rates of evolutionary change during the radiation of late neogene hominoids

Bennett Blumenberg

Faculty of Sciences, L&y Colkgc, 29 Everett St., Cambridge, MA. 02238, U.S.A.

Received 26 October 1979 and accepted 6 March 1980

Keywords: evolutionary rate, hominoids, mandible, morphometric distance, Neogene, phylogeny, Punctuated Equilibria, teeth.

the

The phylogenies of Andrews and Simons provided the descriptive framework within which the pacing of evolutionary change during the radiation of late Ncogene hominoids was investigated. The analysis can be adequately encompassed within a scheme comprised of four rate classes. Rates of change between individual teeth are almost completely uncoupled. Rates of change in mandibular structure and in the few available long bone measurements are of the same order of magnitude as the rate indices that characterize tooth change and are highly correlated with such indices. Within the slow rate classes, the rate of evolutionary change is coupled to the degree of morphometric separation realized. With the moderate and rapid rate classes, no such relationship is revealed. Distantly related taxa are frequently grouped in the same rate class, an observation that suggests similarities in the capacity to respond to selective pressures of comparable intensity within many late Neogene hominoids. Evolutionary rates appear to be more rapid in pongid and hominid phyletic lines than in other hominoid lineages. The Simon’s phylogeny meets a criterion of minimal morphological gaps better than does that of Andrews. Consideration of both elephant and hominoid rate data suggests that a pattern of change that exhibits a slowing down midway through the history of a lineage may be of general significance. Examples of very rapid change support one feature of the Punctuated Equilibria model and raise questions concerning the nature of the evidence for slow rates of morphometric progression.

1. htroduction

Metric data are now available for many late Neogene hominoids and such measurements

are most numerous for dental remains that constitute the majority of the fossil material

(cf Andrews, 1978). Improvements in radiometric dating techniques and geological-

fauna1 correlation during the past 20 years (cf. Bishop & Miller, 1972) now allow for a

preliminary assessment of rates of evolutionary change during the radiation of such

hominoids and for a brief comparative discussion of such results with the limited rate

data published for other mammalian fauna of the Cenozoic. This paper will concern

itself with two of the five issues considered by Campbell (1963) to be priorities for taxo-

nomists; morphological divergence and rates of evolution between populations. Two

widely discussed phylogenetic schemes for late Neogene hominoids, those of Andrews

(1978) and Simons (1972, 1976a,b), will provide the framework for delineating ancestor-

descendant relationships.

The only published information that now exists for rates of evolution between extinct

hominoid taxa is concerned primarily with the nature of endocranial volume (ECV) in-

crease during the evolution of Homo sapiens from the Plio/Pleistocene hominids (Blumen-

berg, 1978; Bon& 1962; Cutler, 1976; KurtCn 1960; Lestrel, 1976; Parenti, 1973). Bone

(1962) was also interested in rates of evolution within the dental complex of Plio/

Pleistocene and later hominids. This study will average indices of morphometric distance

and rates of change over all tooth, mandibular and long bone elements held in common

Journal of Human Evolution (1980) 9, 299-323

0047-2484/8OjO40299-25 $02.00/O @ 1980 Academic Press Inc. (London) Limited

300 B. BLUMENBERG

between taxa. The results of such calculations will be discussed with reference to (a) the magnitude of evolutionary rate; (b) the correlation between results that describe tooth and mandibular evolution, respectively; (c) the degree to which the achievement of a particular degree of morphological distance by a descendant from its presumed ancestor is coupled (i.e. correlated with) to a specified rate of change; (d) the two phylogenies under consideration; (e) available evolutionary rate data for other mammalian fauna of the Cenozoic; and (f) macro or quantum evolutionary processes, particularly as conceptualized by Eldridge & Gould (1972).

While it is beyond the intended scope of this paper to enter into the current debate over the application of cladistic methods to problems of primate systematics, the fact that this study was undertaken at all implies a rejection of such methodology. An important objection to the application of cladism to paleontological reality is the insistence of the method on ignoring the relative age of taxa. Fossils can only be considered once the relationships among living members of the group have been worked out in terms of a cladogram. This approach seeks to unite taxa into monophyletic lineages on the basis of synapomorphy. In a cladogram, at each branching point, a new monophyletic lineage is distinguished by a change in at least one derived character state; paraphyletic lineages are rejected. Sister groups are given identical rank and adjacent lineages, and all those subsequently descended from them, will have the same taxonomic rank as the preceding lineage (Hennig, 1966). Such cladograms fail to take into account ancestor-descendent relationships and fail to attempt to model the process of evolutionary change through time. Age can certainly be considered the fourth dimension of any group or taxon (Szalay, 1977; Van Valen, 1978). Although more imprecise than cladograms, phylogenies and scenarios are to be preferrred because of their attempt to grapple with the existence of paleogeographic history. The method of reconstruction of evolutionary history that underlies the phylogenies under consideration here may best be termed stratophenetic (Harper, 1976, Gingerich & Schoeninger, 1977). While allowing for cladistic considerations, this approach emphasizes the conviction that the reconstruction of evolutionary history must attempt to explain the available morphological, geographic and stratigraphic data; i.e. the totality of the adaptive history of the group must be considered (Van Valen, 1978). For a defense of the cladistic method as it applies to several problems of primate systematics see Delson (1977)) Delson, Eldredge & Tattersall (1977) and Schwartz, Tattersall & Eldredge (1978).

2. Methods

Dental and mandibular metric data have been taken from the following sources. For the Oligocene Fayum primates, Simons ( 1965) was consulted. References for the African Miocene hominoids were Andrews (1978), Andrews & Walker (1976), Fleagle & Simons (1978), and Pilbeam (1969). Papers consulted for measurements on Asian Miocene and Pliocene material were Frayer (1973), Pilbeam (1969), Pilbeam et al.

(19776), Prasad (1968, 1969), Tattersall & Simons (1969), Simons & Chopra ( 1969) and Woo (1957, 1958). Data for European Miocene hominoids were taken from de Bonis et al. (1974) and de Bonis, Bouvrain & Melentis (1975), Hiirzeler (1954), Simons & Pilbeam (1965), Tekkya (1974) and von Koenigswald (1972). It should be noted that the Candir remains are considered to represent Ramapithecus wickeri according to Andrews & Tekkya (1976). Table 1 lists the taxa considered in this analysis.

Table 1.

RATES OF EVOLUTIONARY CHANGE

Taxa and their calibration

301

Taxon n* Age

(X l(ryears) Reference

Oligopithecus sauagei Propliopithecus haeckli Acleopithecus chirobates Aegyptopithecus zeuxis

38 Simons (19766) 35 Simons (19766) 34 Simons (19766) 32 Simons (19766)

Dendropithecus macinnesi Proconsul (R.) gordoni Micropithecus clarki Limnopithecus leg&et Proconsul nyanrae Proconsul afn’canus Proconsul major Ramapithecus wickeri

1 2 2

Miocene-Pliocene 24 10

5 21 19 20 14 4

18-22 18-20

17.8 14-20 14-20 14-20 14-20 11-15

Pliopithecus vindobonensis

Dryopithecus fontani Dryopithecus laietanus

--t 10-16

-I 12.5-14 9.5-15

Dryopithecu.r macedoniensis 2 IO-12

Graecopithecus freybergi 1 8.5 Ramapithecus punjabicus 7 9-13 Dryopithecus ketyuanensis 2 10-12

Sivapithecus indict Sivapithecus sivalensis Gigantopithecus bilaspurensis Gigantopithecus blacki

