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Speciation and Bursts of Evolution Evo Edu Outreach (2008) 1:274–280 DOI 10.1007/s12052-008-0049-4 ORIGINAL SCIENTIFIC ARTICLE Speciation and Bursts of Evolution Chris Venditti & Mark Pagel Published online: 5 June 2008 # Springer Science + Business Media, LLC 2008 Abstract A longstanding debate in evolutionary biology Darwin’s gradualistic view of evolution has become widely concerns whether species diverge gradually through time or accepted and deeply carved into biological thinking. by rapid punctuational bursts at the time of speciation. The Over 110 years after Darwin introduced the idea of natural theory of punctuated equilibrium states that evolutionary selection in his book The Origin of Species, two young change is characterised by short periods of rapid evolution paleontologists put forward a controversial new theory of the followed by longer periods of stasis in which no change tempo and mode of evolutionary change. Niles Eldredge and occurs. Despite years of work seeking evidence for Stephen Jay Gould’s(Eldredge1971; Eldredge and Gould punctuational change in the fossil record, the theory 1972)theoryofPunctuated Equilibria questioned Darwin’s remains contentious. Further there is little consensus as to gradualistic account of evolution, asserting that the majority the size of the contribution of punctuational changes to of evolutionary change occurs at or around the time of overall evolutionary divergence. Here we review recent speciation. They further suggested that very little change developments which show that punctuational evolution is occurred between speciation events—aphenomenonthey common and widespread in gene sequence data. referred to as evolutionary stasis. Eldredge and Gould had arrived at their theory by Keywords Speciation . Evolution . Phylogeny. studying patterns of evolution in the fossil record. It was Punctuational evolution . Charles Darwin well known even by Darwin that, when viewed in the fossil record, many species show rapid bursts of change that are “ As natural selection acts solely by accumulating slight, often followed by longer periods in which little or no change successive, favorable variations, it can produce no great or occurs. However, detractors often argued that the appearance sudden modifications; it can act only by short and slow of punctuated change, and especially of stasis, was an artifact ” steps (Darwin 1859). of the incompleteness or coarseness of the fossil record. The Understanding the range and diversity of organisms we idea is that the vast geological time periods represented by see today is a fundamental challenge for biology. To meet fossil strata obscure many of the smaller changes that occur this challenge, evolutionary biologists attempt to describe on a “real-time” basis. If so, the geological data will be of how and why evolutionary changes accumulate over time. very low resolution, giving the appearance of stasis when in ’ Charles Darwin s 1859 theory of evolution by natural fact gradual changes have occurred (Hecht and Hoffman selection put forward the idea that evolutionary change or 1986;Hunt2007). In reply, Eldredge and Gould maintained divergence accumulates steadily and slowly, in small steps, that patterns consistent with punctuational change were as one species succeeds the previous one in the struggle for observed even when the fossil record was exceptionally existence. It was a revolutionary idea at the time, but now good; this led them in 1977 to coin the phrase “stasis is data” : (see Gould and Eldredge 1977), which has become C. Venditti M. Pagel (*) something of a motto for their theory. School of Biological Sciences, University of Reading, More than 30 years on and the theory of punctuated Whiteknights, Reading RG6 6AJ, UK equilibria remains controversial, with many biologists still e-mail: [email protected] unconvinced as to the value of the theory as a description of Evo Edu Outreach (2008) 1:274–280 275 how evolution proceeds. Much of the controversy sur- rounding it stems from the perception that Eldredge and Gould were in some way suggesting that new non- Darwinian evolutionary processes were required to explain punctuational evolution—that, somehow, the processes of change had become unhooked from the gradual tug of natural selection. These issues are discussed by Niles Eldredge (2008) in the article “The early ‘evolution’ of ‘Punctuated Equilibria’” in a previous issue of this journal. In spite of these clarifications, and although attempts have been made to assess how common punctuated evolution is in the fossil record (see Erwin and Antsey 1995), there remains little consensus as to the generality of the theory. Several years ago, we realized that there might be a way to investigate punctuational evolution without having to resort to observations taken from the fossil record. Our idea was that historical events of punctuational and gradual evolution should leave a distinctive signature on what biologists call phylogenetic trees. In the next two sections, we describe what a phylogenetic