Elsevier Encyclopedia of Geology

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Phanerozoic Diversity Change

Marine biodiversity change The patterns of marine

biodiversity change at the family level during the Pha-

nerozoic emerging from the current databases consist-

ently show a rise from a handful of taxa at the

beginning of the Cambrian to some 1900 families at

the present day (e.g. Figure 1A). This rise included

steep increases in the Early to mid-Cambrian, the

Early and mid-Ordovician, and from the Triassic

to the Holocene. The diversity level attained in the

Ordovician established a plateau, which was essen-

tially maintained until the end Permian mass extinc-

tion event (see Palaeozoic: End Permian Extinctions).

Other mass extinctions in the latest Ordovician and

Late Devonian and at the ends of the Triassic and

Cretaceous periods (see Mesozoic: End Cretaceous

Extinctions) punctuated the overall pattern. The

genus-level curve (Figure 1B) shows a similar overall

pattern but, unsurprisingly, is much more saw-

toothed, and major extinction events (not just the

‘big five’ listed above) had a much more profound

effect at lower taxonomic levels.

The overall diversity curves have been resolved

into three so-called evolutionary faunas (Figure 1),

each characterized by the dominance of a set of

major clades and together representing an increase in

ecological complexity through time (Table 1). The

Cambrian Fauna dominated its eponymous period

but declined thereafter. The Palaeozoic Fauna had its

origins in the Cambrian but rose during the Ordovi-

cian biodiversification event to a dominance that was

largely maintained for the rest of the Palaeozoic. The

Palaeozoic Fauna on the low-latitude Laurentian plate

showed an onshore–offshore pattern of innovation

and expansion of communities during the Ordovician.

The Modern Fauna has been an important component

of the marine biota, initially in nearshore habitats,

since the Ordovician but did not become dominant

until after the end Permian extinction event.

Mass extinctions produced the major troughs in the

diversity curves but represent one extreme of a con-

tinuum of rates of species extinction per million years.

The end-Palaeozoic, end-Mesozoic, and, to a lesser

extent, Late Devonian mass extinctions did not

simply reduce overall biodiversity; they disrupted

the ecological patterns so severely that major changes

in overall community and even ecosystem structure

could occur. However, whilst some significant

changes in the proportions of marine taxa grouped

by fundamental differences in autecology and physi-

ology took place after mass extinctions, many of the

episodes of change or of relative stasis of such fea-

tures did not mirror the concurrent trajectory in

global diversity. Thus, for example, while there were

stepped increases in the proportion of predator taxa

following the end-Permian and end-Cretaceous ex-

tinction events, this proportion then fluctuated only

within a fairly narrow band during the subsequent

rises in overall diversity.

Non-marine biodiversity change The shape of the

non-marine biodiversity curve is markedly different

from that of the marine biodiversity curve (Figure 2),

and its principal components are the plants, insects,

and tetrapod vertebrates.

Whilst the land probably had a microbial flora

extending back into the Precambrian, spores and

phytodebris indicate that land plants were present

Table 1 The major components and ecological structures of the three marine evolutionary faunas

Evolutionary fauna

Community

diversity Food webs Tiering

Suspension

feeders

Detritivores

and

carnivores

Planktonic

food

Animals

in water

column

Modern evolutionary fauna

(demosponges, gastropods,

bivalves, gymnolaemate

bryozoans, malacostracans,

echinoids, vertebrates)

High Highly

complex

Complex Epifaunal

and

infaunal

Common Abundant Many

Palaeozoic evolutionary fauna

(anthozoans, cephalopods,

stenolaemate bryozoans,

‘articulated’ brachiopods,

ostracods, stelleroids,

crinoids, graptolites)

Intermediate Intermediate Develops Epifaunal

common

Common Common Many

Cambrian evolutionary fauna

(hyolithids, monoplacophorans,

‘inarticulate’ brachiopods,

trilobites, eocrinoids)

Low Simple Limited Few Dominant Limited Few

262 BIODIVERSITY

from the mid-Ordovician onwards. These earliest

plants were probably bryophytes; spore evidence sug-

gests that vascular plants or their immediate ancestors

appeared in the Early Silurian. The first unequivocal

land-plant megafossils are known from the mid-Silur-

ian (Wenlock). Following the Silurian and Devonian

development and radiation of early vascular-plant

groups, the plant biodiversity curve (Figure 2B)reflects

the successive acmes of the pteridophytes and early

gymnosperms in the Carboniferous–Permian and

the gymnosperms (see Fossil Plants: Gymnosperms) in

the Triassic–Jurassic and the rise of the angiosperms

from the Cretaceous onwards.