Pongo pygmaeus weidenreichi Symphalangus syndactylus

subfossilis Hylobates sp. subfossilis

15 7-13 9 7-13 1 7-10 3 >l

Pleistocene-Recent 1 ?l

79 ?O.Ol ?O.OO 1

2 ?O.Ol ?O*OO 1

Pongo pygmaeus 349 palaeosumatrensis

Symphalangus syndactylus 23 syndactylus

Hylobates agilis 19 Pan paniscus 73 Pan troglodytes 66 Gorilla gorilla 83 Pongo pygmaeus (Ashton & Zuckerman, 1950) 39 Pongo pygmaeus (Hooijer, 1948) 64

?O.Ol 70.001

-

-

- -

Andrews (1978), Bishop (1972) Andrews (1978), Bishop (192) Bishop (1972) Andrews (1978), Bishop (1972) Andrews (1978), Bishop (1972) Andrews (1978), Bishop (1972) Andrews (1978), Bishop (1972) Andrews & Tekkya (1976), Bishop, Miller & Finch (1969) Andrews (1978), Pilbeam (1972) van Koenigswald (1972) Berggren & Van Couvering (1974), Simons (1972, 1976b) Berggren & Van Couvering (1974), de Bonis, Bouvrain & Melentis (1975), de Bonis et al. (1974) van Koenigswald (1972) Pilbeam et al. (1977a) Berggren & Van Couvering (1974), woo (1957) Pilbeam et al. (1977a) Pilbeam et al. (1977a) Pilbeam et al. (1977a) Eckhardt (1974)

Hooijer ( 1948) Hooijer (1960)

Hooijer (1960)

Hooijer ( 1948)

-

- - -

* Maximum sample size for any single tooth in the fossil material consulted. t Sample size data not available.

302 B. BLUMENBERG

Dental metric data for Pleistocene and subfossil species of Pongo were taken from Hooijer (1948). With respect to P. pygnuw~ palaeosumatrensi’s, Marcus’ (1969) argument for significant differences between samples from Sibrambang Cave and Lida Ajer Cave is not accepted. Using the data on unworn teeth (Hooijer, 1948), Student’s t-test for the significance of the difference between sample means was applied to the length and breadth measurements available for each tooth. Out of 25 such comparisons between the two caves, only four reveal statistically significant differences: P3 B (0.05 > P > 0.02) ; Ml B (0.05 > P > 0.02); P, L (O-01 > P > 0.001) and P, B (O-02 > P > 0.01). Sufficient data were not available to permit between cave comparisons on Is B, Ms L and M, B. All P. pygmaeus palaeosumatrensis material will be considered to represent a single sample for the purposes of statistical analysis. Hooijer (1948) reached the same conclusion. Dental metric data for subfossil species of Symphalangus and Hylobates are taken from Hooijer (1960).

Dental measurements for Go&a gorilla and Pan troglodytes are taken from Almquist (1974) and Pilbeam (1969) ; for Pan paniscus from Johanson (1974) ; for Pongo pygmaeus

from Ashton & Zuckerman (1950) and Hooijer (1948); and for Hylobates agilis and Sym+zlangus syndactylus syndactylus from Hooijer (1960). The data for P. pygmaeus

in Ahston & Zuckerman (1950) and Hooijer (1960) were not pooled and each sample will be treated separately in statistical procedures. Student’s t-test revealed a significant difference between means for nine out of 28 measurements compared between the two samples: I1 B (O-05 > P > O-02); C L (O-01 > P > 0.001); Pa L (0.01 > P >

0.001) ; Ps B (P = 0.01) ; C B (0.02 > P > 0.01) ; Pa L (P < 0.001) ; P, B (O-01 > P >

0.001); P, L (P < O-001) and P, B (O-02 > P > 0.01). Selected ECV and postcranial skeletal measurements were obtained from Fleagle &

Simons (1978), Fleagle, Simons & Conroy (1975), Pilbeam & Simons (1971), Radinsky (1974, 1975), Simons (1972), Tobias (1971) and Zihlman & Cramer (1978).

The error inherent in drawing measurements from such diverse sources must be appraised. Aside from alterations in tooth dimensions introduced by wear and attrition, such error can be divided into three components; inter-observer, intra-observer and methodological. Tooth length and breadth were the only dental dimensions used in this analysis. When a choice was available, dimensions quoted as maximum were used. Tooth breadth is standardized as (maximum) dimension perpendicular to length. However, two slightly different methods of defining molar crown length exist, maximum tooth length and the middle axis tooth dimension. Frayer (1978) estimates the differences revealed by these two methods to be of the same magnitude as the intra-observer error with anterior teeth least susceptible to this source of error. Johanson (1974) and Leutenegger (1971) report an average error of 0.2 mm on material measured repeatedly by the same worker. Wolpoff (1971a, p. 17) reports a relative measurement error for length determinations of 1% (0.09 f 0.22 mm) ; for breadth measurements of 0.5 “/, (0.08 f 0.16 mm) and for area determinations of 1.9% (1.3 mm2) in his own work and an error range of 0*001--0*2 mm in other studies.

Because this study draws upon measurements determined by a number of different workers, the problem of inter-observer error is particularly relevant. Wolpoff (197la) surveys a number of reports and concludes that a reasonable range for inter-observer error is 0*05--0*15 mm. He reports one glaring exception with an error range of 0.9-l cm ( !) that fortunately for this exercise involves Plio/Pleistocene hominids (Robinson 1965 ; Tobias & von Koenigswald, 1964). Frayer (1978) determined the level of inter-observer

RATES OF EVOLUTIONARY CHANGE 303

error between himself and Lefevre (1973) to be 04-0*9 % for mandibular measurements and 2.2 o/0 for maxillary dimensions. It seems safe to assume that inter-observer error is slightly higher than the uncertainties introduced by repeated measurements of a single worker. Wolpoff (1971~) believes that when the data used for a single taxon are culled from several publications, the errors introduced will tend to cancel each other and will therefore not affect the comparisons developed. It seems unlikely that the total error introduced by surveying the literature for metric data exceeds 5 %.

Not all available dental metric data have been used. Measurements were selected that appear to have functional and metabolic significance; i.e. significant components of functional complexes have been deliberately selected for quantitative study as recom- mended by Campbell (1963). S UC h a selection of metric data, while not employing all measurements available, attempts to avoid the pitfall of weighting with extreme subjec- tivity that results from emphasizing only one or two features of the dentition (Campbell, 1963). Nonetheless, where the fossil material that exists is very sparse (0. savagei, P.

haeckli, D. fontani, G. freybergi, H. subfossilis, P.p. weidenreichi) such a narrow and distorted focus in the data analysis is unavoidable.

