Check out this answer on Socratic to read about some of the theories behind this observed pattern. There's also an excellent link here for further reading. How can latitude affect species diversity? Kate M.
Dec 20, Explanation: This idea is also referred to as the latitudinal diversity gradient, meaning that as you move from the equator towards the poles, diversity lessens. Related questions What is the Law of Tolerance?
Can you explain how evolution and biodiversity are related? A twofold role for global energy gradients in marine biodiversity trends. Science New York, N. Deep-Sea Biodiversity: Pattern and Scale. Harvard University Press, Woolley, S. Deep-sea diversity patterns are shaped by energy availability. McClain, C. Energetics of life on the deep seafloor. Temporal latitudinal-gradient dynamics and tropical instability of deep-sea species diversity.
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Hansen, J. Global temperature change. Large-scale biogeographic patterns in marine mollusks: A confluence of history and productivity? Patterns and controlling factors of species diversity in the Arctic Ocean. Download references. You can also search for this author in PubMed Google Scholar. Correspondence to Hanieh Saeedi. Reprints and Permissions. Sci Rep 9, Download citation. Received : 20 December Accepted : 12 June Published : 26 June Anyone you share the following link with will be able to read this content:.
Sorry, a shareable link is not currently available for this article. Provided by the Springer Nature SharedIt content-sharing initiative. If so, observed differences in rates different branch lengths cannot result from PE unless speciation PE 2 and extinction have occurred only on the branch leading to the tropical species. Hidden or incipient speciation PE 3 could also explain the observed faster rate of molecular evolution if it was occurring more frequently in the tropical lineages than the temperate lineages.
Species are selected from sister lineages with different species numbers to examine whether or not lineages with higher cladogenesis have faster rates for example, Lanfear et al.
Relatively large data sets are pruned X to a pair of contrasting lineages, if more sequence data are available for one family than another, to avoid the node density effect. Lineage extinction dashed lines undermines confidence in inferences about the directionality of effects.
Insufficient knowledge of extant lineages produces the same effect as extinction suggesting that this method would only work for well-studied groups. It has been suggested that the LBG arose in response to a latitudinal gradient in kinetic energy influencing rates of molecular evolution in ectotherms Allen and Gillooly, ; Gillooly and Allen, Under this model, ectotherms in warmer areas have an increased metabolic rate governing consumption of oxygen and production of oxygen-free radicals that potentially increase mutation rate by damaging DNA Allen et al.
Comparing species with different metabolic rates is a proxy for comparison of different rates of oxygen-free radical formation and inferred mutation , which until recently could not easily be measured directly. New sequencing technology now allows for comparison of whole genomes of parents and their offspring, which can be used to estimate de novo mutation rates of species see below.
In comparison to other putative divers, the concept of metabolic rates and the LBG yields clearly testable predictions, but has been heavily contested since its origin Algar et al.
Unexpectedly, elevated rates of molecular evolution in the tropics compared with temperate regions have also been observed in some endotherms for example, mammals; Gillman et al. Metabolic rates in endotherms are closely linked to body size, with larger bodies having higher absolute metabolic rates but lower rates per unit mass.
But Gillman et al. There is, however, evidence from comparative studies that basal metabolic rate is de-coupled from molecular evolution in the bird and mammal mitochondrial DNA mtDNA that are widely used in these studies Lanfear et al.
Thus, the proxy for oxygen-free radicals metabolic rate may inform little on rates of mutation or molecular evolution, and we are no closer to understanding the driver of variation in the rate of molecular evolution and the formation of the LBG. Generation times are generally assumed to be shorter in tropical organisms than their temperate counterparts.
Having a short generation time could cause lineages to accumulate more mutations in a given time period compared with lineages with long generation times Thomas et al. Although under-examined, the predicted correlation between generation time and rates of molecular evolution has been observed in some groups of invertebrates, flowering plants, host—parasite systems and mammalian nuclear sequences Nikolaev et al.
Ohta suggested that variation in generation time and population size was the cause of the discrepancy between observed rates of protein and nucleotide evolution.
Plants especially large, long-lived species can be problematic subjects with regard to generation time, because mutations in reproductive tissue are also influenced by mutagenesis of somatic cells Gaut et al.