The diversity curve for the insects (Figure 2C)

shows a rather saw-toothed exponential rise from the

first appearance of the group in the Devonian. The

tetrapods (Figures 2D and 3A) also show an essen-

tially exponential pattern, with a long (Devonian to

mid-Cretaceous) period of weak diversity increase

followed by a steep rise. The curve primarily reflects

the radiations and acmes of three major sets of groups:

the basal tetrapods and synapsids in the Palaeozoic,

archosaurs in the Mesozoic, and lissamphibians,

lepidosaurs, birds, and mammals in the Cenozoic.

The colonization of freshwater and the increase

in diversity therein was achieved both by air-breathing

groups and by taxa from several major marine clades

that independently became adapted to reduced salinity

by movement upstream through estuarine environ-

ments. However, the occupation by invertebrates of

freshwater habitats, especially those within the sub-

strate, took a remarkably long time, and it was not

until the Mesozoic that a significant range of originally

marine groups developed the necessary osmoregula-

tory, reproductive, and dispersal capacities to occupy

freshwater environments.

Figure 2 Terrestrial biodiversity through the Phanerozoic. (A) All multicellular families (lower curve represents families undoubt-

edly present within each stage and restricted to the non-marine environment; upper curve includes less stratigraphically well-

attributed families and those that include environmentally equivocal that include marine or environmentally equivocal taxa); (B)

vascular land plants; (C) insects; and (D) non-marine tetrapods. (Adapted from Benton MJ (2001) Biodiversity through time. In: Briggs

DEG and Crowther P (eds.) (2001)

Palaeobiology II, pp. 211–220. Oxford: Blackwell Publishing.)

BIODIVERSITY 263

Understanding Biodiversity Curves

If the existing curves are even an approximation of

the true pattern, the great challenge is to interpret

them in terms of the processes leading to biodiversity

change through geological time. At the simplest level,

the curves reflect the complex interplay of species

originations, extinctions, and stasis, with the major

changes in trajectory representing episodes of radi-

ation punctuated by mass extinctions. Although the

traditional assumption has been that radiations were

‘adaptive’, representing the acquisition of new char-

acters by the radiating group that gave them superior-

ity over their competitors, there is increasing evidence

for the expansion of many clades into previously

unoccupied ecospace. This is perhaps most strikingly

illustrated by the tetrapods, for which it has been

calculated that a maximum of only 13% of families

could have originated by competitive replacement of

earlier taxa (Figure 3). It has also been argued that

clade radiations could arise randomly, without being

driven by any deterministic cause such as competition

or expansion associated with a new adaptation or

opportunity.

The shapes of biodiversity curves Several attempts

have been made to model mathematically the shapes

of global Phanerozoic diversity curves and therefore

to determine whether they reflect overarching global

processes and patterns of evolution.

Continental biodiversity, either as a whole or of

individual large clades (Figure 2), seems to show

an exponential rise without a sustained levelling off.

In contrast, the family-level marine curve has been

modelled fairly closely in terms of logistic curves rep-

resenting the Cambrian, Palaeozoic, and Modern

Faunas perturbed by the major mass extinction events

(Figure 4). In this coupled logistic model, each evolu-

tionary fauna showed a slow initial increase in diver-

sity, followed by a steep increase to a plateau and then

a slow decline corresponding to the rise of the follow-

ing evolutionary fauna (which had a lower initial rate

of diversification and a higher maximum diversity

level). Within such a model, the trajectory of the

curve for the Modern Evolutionary Fauna is currently

still in its exponential phase, but its roughly convex

shape suggests that an asymptotic stage could be pro-

jected in the future. The effects of the mass extinction

Figure 3 Biodiversity change in 840 non-singleton tetrapod

families, broken down into (A) the component groups and (B) the

styles of origination of new families. Overlap candidate competi-

tive replacements are those in which the range of an originating

family overlapped with that of another family rather than re-

placing it in the fossil record. (Adapted from Benton MJ (1999)

The history of life: large databases in palaeontology. In: Harper

DAT (ed.)