Incisor size, as measured by the width of the maxillary incisor tooth row, has been found to be highly correlated with body weight. Deviations from the regression line are explained by the observation that anthropoids whose diets consist primarily of large, tough fruits have larger incisors than those that eat leaves, grass or berries (Goldstein, Post & Melnik, 1978; Hylander, 1975). The width of the maxillary tooth row is rarely available for fossil material so in this analysis the width of individual incisors has been substituted. Female upper canine size is also highly correlated with body weight with females of monogamous species and those living in multi-male groups having larger canines than those living in harem groups (Harvey, Kavanagh & Clutton-Brock, 1978a,b). Due to imperfect canine preservation, comparisons between 0ligopithecu.r and D. ma&m&,

and Oligopithecus and L. legetet substitute canine length for area (Simons, 1965). Post- canine area in primates has been shown to be correlated with body weight also (Kay, 1975). Variations in the allometric relationship are attributable to dietary differences between the three fundamental classes of insectivores, frugivores and folivores with leaf eating species tending to have larger teeth for their body size than fruit eating species (Goldstein, Post & Melnik, 1978). Maxillary postcanine area shows a highly significant allometric relationship with adult skull length, an observation suggesting that the amount of mastication required by preferred food is an important factor selecting for tooth size (Pirie, 1978). Individual tooth areas and summed postcanine areas in both the maxillary and mandibular dentition have been found to be highly correlated with several indices descriptive of the femur and humerus in a population of Rhesus Monkeys (Lauer, 1975). Area measurements have been employed as a functional measure of the posterior dentition (Wolpoff, 197 1 b) . Because the lower third premolar in apes is a mesio-distally elongated tooth set at an angle to the tooth row and not directly involved in grinding and crushing, summed areas of P4-M3 have been used instead of P3-M3 (Frayer, 1973). Indices formulated using summed P4-M3 areas are highly correlated with those computed using summed P3-M3 areas (Frayer, 1973). This decision allows for an increase in the number of comparisons possible when limited fossil material exists; remains of P3 are far less common than those of P4. Thii consideration is of particular relevance in comparisons involving Propliopithecus and Aegypopithecus.

The nature of the fossil material precludes consideration of problems relating to sexual

304 B. BLUMENBERG

dimorphism. Sexed data from living anthropoids have been pooled in order to arrive at figures that are representative of populations means. Presumably fossil samples of reasonable size, such as exist for D. macinnesi, L. legetet, P. aafricanus and P. nyanzae (Andrews, 1978), contain representatives of both sexes. To the extent that extinct taxa exhibited considerable sexual dimorphism and the paucity of available remains is biased towards one sex, partially determines the degree to which analyses are based on atypical members of the group. Such considerations aside, relatively small sample sizes suffice to detect a 25 % or 50 % difference between means; 22-23 individuals must be examined in order to detect a 10 % difference between means at the 95 % confidence level (Blumenberg, 1978; Sokal & Rohlf, 1969, pp. 246-249).

Two indices of morphometric distance have been computed. The Coefficient of Divergence of Clark (1952) as discussed by Sneath & Sokal (1973) is defined as:

C.D. = 1 n (X, - X2)

C( nj=l (Xl + x2> (1)

where X, is the character measurement in taxon 1 and X2 is the same character measurement in taxon 2.

A standardized measure of morphometric distance (i.e. takes into account the variability inherent in the characters under investigation) as defined by Lerman (1965) has been calculated according to the formula

D = (2(X2 - Xl)2/s2)+

where X2 and XI are as defined above and s2 is the variance of the morphometric element - The advantages and disadvantages of standardization are discussed by Sneath & Sokal (1973). This index was averaged only over dental elements due to the difficulty of

deriving reliable variance estimates for measurements on skeletal features. Variances for each tooth in the dentition were taken as the square of the midpoint of the standard deviation ranges quoted in Ahnquist (1974) and Pilbeam (1969). Such values are based upon studies of Pan and Gorilla and may underestimate the variability inherent in the dentition of the Pongo lineage (Ashton & Zuckerman, 1950; Hooijer, 1948) while over- estimating such variability within the Hylobatidae (Hooijer, 1960). Rather than arbitrarily assign each extinct taxon to one of these three variance classes, it was deemed preferable to use estimates that represent the midpoint of the contemporary hominoid range. To the extent that a particular ancestor-descendant pair of extinct taxa possessed a variability in metric dental characters more like living Pongo or hylobatids than Pan or Gorilla, estimates of D may be slightly inflated or reduced, respectively. D is the straight line distance (relative measure of similarity) between two points plotted in orthogonal coordinates where each coordinate axis corresponds to one character and is scaled in terms of its standard deviation. D2 is a particular case of Mahalanbois’ function Da for uncorrelated characters (Lerman, 1965). Both C.D. and D are examples of Campbell’s (1963) “recent type” of distance indices that do not take into account the correlations between characters. The problem of possible correlations between tooth dimensions is quite complicated (Townsend & Brown, 1979) and its investigation should encompass an analysis of both functional and ecological correlates (Cain & Harrison, 1958). Cain &

Harrison (1958) believe such correlations to be irrelevant to the working out of overall


similarity between forms but of significance when phylogenetic relationships are being deduced.

Two measures that characterize rate of evolutionary change between taxa have been computed. According to Haldane (1949), the mean value of proportional change may be ,conceptualized as (l/x) (dx/dt) or [(d/dt)] [logex)] or

darwins = (logeX, - logeX1)/t (3)

where X2 is the linear measurement of structure X in population 2 after time t has elapse. Square root and cube root are use for area and volume measurements respectively. Elapsed time is here conceived of as the interval between the midpoints of the stratigraphic (or chronometric) ranges of the taxa being compared (Blumenberg, 1978; Lerman, 1965). Such a definition seems compatible with the interpretive problems posed by the nature ,of morphological change (Lerman, 1965). Blumenberg’s (1978) labelling of the midpoint of a taxon’s stratigraphic or chronometric range as median population age is, perhaps, misleading.

D divided by t may be regarded as the average rate of increase in the statistical distance from 0 to D between two taxa, one presumably evolving from the other. Such rates are always positive, in contrast to Haldane’s (1949) conceptualization of the darwin which allows for negative rates.

The reliability that may be inferred for measures of rate of change is directly related to the quality of the chronometric determinations. Table 1 lists the material used and presents the calibration data that are available for the taxa under scrutiny. The radiometric dates that appear to possess good reliability are those available for the African Miocene sites (Bishop, 1972; Bishop & Miller, 1969; Bishop, 1972). The remaining dates represent current considerations of radiometric data supplemented by fauna1 and geological information and may be most inaccurate with respect to G. freybergi, D. keiyuanensis,

Gigantopithecus species and the Pleistocene and subfossil Pongo and Hylobatidae material. The assignment of P. pygmaeus weidenreichi to 1 mya lies in the realm of guesswork, although fauna1 considerations make an older date seem unlikely. The subfossil Asian material may prove impossible to date accurately and has been arbitrarily assigned an age of 10,000 years BP. Living species of anthropoids have been arbitrarily assigned an age of 0.5 mya for some computations in an attempt to take in to account Kurten’s (1959) ideas about species longevity. For calculations, Asian apes were arbitrarily assigned an age of 10,000 years BP when transitions from known Pleistocene and subfossil material were investigated (Hooijer, 1948, 1960). As will be seen, the indices measuring rates of change employed in this exercise are quite robust to such sources of error, ignorance and in- decision in determining elapsed time that are in the order of lo-15 %.

It is not the intention of this paper to enter into matters of taxonomic attribution or the current debate surrounding the hominid status of Gigantopithecus (Corruccini, 1975a,b;

Eckhardt, 1973, 1974; Frayer, 1973; Pilbeam, 1970; Robinson & Steudel, 1973) or Ramapithecus (Andrews & Tekkya, 1976; Conroy, 1972; Frayer, 1974; Gantt, Pilbeam & Steward, 1977; Greenfield, 1975).

It is also not the purpose of this exercise to develop and defend one phylogenetic scheme over another, although a few comparative comments will be made. The phylogenetic ideas of Andrews (1978) and Simons (1972, 1976a,b) are used to provide plausible descriptions of evolving hominoid lineages within which an inquiry into rates of change may be made. One major difference between the two phylogenies lies in the interpretation

306 B. BLUMENBERG

of the adaptive radiation that originated from the Fayum primates. Andrews views Proplio-

pithccus as a possible ancestor to several Miocene lineages while Simons believes AegyPtopithecus is a probable ancestor for the Miocene apes of the genus Proconsul and Aeleopithecus represents a likely beginning to hylobatid-like lineages. Less probable, perhaps, is the possibility that Aegyptopithecus lies at the base of all succeeding radiations. These ideas are summarized in Figure 1.

