitat. But such factors, although not setting ceilings, may well modulate the gradients.

Recent studies

A considerable number of recent studies support the hypothesis. [8] Thus, diversity of marine benthos, interrupted by some collapses and plateaus, has risen from the Cambrian to the Recent, and there is no evidence that saturation has been reached. [9] Rates of diversification per time unit for birds and butterflies increase towards the tropics. [10] Allen et al. found a general correlation between environmental temperature and species richness for North and Central American trees, for amphibians, fish, Prosobranchia and fish parasites. They showed that species richness can be predicted from the biochemical kinetics of metabolism, and concluded that evolutionary rates are determined by generation times and mutation rates both correlated with metabolic rates which have the same Boltzmann relation with temperature. They further concluded that these findings support the mechanisms for latitudinal gradients proposed by Rohde. [11] Gillooly et al. (2002) described a general model also based on first principles of allometry and biochemical kinetics which makes predictions about generation times as a function of body size and temperature. [12] Empirical findings support the predictions: in all cases that were investigated (birds, fish, amphibians, aquatic insects, zooplankton) generation times are negatively correlated with temperature. Brown et al.(2004) further developed these findings to a general metabolic theory of ecology. [13] Indirect evidence points to increased mutation rates at higher temperatures, [14] [15] and the energy-speciation hypothesis is the best predictor for species richness of ants. [16] Finally, computer simulations using the Chowdhury ecosystem model have shown that results correspond most closely to empirical data when the number of vacant niches is kept large. [17] Rohde gives detailed discussions of these and other examples. [8] [18] Of particular importance is the study by Wright et al. (2006) which was specifically designed to test the hypothesis. It showed that molecular substitution rates of tropical woody plants are more than twice as large as those of temperate species, and that more effective genetic drift in smaller tropical populations cannot be responsible for the differences, leaving only direct temperature effects on mutation rates as an explanation. [19] Gillman et al. (2009) examined 260 mammal species of 10 orders and 29 families and found that substitution rates in the cytochrome B gene were substantially faster in species at warm latitudes and elevations, compared with those from cold latitudes and elevations. A critical examination of the data showed that this cannot be attributed to gene drift or body mass differentials. The only possibilities left are a Red Queen effect or direct effects of thermal gradients (including possibly an effect of torpor/hibernation differentials). Rohde (1992, 1978) had already pointed out that “it may well be that mammalian diversity is entirely determined by the diversity of plants and poikilothermic animals further down in the hierarchy”, i.e., by a Red Queen effect. He also pointed out that exposure to irradiation including light is known to cause mutations in mammals, and that some homoiothermic animals have shorter generation times in the tropics, which - either separately or jointly - may explain the effect found by Gillman et al. [20] Gillman et al. (2010) extended their earlier study on plants [21] by determining whether the effect is also found within highly conserved DNA. They examined the 18S ribosomal gene in the same 45 pairs of plants. And indeed, the rate of evolution was 51% faster in the tropical than their temperate sister species. Furthermore, the substitution rate in 18S correlated positively with that in the more variable ITS. These result lend further very strong support to the hypothesis. [22] Wright et al. (2010) tested the hypothesis on 188 species of amphibians belonging to 18 families, using mitochondrial RNA genes 12S and 16S, and found substantially faster substitution rates for species living in warmer habitats at both lower latitudes and lower elevations. [23] Thus, the hypothesis has now been confirmed for several genes and for plants and animals.

Vázquez, D.P. and Stevens, R.D. (2004) conducted a metanalysis of previous studies and found no evidence that niches are generally narrower in the tropics than at high latitudes. [24] This can be explained only by the assumption that niche space was not and is not saturated, having the capacity to absorb new species without affecting the niche width of species already present, as predicted by the hypothesis.

Depth gradients

Species diversity in the deepsea has been largely underestimated until recently (e.g., Briggs 1994: total marine diversity less than 200,000 species). [25] Although our knowledge is still very fragmentary, some recent studies appear to suggest much greater species numbers (e.g., Grassle and Maciolek 1992: 10 million macroinvertebrates in soft bottom sediments of the deepsea). [26] Further studies must show whether this can be verified. [27] A rich diversity in the deepsea can be explained by the hypothesis of effective evolutionary time: although temperatures are low, conditions have been more or less equal over large time spans, certainly much larger than in most or all surface waters.

Related Research Articles

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Bergmanns rule Biological rule stating that larger size organisms are found in colder environments

Bergmann's rule is an ecogeographical rule that states that within a broadly distributed taxonomic clade, populations and species of larger size are found in colder environments, while populations and species of smaller size are found in warmer regions. Bergmann's rule only describes the overall size of the animals, but does not include body parts like Allen's rule does.