Numerical Palaeobiology, pp. 249–283. Chichester: Wiley.)

Figure 4 Three-phase coupled logistic model of changes in

family-level marine biodiversity during the Phanerozoic, includ-

ing perturbations simulating the mass extinctions at the end of the

Ordovician (O), in the late Devonian (D), and at the ends of

the Permian (P), Triassic (Tr), and Cretaceous (K). The numbers

1, 2 and 3 refer to the Cambrian, Palaeozoic and Modern evolu-

tionary faunas shown on Figure 1. Dotted lines represent the

trajectories of the unperturbed three-phase system. (Adapted

from Sepkoski JJ (1984) A kinetic model of Phanerozoic taxo-

nomic diversity. III. Post-Paleozoic families and mass extinctions.

Paleobiology 10: 246–267.)

264 BIODIVERSITY

events enhance the fit of the theoretical curves to the

measured global patterns.

By extending the theory of island biogeography

developed in the 1960s to the global scale, the logistic

curves applied to marine familial diversity have been

interpreted as reflecting evolution into new habitats

and empty niches until a level of dynamic equilibrium

is reached at the carrying capacity of the environ-

ment. This theory has received widespread, but not

universal, acceptance. It has been argued, for exam-

ple, that the shapes of the curves may represent not

global-scale biotic interactions but rather the total

effect of physical perturbations operating at a wide

range of geographical and temporal scales. Moreover,

the evidence for equilibrium, even on an ecological

rather than a geological time-scale, and the upper

limit on diversity that it would impose have also

been widely contested. The Palaeozoic diversity plat-

eau, taken to be strong evidence for equilibrium, may

have been maintained by factors other than biotic

interactions and may even be an artefact of the taxo-

nomic level of the data. Importantly, it is clear that

different clades or parts thereof may show very dif-

ferent patterns of biodiversity change.

The causes of biodiversity change Despite consider-

able research effort, there has been only limited success

in identifying ‘rules’ governing species and community

diversity and the relative abundances of species within

communities. It is highly unlikely, even for mass extinc-

tion events, that a single factor can be invoked to

explain global changes in biodiversity. In addition to

factors intrinsic to individual clades, a host of inter-

related extrinsic factors undoubtedly influence the evo-

lution, distribution, and diversity of organisms across

the spectrum of spatial and temporal scales. Crucially,

the factors operating at one scale may be very different

from those operating at another. Intriguingly though,

there is some evidence to suggest that the family-level

global diversity curve has properties of self-organized

criticality, and, thus, Phanerozoic diversity change may

be driven by the internal dynamics of life itself, as well

as responding to external factors.

The present-day concerns over biodiversity change

stem from the recognition of the deleterious conse-

quences of anthropogenic activities, including habitat

destruction, pollution, and influences on global cli-

mate. The fossil record provides a time perspective

not only on the patterns of biodiversity change but

also on the natural physical factors that drive it, from

local fluctuations in environmental conditions to plate

tectonics, eustasy, ocean circulation patterns, and cli-

mate changes. Heightened awareness of the quality

and detail of the data involved in the generation and

analysis of diversity curves at all spatial and temporal

scales should result in greater confidence in the

conclusions drawn from them.

Further Reading

Bambach RK, Knoll AH, and Sepkoski JJ (2002) Anatom-

ical and ecological constraints on Phanerozoic animal

diversity in the marine realm. Proceedings of the Na-

tional Academy of Sciences USA 99: 6854–6859.

Benton MJ (1999) The history of life: large databases in

palaeontology. In: Harper DAT (ed.) Numerical Palaeo-

biology, pp. 249–283. Chichester: Wiley.

Benton MJ (2001) Biodiversity on land and in the sea.

Geological Journal 36: 211–230.

Briggs DEG and Crowther P (eds.) (2001) Palaeobiology II.

Oxford: Blackwell Publishing.

