REVIEWS
The demographic transition:
are we any closer to an
evolutionary explanation?
Monique Borgerhoff Mulder
D
emographic transitions The radical shift in human reproduction in reflects historical demographers’
normally entail two feathe late 19th century, known as the
recognition that the 19th century
tures. First, there is a radidemographic transition, constitutes a
fertility decline coincided with two
cal decline in the number
major challenge to evolutionary
processes: the devaluation of child
of offspring that parents produce,
approaches to human behaviour.
labour in cottage industry and its
despite overall increases in the
Why would people ever choose to limit
banning in factories, and the growavailability of resources1. This is their reproduction voluntarily when, at the ing benefits associated with the
particularly true of the 19th cenpeak of the Industrial Revolution,
education of children for employtury European transition, where
resources were apparently so plentiful?
ment in an increasingly competitive
marital fertility halved in less than
Can the transition be attributed to
open-market economy11. It is also
30 years in some countries (Fig. 1),
standard life history tradeoffs, is it a
linked to the observation that parbut also characterizes some 20th
consequence of cultural evolutionary
ents in land-limited agrarian comcentury transitions in the develprocesses, or is it simply a maladaptive
munities selectively abandoned
oping world, such as in Thailand2.
outcome of novel and environmental
later-born sons to reduce the numSecond, rich (and often aristo- social conditions? Empirical analyses and ber of potential inheritance claims
cratic) families reduce their fertility
new models suggest that reproductive
on the estate12, thereby avoiding
earlier, and often more markedly,
decision making might be driven by a
impoverishment and loss of family
than the rest of the population3,
human psychology designed by natural
rank. This hypothesis poses the
such that the positive correlations
selection to maximize material wealth.
quantity and/or quality trade-off
commonly found between wealth
If this is the case, the mechanisms
(first pioneered for clutch size
and fertility in predemographic governing fertility and parental investment variation by Lack6) that has strucpopulations disappear: censuses are likely to respond to modern conditions tured much subsequent thinking
from England and Wales in 1911 with a fertility level much lower than that in life history analysis, including
(Ref. 4) demonstrate this point
that would maximize fitness.
work on humans13,14.
(Table 1).
Attractive as this hypothesis
The fact that people in an
is, it has no direct support. Kaplan
Monique Borgerhoff Mulder is at the
increasing number of societies
et al. tested its central prediction
Dept
of
Anthropology,
University of California at Davis, that the number of grandchildren
worldwide voluntarily reproduce
Davis, CA 95616, USA
at lower levels than would apparwould peak at an intermediate level
(mborgerhoffmulder@ucdavis.edu).
ently maximize their lifetime reproof fertility15. Using data on conduction poses a major challenge
temporary men in the state of New
to evolutionary anthropologists.
Mexico (USA), they did not find a
Indeed, the sociologist Daniel Vining identified this puzzling curvilinear relationship (Fig. 2). Indeed, although it is well
trend as the ultimate challenge to evolutionary approaches known that higher parental fertility in modern societies is
to human behaviour4. In some senses, this view is naive: linked to lower investment in individual children, the low
many social scientists fail to appreciate that natural selec- income of children from large families does not decrease
tion favours different optimal allocations of effort to repro- their fertility. In short, although birth rates can respond alduction in different ecological and social environments and most instantaneously to changes in the costs of children
hence that lower-than-maximum levels of fertility might be (such as was seen in the precipitous decrease in fertility in
optimal. Nevertheless, behavioural ecologists are puzzled East Germany once state benefits contingent on children
by the dramatic nature of the decline. They are also deeply were withdrawn at reunification16), the overall fitness benpuzzled by the emerging negative correlations between efits of such adjustments remain elusive.
wealth and reproduction, when evidence that the wealthy
Until recently, attempts to support this hypothesis analytioutreproduce the poor is so prevalent in predemographic cally have also failed. Investigators modelling the competitransition populations5. Consequently, a range of hypoth- tive environments in which high levels of parental investeses are now being explored to explain why parents with ac- ment are critical to offspring success have been unable to
cess to plentiful resources choose low fertility rates (Box 1). simulate the classic features of demographic transitions17.