Figure 1. Two phylogenetic schemes. (a) After P. Andrews, with additions; (b) after E. L. Simons, with additions.

(a)

OLIGOCENE MIOCENE PLIOCENE REISTOCENE RECENT

+ + t t

The following additions to these phylogenies have been made in an attempt to encompass recent or obscure finds. Each of the aforementioned Fayum primates (apes?) is explored as a possible ancestor for M. clurki (Fleagle & Simons, 1978). P. major is considered a probable form out of which evolved D. maceaimienris and G. freybergi (Andrews, 1978, p. 212; Andrews & Tekkya, 1976; Simons, 19766; von Koenigswald, 1972). Both P. major and P. africanus are explored as possible ancestors to D. keiyuanensi.r (Woo, 1957, 1958). The thinking of Simons is followed in choosing possible ancestors for Pan and Gorilla. The proposal of D. macinnesi as the ancestor of living Hylobates and Sjwaphalangus follows a suggestion advanced by Andrews (1978, pp. 209-210). The placing of the thick enamelled S. indicus on the phyletic line leading to Pongo is the most speculative of these recent additions to the phylogenetic milieu and violates ideas presented in Simons (1976qb). A d n rews (1978, p. 213) does not believe P. major is ancestral to Pongo. All of these ideas about lineages, ancestors and descendants, ranging from those most acceptable


to those most speculative, will be explored for the information they provide about morphometric separation and rates of evolutionary change.

All calculations were performed on Texas Instruments SR-52 and TI 59 calculators using available software and programs written expressly for this exercise.

3. Results

Kurt&n (1960), p u on consideration of rates of evolution in Tertiary and Quaternary mammals, defines three distributions of darwin rates. A rates have a mean value of over 1000 millidarwins. They are believed by Kurt&n to equate with Simpson’s (1953) tachy- telic rates; bursts of rapid and quantum evolutionary change over relatively short intervals of time that often correlate with major shifts in adaptive zones. B rates are characterized by a magnitude that rarely exceeds 1 darwin (mean 500 millidarwins) and C rates are considerably below 100 millidarwins. Both are equated by Kurt&r (1960) to Simpson’s (1953) horotelic rates. I wish to propose a fourth rate classification labelled D rates. This group shows a distribution within a narrow range and an upper limit that rarely exceeds 10 millidarwins. The D rate class should not be equated with Simpson’s bradytely because this concept of low evolutionary rate (or “arrested evolution”) focuses upon taxa that seemed to undergo little morphological change over periods of time that are of the order of at least 100 million years.

Appendices A through E present the evolutionary rate results in terms of millidarwins (equation 1) for each structural element in each evolutionary transition. Examples of A and B rates are rare with respect to dental features and non-existent with respect to the limited ECV, mandibular and postcranial skeletal data available. As seen in the appendice data within each rate class, rates of change between individual teeth appear to be uncoupled (i.e. uncorrelated) although some weak trends are discernible. Within the few examples comprising class A rates, Ml and P, appear to evolve at the slowest pace while Ms and M, seem to exhibit the greatest rates of change. The few class B results do not permit comment about possible trends. Likewise, the C class maxillary observations do not reveal any systematic patterns of change. Excluding the thick enamalled apes, possible hominids and living pongids, the C class mandibular molar results seem to suggest that M, often exhibits the lowest rate of change while M, evolves at the most rapid rate. Within the D rate class, the maxillary results reveal no obvious trends, while the mandibular results suggest low rates for Ms when overall structural increase charac- terizes the transition but high rates for M, and low rates for P, and M, when overall structural decrease describes the change between taxa. Such observations suggest that little in the way of mathematical correlations can be detected in rates of evolution of individual teeth between presumed ancestor and descendant taxa.

Tables 2 through 5 present morphometric distance (separation) measures and rate of change indices that are averaged over all elements held in common for the ancestor- descendant relationships postulated by the phylogenies under scrutiny; these are computed for morphometric distance (D), coefficient of divergence (C.D.), and rate of change indices millidarwins and D/t. Osborne, Horowitz & De George (1958) report a strong genetic component to the mesio-distal dimension of permanent anterior teeth H. s&ens (U.S.A.). Alvesalo, Osborne & Kari (1975) suggest a positive influence of the Y chromosome on tooth size in a small group of Finns and Americans, a conclusion that was also reached by Sirianni & Swindler (1974) in studies on Macaca nmestina. HOW-

308 B. BLUbtENBERG

Transition t x l(r

k,’ kst years m.d. D C.D. Dlt

Structural decrease H. subfossilis-H. agilis S. s. subfossilis-S. syndactylus P. p. pnlaeosumatrensis-P. pygmaeus

(Hooijer) P. p. palaeosumatrensis-P. pygnuaeus

(Ashton and Zuckerman)

5 0 0.009 - 6806 1.30 0.034 144.87 12 0 0.009 - 9727 3.45 0.046 382.90 18 0 0.009 - 7351 6.17 0.036 685.74

18 0 0.009 - 11243 8.95 0.053 994.18

* Number of tooth characteristics held in common between the two taxa. f Number of mandibular features and long bone measurements held in common between the two taxa.

Table 3 Class B evolutionary rates*

Transition t x 10”

k, ks years m.d. D C.D. D/t

Structural increase S. indicus-G. bilaspurensis

Structural decrease P. p. weidcnreichi-P. pygmaeus

(Ashton and Zuckerman) P. p. weidenrcichi-P. pygmaeus

(Hooijer)

7 0 I.5 + 157.0 15.19 0.123 10.13

9 0 0.999 - 137.2 8.03 0.075 8.04

9 0 0.999 - 105.1 6.90 0.063 6.91

* See notes to Table 2.

ever, Townsend & Brown (1978a) could not confirm sex chromosome involvement in determining tooth size in a study population of Australian aborigines. In their study sample, Townsend & Brown (19786) attribute 64 oA of the total variability in tooth size to genetic factors. In the light of such reports, the values for the morphometric measures D and C.D. that are averaged over tooth elements may be loosely considered to represent genetic (polygenic) distances between taxa.

Discussion I: Skeletal evolution

As can be seen in Tables 4 and 5, when metric data on mandibular and postcranial elements formed part of the data base available to compute overall morphometric distances and evolutionary rates between taxa, the results do not differ appreciably from those obtained when relying solely upon tooth measurements. Rates of evolutionary change in the tooth complex and in these highly selected groupings of mandibular and postcranial skeletal elements are not only of the same magnitude, but are highly correlated when 20 transitions are examined (Y = 0.898, P ( 0.001 for 18 d.f.). Inspec- tion of Table 6 will reveal that the greatest degree of uncoupling between tooth and skeletal evolutionary rates occurs in several class D transitions: P. major to S. indicus;

P. major to S. sivalensis and D. macinnesi to H. agilis.


Table 4 cl8r c evolhry trtes*

Transition (AY

k, k, yeam) m.d.t m.d.S D C.D.t C.D.: Dlt

Structural increase 0. savage&P. haeckli 0. savagei-D. ma&r& 0. savage&L. leg&et P. haeckli-L. lege#et P. haeckli-P. nyanzac P. hacckli-P. major P. hacckli-P. africanns P. haeckli-P. (R.) gordoni P. haeckli- D. macinncsi A. rcuxis-P. afriGanus A. zewis-P. major A. zcuxis-P. nyanrae A. ret&-P. (R.) gordoni A. chirobates-D. macinnesi A. chirobates-M. ctarki A. chirobates-L. legetet P. major-D. macedoniensis P. of&anus-D. keiManensis P. &cams.+R. &keri R. wicke+R. punjabicus G. bilaspwensis-G. blacki P. major-G. gorilla P. af&anus-P. troglodytes S. indicus-P. 6. weidenreichi S. indicu-P. ;b.