Crame JA and Owen AW (eds.) (2002) Palaeobiogeography

and Biodiversity Change: The Ordovician and Mesozoic–

Cenozoic Radiations. Special Publication 194. London:

Geological Society.

Gaston KJ and Spicer JI (1998) Biodiversity: An Introduc-

tion. Oxford: Blackwell Science.

Hewzulla D, Boulter MC, Benton MJ, and Halley JM

(1999) Evolutionary patterns from mass originations

and mass extinctions. Philosophical Transactions of the

Royal Society of London Series B 354: 463–469.

Knoll AH (1994) Proterozoic and early Cambrian protists:

evidence for accelerating evolutionary tempo. Pro-

ceedings of the National Academy of Sciences USA 91:

6743–6750.

Levin SA (ed.) (2001) Encyclopedia of Biodiversity, 5 vols.

San Diego: Academic Press.

May RM (1992) How many species inhabit the Earth?

Scientific American October 1992: 18–24.

Miller AI (2000) Conversations about Phanerozoic global

diversity. Paleobiology 26(4 Suppl.): 53–73.

Sepkoski JJ (1984) A kinetic model of Phanerozoic taxo-

nomic diversity. III. Post-Paleozoic families and mass

extinctions. Paleobiology 10: 246–267.

Sepkoski JJ (1997) Biodiversity: past, present, and future.

Journal of Paleontology 71: 533–539.

Smith AB (2001) Large-scale heterogeneity of the fossil

record: implications for Phanerozoic biodiversity studies.

Philosophical Transactions of the Royal Society of

London, Series B 356: 351–367.

Ward BB (2002) How many species of prokaryotes are

there? Proceedings of the National Academy of Sciences

USA 99: 10234–10236.

BIODIVERSITY 265

a0005 BIOLOGICAL RADIATIONS AND SPECIATION

P L Forey, The Natural History Museum,

London, UK

Introduction

This entry discusses the evidence for speciation and

the patterns of species multiplication and associ-

ated morphological change as revealed in the fossil

record. Speciation is regarded as the key stage in

the generation of biological diversity as it is the

point at which reproductive isolation is achieved –

the hallmark of the biological species (see Evolution).

Many studies of modern species concern themselves

with the short-term genetic fluctuations between

populations of a single species, how those fluctu-

ations may come about and how they are maintained.

Such studies are not possible in the fossil record. On

the other hand, the fossil record documents extended

timescales and may, therefore, identify patterns of

long-term changes relevant to different theories of

speciation that are unavailable in the Recent world.

Irrespective of the mode of speciation, the fossil

record demonstrates repeated instances, geologically

speaking, of sudden increases in the numbers of species

and these are usually associated with rapid morpho-

logical divergence. Such events are commonly referred

to as radiations, more usually adaptive radiations, and

are inferred to have come about by causes intrinsic to

the organisms and/or extrinsic causes. Although stud-

ies of radiations involving populations and closely

related species are studied in the Recent world,

broad-scale patterns can only be studied through the

time dimension supplied by the fossil record.

Species, Species Recognition and

Speciation in the Fossil Record

There is a plethora of species definitions and criteria

for recognizing species in the Recent world, but

few are applicable to the fossil record where only a

limited amount of morphological data, usually the

skeleton, is available for study. Three such definitions

that rely on some underlying process of speciation are

shown in Figure 1. The biological species concept of

Ernst Mayr (Figure 1A) is probably that most

favoured by people studying the Recent world.

Species status is achieved when populations diverge

genetically to such an extent that reproductive

cohesion is broken. There may be several reasons for

fragmentation of reproductive continuity (geography,

ecology, and behaviour are most commonly cited),

although Mayr stressed geographical isolation. The

parental species may continue to live alongside the

daughter species. In practice, most modern species

are not recognized on reproductive criteria: instead

some measure of morphological, genetic, behavioural

or ecological difference is used as a surrogate for

reproductive incompatibility. As such, palaeonto-

logical species, recognized almost exclusively on

morphological differences, are equally as valid as

modern species. In fact, there has been empirical jus-

tification for equating genetic differences with mor-

phological differences between modern species of

some groups such as bryozoans and Darwin’s finches.

It is, however, recognized that morphological differ-

ence may not always indicate species differences (e.g.,

sibling species or polymorphic species including

mimetic species).