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Macroecology is the subfield of ecology that deals with the study of relationships between organisms and their environment at large spatial scales to characterise and explain statistical patterns of abundance, distribution and diversity. The term was coined in a small monograph published in Spanish in 1971 by Guillermo Sarmiento and Maximina Monasterio, two Venezuelan researchers working in tropical savanna ecosystems and later used by James Brown of the University of New Mexico and Brian Maurer of Michigan State University in a 1989 paper in Science.

Evolutionary ecology Interaction of biology and evolution

Evolutionary ecology lies at the intersection of ecology and evolutionary biology. It approaches the study of ecology in a way that explicitly considers the evolutionary histories of species and the interactions between them. Conversely, it can be seen as an approach to the study of evolution that incorporates an understanding of the interactions between the species under consideration. The main subfields of evolutionary ecology are life history evolution, sociobiology, the evolution of interspecific interactions and the evolution of biodiversity and of ecological communities.

Rapoport's rule is an ecogeographical rule that states that latitudinal ranges of plants and animals are generally smaller at lower latitudes than at higher latitudes.

Intermediate disturbance hypothesis Model proposing regional biodiversity is increased by a moderate level of ecological disturbance

The intermediate disturbance hypothesis (IDH) suggests that local species diversity is maximized when ecological disturbance is neither too rare nor too frequent. At low levels of disturbance, more competitive organisms will push subordinate species to extinction and dominate the ecosystem. At high levels of disturbance, due to frequent forest fires or human impacts like deforestation, all species are at risk of going extinct. According to IDH theory, at intermediate levels of disturbance, diversity is thus maximized because species that thrive at both early and late successional stages can coexist. IDH is a nonequilibrium model used to describe the relationship between disturbance and species diversity. IDH is based on the following premises: First, ecological disturbances have major effects on species richness within the area of disturbance. Second, interspecific competition results in one species driving a competitor to extinction and becoming dominant in the ecosystem. Third, moderate ecological scale disturbances prevent interspecific competition.

Latitudinal gradients in species diversity Global increase in species richness from polar regions to tropics

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Klaus Rohde German biologist

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Index of biodiversity articles

This is a list of topics in biodiversity.

Elevational diversity gradient Ecological pattern in which biodiversity changes with elevation

Elevational diversity gradient (EDG) is an ecological pattern where biodiversity changes with elevation. The EDG states that species richness tends to increase as elevation increases, up to a certain point, creating a "diversity bulge" at middle elevations. There have been multiple hypotheses proposed for explaining the EDG, none of which accurately describe the phenomenon in full.

Avian clutch size

Clutch size refers to the number of eggs laid in a single brood by a nesting pair of birds. The numbers laid by a particular species in a given location are usually well defined by evolutionary trade-offs with many factors involved, including resource availability and energetic constraints. Several patterns of variation have been noted and the relationship between latitude and clutch size has been a topic of interest in avian reproduction and evolution. David Lack and R.E. Moreau were among the first to investigate the effect of latitude on the number of eggs per nest. Since Lack's first paper in the mid-1940s there has been extensive research on the pattern of increasing clutch size with increasing latitude. The proximate and ultimate causes for this pattern have been a subject of intense debate involving the development of ideas on group, individual, and gene-centric views of selection.

The term phylogenetic niche conservatism has seen increasing use in recent years in the scientific literature, though the exact definition has been a matter of some contention. Fundamentally, phylogenetic niche conservatism refers to the tendency of species to retain their ancestral traits. When defined as such, phylogenetic niche conservatism is therefore nearly synonymous with phylogenetic signal. The point of contention is whether or not "conservatism" refers simply to the tendency of species to resemble their ancestors, or implies that "closely related species are more similar than expected based on phylogenetic relationships". If the latter interpretation is employed, then phylogenetic niche conservatism can be seen as an extreme case of phylogenetic signal, and implies that the processes which prevent divergence are in operation in the lineage under consideration. Despite efforts by Jonathan Losos to end this habit, however, the former interpretation appears to frequently motivate scientific research. In this case, phylogenetic niche conservatism might best be considered a form of phylogenetic signal reserved for traits with broad-scale ecological ramifications. Thus, phylogenetic niche conservatism is usually invoked with regards to closely related species occurring in similar environments.

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Countergradient variation is a type of phenotypic plasticity that occurs when the phenotypic variation determined by a biological population's genetic components opposes the phenotypic variation caused by an environmental gradient. This can cause different populations of the same organism to display similar phenotypes regardless of their underlying genetics and differences in their environments.