palaeosumatrensis Structural decrease

A. zeuxis-M, clarki P. nyanzae-D. la&anus P. major-D. keayuanensis P. major-l. wickeri P. p. &i&nreichi-P. p.

palaeosumatrensis

3 + 63.6 - I8 + 30.1 - 21 + 17.4 - 18 + 10.6 + 10.8 18 + 43.7 + 44.6 18 + 54.3 + 54.1 18 + 32.8 + 31.8 16 + 34.1 + 37.4 15 + 22.3 + 22.3 15 + 19.7 + 21.1

1.88 7-99 4-93

0.095 O-268 0.185 0.100 0.376 0.455

0.626 o-444 0.235

3 5 5 5 5 5 5 5 5 6 5 5 5 7

; 9 7

11 9 7

18 18 8

16

5 6 7

11 9

0 0 0 2 2 2 2 2 2 2 2 2 2 0 0 0 0 0 0 0 0 0 3 0 0

0 3 0 0 0

-

O.li 0.383 0.452

4.00 22.13

0.222 1.23

30.79 1.71 14.85 0.289 13.36 0.268

0.281 0.295

0.825 0.835

7.25 6-81

0.168 0.148 0.329 0.241 0.127 0.242 0*121 0.172 0.076

0.484 0.454 1.60 1.02 0.508

- 0.165 0.330 0.253 0.159

-

15 + 45.6 + 45.5 23.93 15 + 32.7 + 34.1 15.28

6.60 IO.37 4.31 6.77 9.13

13 + 19.3 + 23.6 14 + 34.3 - 16.2 + 13.0 - 17 + 18.8 - 6 + 18.6 - 6 + 46.2 - 3.8 + 44.5 - 2.2 + 15.2 - 6.5 + 22.7 + 20.0

16.5 + 10.7 - 16.5 + 16.9 + 18.3 9 + 14.8 - 9.99 + 13.1 -

0.740 0.266 - 0.398 1.52 2.21 2.10 1.59 1.60 1.11 0.967 0.808 1.06

- -

13.23 0.151 7.99 0.094

- -

3.51 0.034 10.38 0.075 18-26 0.101 15.96 0.147 7.27 0.075

10.55 0.081

- 0.076

-

0.173 - -

17.2 - 16.1 - 6 - 29.4 - 25-4 6 - 11.1 - 3.8 - 63.7 - 0.99 - 41.9 -

5.59 0.120 o-393 1.20 1.09 4.2 1 4.35

- 0.089

- 7.22 o-097 6.51 0.055

15.98 0.140 4.31 0.040 -

* See notes to Table 2. t Averaged over k,. t Averaged over k, + k,.

Discussion II: Morphometric separation and rate of change As can be seen in Table 7 in the 55 transitions investigated, the two measures of morphometric distance, unstandardized (D) and standardized (C.D.), are highly correlated (r = O-798, P < 0.001 for 53 d.f.). Sneath & Sokal (1973) have observed that standardization frequently has little effect upon distance coefficients. The two indices measuring rate of change (millidarwins and D/t) were also highly correlated (r = O-910, P < O-001 for 53 d.f.). Both over the entire spectrum of transitions, and within a combination of the A and B rate classes, there was no correlation between the magnitude of morphometric separation between taxa and the magnitude of the rate of change that produced such divergence. However, within a combination of the C and D rate classes, there is a significant correlation (P < 0X101 for 46.d.f.) between morphometric distance between taxa and the degree of evolutionary rate that produced such separation for three of four

310 B. BLUMENBERG

Table 5 Class D cvolutioaary rates*

Transition (AO,

k, ks years) m.d.* m.d.* D C.D.+ C.D.* D/t

Structural increase P. haeckli-M. clarki A. zeuxis-D. macinucsi D. macinneti-P. vinahbonensis P. nyantae-D. fontani P. africanus-P. pan&s D. macinnesi~. s. spuhcQlu.s D. macinnesi-S. s. syndac@us D. macinnesi-S. s. subfossilis S. indicus-P. pygmaeus

(Ashton and Zuckerman) S. indicus-P. pygmaeus

(Hooijer) S. it&us-P. Pygmaeus

(Hooijer) Structural decrease

A. me&s-L. legetet P. major-S. indicus D. macinne&H. agilis D. macinuesi-H. agilis D. macinnesi-H. sp.

subfossilis P. major-s. sivaleusis P. major-G. freybergi

17.2 + 3.8 - 1.59 0.044 12.0 + 3.3 + 3.6 1.44 0.042 7.0 + 4.2 + 2.6 3.22 0.044 4.0 + 7.0 - 1.01 0,029

16.5 + 8.8 + 10.5 8.84 0.089 19.5 + 4.8 - 4.83 0.052 19.999 + 4.7 - 4.83 0.052 19.99 + 9.4 - 7.99 0.095 9.5 +2.9 - 5.68 0.047

- 0.042 0.042

-

0.123 - - - -

0.092 0.120 0460 0.252 0.536 0.248 0.242 0400 0.598

+ 7.0 - 7.34 0.061 - 0.773

5 5

18 2

18 18 18 12 16

16

16

0

0

9.5

9.999 + 6.7 - 7.34 0,061 0.734

15 - 6.9 - 6.4 3.12 0.064 0.066 0.208 7 - 2.6 - 0.6 7.25 0.052 1.054 1.04

19.5 - 5.8 - 5.0 5.43 0.062 0.064 0.278 19.999 - 5.7 - 3.9 5.43 0.062 0.070 0.272 19.99 - 4.7 - 1.95 0.048 - 0.098

3 7.0 -6.9 -11.3 4.42 0.036 0.057 0.631 0 8.5 - 6.9 - 2.93 0.044 - 0.345

5 16 18 18 5

12 4

* See notes to Tables 2 and 4.

Table 6 Coupling of teeth and skeletal element evolutionary rates

Transition Teeth* Skeletont

rates (m.d.) rates (m.d.)

P. haeckli-L. legetet + 11.3 + 10.6 P. haeckli-P. nyanrae + 47.0 + 43.7 P. haeckli-P. maj’or + 53.4 + 54.3 P. haeckli-P. africanus + 29.3 + 32.8 P. haeckli-P. (R.) gordoni + 45.5 + 34.1 P. haeckli-D. macinnesi + 22.3 + 22.3 A. zewis-P. africanus + 16.4 + 19.7 A. zeuxis-P. major + 45.4 + 45.6 A. ret&-P. nyanzae + 37.7 + 32.7 A. reuxis-P. (R.) gordoni + 34.4 + 19.3 A. Zeus&-L. legetet - 5.2 - 6.9 A. zeuxis-D. macinnesi + 4.5 + 3.3 D. macinnesi-P. vindobonensis + 0.52 + 4.2 P. nyanzae-D. laietanus - 17.4 - 29.4 P. major-S. indicus + 7.4 - 2.6 P. major-S. sivalensis - 28.7 - 6.9 G. bilaspurensis-G. blacki + 10.8 + 22.7 P. africanus-P. troglodytes + 31.3 + 16.0 P. africanus-P. paniscus + 22.4 + 8.8 D. macinnesi-H. agilis + 11.6 - 5.7

RT = 0*855R - 0.299; r = O-897734; P c O*OOl for 18 d.f. * Averaged over all teeth held in common between the two taxa. t Averaged over all mandibular featnres and long bone measnrements held in common between tke two taxa.