The evolutionary concept of George Gaylord

Simpson (Figure 1B) defines a species as an an-

cestor-descendent sequence of populations changing

through time with their own trends and tendencies.

Figure 1 Three concepts of species. (A) Mayr’s biological

species. Populations reach species distinction when reproduct-

ive cohesion is broken, usually by the imposition of some isolat-

ing mechanism such as geographic separation of populations

(triangle). The original species (X) may or may not continue to

live contemporaneously with the daughter species (Y). (B) Simp-

son’s evolutionary species concept. Species are ancestor-

descendent sequences of populations changing through time.

A species may change gradually into another (X ! Z) through a

process of anagenesis or the lineage may split (X ! Y) through

cladogenesis. (C) The Hennigian species concept recognizes

species as lineage segments. As soon as a cladogenetic

event occurs there are automatically two new species (Y and

Z), regardless of any perceived morphological change. Stars

represent speciation events.

266 BIOLOGICAL RADIATIONS AND SPECIATION

This definition embraces asexual species. Such a def-

inition allows for one species to change gradually

into another (anagenesis) as well as instances where

a species is budded off or where one species splits into

two or more (cladogenesis).

The Hennigian species concept (Figure 1C) (named

after the German entomologist Willi Hennig), like the

biological species concept, is based on the criterion

of reproductive isolation. However, the species limits

are recognized only at the point where one species

splits to two or more and the interbreeding pattern of

gene flow is disrupted, at which point the ancestral

species cannot by definition live alongside the des-

cendants. This species definition is strictly dependent

upon the shape of the phylogenetic tree.

Recognition of Species

Species in the fossil record are nearly always recog-

nized on morphological criteria. There are some

exceptions, such as instances where species are distin-

guished from one another purely on the basis of

stratigraphic occurrence or geographic location, but

these are usually accepted as stopgap measures; to be

revised when more information about morphology

becomes available.

Monophyletic species are recognized on the basis

of possessing one or more unique morphological

characters and this corresponds most closely with

the Hennigian species concept. Species viewed in

this light have the same ontological status as higher

taxa such as genera, families, orders, etc. In reality,

very few species can be recognized in this way and

logically it denies the existence of ancestors since

ancestors have no unique features.

Typological species were once commonly recog-

nized, whereby a single specimen (usually the first dis-

covered or the most complete) was chosen as the

Linnean type and this formed the nucleus of morpho-

logical variation considered worthy of species status.

How much variation was to be allowed before a new

species was recognized was never stated. Such species

are usually frowned upon now as being steeped in

the philosophy of pre-Darwinian essentialism. Yet,

there remain many instances where such species are

still used.

Phenetic species arose out of numerical taxonomy.

Here species are measured for as many morphologic-

ally continuous variables as possible. The variables

are then subjected to multivariate analysis, and clus-

ters – (the species) – are then recognized. Although

this may appear to be objective and theory free, it is

heavily dependent upon the original samples analyzed

and the multivariate analysis algorithms used. It is

not always easy to identify which of the individual

variables is contributing to the differences and, there-

fore, it is difficult to diagnose the species.

Phylogenetic species are the smallest diagnosable

assemblages of specimens showing a unique combin-

ation of characters. Combination is the operative

word here since there is no requirement for identifying

unique characters (cf. monophyletic species recogni-

tion). Of all the recognition criteria this most faith-

fully agrees with all of the process-based definitions

of species, as well as agreeing with the day-to-day

practice of palaeontologists.

Estimates of species longevity may depend on which

of the above ways the species have been recognized.

Surveys across various fossil groups give average figures

that range from 1 million years (for small mammals

where generation time is very short) to 13 million

years for dinoflagellates (which are largely asexual).

Speciation in the Fossil Record

It is generally accepted that most speciation events,

with the possible exception of those caused by hybrid-

ization and polyploidy (chromosomal duplication),

are too protracted to be detected in the Recent

world and too brief to be seen in the fossil record.

The lower limits of time that can be successively

sampled in the fossil record appear to be in the order

of 5000–10 000 years and such deposits are very rare,

restricted to lake deposits of relatively short duration

and small geographic range, and some deep-sea de-

posits. Nevertheless, there have been many studies

seeking to establish the precise pattern of speciation.