Little direct testing of generation times across latitudes has been undertaken, probably because the basic biology of many tropical species is not known. Linked to this problem is the issue of longevity, which refers to the number of cell cycles an individual undergoes rather than the life expectancy of the organism. This is another life history hypothesis that has yet to be implicated in the LBG, but has a similar relationship to rates of molecular evolution as generation time.
Longer-lived species with more cell cycles per individual are expected to be more disadvantaged by higher mutation rate, as premature aging could result from a high somatic DNA mutation rate Nabholz et al. UV is a mutagen and as a component of insolation is most constant and intense at the equator Figure 1; Rastogi et al. The proposed role of UV as a producer of heritable mutations is perhaps more straightforward in animals where exposure to UV could occur during embryonic development Epel et al.
In plants, UV has been proposed as a potential mutagen of pollen grains and experimental studies have shown its effects Ahmad et al. In mammals, a role for UV in germline mutations is less obvious but not incomprehensible Wilson Sayres and Makova, UV levels are generally higher in the tropics than in temperate regions, however, UV radiation, unlike temperature, does not decrease with altitude and may even increase Korner, ; Barry, A number of studies exploring differences in rates of molecular evolution have considered sister species pairs from different altitudes Wright et al.
In all cases plants, mammals and amphibians , inferred rates of molecular evolution were higher at lower altitudes, contrary to the prediction of UV as a biodiversity driver. The effects of population size are complex and important as they may interact with other putative drivers of LBG Charlesworth, There is an extensive literature relating to the influence of population size, comprising many contrasting views and interpretations.
Population size is often postulated as a component of LBG hypotheses, but is variously assumed to be larger and smaller in the tropics, although evidence for either view is limited Wright, ; Currie et al. Land area of islands is commonly used as a proxy for population size in studies of rates of molecular evolution in terrestrial organisms Woolfit and Bromham, ; Wright et al.
Although there is some limited evidence for latitudinal gradients in population size on a global scale, the topic is controversial because of the contested influence of population size on rates of molecular evolution. This is frequently confounded by a lack of clarity about what estimate of molecular evolution is being used: relative branch length or population variation. Theory predicts that large populations will have more genetic diversity at any one time compared with small populations Ellstrand and Elam, This is because smaller populations are more sensitive to genetic drift so that mutations are likely to go to fixation more quickly resulting in lower net genetic variation at the population level.
Furthermore, the fewer individuals there are in absolute terms the fewer mutations there will be in the population. Thus, large populations might contain the genetic diversity required for ecological speciation.
The connection between population size and relative branch length is less clear Johnson and Seger, ; Woolfit and Bromham, ; Nikolaev et al. Branch lengths in a phylogeny, expressing local rates of molecular evolution, are influenced by mutation rate and fixation rate. Under neutral theory the ratio of non-synonymous nucleotide mutations dN to synonymous nucleotide mutations dS will be equal to their respective proportions in the genome Kimura, Therefore, the rate of molecular evolution would not be influenced by population size under a neutral model.
Synonymous nucleotide mutations do not result in amino-acid substitutions and are usually neutral with respect to natural selection, having no phenotypic effect. In contrast, non-synonymous nucleotide mutations result in amino-acid substitutions and are therefore frequently not neutral. Almost all studies investigating population size and the rate of molecular evolution violate the assumption of neutrality by studying areas of the genome that are known to be constrained by selection for example, coding regions of mitochondrial genomes such as COI and Cyt b.
Nearly-neutral theory was developed in response to observations of a mismatch between inferred short-term and long-term mutation rates, and predicts that slightly deleterious mutations, which would be eliminated in large populations, may tend to persist longer in smaller populations where purifying selection is more relaxed Ohta, Thus, the nearly-neutral model predicts more non-synonymous substitutions in small populations, but synonymous substitutions should be independent of population size Ohta, Some empirical evidence agrees with this model Johnson and Seger, ; Woolfit and Bromham, ; Bakewell et al.