Tab

le

7 R

ektio

r~sh

ip

with

in

md

b

etw

eea

ia

dic

as

of

mo

rph

om

etri

c d

.i&

ma

ce m

d

rate

s o

f ev

olu

tio

xm

y

cha

ng

e

All

rate

cl

asse

s C

lass

es

A a

nd

B c

ombi

ned

Cla

sses

C

and

D

com

bine

d

I. M

o@w

mek

ic

dist

ance

ucr

sus r

ak o

f ch

ange

D

=

- 0*

0003

24 I

m.d

.1

f 8.

429

r =

0.

127,

P

>

0.1

for

d.f.

=

53

D.D

. =

-

OG

OO

OO

8 Im

.d.1

+

0.

1197

r

=

0.20

5,

P >

O

-1 fo

r d.

f. =

53

D

=

- 0@

0210

8 (D

/t)

+

8.30

32

r =

0.

061,

P

>

0.1

for

d.f.

=

53

C.D

. =

-

0.00

0099

(D

/t)

$ 0.

118

r =

0.

179,

P

>

0.

1 fo

r d.

f. =

53

II.

Indi

ces

of m

mph

omtt

ric

dist

ance

D

=

50.8

4 C

D.

+

2.41

r

=

O-7

98, P

<

O+

lOl f

or d

.f.

=

53

D =

-

OW

O43

1 Im

.d.1

+

9.

339

I =

0.

474,

P

>

0~1

for

d.f.

=

5 C

.D.

=

- O

*OO

OO

O4 Im

.d.1

+

0.

083

r =

0.

677,

0.

05 <

P

<

0.1;

d.

f. =

5

D =

-

0.00

114

(D/t)

+

7.50

5 r

=

0.10

0,

P >

0-

l fo

r d.

f. =

5

C.D

. =

-

OG

OO

O38

(D/t)

+

O

-074

r

=

0.48

5,

P >

0.

1 fo

r d.

f. =

5

D =

12

9.07

C.D

. +

7.

87

r =

0.

898,

0.

01

>

P

>

0.00

1;

d.f.

=

5

D =

O

-214

2 jm

.d.1

+

4.

091

r =

0.

579,

P <

0.

001

for

d.f.

=

46

C.D

. =

0.

0037

Im

.d.1

+

0.

0478

r

=

O-6

37, P

<

O+

Ol

for

d.f.

=

46

D =

2.

838

(D/t)

$

5.82

0 r

=

0408

, 0.

01

>

P

>

OG

Ol

for

d.f.

=

46

C.D

. =

0.

0154

3 (D

/t)

+

0.10

8092

r

=

O-1

41,

P >

0.

1 fo

r d.

f. =

46

D =

51

.38

C.D

. f

2.10

r

=

0.80

9,

P <

O

-001

for

d.f.

=

46

III.

In

dice

s of

rat

es o

f ch

ange

(D

/t)

=

O-0

6534

Im

.d.1

-

1.96

6 r

=

O-9

10, P

<

0.00

1 fo

r d.

f. =

53

(D

/t)

=

0.06

803

1 m.d

. I

- 26

.3

(D/t)

-

0.03

481

Im.d

.1

+

O-2

034

r =

0.

848,

0.

02

>

P >

0.

01;

d.f.

=

5 r

=

0.65

5,

P

<

0.00

1 fo

r d.

f. =

46

Ap

pea

dls

A

Tra

nsiti

on

Cla

ss

A e

vo

luti

on

ary

ra

tes

for

ind

ivid

ua

l te

eth

(m

illi

drt

wir

u)

IlB

I2

B

CA

P3

A

P4A

M

IA

M2A

M

3A

CW

-M3A

Max

illa-

stru

ctur

al

decr

ease

H

. su

bfos

sili

s-H

. a&

s P

. p.

fm

iaao

sum

atr~

nsis

. py

gmaa

us

(Hoo

ijer,

19

48)

P.

p. j

w.a

eosu

mat

ranr

is-P

. pyg

mac

us

(Aah

ton

& Z

ucke

rman

, 19

50)

s. su6fsdti4.

s. synah~lus

Man

dibl

catr

uctu

ral

decr

ease

H

. su

bfdi

s-H

. a&

is

s. i

ubfo

ssil

is-s

. s.

syn

dacc

tylu

c P

. p.

#+

&w

umat

rmls

is-P

. py

gmae

ur

(Hoo

ijer,

19

48)

P. p.

$&

&w

wna

keti-

P.

pygm

acus

(A

shto

n SK

Zuc

kerm

an,

1950

)

- 53

51

- 12

22

- 69

92

- 17

64

- 10

938

- 98

86

- 55

98

- 76

48

- 39

02

- 12

178

- 35

84

- 69

32

- 10

258

- 15

767

- 18

008

- 11

672

- 90

05

- 58

35

- 15

131

- 46

28

- 74

33

- 12

73

- 99

74

- 13

64

- 13

43

- 11

37

- 12

23

- 10

129

- 10

335

- 81

36

- 34

65

- 71

90

- 87

93

- 85

14

- 72

51

- 79

00

- 10

692

- 98

64

- 75

84

- 45

4 -

7733

-

1223

0 -

6229

-

7230

- 74

33

- 11

480

- 15

769

- 14

283

- 68

06

- 99

70

- 15

879

- 10

442

- 11

183

312 B. BLUMENBERG

Appasdix B Class B evolutioaary~rates for individual teeth (millidarwiriir)

Transition CA P3A P4A MIA M2A M3A BP4M3k

Maxilla-structural decrease P. p. wet&r&hi-P. jygmaeus - 22 - 174 - 120 - 191 - 116 - 150

(Ashton & Zuckerman, 1950) P. p. waidnareichi-P. pygmacur + 33 - 162 - 102 - 165 - 106 - 134

(Hooijer, 1948)

Mandible-structural increase S. indims-G. bilaspurensis -I- 60 + 106 + 207 + 211 + 164 + 168 + 183

Mandible--structural decrease P. p. wcidearcichi-P. pygmacus - 60 - 181 - 221

(Ashton & Zuckerman, 1950) P. p. weidcnraichi-P. pygmaeus - 7.1 - 121 - 183

(Hooijer, 1948)

possible relationships (Table 7). In spite of the small samples sizes of the A and B rate classes, these results suggest that only in the high rate classes is the degree of morphometric distance achieved by a descendant from its “immediate” ancestor (as defined at the species level) independent of the speed at which that change occurred.

The functional demands upon individual teeth that result from shifts in adaptive zone and diet should be the predominant selective pressures influencing rates of change as taxa evolve and change through time. The results presented in the appendices suggest that within most taxa, a different response of each tooth to such selective pressures is the rule rather than the exception. Association of distantly related taxa in the same evolutionary rate class may only indicate similarities in the overall intensity of selective pressures during critical periods of time and similarities in the speed with which different taxa could respond to such pressures. Such rate class groupings reveal nothing about the specific characteristics of such selective pressures nor do they imply morpholological and/or functional similarities between genera or species. The diversity of taxa in rate classes C and D is self evident.

