Understanding the Dynamics of Trends Within Evolving Lineages
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Paleobiology, 26(3), 2000, pp. 319±329 MATTERS OF THE RECORD Understanding the dynamics of trends within evolving lineages John Alroy National Center for Ecological Analysis and Synthesis, University of California, 735 State Street, Santa Barbara, California 93101. E-mail: [email protected] Accepted: 3 May 2000 Introduction between underlying hypotheses and methods for testing these hypotheses has sometimes The study of evolutionary trends is one of been weak. Although paleontologists have the oldest and most intriguing topics in evo- long since embraced the use of null models, lutionary biology and paleobiology (McNa- they still do not employ a single, straightfor- mara 1990). Workers since Cuvier, Lyell, and ward de®nition of ``randomness'' (Eble 1999). Owen have wanted to know if the fossil record As a result, different authors use different null demonstrates ``progression'' within temporal hypotheses, and depending on their concep- sequences of related organisms. Regardless of tual outlooks they may interpret the same whether changes in the average values of mor- kinds of patterns as either con®rming or re- phological characters are progressive in any futing the existence of trends. Considering the meaningful sense, these changes are still of volume of literature and the century-long ge- great interest. In practice, questions about nealogy of the subject, this lingering panoply trends are most commonly framed by pale- of viewpoints may come as a surprise. ontologists in terms of ``complexity'' (however Two points of confusion are responsible for de®ned) or body size (McShea 1998a). the lack of agreement. First, it has been rec- Research on evolutionary trends has inten- ognized for more than two decades now that si®ed over the last few years, bringing several morphological trends may result from pro- fundamental conceptual issues to a head. cesses operating at different levels of the phy- Here I analyze these conceptual problems, logenetic hierarchy (Stanley 1975; Hull 1980). concluding that paleontologists should large- However, it is not always easy to say whether ly abandon a key method used in most stud- a particular pattern shows that nonrandom ies: comparing among-species morphological evolution is occurring at a particular hierar- distributions in successive time slices, irre- chical level (Hull 1980; Hoffman 1989). Sec- spective of phylogenetic patterns. These data ond, most workers have tried to ®t evolution- supposedly may distinguish random evolu- ary patterns into binary categories of random- tion, constant directional trends, and diffu- ness and nonrandomness. However, nearly sion away from morphological boundaries. identical evolutionary trends may be generat- However, many other simple evolutionary dy- ed by a surprisingly large number of qualita- namics may result in the same nonrandom tively distinct mechanismsÐeven within a trends, making it dif®cult to distinguish qual- single hierarchical level. itatively distinct mechanisms using time-slice The real question, then, is not whether evo- data. Thus, I will try to show that comparisons lution is random, but instead exactly what between ancestral and descendant morphol- mechanisms do govern demonstrable trends, ogies need to be made instead. and at what hierarchical level these mecha- The fundamental problem here is not so nisms operate. The ancestor-descendant meth- much mathematical or statistical as conceptu- od advocated here may solve both problems at al. Null hypotheses have not always been once. It should often point to a proximate framed rigorously, and the logical connection mechanism that generates a trend if there is q 2000 The Paleontological Society. All rights reserved. 0094-8373/00/2603-0001/$1.00 320 JOHN ALROY one, illustrating such a mechanism clearly nonrandom evolution within lineages after even if it is quite complex. Because the data controlling for apparent sorting among line- only apply to within-lineage patterns, the ap- ages (Wagner 1996). Likewise, recent results proach also avoids the common error of con- on body mass in mammals (Alroy 1998) and ¯ating processes operating at different hier- suture complexity in ammonites (Saunders et archical levels. al. 1999) also can only be interpreted as show- ing nonrandom evolution within lineages. Within- and Among-Lineage Trends Still, though, these three studies didn't try to There are many ways to get from one dis- quantify the relative importance of within- tribution of morphologies at one time to an- and among-lineage factorsÐthey merely other at a later time (Stanley 1975; McNamara sought to establish the fact that within-lineage 1990; McShea 1998a,b). For example, if most trends are real. members of a clade