Although they can show that low fertility is favoured when
Three hypotheses for demographic transition
the costs of raising children are high18 and when the effects
In the first hypothesis, evolutionary anthropologists sug- of parental investment on offspring quality diminish slowly19,
gest that, in modern societies, lowered fertility rates are they cannot simulate situations in which high-income groups
optimal (with respect to fitness) because of the competitive maximize their fitness by producing fewer offspring than
environment in which offspring are raised – a world in which low-income groups17,20. The failure of both empiricists and
high levels of parental investment are critical to a child’s modellers to support Lack’s hypothesis is disappointing, as
success9 and costly to the parent10. This hypothesis closely it had seemed such a good candidate.
266
Copyright © 1998, Elsevier Science Ltd. All rights reserved. 0169-5347/98/$19.00 PII: S0169-5347(98)01357-3
TREE vol. 13, no. 7 July 1998
REVIEWS
Enter evolutionary psychology
Perhaps because of difficulties with each of these hypotheses, attention has turned recently to the psychological
TREE vol. 13, no. 7 July 1998
0.9
Belgium
Netherlands
0.8
Bulgaria
0.7
Russia
Index of marital fertility (Ig)
The second hypothesis (Box 1) is that lowered rates of
fertility are a consequence of darwinian, but nongenetic,
mechanisms of inheritance. This view stems from the recognition that, as a result of cultural evolutionary processes,
not all human traits and institutions need to be seen as
serving individual reproductive interests8. The argument is
that traits that do not necessarily enhance genetic fitness
can spread through a population as a result of imitation,
especially if these traits are expressed by people who are
otherwise very successful. Because, in a competitive market
economy, childless individuals (or people with fewer than
the average number of children) are often highly successful
in careers as teachers, communicators or politicians, these
people serve as models for others in the population. In the
model, if a suite of traits are imitated indiscriminately, low
fertility might spread. Boyd and Richerson8 have used mathematical models to show how this process (which they call
indirect bias) might arise through ordinary evolutionary
processes if the costs associated with reaching adaptive
solutions are high.
This idea is intriguing. It may well account for the rapid
spread of fertility-limiting behaviour through populations
(as, for example, in Sweden21) and is intricately linked to the
notion, now popular among demographers and social scientists, that it is changes in ideas (rather than changes in
the economy) that cause fertility transitions22. However, the
cultural evolutionary hypothesis poses problems. First,
there is no clear reason why influential trendsetting individuals choose lower fertility in the first place. Granted,
there will be trade-offs between seeking socioeconomic status and reproducing early and often. But why reproduction
is sacrificed to such extremes still needs to be explained or
at least raises questions about how such status-seeking becomes stable in a population where there may be countervailing selection for high fertility. Second, theorists working
on cultural inheritance build their models with very different
assumptions concerning the mechanisms of evolutionary
processes than do behavioural ecologists. Such abandoning
of the basic organic evolutionary model may still be premature, although the potential importance of such mechanisms
is pointed to in newer work, outlined here.
The third hypothesis (Box 1) is that low fertility is maladaptive – a by-product of changes in our environment that
serves no adaptive function. The commonest cause cited here
is the availability of cheap and efficient birth control methods.
Thus, Perusse23 has shown that wealthier men in his Canadian
sample achieve higher copulation rates than their poorer
counterparts: without the availability of contraceptives, the
wealthy would outreproduce their less wealthy competitors.
This is an interesting finding but not a sound explanation for
the demographic transition. The European transition started
long before effective birth control technology was available.
Furthermore, in many contemporary African nations the
transition fails to occur, despite the availability of contraceptives. Finally, this perspective does not explain why some
sectors of the population adopt contraceptive use before
others. Most importantly, the hypothesis that low fertility is
a maladaptive by-product of rapid social change is not an
explanatory theory at all because it fails to specify precisely
what has changed in the environment, why these changes
lead to lowered fertility and even what kinds of evolved
mechanisms might underlie this response15. The overall justification of an evolutionary approach is given in Box 2.