However, some of these studies have found the results to be weak, with one finding the reverse dN and dS were both lower in smaller than larger populations; Wright et al. Positive selection complicates matters, as the efficiency of selection in larger populations results in the retention of beneficial mutations, whereas in smaller populations beneficial mutations act more like neutral mutations Charlesworth, In addition, these studies have tended to use a limited number of markers; often a single-mtDNA sequence, which is subject to the Hill-Robertson effect due to selection and lack of recombination Charlesworth, The demographic influence of population size on the gradient of molecular evolution remains unresolved, due to the continued debate and conflicting empirical evidence on the influence of population size on molecular evolution rates and uncertainty about actual population size differences between high and low latitude species.
Evidence for differential rates of molecular evolution being correlated with latitude are compelling yet they lack one key feature as a model to explain the biodiversity gradient; no study has yet convincingly linked differences in rates of molecular evolution to differences in speciation, extinction or immigration rates Wright et al. Although the evidence for elevated rates of molecular evolution at the tropics is substantial, it remains only a pattern.
Many traits that have been linked to the biodiversity gradient, such as niche conservation, similarly describe a pattern an outcome rather than the process driver involved in generating the gradient Wiens and Graham, ; Losos, ; Crisp and Cook, Rates of molecular evolution and numbers of species show latitudinal clines, but are they directly related to one another, or is a third independent variable implicated?
For example, differences in branch length between high- and low-latitude phylogenetically independent angiosperms show higher rates of molecular evolution at low latitude, but this does not correlate with higher speciation rates.
Instead, the two rates were each independently linked to an environmental variable temperature or UV; Davies et al. Therefore, an indirect link might explain the observed relationship between rates of molecular evolution and biodiversity, or one of these two traits might drive the other.
Contrary to the idea that elevated rates of molecular evolution lead to increased speciation, some studies infer that elevated molecular evolution is a result of more speciation, referred to as punctuational evolution Pagel et al. Thus, the increased rate of molecular evolution in the tropics may be a product of speciation rather than the cause of it Webster et al.
Evidence of a link between rates of molecular evolution and net-diversification has been reported Barraclough and Savolainen, ; Jobson and Albert, ; Eo and Dewoody, ; Lancaster, ; Lanfear et al.
However, none of these studies have conclusively shown that increased rates of molecular evolution are responsible for elevated rates of speciation. Evidence that the rate of molecular evolution is driving speciation rate has been found in plants Lancaster, , but the inference relies on the approach used to model extinction Quental and Marshall, Also, it is unclear whether results from a multicopy nuclear marker that evolves via concerted evolution Internal Transcribed Spacer ITS can be meaningfully extrapolated to the rest of the genome.
An extensive sequence data set from extant birds Lanfear et al. Lanfear et al. This suggests that selection and population size were not influencing net-diversification rates, but that the rate of molecular evolution was the most important influence on the total number of species within a clade.
Nevertheless, this result does not exclude the possibility that speciation itself accelerated the rate of molecular evolution in these birds, or even that extinction which contributes to net-diversification is lower where molecular rate is high Figure 3. As noted, analyses of molecular genetic data are limited by their inability to directly identify extinction and its effect Weir and Schluter, ; Lancaster, ; Quental and Marshall, Molecular studies are not well suited to examining extinction rates because extinct taxa are not available for sampling.
Molecular phylogenies show inferred ancestral relationships among extant sampled taxa and do not show how extinction or failed sampling influence observed branch lengths. Varying rates of speciation and extinction can leave similar molecular phylogenetic patterns Rabosky and Lovette, ; Crisp and Cook, , so methods such as lineage through time plots may be misleading.
It is therefore difficult to disentangle the relative contribution to diversity of speciation and extinction without a good fossil record Figure 3 ; Quental and Marshall, Evidence from palaeontological studies suggests a combination of lower extinction and higher speciation rates in the tropics has driven the biodiversity gradient Jablonski et al. If rates of molecular evolution are faster in low latitudes as many have found Davies et al. A strict clock rate is inappropriate when it is probable that rates of molecular evolution vary among lineages.
Indeed, any variation in diversification rates among lineages, which is likely to be the rule rather than the exception, will interfere with estimations of true extinction rates Rabosky, Direct linkage of extinction rates to rates of molecular evolution has also been suggested; increased rates of molecular evolution could reduce extinction risk by increasing the number of beneficial mutations that enable lineages to persist longer Lanfear et al.