There are two polarized theories on the tempo and

pattern of speciation (Figure 2), punctuated equilibria

and phyletic gradualism. Both have support from the-

oretical and empirical studies in the modern world

(see legend to Figure 2) and it is likely that both modes

of speciation can happen.

Phyletic gradualism Phyletic gradualism holds that

morphological evolution is gradual and occurs inde-

pendently of speciation. Most of the studies support-

ing this mode of evolution involve organisms that

can be minutely sampled through continuous rock

sequences.

Figure 3 shows an example of one gastropod species

(see Fossil Invertebrates: Gastropods) changing or re-

placing another in an ancestor-descendent sequence

(gradual speciation by anagenesis) by a shift in the

modal shell form as measured by morphometric dis-

criminant analysis. In this example the gradual

change has been correlated with increasing depth

of water. The populations intermediate between

the two species were presumed to have lasted for

73 000–250 000 years. In this pattern there is no

BIOLOGICAL RADIATIONS AND SPECIATION 267

rapid shift in morphology and, in studies of this kind,

the species limits are usually recognized by comparing

the morphological variation over a period of time

with morphological variation of modern species and

dividing up the continuum accordingly.

In another study (Figure 4) involving radiolarians

recovered from Pacific Ocean deep-sea drilling cores,

an instance of speciation was detected occurring

during a period of about 500 000 years. In this case

both the ancestral and daughter species showed

marked deviation in many aspects of skeleton form.

Further, it is interesting to note that morphological

deviation, although slightly accelerated during the

speciation event, continued for some time after the

separation of morphotypes, suggesting strongly that

morphological evolution is decoupled from speci-

ation. In this instance both parental and daughter

species showed morphological evolution and it may

have been possible to suggest that a single ancestral

species gave rise to two daughter species. The author

of this study refrained from this because the ancestral

species, which still lives today, shows a high degree of

polymorphism and geographic variation, the scale

of which is comparable to the variation seen in the

lineage in this study.

It needs to be pointed out that many (but not all)

of the studies that support phyletic gradualism, report

information on continuously varying morphologies

such as lengths and shapes. These parameters are in-

herently gradualist in their measurement, unlike many

characters used in cladistic analysis such as presence/

absence or four toes versus five toes in which there can

be no intermediates.

Punctuated equilibrium Speciation following the

punctuated equilibrium model (Figure 2B) has been

claimed for many groups of organisms. This theory

suggests that species themselves exhibit morpho-

logical stasis and that speciation is, geologically spe-

aking, instantaneous and is accompanied by rapid

morphological shifts. In order to demonstrate this at

least three objections raised by advocates of phyletic

gradualism must be overcome. The first objection is

that what appear to be sudden changes only seem so

because either the sampling intervals are too wide or

the fossil record is lacking in the key years where

intermediate populations might have lived. There-

fore, it is necessary to demonstrate that the sampling

is dense. A second objection is that it is possible

that an isolated population, destined to become a

new species, was accumulating genetic and morpho-

logical distinctiveness in a gradualist manner, but that

the population was either so small that fossilization

potential was virtually zero, or that it was living

elsewhere. The sudden appearance is, therefore, ex-

plained not as any intrinsic property of speciation, but

by the new species population growing large enough

to leave a fossil record, or by immigration into the

area of study. The third objection is a taxonomic

argument. Many of the claims of punctuated equilib-

rium involve species recognized by qualitative charac-

ters where there are no intermediate states (see

above). If this is true then the sudden appearance of

morphologic changes must coincide with speciation

and must also remain static until the next ‘speciation’.

Despite the objections there are cases where punc-

tuated equilibrium can be demonstrated. Figure 5

illustrates the case of speciation in the bryozoan

genus Metrarhabdotus during the Neogene and Qua-

ternary of tropical America (see Fossil Invertebrates:

Bryozoans). The species were distinguished on both

continuously varying characters, as well as qualitative

differences. A stratophenetic tree was constructed.

There was particularly dense sampling of the fossil

record in the Dominican Republic between 8 and

4 million years ago, within which time there was the

successive origin of 12 species. Each of the species

appeared suddenly, accompanied by substantial

morphological differences. The species themselves ex-

hibited little or no change throughout their individual

Figure 2 Two theories of speciation modes in the fossil record.