Italy
England
Hungary
0.6
Ireland
0.5
0.4
France
Spain
Portugal
0.3
0.2
0.1
1860
1880
1900
1920
1940
1960
1980
Date
Fig. 1. Index of marital fertility (Ig) in selected national populations of Europe. From
Ref. 1, with permission.
Table 1. Surviving children per married couple
(where wife’s age exceeds 45 years) classified by
social status, 1911, England and Walesa
Social class
Professional
Lower white collar
Skilled manual
Semiskilled manual
Unskilled
Textiles
Coal mining
Agricultural labourers
a From
Surviving children per married couple
2.94
3.38
3.82
3.79
3.88
3.31
4.45
4.57
Ref. 4, with permission.
mechanisms underlying reproductive behaviour. Can mechanisms be identified (and we are considering here primarily
psychological mechanisms) that would have been favoured
in pretransition human populations and can also account
for the fertility patterns seen in modern societies?
Rogers20 became interested in identifying the conditions
in which material motivations might be selected over reproductive motivations, an important issue considering that
consumption of material goods seems to compete with childbearing. His first model showed that, in environments where
wealth is heritable, an individual’s wealth is a better predictor of the number of second and third generation descendants produced than is fertility17. Under such conditions, material motivations could be selected over pure reproductive
motivations through conventional darwinian processes. Unfortunately, a more realistic diploid version of the model20
could simulate no such equilibrium.
267
REVIEWS
Box 1. Evolutionary hypotheses
for demographic transitions
Box 2. Why an evolutionary theory of fertility
transitions is required?
Lowered rates are:
(1) Optimal because of the competitive environment in which offspring are
raised. In conditions where high levels of parental investment are critical to offspring and costly to parents, parents optimize fitness by producing a few children with high levels of investment rather than many with less investment per
capita. This hypothesis implies a trade-off between offspring quantity and
quality6,7.
(2) A consequence of darwinian but nongenetic mechanisms of inheritance.
Boyd and Richerson8 suggest that traits (such as low fertility) associated with
successful individuals are preferentially imitated by others in the population.
Indiscriminate imitation of the traits of successful models might evolve through
ordinary organic adaptive processes if the costs to an individual reaching the
local optimal value for traits through experimentation and individual learning
are high.
(3) A maladaptive by-product of rapid environmental change that has no adaptive value. Owing to the radical changes in the social, economic, political and
ecological conditions that modern humans now experience, evolved mechanisms (either psychological or physiological) no longer generate appropriate
responses to external conditions. As a result, maladaptive levels of fertility
are observed.
A theory of fertility should specify what causes both the overall reduction in fertility and the observed relationships between independent
variables (such as wealth) and fertility. It must also account for why
these changes produce a demographic transition on the basis of a
broader model of fertility determinants. Demography, as a discipline,
has little theory of its own; accordingly, Keyfitz24 observes that demography ‘has withdrawn from its borders and left a no-man’s land
which other disciplines have infiltrated’. Evolutionary theory is one such
intruder25. Another is economics, but recent overviews suggest that
economic theories of fertility suffer from several conceptual weaknesses. Most notably, demand-orientated models (‘how many children
do people want?’) are poorly linked to broader explanations for why
humans behave as they do26.
So what can evolutionary theory offer? Most fundamentally, it provides a justification for why organisms, including humans, are designed
by natural selection to maximize fitness – economics offers no such causal closure. As regards fertility, models can be formulated of how fertility
is likely to be affected by extrinsic features of the environment, in conjunction with evolved physiological and psychological mechanisms25,27;
this work is often informed by work on other species28. Although it is
increasingly recognized that not all aspects of human behaviour can be
characterized as adaptations, variability in fertility is one trait for which
simple adaptive models are likely to be valuable, at least as a starting
point [Box 1 (1)], because fertility differentials are so exposed to selection. Where such models fail, evolutionary, but less explicitly adaptationist, perspectives are being investigated [Box 1 (2), (3)].