The effect of mutation rate on extinction is largely unknown, although excessive mutation rates will tend to yield a high genetic load Butlin et al.
The future role of molecular research investigating the biodiversity gradient is uncertain as studies have proven to be method-sensitive and open to variable interpretation. Considered individually, many molecular studies appear to provide compelling and relevant evidence, however, the associations they reveal rarely constitute direct evidence of a primary driver.
Many studies of the LBG run parallel to one another and cannot be directly compared because they employ very different methods. However, a consensus is developing that there is a correlation between rates of molecular evolution and species diversity; the challenge now is to identify their causal relationship. Palaeontology has the potential to track diversity trends through time and is therefore well suited to estimate extinction, speciation and immigration rates.
A recent test of the influence of seasonality on the biodiversity gradient shows the potential for palaeontology to narrow the field of possible drivers of the LBG Archibald et al.
Given that sampling of DNA sequence data is limited to extant species and there are known problems with estimating extinction rates from molecular data Rabosky, , it might be argued that molecular data have a limited role in examining extinction rates, a processes central to the biodiversity gradient debate. However, analytical tools are available if appropriate questions are asked Nee, , and our ability to incorporate extinction into phylogenetic models is improving Etienne and Apol, ; Morlon et al.
The use of data from measurably evolving populations for example, in rapidly evolving disease causing microorganisms; and other populations using ancient DNA approaches will continue to enhance our ability to quantify rates of molecular change and strengthen theoretical developments Drummond et al. Inferences about regional biodiversity and choice of entities for data sampling are both influenced by current taxonomy. In some studies, unrecognised species probably influence the conclusions drawn.
It is possible that differences among the extinction and speciation rates inferred in Weir and Schluter were, in part, artefacts of more intense taxonomic splitting of high-latitude versus low-latitude biota Tobias et al. Higher within-species genetic diversity has been found in plants and vertebrates from lower latitudes, and this could be taken to mean that there are more undescribed species in the tropics, or that the tropical taxa have larger population sizes Martin and Mckay, ; Eo et al.
Incorporation of phylogenetic information into estimates of biodiversity phylogenetic distinctiveness metrics is increasing Purvis and Hector, ; Davies and Buckley, , but phylogenetic approaches to estimating biodiversity rely in part on the same information that is, branch length and branching pattern being used to assess rates of molecular evolution. This circularity weakens inferences about drivers of the LBG that might be further influenced by lineage extinction and local biogeographic history.
Mitochondrial DNA has been used in several studies comparing rates of molecular evolution among lineages with speciation rates. There is some evidence that molecular evolution rates from the mitochondrial genome and the nuclear genome are subject to different pressures that might reflect a difference in proximity to cell metabolic processes or natural selection Welch et al. Mitochondrial DNA is frequently used in analyses that assume neutrality in terms of molecular evolution, but this is unlikely to be realistic for review see Galtier et al.
Given the metabolic role of mitochondria, the mitochondrial genome could be implicated in climate adaptation, and thus far from neutral in the context of the LBG. This idea has been mooted for human mitochondrial genetics Ruiz-Pesini et al.
This could be particularly problematic in plants where hybridisation has an important role in species formation and exchange among lineages Hegarty and Hiscock, To date, selection of DNA sequence loci has been strongly influenced by the availability of data and universal PCR primers, a problem which is being alleviated by the use of next generation sequencing NGS technology.
It is clear that we need approaches that avoid reliance on uncertain proxies. Tests of associations between putative drivers and speciation rates currently rely heavily on substitutes that are assumed to correlate well with the characters of interest. Many of these correlations first need to be independently verified in the organisms of interest. Also, the design of many studies reflects methodological and sampling limitations, and has often dealt with comparison of just two alternative conditions for example, big population versus small population, high latitude versus low latitude rather than analysing gradient or transect data.
Although continuous traits are often measured for example, population size , they are then categorised into two alternatives for example, big versus small in order for inferences to be drawn from paired comparisons for example, sign tests.
Mutation rates vary between species, between individuals and within the genome itself, so disentangling all the drivers of mutation rates is unlikely to be easy Hodgkinson and Eyre-Walker, Clarity of assumptions and experimental design and application of methods at the appropriate evolutionary level are essential, and here we suggest some directions that might take us forward.