(A) Phyletic gradualism suggests that speciation is a by-product

of changing ratios of gradual morphological variation (implied

gene frequency) within populations through time and that speci-

ation may happen through anagenesis (species X ! Y) as well as

cladogenesis (X ! Z). Phyletic gradualism agrees with Simp-

son’s evolutionary species concept (Figure 1B), coupled with

Fisher’s notion of ‘directed’ selection of gene frequencies within

populations. (B) Punctuated equilibrium claims that individual

species remain morphologically constant throughout time and

that all or most of the morphological (and implied genetic)

change is associated with cladogenesis: the speciation event.

This theory agrees with Mayr’s biological species concept

(Figure 1A) coupled with Sewell Wright’s idea of stabilizing

selection. In these diagrams the species are denoted by letters

and different shadings and the speciation events by stars.

268 BIOLOGICAL RADIATIONS AND SPECIATION

existence and this is a key feature of the punctuated

equilibrium model. Although the possibility of immi-

gration cannot be completely dismissed this is un-

likely because the basal species of this lineage are

morphologically very similar to species occurring

earlier in the same area.

Therefore, it appears as though there is evidence for

both patterns of speciation in the fossil record.

Indeed, there may be instances where both kinds can

be recognized within the same sequence, affecting

closely related organisms. More commonly, species

within the same lineage may show gradualism at

some times and punctuated patterns at others (but

here there is always the objection of differential sam-

pling). Rather than trying to prove that all speciation

complies with one or the other model it may be more

productive to try to determine when and why one or

the other is predominant.

Radiations

Radiations are episodes of increased rates of speci-

ation relative to extinctions resulting in rapid net

increases in diversity. They may be accompanied by

evolution of more diverse body forms than in normal

periods when extinction and origination rates are

approximately equal. Radiations are nearly always

explained in terms of adaptation, hence ‘adaptive ra-

diations’, the latter being defined by Dolph Schluter as

the evolution of ecological and morphological diver-

sity within a rapidly speciating lineage. There are diffi-

culties with defining both ecological and phenotypic

diversity and it is not always easy to describe precisely

the adaptation and even more difficult to isolate the

cause, but the following factors have been suggested as

triggers for radiations (adaptive or otherwise):

1. Abiotic causes – such as changing levels in O

/CO

concentrations or significant changes in ambient

temperature, changes in ocean current circulation,

fragmentation of landmasses and consequent

changes in both the length and ecological differen-

tiation of coastlines, and changes in latitudinal

distributions.

2. Biotic factors – such as the evolution of a particu-

lar character or characters that may change

aspects of life history leading to rapid speciation

and/or ecological differentiation.

Figure 3 Speciation through anagenesis. In this study a species of marginellid gastropod – (Prunum coniforme) – living in the Mio-

Pliocene of the Dominican Republic was seen to show strong directional selection in shell morphology towards a new species –

(

Prunum christineladdae). (A) Histogram of individuals on the discriminant axis that best distinguishes the two species (endpoints of

sampling) collected from successively higher stratigraphic levels within the section at Rio Gurabo, Dominican Republic (section

thickness marked in meters at left). Notice that at level F there appears to be a mixed population which appears to have occupied

0.6–2.5% of the entire range of the ancestral species – (

P. coniforme). (B) Over a longer time scale the species are distributed in

sediments inferred to have been deposited in increasingly deeper water. Part of the history at Rio Gurabo is repeated at neighbouring

Rio Cana. Reproduced from Nehm RH and Geary DH (1994) A gradual morphologic transition during a rapid speciation event

in marginellid gastropods (Neogene; Dominican Republic).

Journal of Paleontology 68: 787–795.

BIOLOGICAL RADIATIONS AND SPECIATION 269

3. Interactions between organisms – such as the ex-

tinction of one kind allowing ecological and

phenotypic diversification in another. Alterna-

tively it may be true that the morphological and

diversification of one group allows similar phe-

nomena in another (e.g., insects and plants). For

example, in fishes, it has long been noted that the

evolution of many durophagous (mollusc crush-

ing) lineages coincides with the Mesozoic diversifi-

cation of bivalve and gastropod molluscs. Many of

these ‘associations’ are anecdotal and, while they

may well be true, they need firm experimental

evidence for their justification.