are small early on, but most are large at a later date, that could be a ``Passive'' and ``Driven'' Trends random pattern, or it could be because either Over the last decade there have been many (1) natural selection (or conceivably mutation other studies of trends in morphospaces (i.e., pressure, or an ecophenotypic response) has distributions of species across quantitatively operated within evolving populations to in- de®ned morphological gradients). The mor- crease mean body size; (2) species with small phospace tradition was initially inspired by average body size are unlikely to give rise to Raup (1966), and it has ¯owered greatly with daughter species; or (3) small species are more improvements in multivariate statistical meth- likely to go extinct. ods and computational technology. However, These scenarios break down into two even most morphospace studies have yielded rela- narrower categories: trends (random or not) tively ambiguous results. I would argue that that operate within evolving populations, and the reason is a conceptual hurdle. Instead of trends that operate by sorting among species. focusing on within- vs. among-lineage pat- The same within- vs. among-lineage distinc- terns, the literature on morphological trends tion was recognized decades ago and was the typically invokes a very different argument subject of intense theoretical debate in the presented by Stanley (1973), Fisher (1986), and 1980s (Hull 1980; Hoffman 1989; McNamara McShea (1994). 1990). Surprisingly, relatively little empirical As McShea (1994) explains so clearly, a non- research followed directly from this concep- random trend could result from one of two tual advance. process. First, there could be a constant force One possible explanation is the dif®culty of (such as a bias in the probability of change) obtaining high-quality data. To show un- that operates across all morphologies (an ``ac- equivocally that a nonrandom pattern result- tive'' or ``driven'' trend). Second, there could ed mainly from a mechanism operating with- be a force that is variable across the range of in, but not among, lineages, one would have morphologies (such as an absorbing or re¯ect- to not only examine changes within lineages ing boundary), which might truncate the dis- but quantify speciation and extinction rates tribution of morphologies (``passive diffu- across the same clade. So one would have to sion''). For example, there may be an absolute study a clade with many species, because oth- minimum body size that pertains to all mem- erwise quantifying speciation and extinction bers of a clade. Some workers have indeed rec- rates precisely would be impossible. ognized that such a boundary could be main- Very few studies have met these criteria. tained by either within- or among-lineage fac- Norris's work on foraminiferans came close, tors (e.g., McShea 1998b), but for the most part but he did not have access to phylogenetic they have not stressed the point. Even more data that would have allowed direct analyses importantly, many workers have interpreted of evolving lineages (Norris 1991). However, McShea's 1994 paper to mean that passive dif- Wagner's later study of early Paleozoic snails fusion is no more interesting than purely ran- did include phylogenetic data, and it did show dom evolution, leaving only driven trends as MATTERS OF THE RECORD 321 evidence of genuinely interesting evolutionary processes. McShea (1994) proposed a ``subclade'' test that attempted to distinguish passive and driven trends by contrasting static distribu- tions of morphologies observed at different times. One strength of this test is that it does not require a detailed phylogeny, unlike the ancestor-descendant comparisons that Mc- Shea recognized as the alternative basis of FIGURE 1. A simple case in which a trend within line- ages plays out across a phylogenetic tree. The trend con- such analyses. Although most obviously ap- stitutes a bias in the direction of change across all plicable to paleontological data sets, McShea's branches of a tree. The ``small'' or ``simple'' morpholo- test has drawn so much attention that it has gies at the left-hand side of a morphological spectrum tend to move quickly toward more ``large'' or ``com- even been employed by comparative biolo- plex'' morphologies to the right (A); but once those mor- gists using neontological data (e.g., Maurer phologies are attained, there is little additional change. 1998). The fact that the same pattern is seen across indepen- dent lineages shows that the trend is not random, but Many studies have operated within the pas- instead the deterministic result of underlying dynamic sive/driven trend paradigm, but few of them rules. The rules are best illustrated by ``breaking'' the have found conclusive results. Passive diffu- branches to produce a plot (B) of