90
80
Grandchildren
70
60
50
40
30
20
10
0
0
2
4
6
8
10
12
Father's children
14
16
18
20
Fig. 2. The relationship between the fertility of Albuquerque men (number of offspring
produced) and their production of grandchildren. If the number of grandchildren
peaks at an intermediate level of fertility (as in a tradeoff model), a second order
polynomial regression should yield a positive linear term and a negative squared
term. No such effect is found. The model (reduced to include only significant effects)
shows that the number of children produced is by far the strongest predictor of variation in the numbers of grandchildren produced. In other words, men with the most
children have the highest number of third generation descendants, contrasting
sharply with the observed modal fertility of two children in this population. From Ref.
15, with permission.
Kaplan also focuses on psychological mechanisms25. He
retains the assumption that, as in all organisms, human fertility is the outcome of an allocation decision between the
number and the quality of offspring and, therefore, a function
of the effects of resources on reproductive success. However, because optimizing fertility relative to the impact of
parental investment on the reproductive outcomes of the
next generation is such a difficult problem to solve, simple decision making rules (‘rules of thumb’) could have evolved15.
Specifically, psychological mechanisms might have been
designed by natural selection to maximize the sum of the
energetic resources stored by individuals and their descendants. Kaplan and Lancaster are starting to test elements of
this model using empirical data from their sample of contemporary New Mexico men, with considerable success29.
268
But what about pretransition societies? Social anthropologists and historical demographers commonly argue that
parents exhibit fertility schedules that serve to protect the
integrity of a family’s material property. For example, primogeniture and unigeniture (whereby the complete inheritance
is given to the first or last born son, respectively) have long
been interpreted as strategies for preventing impoverishment and loss of family rank, and infanticide has been interpreted as a means of shaping appropriate sets of heirs30,31.
However, in highly pronatal communities, such as many parts
of contemporary Africa (and indeed most pre-transition
populations), it is generally assumed that people have as
many children as they can (or at least as many as women
can subject to the trade-off between offspring survival and
interbirth interval13). A recent collaborative project uses
empirical data to explore whether reproductive or material
motivations better predict the marital and reproductive decisions of men in a highly pronatal Kenyan Kipsigis community32. The results (Box 3) show that material motivation is
pre-eminent, even though no obvious heir-shaping strategies are evident in this society. In summary, there are now
intriguing arguments and scattered seeds of evidence that a
highly materialistic human psyche might be evolutionarily
stable and deeply rooted in our past, and not just an aberrant outcome of modern society.
Closing in on an explanation
So why, within societies, do richer people have fewer
children? Several studies are closing in on an explanation for
this second puzzle posed by the demographic transition.
Assuming there is a general human psychology designed
to maximize the sum of the incomes of all descendants produced, Kaplan et al. argue that rich, highly skilled parents
have fewer children than poorer, less skilled parents because the time and resources the former put into their offspring are intrinsically more valuable (i.e. produce more
skills) than the time and resources invested by the latter15,25.
It is widely recognized that the rate at which a child learns
depends on the knowledge and skills that it already possesses. If such a nonlinear effect is real, then the opportunity
TREE vol. 13, no. 7 July 1998
REVIEWS
Conclusion
The demographic transition that has, or is, affecting so
many parts of the world is a highly complex phenomenon
with no single form and no common set of causes36. At least
two phenomena need to be explained: the reduced levels
of fertility despite relative material plenty; and the erosion
of the widespread correlations between resources and fertility characterizing predemographic transition populations.
The contribution of behavioural ecology to this perplexing issue consists of disentangling the notion of ‘the transition’ and
tackling its various components with appropriate theory.
Although rapid declines in fertility may ultimately prove to
be a maladaptive by-product of massive changes in the market place (because no data currently attest to long-term fitness payoffs), our explanations increasingly specify precisely what has changed in the environment and what kinds
of evolved mechanisms might underlie this phenomenon.
More broadly, these developments point to how evolutionary psychology, behavioural ecology and cultural inheritance models – three approaches once characterized as
mutually incompatible – each stand poised to contribute to
an evolutionary explanation for the complexities of human
behaviour.
Acknowledgements
Thanks to Tim Caro, Sarah Hrdy and Eric Smith for
comments and discussion, and to Alan Rogers for
permission to cite an unpublished result.