It is rarely possible to isolate a single cause and in

all probability complex interactions between two

or more factors are ultimately responsible for radi-

ations. Therefore, the study of these evolutionary

phenomena is both frustrating and challenging,

demanding lengthy and detailed data collection.

Factors which may distort our view of radiations

include the following:

1. Imperfections of the fossil record. Large hiatuses or

differential preservation will inevitably distort our

perception of radiations. For example, the sudden

appearance and apparent radiation of many cat-

fishes (most of which are freshwater) in the Early

Tertiary must be judged against the knowledge that

there are very few freshwater fish-bearing deposits

in the underlying Upper Cretaceous.

2. Appearance of many diverse animals and plants

associated with Lagersta

tten deposits. Such deposits

dramatically increase the numbers and morpho-

logical diversity of species that may, in reality,

have had a long unrecorded history.

Figure 4 Speciation and the gradualist model. The present day radiolarian species Pterocanium charybdeum extends well back into

the Miocene where a subspecies (

P. c. allium is recognized). At about 4 million years ago a new species, P. prismatium arose and

subsequently went extinct about 1.8 million years ago. To compile this diagram samples representing populations at 50 000 year

intervals were taken from piston cores during the key 4.5–3.5 million year interval (much more widely spaced outside of this time

band). The radiolarian tests were measured for 32 morphological characters such as length, breadth, angles and outlines and

subjected to a multivariate discriminant analysis. The x-axis of the diagram shows the value of the discriminant function that best

separates the species. The boxes are the population means  1 standard error. The histograms on the right show discriminate scores

at the point of the intermediate population (lower diagram) and the sampling level 50 000 years later (upper diagram), showing clear

separation at this time; these two documenting the speciation event. Notice that both lineages continue to diverge morphologically in a

gradualist manner long after the speciation event. Reproduced from Lazarus DB (2001) Speciation and morphological evolution. In:

Briggs DE and Crowther PR (eds.)

Palaeobiology II, pp. 133–137. Oxford: Blackwell Scientific.

270 BIOLOGICAL RADIATIONS AND SPECIATION

3. Taxonomic artifact. Many diversity studies are

based on estimates of numbers of genera or fam-

ilies. Dependent on how those taxa are recognized

in successive stratigraphic levels, the expansion (or

decrease) in diversity may be exaggerated or dis-

torted. For example, it has been pointed out that

the apparent sudden increase in new trilobite fam-

ilies in the earliest Ordovician may be an artifact

of taxonomy of Cambrian trilobites, since many

phylogenetic lines can now be drawn across the

boundary linking families that were previously

thought to be quite distinct.

Despite these problems there are good examples of

association of cause with radiations.

Environmental Shift

One general pattern that has been identified after

extensive collection of both taxic and geological

data is that many invertebrate clades originated in

nearshore marine environments and subsequently

expanded in species numbers to occupy deeper

water, at the same time becoming morphologically

more diverse. Figure 6 shows one example of the

Figure 5 Punctuated equilibrium in Neogene bryozoans. The cheilostome genus Metrarhabdotos is widely distributed in Miocene and

Pliocene deposits of the Caribbean. Sections in the Dominican Republic are particularly rich and uninterrupted, such that is possible to

sample extensively. This study used 46 measurements, counts or codings of colony morphology of the species (some were left un-

named). These were subjected to multivariate analysis and the stratophenetic tree shown here was produced. The dots are sampling

points. The x-axis is a measure of the morphologic distance. Notice the sudden shift in morphological variation at points of

cladogenesis. The within species variability is very small compared with the between species differences as predicted by the

punctuated equilibrium model, as is also the prediction that the ancestral species continues to live alongside the descendant. The

age, epoch and nannoplankton zones are shown on the y-axis. Reproduced from Cheetham AH (1986) Tempo of evolution in a Neogene

bryozoan: rates of morphological change within and across species boundaries.

Paleobiology 12: 190–202.

BIOLOGICAL RADIATIONS AND SPECIATION 271

Elsevier Encyclopedia of Geology - vol I A-E

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