These indices of rates of change (millidarwins, o/t) which are expressed per million years, may be converted to units per generation if one is willing to speculate upon generation time in extinct taxa. As presented, all results assume a generation time of one year. Such an assumption is clearly unreal&tic for most, if not all, of the taxa under scrutiny here. To obtain a rough measure of change per generation, one might assume the generation time of the Fayum primates to be 2-4 years; those of the Hylobatidae to be 4-6 years and those of the Pongidae to be 8-10 years. Multiplying the rate indices in Tables 2-5 by the appropriate (?) generation time yields a speculative guess as to the rate of change per generation. Rates in pongid and presumed hominid lineages emerge as considerably faster than those in other phyletic lines.

In terms of morphometric distances, the Simons phylogeny reveals a closer relationship between the Fayum ape (A. zeuxis) believed to give rise to the three Miocene Proconsul species than does the Andrews phylogeny which places P. huakli at the base of this radiation. Figures 1 and 2 depict each phylogeny in terms of C.D. and D units, respectively, between taxa. For those taxa held in Common between the two phylogenies, comparative assessment does not yield any additional differences of significance. G. bilas~urmsis appears far removed from its presumed ancestor (S. indicus), a reflection of its large size.

The two species of Ramapithectu appear closely related. R. wickeri is moderately distant

App

eneI

ix

c;

Tra

nsi

tion

C;1

aam

0 cv

olu

tion8ry

r8

tes

for

indiv

idu81

teetn

(m

wra

irw

msj

IlB

IZ

B

CA

P

3A

P4A

M

lA

MZ

A

M3A

X

P4-

M3A

Max

illa

--st

ruct

ura

l in

crea

se

P.

afri

canu

s-R

. w

icke

ri

R.

wic

ker&

R.

pwja

bicu

c P

. m

&@

-G.

gori

lla

P.

afia

nus-

P.

trog

loay

tes

S.

idcu

s-P

. p.

w

eia%

nrei

chi

S.

it&us

-P.

p. p

alae

osum

atre

nsi

Max

illa

stru

ctu

ral

decr

ease

P

. m

&r-

R.

wic

keri

P

. p.

w

eia&

reic

hi-P

. p.

pah

cosu

mat

rens

is

Man

dibl

ostr

uct

ura

l in

crea

se

0.

sava

gei-

P.

haec

kli

0.

sava

ge&

D.

mae

inne

si

0.

sam

qgei

-L.

kget

et

P.

haee

kli-

L.

kget

et

P.

/me&

i-P

. nt

zae

P.

ha&

l-P

. m

ajor

P

. ha

eckl

i-P

. af

ianu

s P

. ha

ed&

-P.(

R.)

go

dni

P.

haec

kl-D

. m

acin

nes

i A

. ze

uxis

-P.

afia

nas

A.

zeux

is-P

. m

ajor

A

. ze

uxis

-P.

nyan

zac

A.

uwri

r-P

. (R

.)

gord

oni

A.

chir

&&

s-D

. m

acin

nesi

A

. ch

irob

ates

-M.

clar

ki

A.

chim

bate

s-L

. kg

&t

P.

nujo

r-D

. m

aco&

ien

sis

P.

aJiiC

Onu

s-D

. kx

n~ne

nsis

P

. &

ican

u.-R

. w

icke

ri

R.

wid

er&

R.

pwrj

abic

us

G.

bi&

spur

enri

s-G

. bl

acki

P

. m

ajor

-G.

god&

P

. af

ricn

us-P

. tr

ogtiy

tes

S.

i&-P

. p.

w

ei&

nrei

.ehi

S.

id

&us

-P.

p. p

alae

vsum

atre

nsis

Man

dibl

oatr

uct

ura

l de

crea

se

A.

.xou

.&-M

. cl

ark

i P

. n

yan

zae-

D.

laio

tan

us

P.

ma*

-D.

keiy

uane

nsis

P

. m

qjor

-R.

wic

ker

i P

. p.

u

wi&

nre

ich

i-P

. p.

pal

acos

um

arre

nsi

s

+

7.8

+

4.8

+

27.7

+

23

.5

+

37.9

f

24.7

- 1.

5 +

36

.2

+

15.0

+

8.

8 +

34

.9

+

24.2

+

18.4

+

37

.7

+

76.4

-

15.0

-

22.2

-

3.6

+

16.0

+

18

.6

+

16.7

+

12

.6

+

10.8

+

4.

4 +

29

.8

+

21.6

+

12

.3

+

17.5

- 15

7 -

45.8

+

83

.9

+

40-o

. +

26

.5

+

23-O

* +

12

.9

+

22.7

+

40

.0

+

1.6

+

18.5

+

6.

6 +

20

.9

- 12

.9

+

6.7

+

64.3

+

11

.7

+

70.3

+

3.

0 +

17

.6

+

30.3

+

10

.2

+

14.0

+

21

.2

+

19.4

+

22

.2

+

19.6

+

14

.8

- 45

.7

- 12

.7

- 98

.9

- 51

.3

+

82.5

-

52.6

- 93

.9

+

57.8

+

26

.2

+

16.4

+

9.

5 +

40

.0

+

48.1

+

28

.0

+

30.0

+

19

.9

+

24.4

+

48

.4

+

37.5

+

26

.2

+

36.9

+

15

.1

+

22.8

+

34

.0

+

61.5

+

64

.0

+

47.4

+

28

.1

+

18.4

+

17

.4

+

14.1

- 7.

9 -

21.9

+

I.

4 -

31.0

+

65.5

-

19.5

+

14

.0

+

1.3.

0 +

15

.5

+

7.2

- 10

.1

- 67

.6

+

69.0

+

25

.3

+

13.9

+

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316 B. BLUMENBERG

Figure 2. Morphometric distances in terms of D units between taxa. Distances should be measured along the arrows and are nof additive between three or more taxa. There is no time dimension to the figure. (a) After P. Andrews, with additions; (b) after E. L. Simons, with additions.

(b)

rkci

;

from P. africanus and shows even less affinity to P. major. While such observations do:not comment directly on its presumed hominid status, they do establish its distinctiveness from Proconsul. P. major emerges as a reasonable ancestor for the “thick enamelled” apes. P. nyanrae can be plausibly defended as lying at the base of the radiation of European Dryopithecus. Based upon the limited material available, D. macinnesi is seen to be very close to A. zcuxis in terms of morphometric distance and a good choice for the ancestor of subfossil and living hylobatids. P. africanus is seen to be moderately distant from living species of Pan. On the scale of D units, P. major is far removed from G. gorilla. Except for the class A and class B rate change transitions (Tables 2 and 3) of which S. indicus-


G.. bila.@ren..& is the only pre-Pleistocene event, Figures 1 and 2 which are drawn to ‘indicate the morphometric distances between.pairs of taxa also represent the magnitude of the rate of evolutionary change between ancestor-descendant pairs of taxa. Within the C and D rate classes, there is a highly signiicant correlation between the morphometric separation achieved between taxa and the rate of evolutionary change that produced such divergence as discussed above. Harper (1976) has proposed an ordering principle of “minimal morphological gaps” as one of several that should govern the construction of preferred phylogenies. The Simons phylogeny is better able to meet this criterion than that of Andrews for taxa held in common between the two phylogenies.

Discussion III: Comparisons with other mammalian fauna

As Could & Eldredge (1977) note, there have been few studies that have attempted to quantify evolutionary rates and as a result there is a paucity of information available for comparative discussion. Class D darwin rates have been presented for ectoloph change between Merychippus and Neohipparion in Haldane (1949). Data on various molar measurements in Miocene Merychippus produced class C and B rates (Downs, 1961) and similar rates also characterized paracone height and ectoloph length of MS during the evolution of five extinct horse species (Haldane, 1949) and for the pace of ECV change during the evolution of H. sapiem from Plio/Pleistocene hominids (Blumenberg, 1978). Lerman ( 1965) presents morphometric distances and rates of evolutionary change for four pair- wise comparisons of oreodont species that fall in the danvin Crate class as judged by the correlation between millidarwins and D/t that is discussed above.