References
1 Coale, A.J. and Treadway, R. (1986) A summary of the changing
distribution of overall fertility, marital fertility, and the
proportion married in the provinces of Europe, in The Decline of
Fertility in Europe (Coale, A.J. and Watkins, S.C., eds), pp. 31–181,
Princeton University Press
2 Knodel, J. et al. (1982) Fertility in Thailand: Trends, Differentials, and
Proximate Determinants, National Academy of Sciences
3 Livi-Bacci, M. (1986) Social-group forerunners of fertility control
in Europe, in The Decline of Fertility in Europe (Coale, A.J. and
Watkins, S.C., eds), pp. 182–200, Princeton University Press
TREE vol. 13, no. 7 July 1998
Box 3. Dynamic state models: do Kipsigis men
marry for money or love?
Dynamic state models33 are increasingly used in studies of human fertility 18,34 to identify how environmental factors affect reproductive decisions. In a study of a Kenyan Kipsigis community32, a dynamic model
is used to determine the precise nature of the terminal payoff (wealth
or children) individual men appear to be maximizing, as judged by their
life history decisions.
A model was built of men’s marital careers, with the aim of predicting when they should take subsequent wives. In the model, men
are characterized by different states – their capital (livestock and land),
the number of their wives and the number of their children. Their economic productivity depends on their capital assets – the labour of their
wife (or wives) and various stochastic factors. Net food surpluses, after
family consumption, are invested in livestock. Children are born and entail costs (illnesses, education) that are met by livestock sales. Marrying
a wife is costly (the bride-wealth payment) and each year every man gets
the option to choose another wife (there is no divorce in the population).
The analysis was in three steps. First, an array of terminal functions (or maximands) were constructed, based on differently weighted
combinations of offspring and wealth-per-offspring (capturing a continuum of reproductive to material motivation). Second, using backward
runs of the model33, the optimal marital career was determined for
each weighting of the terminal function. This showed that, given the
same initial starting conditions, men concerned with maximizing wealth
per child married fewer wives than men concerned with maximizing the
numbers of their children. The former also had a higher wealth threshold at which they took additional wives. The third and key step entailed
comparing each set of optimal strategies (predicated on different terminal functions) with what Kipsigis men actually do. The figure shows
that the best fit between the model and the observed behaviour occurs
where the terminal function is heavily weighted towards accumulating
a lot of wealth for each child. In other words, a Kipsigis man’s behaviour seems optimally designed to accumulate material resources over
the lifespan, resources that are divided among his sons at death.
6000
5800
5600
Total SSQ
costs of producing an additional child are greater among the
rich than they are among the poor, driving the negative or
curvilinear relationships between parental wealth and fertility noted by Vining. Mace35 models a similar process in which
different strata in society optimize fitness (with different fertility levels) as a result of following different decision rules.
Therefore, although wealth and fertility may be positively
correlated within strata, the relationship can break down
between strata. Finally, Rogers, who was until recently unable to simulate an environment in which optimal fertility
decreases with wealth20, now reports that in environments
in which inheritance greatly boosts an individual’s ability to
earn an income (each dollar inherited generates on average
two dollars of earned income) wealthy parents can attain
higher long-term fitness at equilibrium than poorer parents
by producing fewer children (A.R. Rogers, unpublished).
In summary, the rich and skilled may produce fewer children than the poor and unskilled because different groups
within a society vary in the learning trajectories of their offspring, in the effects of inherited wealth on earning power,
and in wealth. This outcome arises purely from mechanisms shaped by ordinary adaptive processes. Furthermore, once such fertility reductions appear among the rich,
they can potentially spread to other social classes, even if
the appropriate conditions do not exist in these groups, by
the process of indirect bias posited by cultural evolutionists
[Box 1 (2)].
5400
5200
5000
4800
0
0.2
0.4
0.6
0.8
1
Gamma
The total sum of squares (total SSQ) measures the error between the
optimal strategy predicated on a particular terminal function and the
observed behaviour of real men. The terminal function (gamma) represents the conflicting ‘motivation’ to maximize children (gamma = 0) or
wealth per child (gamma = 1). The values of gamma are shown by filled
circles and the line is interpolated for ease of viewing. Results proved
robust to sensitivity analyses. From Ref. 32, with permission.