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References

  1. 1 2 K. Rohde: Latitudinal gradients in species diversity: the search for the primary cause, Oikos, 65, 514-527,1992.
  2. ↑ K. Rohde: Latitudinal gradients in species diversity and their causes. I. A review of the hypotheses explaining the gradients. Biologisches Zentralblatt 97, 393-403, 1978a.
  3. ↑ K. Rohde: Latitudinal gradients in species diversity and their causes. II. Marine parasitological evidence for a time hypothesis. Biologisches Zentralblatt 97, 405-418, 1978b.
  4. ↑ B. Rensch: Neuere Probleme der Abstammungslehre. Die transspezifische Evolution. Encke, Stuttgart, 1954.
  5. ↑ R.E. Ricklefs: Ecology. Nelson and Sons, London, 1973.
  6. ↑ F.G. Stehli, E.G. Douglas and N.D. Newell: Generation and maintenance of gradients in taxonomic diversity. Science 164, 947-949, 1969.
  7. ↑ N.W. Timofeeff-Ressovsky, K.G. Zimmer und M. Delbrück: Über die Natur der Genmutation und der Genstruktur. Nachrichten aus der Biologie der Gesellschaft der Wissenschaften Göttingen I, 189-245, 1935.
  8. 1 2 K. Rohde: Nonequilibrium Ecology , Cambridge University Press, Cambridge, 2005b, 223 pp. ISBN   0-521-67455-7.
  9. ↑ D.Jablonski: The future of the fossil record, Science 284, 2114-2116, 1999.
  10. ↑ M. Cardillo: Latitude and rates of diversification in birds and butterflies. Proceedings of the Royal Society London 266, 1221-1225,1999.
  11. ↑ A.P. Allen, J.H. Brown, and J.F. Gillooly: Global biodiversity, biochemical kinetics, and the energetic-equivalence rule. Science, 297, 1545-1548, 2002.
  12. ↑ J.F. Gillooly, E.L. Charnov, G.B. West, M.Van Savage, and J.H. Brown: Effects of size and temperature on developmental time. Nature 417, 70–73, 2002.
  13. ↑ J.H. Brown, J.F. Gillooly, A.P. Allen, M. Van Savage, and G.. West,. (2004). Toward a metabolic theory of ecology. Ecology 85, 1771-1789.
  14. ↑ C. Bazin, P. Capy, D. Higuet, and T. Langin, T.: Séquences d’AND mobiles et évolution du génome. Pour Sci., Hors. Sér. Janvier 97, 106-109., 1997 (zitiert in. Harmelin-Vivien 2002).
  15. ↑ M.L. Harmelin-Vivien: Energetics and fish diversity on coral reefs. In: Sale, P.F. Hrsg. Coral reef fishes. Dynamics and diversity in a complex ecosystem. Academic Press, Amsterdam, pp. 265-274, 2002.
  16. ↑ M. Kaspari, P.S. Ward and M.Yuan: Energy gradients and the geographical distribution of local ant diversity. Oecologia 140, 407-413, 2004.
  17. ↑ K. Rohde and D. Stauffer: "Simulation of geographical trends in Chowdhury ecosystem model", Advances in Complex Systems 8, 451-464, 2005.
  18. ↑ K. Rohde: Eine neue Ökologie. Aktuelle Probleme der evolutionären Ökologie". Naturwissenschaftliche Rundschau, 58, 420-426, 2005.
  19. ↑ S. Wright, J. Keeling and L. Gillman 2006. The road from Santa Rosalia: a faster tempo of evolution in tropical climates. Proceedings of the National Academy of Sciences 103, 7718 –7722.
  20. ↑ Gillman, L.N., Keeling, D.J., Ross,H.A. and Wright, S.D. 2009. Latitude, elevation and the tempo of molecular evolution in mammals. Proceedings of the Royal Society B 2009.
  21. ↑ S. Wright, J. Keeling and L. Gillman. 2006. The road from Santa Rosalia: a faster tempo of evolution in tropical climates. Proceedings of the National Academy of Sciences 103, 7718 –7722.
  22. ↑ Gillman L.N., Keeling D.J., Gardner R.C., Wright S.D. Faster evolution of highly conserved DNA in tropical plants. Journal of evolutionary Biology 23, 1327-1330,2010
  23. ↑ Wright S.D., Gillman L.N., Ross H.A. and Keeling D.J. Energy and tempo of evolution in amphibians. Global Ecology and Biogeography 2010
  24. ↑ Vázquez, D.P. and Stevens, R.D. The latitudinal gradient in niche breadth: concepts and evidence. American Naturalist 164, E1- E19, 2004.
  25. ↑ J. C.Briggs. Species diversity: land and sea compared. Systematic Biology 43, 130-135, 1994.
  26. ↑ J F. Grassle and N .J. Maciolek: Deepsea species richness: regional and local diversity estimates from, quantitative bottom samples. American Naturalist 139, 313-341, 1992.
  27. ↑ K. Rohde: Ecology and biogeography of marine parasites. Advances in marine biology 43,1-86, 2002.
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