Maglio (1973) presents evohuionary rates that encompass classes B, C and D for eight molar characteristics in the Elephantidae lineages Primelephas gomphotheroicies-Mammuthus

primigenius (figure 43) and P. gomphotheroides-Ehphas iobnsis (figure 44). The pattern of change shown on these graphs for each molar characteristic strongly suggests that for any evolutionary progression, the rates of change between different features of the same tooth are uncoupled. It seems unlikely that selection pressures would be applied in a homogeneous fashion to each functional characteristic of a particular tooth. Maglio’s (1973) figures 43 and 44 are asymmetrically hourglass shaped. This pattern suggests that midway through the history of these two evolving lineages, the variability in evolutionary rate momentarily decreases and the rate of change itself slows, soon to be followed by a dramatic increase in both variability and overall magnitude of rate of change, In the hominoid phylogenies under consideration here, the proposed lineage containing the most species is P. haeckli-P. major-S. indicus-G. bilaspurensis-G. blacki. In terms of darwin rate classes, the four progressions exhibit the sequence C-D-B-C which is similar to the pattern of change in evolutionary rate exhibited by the two elephant lineages analyzed by Maglio (1973). The magnitude of evolutionary rate, however, is lower in the hominoid lineage. Such a similarity in the pacing of evolutionary change within Neogene lineages of two unrelated orders of mammals raises the possibility that each order is manifesting a similar response to some broad based, but as yet unspecified, selective pressures.

Discussion IV: Punctuated equilibria

Kurt& (1960) presents results suggesting that several postglacial mammalian lineages (Gulo, Mmtes, Meles, Ursus) are characterized by class A darwin rates. The postglacial dwarfing of U. arctos provides evidence for an evolutionary rate of - 20,000 millidarwins

318

(0)

B. BLUMENBERG

Figure 3. Morphomctric distances in terma of C.D. units between taxa. Distances should be measured along the arrows and are it additive between three or more taxa. There is no time dimension to the figure. (a) After P. Andrews, with additions; (b) after E. L. Simons, with additions.

(b)

Stole k22 units)

(Kurt&r, 1955). Although larger in absolute terms, this is the magnitude of the rate of change suggested in this analysis for the progression from subfossil Hylobates, Sym&ahgus and Pongo to living species of these genera (Table 2). Campbell (1963) has questioned the assumption that ancestor-descendant populations have been investigated by Kurt& in this case.

Theories of macro or quantum evolutionary change are both plentiful and not new and it is beyond the scope of this discussion to consider the diversity of such proposals. However, the theory of Punctuated Equilibria (Eldredge & Gould, 1972; Gould & Eldredge, 1977) has one feature that these results offer comment upon. Eldredge & Gould (1972) have applied the tenets of allopatric speciation to a consideration of the


fossil record. Assuming that (a) new species arise by the splitting of lineages; (b) new species develop rapidly; (c) small subpopulations of ancestral forms give rise to new species, and (d) new species originate in small isolated areas at the periphery of the habitat, then (i) a local stratigraphic section containing ancestor and descendant species should reveal a sharp morphological break between the two, and (ii) many such breaks in the fossil record are real given the rarity of intermediate transitional forms between taxa. The point of relevance to these calculations is the proposed rapidity of evolutionary change (tenet b). According to this theory, long periods of slow change, that are commonly perceived in the fossil record, are but artefactual and due to the averaging of a very short period of punctuated equilibria with a long succeeding period of little or no change. The implication of these ideas of punctuated equilibria for the study of quantified rates of evolutionary change is clear; actual periods of change will be difficult to detect because of their rapidity unless extremely fine-grained stratigraphic resolution can be achieved.

While no example of ancestor-descendant related taxa closely following one another in adjacent stratigraphic layers is provided by the fossil material under examination here, evolutionary transitions separated by small intervals of elapsed time have been proposed. Such situations can serve as a crude substitute for close stratigraphic association at a single site and provide the basis for some preliminary comment. If living species of Spfihalangus, Hylobates and Pongo can be assumed to be descended from the subfossil forms of these genera discovered by Hooijer (1948, 1960), then they represent another example of postglacial dwarfing that proceeded at a rapid rate (Table 2). Note that the magnitude of the morphological change that resulted is not particularly large; rate of change is not coupled to magnitude of change in darwin rate classes A and B (see discussion II above). Harper’s (1976) phylogenetic ordering principle of ‘minimal morphological gaps” is not violated by these observations. To the extent that ideas of Punctuated Equilibria are attractive and accepted (proven 3) as representing real evolutionary process, a parsimonious approach to phylogeny construction would do well not to employ a criterion of minimal rates of change between ancestor and descendant taxa.

The elapsed time interval between taxa in rate class A is approximately lo4 years; in rate class B l-1.5 x lo6 years; and in rate classes C and D l-22 x lo6 years (with a mean of 13 x 106 years). These observations recall the question raised by Eldredge & Gould (1977). Are slow evolutionary rates merely an artefact of a necessarily coarse- grained stratigraphic approach to sampling the fossil record? If it were possible to reduce stratigraphic and/or chronometric intervals between extinct taxa to circa lo4 or lo6 years, would the pace of evolutionary change be perceived as frequently proceeding at the rapid rate characteristic of darwin rate class A?

4. Conclusions and Summary

(1) Rates of evolutionary change in the teeth complex and mandible during the radiation of late Neogene hominoids are significantly correlated. For the few long bone measurements available, there is also a high correlation with rates of tooth change.

(2) There is no correlation between the magnitude of morphometric separation achieved between ancestor and descendant taxa and the rate of change that produced such divergence in rapid evolutionary rate classes.

(3) The association of distantly related taxa in the same evolutionary rate class is taken

320 B. BLUMENBERG

to indicate only similarities in the speed of the adaptive response to selection pressures of comparable intensity.

(4) When evolutionary rates are considered as unit change per generation, they are greater in pongid and hominid lineages than in other hominoid phyletic lines.

(5) The primary difference between the two phylogenetic schemes considered is the closer relationship in the Simons phylogeny of the Fayum ape believed ancestral to a Miocene radiation to its descendant Proconsul species. The Simons phylogeny meets a criterion of “minimal morphological gaps” between ancestor and descendant taxa better than does the phylogeny of Andrews.

(6) Two elephant lineages and one hominoid lineage of the Neogene, each consisting of five species, reveal a similar pattern in evolutionary rate sequence. Midway through the history of each evolving lineage, there is a significant slowing of the rate of change. Such a similarity in pattern between two unrelated groups of mammals raises the possibility that this observation may be of some general significance. It may reflect similarities in the speed of the adaptive response to broad based, but as yet unspecified, selective pressures.

(7) The evolution of living species of the Hylobatidae and Pongo from subfossil forms appears to represent examples of Punctuated Equilibria in that the rate of evolutionary change involved is very rapid. The rate of morphometric separation achieved by these contemporary species from their presumed immediate ancestors is not, however, particularly large.

(8) Thii analysis reveals a negative correlation between evolutionary rate class and elapsed time, i.e. the more rapid the rate of change, the smaller the associated time interval. The suggestion is made that (many ?) observations of slow evolutionary rates may be artefacts of a coarse-grained approach to the stratigraphic and/or chronometric record.

This work was conducted in the absence of the solicitation of funds from any foundation, public or private. Bernard Campbell and Bernard Wood contributed helpful suggestions and comments.

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rates of evolutionary change during the radiation of late neogene hominoids

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