4 Vining, D.R. (1986) Social versus reproductive success – the central
theoretical problem of human sociobiology, Behav. Brain Sci. 9,
167–260
5 Cronk, L. (1991) Wealth, status and reproductive success among
the Mukogodo, Am. Anthropol. 93, 345–360
6 Lack, D. (1947) The significance of clutch size, Ibis 89,
302–352
7 Smith, C.C. and Fretwell, S.D. (1974) The optimal balance between
size and number of offspring, Am. Nat. 108, 499–506
8 Boyd, R. and Richerson, P.J. (1985) Culture and the Evolutionary
Process, University of Chicago Press
269
REVIEWS
9 Irons, W.G. (1983) Human female reproductive strategies, in Social
Behavior of Female Vertebrates (Wasser, S.K., ed.), pp. 169–213,
Academic Press
10 Turke, P. (1989) Evolution and the demand for children, Popul.
Dev. Rev. 15, 61–90
11 Lesthaeghe, R. and Wilson, C. (1986) Modes of production,
secularization, and the pace of fertility decline in Western
Europe, 1870–1930, in The Decline of Fertility in Europe (Coale, A.J.
and Watkins, S.C., eds), pp. 261–292, Princeton University Press
12 Voland, E. and Dunbar, R. (1995) Resource competition and reproduction: the relationship between economic and parental strategies
in the Krummhorn population (1720–1874), Hum. Nat. 6, 33–49
13 Blurton Jones, N. (1987) Bushman birth spacing: direct tests of
some simple predictions, Ethol. Sociobiol. 8, 183–203
14 Hill, K. and Hurtado, A.M. (1996) Ache Life History, Aldine de Gruyter
15 Kaplan, H.S. et al. (1995) Fertility and fitness among Albuquerque
men: a competitive labour market theory, in Human Reproductive
Decisions (Dunbar, R.I.M., ed.), pp. 96–136, St Martin’s Press
16 Conrad, C., Lechner, M. and Werner, W. (1996) East German fertility
after unification: crisis or adaptation, Popul. Dev. Rev. 22, 331–358
17 Rogers, A.R. (1990) The evolutionary economics of human
reproduction, Ethol. Sociobiol. 11, 479–495
18 Beauchamp, G. (1994) The functional analysis of human fertility
decisions, Ethol. Sociobiol. 15, 31–53
19 Pennington, R. and Harpending, H. (1988) Fitness and fertility
among Kalahari !Kung, Am. J. Phys. Anthropol. 77, 202–319
20 Rogers, A.R. (1995) For love or money: the evolution of
reproductive and material motivations, in Human Reproductive
Decisions (Dunbar, R.I.M., ed.), pp. 76–95, St Martin’s Press
21 Carlsson, G. (1966) The decline of fertility: innovation or
adjustment process, Popul. Stud. 20, 149–174
22 Watkins, S.C. (1990) From local to national communities: the
transformation of demographic regimes in western Europe,
1870–1960, Popul. Dev. Rev. 16, 241–272
23 Perusse, D. (1993) Cultural and reproductive success in industrial
societies: testing relationship at the proximate and ultimate
levels, Behav. Brain Sci. 16, 267–322
24 Keyfitz, N. (1984) Introduction: biology and demography, in
Population and Biology (Keyfitz, N., ed.), pp. 1–7, Ordina Editions
25 Kaplan, H.S. (1996) A theory of fertility and parental investment in
traditional and modern human societies, Yearb. Phys. Anthropol.