Many of the life-history traits that have been implicated as driving rate changes are not independent of one another and work needs to focus on taxa that allow the elimination of some of these variables. The sister species approach has some benefits over the large clade approach, but for life-history traits that cannot be adequately represented across sister species pairs, additional methods will be necessary see Lanfear et al.
Quantification of life history traits in specific organisms of interest is required to avoid reliance on rules of thumb and generalised concepts that are often based on limited direct evidence. An emphasis on regionally focused examples should enhance opportunities for gathering accurate life-history data, and inclusion of specific geophysical information when accommodating effects of local biogeographic history. Until recently, some studies of rates of molecular evolution across the LBG have used single genes from the mtDNA genome and a single branch from each species.
NGS technologies now provide multilocus data that improve confidence in estimates of relative rates of molecular evolution across the genome in sister species comparisons, even for non-model organisms.
In the past, when more than one sequence was available per species the shortest branch was routinely chosen for example, see Wright et al. NGS allows dense population sampling of sister species pairs that allows analysis of the effects of choosing the shortest branch versus other metrics, such as average branch length or random branch selection. Studies, particularly those involving high genetic divergence, may also benefit from an out-group comparison Hugall and Lee, Population level studies have the potential to inform on speciation processes at a stage equivalent to the node in a species tree, instead of relying on inferences about past speciation from tree tips.
For understanding the LBG, determining population size variation between latitudes is important. Thus, a focus on multi-locus data for taxa with sedentary lifestyles would facilitate population density comparisons. If a consistent difference in population size among latitudes exists then the interaction between population size and rates of molecular evolution becomes extremely important to the LBG debate.
Multi-locus data sets exist for model organisms for example, humans and Drosophila ; Bakewell et al. However, population size may not be constant through time and methods for inferring past population size that are independent of rates of molecular evolution will be required. If speciation occurs predominantly in small isolated populations Venditti and Pagel, , signal about the size of the populations during speciation may be lost through processes such as back-mutation during subsequent population expansion Charlesworth and Eyre-Walker, ; Lanfear et al.
It will also be possible to estimate how long after a speciation event the population genetic signature remains detectable. Analysis of recent-past population diversity using ancient DNA methods could enhance modelling of rates of molecular evolution and population size. Similarly, studies involving ancestral gene reconstruction and resurrection will likely illuminate our understanding of the respective roles of genetic drift and selection on gene evolution Thornton, Expansion of studies involving lineages represented in the fossil record and neobiota will contribute broadly to meshing information on speciation, extinction and migration with rates of molecular evolution.
In particular, such data will help explore the directionality of the relationship between rates of molecular evolution and speciation. Sequencing multiple loci from extant species that are members of fossil-rich lineages will allow extinction rates to be correlated with both speciation and molecular evolution rates.
This has the potential to isolate speciation from net-diversification, an important step towards resolving the link between rates of molecular evolution and speciation. Fossil data can be used to constrain molecular clock analyses and allow rate comparisons that are not limited to sister species approaches. Measures of extinction could reveal any relationship between rates of molecular evolution and extinction.
Sequencing of parents and their offspring allows species-specific estimates of mutation rate, and if the necessary multiple comparisons and multiple tissues are included, the population variance in individual de novo mutation rate can be known Hodgkinson and Eyre-Walker, Two experimental approaches using this method could contribute directly to the study of LBG. First, the effect of temperature and UV on mutation rate could be assessed using lab-based model organisms in controlled environments.
This would allow discrimination between the two components of molecular evolution, variation in rates of mutation and variation in rates of fixation Ho et al. In addition, mining pedigree data sets that exist for applied research in agriculture, horticulture and conservation will allow tests for a link between longevity of individuals within species and mutation rate.
Differences in rates of molecular evolution between global regions are fascinating and warrant much more focused research. But perhaps the priority should be to find the directionality of the link, if any, between speciation and molecular evolution. Without this fundamental information the evolutionary speed hypothesis cannot provide compelling predictions to test putative drivers of the LBG. Haploid culture and UV mutagenesis in rapid-cycling Brassica napus for the generation of resistance to chlorsulfuron and Alternaria brassicicola.
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