tree is and how biologists use them. We then go on to discuss how we employed them to search for evidence of punctuational bursts of evolution. Fig. 1 Charles Darwin’s first known sketch of a tree describing how organisms are related. The sketch appears in Darwin’s first notebook on transmutation of species dating from 1837. (Image credit: © Auckland Museum.) Phylogenetic Trees In publishing his theory of evolution by natural selection, Phylogenetic trees can tell us more about the patterns Charles Darwin showed that groups of species diverge over and process of evolution than is at first obvious. Phylog- time from a common ancestral species by a process of what enies, by tracing the evolutionary history of a group of he called “descent with modification.” Darwin was among organisms over a long period of time, can be used to infer the first to use the metaphor of a tree to illustrate this idea. the historical processes of evolution that gave rise to the In much the same way that we often draw family trees or diversity we see today. The nodes or points on the tree genealogies to represent the lines of descent within a where a split occurs represent the historical event of an family, phylogenetic trees describe how, over evolutionary ancestral species giving rise to daughter or descendant time, ancestral species give way to new descendant species. species. The branches between these nodes represent Darwin repeatedly used tree diagrams to convey his periods of evolution between speciation events. It is natural thoughts (Fig. 1), and as a measure of the importance he to think of these branches as recording the amount of time assigned them, the only figure in the Origin of Species between speciation events. However, if the phylogeny is depicts an idealized tree showing species, speciation and constructed from gene-sequence data, the length of the extinction events. branches records the amount of molecular or gene-sequence Today, biologists reconstruct evolutionary history to evolution that has occurred over the time period the branch reveal how species are related to each other. Like Darwin, represents (Fig. 2); that is, it records how different in an they represent this information in a tree-like form. The field evolutionary genetic sense the daughter species is from its of study for inferring phylogenies is called phylogenetics ancestor. In a previous issue of this journal, Gregory (2008) and the trees produced are called phylogenies or phyloge- provides a more in-depth explanation of phylogenic trees netic trees. The term phylogeny literally means the origin or and how to interpret them. genesis of groups. Up until recently, phylogenies were inferred from the analysis of the similarities and differences among species based on a suite of morphological character- Punctuated Molecular Evolution istics. It is now relatively easy to obtain gene-sequence information, and genetic data have largely replaced mor- The possibility of a link between the rate (or amount) of phological characters as the data of choice for inferring evolution and speciation has interested biologists for over trees of contemporary species. 50 years (Simpson 1944; Mayr 1954). In contrast to a 276 Evo Edu Outreach (2008) 1:274–280 Species 1 separates the common ancestral species from its descend- Species 2 ants. The second value we record is the number of nodes or Species 3 historical splitting events along each path through the tree. Species 4 Species 5 Some lineages, such as many birds or monkeys living Species 6 today, have gone through many speciation events since Root Species 7 their respective common ancestors. By comparison, it Species 8 would be interesting if, for example, species we think of Species 9 today as “living fossils” have had few speciation events Species 10 since the time of their last ancestor millions of years ago. Figure 2 shows how the two values that we measure can Amount of evolution be obtained from a phylogenetic tree. If evolution proceeds Fig. 2 A phylogenetic tree, the filled circles are called nodes and represent an event of speciation in which—moving from left to right— gradually and independently of speciation events, there one species becomes two. The horizontal branches measure the amount would be no association or correlation between the number of genetic evolution between speciation events. The vertical lines have of nodes (historical speciation events) and path length (see ‘ ’ no meaning and are just for spacing. What we refer to as a path (see Fig. 3a,c). That is, if the process of speciation plays no role text) is highlighted in red through the tree. The amount of evolution along a path (sum of the horizontal branches) is called the path length. itself in causing change, then we do not expect that modern The red circles identify the speciation events along that path species at the end of paths through the tree with more speciation events will have diverged more from their gradualistic view of evolutionary change, the existence of a common ancestor at the base of the tree.
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