39, 91–135
26 Robinson, W.C. (1997) The economic theory of fertility over three
decades, Popul. Stud. 51, 63–74
27 Ellison, P.T. et al. (1993) Population variation in ovarian function,
Lancet 342, 433–434
28 Wasser, S.K. (1994) Psychosocial stress and infertility, Hum. Nat. 5,
293–306
29 Kaplan, H.S. and Lancaster, J.B. The life histories of men in
Albuquerque: an evolutionary–economic analysis of parental
investment and fertility in modern society, in Human Behavior and
Adaptation: an Anthropological Perspective (Cronk, L.,
Chagnon, N.A. and Irons, W., eds), Aldine de Gruyter (in press)
30 Wrigley, C. (1978) Fertility strategy for the individual and the
group, in Historical Studies of Changing Fertility (Tilly, C., ed.),
pp. 135–154, Princeton University Press
31 Hrdy, S.B. and Judge, D.S. (1993) Darwin and the puzzle of
primogeniture, Hum. Nat. 4, 1–45
32 Luttbeg, B., Borgerhoff Mulder, M. and Mangel, M. To marry
again or not? A dynamic model for demographic transition, in
Human Behavior and Adaptation: an Anthropological Perspective
(Cronk, L., Chagnon, N.A. and Irons, W., eds), Aldine de Gruyter
(in press)
33 Mangel, M. and Clark, C.W. (1988) Dynamic Modeling in Behavioral
Ecology, Princeton University Press
34 Mace, R. (1996) When to have another baby: a dynamic model of
reproductive decision-making and evidence from Gabbra
pastoralists, Ethol. Sociobiol. 17, 263–273
35 Mace, R. An adaptive model of human reproductive rate:
why people have small families, in Human Behavior and
Adaptation: an Anthropological Perspective (Cronk, L., Chagnon, N.A.
and Irons, W., eds), Aldine de Gruyter (in press)
36 Low, B.S. (1994) Men in demographic transition, Hum. Nat. 5,
223–254
Molecules remodel the mammalian tree
T
Wilfried W. de Jong
hese are exciting times for
DNA sequences provide a direct record
and Lagomorpha (rabbits, pikas,
those who are interested in
of the genealogy of extant species.
etc.); Archonta, grouping Primammalian relationships. This tremendous reservoir of phylogenetic mates, Scandentia (tree shrews),
Changes are rapid, and coninformation is only beginning to be
Dermoptera (flying lemurs) and
troversies abound. Just six years
exploited. If progress in molecular
Chiroptera (bats); and an ungulate
ago, an influential review summaphylogeny is being made, it should be
clade comprising Artiodactyla
rized the state of the art in mammost conspicuous amongst mammals,
(even-toed ungulates), Cetacea
malian phylogeny1. Combining moras their evolution is probably the
(whales), Perissodactyla (oddphological and some molecular
best-studied. Indeed, surprising changes
toed ungulates), the paenungulate
evidence, it presented a rather well
have recently been proposed for the tree
orders Proboscidea (elephants),
resolved tree of the 18 currently
of mammalian orders. These range from
Sirenia (sea cows) and Hyracoidea
recognized orders of placental
grouping whales with hippos, to placing
(hyraxes), and the monospecific
mammals (Fig. 1a). This tree incorAfrican golden moles closer to elephants
order Tubulidentata (aardvark).
porated the most prevalent and
than to their fellow insectivores.
The relationships of Insectivora
dominant opinions of the time.
and Carnivora with other extant
Monotremata – the egg-laying Platyorders remained enigmatic, and the
Wilfried de Jong is at the Dept of Biochemistry,
pus and Echidna – and Marsupialia
Macroscelidea (elephant shrews),
University of Nijmegen, PO Box 9101,
successively branch off from the
Pholidota, Artiodactyla and Hyra6500 HB Nijmegen, and the Institute for Systematics
main stem towards the Eutheria,
coidea were considered to have
and Population Biology, University of Amsterdam,
the placental mammals. Edentata
relatively uncertain relationships.
1090 GT Amsterdam, The Netherlands
(or Xenarthra: sloths, anteaters
Thus, it would appear that
(w.dejong@bioch.kun.nl).
and armadillos), probably together
after more than a century of palewith Pholidota (pangolins), constiontological and morphological intute the sister group of all other
quiry, complemented with more
placental orders. Among the latter, some widely accepted recent molecular findings, progress was being made towards
superordinal clades are shown: Glires, combining Rodentia a gradually growing consensus, reflected by the topology in
270
Copyright © 1998, Elsevier Science Ltd. All rights reserved. 0169-5347/98/$19.00 PII: S0169-5347(98)01335-4
TREE vol. 13, no. 7 July 1998