Bio1.J. Linn. SOC.,1,pp. 135-148. With 1 plate and4figures
April 1969
Evolution and speciation in a tropical high mountain flora
OLOV HEDBERG
Institute of Systematic Botany, University of Uppsala, S w e h
Few ecosystems provide better opportunities for the study of evolution and speciation than
those inhabiting the uppermost parts of the high east African mountains. These mountains are
mainly of volcanic origin and lie widely scattered across the wide plateaux of east Africa,
several of them reaching altitudes between 3500 and 6000 m. Their vegetation deviates very
much from that of the intervening lower country, displaying a marked zonation with a montane
forest belt, a (subalpine) ericaceous belt, and an afroalpine belt. T h e flora of the latter, the
afroalpine flora, is of exceptional interest in this connection.
The afroalpine flora is famous for its large numbers of geographically vicarious taxa-its
Giant Senecios and Giant Lobelias are as renowned as the finches of the Galapagos
Islands. Ecologically, the afroalpine biota is indeed also an island biota-the high mountain
summits protrude as isolated temperate islands above the warm surrounding plains. These
mountains have evidently stood isolated from each other since their origin. Pleistocene climatic
changes have certainly modified their vegetation zonation to a considerable extent, but direct
contacts between their afroalpine enclaves during the Pleistocene or earlier seem most improbable. These enclaves must therefore have been isolated from each other and from other
high mountain areas for avery long time, and dispersal of plants between them must presumably
have occurred mainly by long distance transport, possibly facilitated by cyclones.
Some 80% of the afroalpine species of vascular plants are endemic to the high mountains
of tropical east Africa and Ethiopia. Vicarious taxa occur of different status. In some cases one
species occurs on all or most of the east African mountains with a vicariad in other parts of the
world, as exemplifiedby Subulariamonticola(afroalpine) and S . aquatica (circumboreal). In other
cases each of two species is conlined to one group of mountains, as in the species pair Lobelia
wollastonii (Virunga Volcanoes and Ruwenzori) and L. telekii (Elgon, Aberdare, M t Kenya).
Finally, there are several groups of vicarious taxa where each taxon is confined as a rule to a single
mountain, as in the Lobelia deckenii group with six cognate species. Numerous similar cases
occur among spiders and insects.
The differentiation of these vicarious species is assumed to have occurred through natural
selection in connection with genetic drift, acting upon geographically isolated and originally
very small random samples of the gene pools concerned. The amount of differentiation between
different mountain populations differs considerably between different groups-numerous intermediate stages exist between morphologically indistinguishable populations and full-fledged
vicarious species. The rate of evolutionary change seems to differ considerably between
different genera and families. Whether internal barriers to interbreeding exist between these
vicarious taxa is in most cases unknown.
No less interesting than the geographically vicarious taxa mentioned above are some cases
of altitudinal vicariism. Some afroalpine species appear to have evolved from afromontane forest
species through progressive adaptations favouring survival in the inhospitable afroalpine
climate. The most remarkable examples are provided by the strangely specialized Giant
Senecios and Giant Lobelias.
CONTENTS
Introduction .
Endemism in the afroalpine flora
.
Age of isolation of the montane enclaves
.
Age of isolation of afroalpine enclaves
Degree of differentiation in various groups
Genetic systems, pollination biology, polyploidy
Mode of evolution
.
References
.
.
.
.
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.
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136
136
136
141
143
144
. 144
. 147
136
OLOVHEDBERC
INTRODUCTION
Few ecosystems provide better opportunities for the study of evolution and speciation
than those inhabiting the uppermost parts of the high east African mountains. The
latter are mainly of volcanic origin and lie scattered across the wide plateaux of east
Africa, several of them reaching altitudes between 3500 and 6000 m. The most important are the Virunga Volcanoes, Ruwenzori, Elgon, Aberdare, Mt Kenya, Kilimanjaro and Mt Meru. Their vegetation deviates profoundly from that of the intervening lower country, displaying a marked zonation with a montane forest belt, a (subalpine) ericaceous belt, and an afroalpine belt (Hedberg, 1951). The flora of the latter,
the afroalpine flora, is of special interest in this connection.
The climate of the afroalpine belt is rather harsh, with 'summer every day and
winter every night', and its flora is poor in species. This afroalpine flora is, however,
extremely interesting because of the striking ecological adaptations displayed by many
species (Hedberg, 1964) and its large amount of geographically vicarious taxa. Its
Giant Senecios and Giant Lobelias are as renowned in this respect as the finches
of the Galapagos Islands. Ecologically, the afroalpine biota is indeed also an island
biota-the high mountain summits protrude as isolated temperate islands above the
warm surrounding plains.
ENDEMISM IN THE AFROALPINE FLORA
Some 80% of the afroalpine species of vascular plants are endemic to the high mountains of east Africa and Ethiopia (Hedberg, 1961). Vicarious taxa occur of different
status. In some cases one species occurs on all or most of the east African mountains
with a vicariad in other parts of the world, as exemplified by Subularia monticola"
(afroalpine) and S. aquaticu (circumboreal). In other cases each of two species is confined to one group of mountains, as in the species pair Lobelia wollastonii (Virunga
Volcanoes and Ruwenzori) and L. telekii (Elgon, Aberdare, Mt Kenya). Finally, there
are several groups of vicarious taxa where each taxon is confined as a rule to a single
mountain, as in the Lobelia deckenii group with six cognate species. Numerous similar
examples occur among spiders and insects.
The vicarious species of the afroalpine flora have evidently arisen under the
influence of geographical isolation. The mountains harbouring them are of unequal
ages (Miocene to late Pleistocene) and must have stood isolated from each other since
their origin (cf. Hedberg, 1961). Their afroalpine enclavesare at present very efficiently
isolated from each other. In order to investigate how long this isolation may have
lasted we must mobilize the evidence available concerning the Pleistocene climatic
and vegetational history of east Africa. A considerable part of this paper must therefore
be devoted to evaluation of such evidence.
AGE OF ISOLATION OF THE MONTANE ENCLAVES
That considerable climatic changes have occurred in east Africa during the Pleistocene is evidenced by traces of extensive earlier glaciations-terminal moraines have
+ The botanical nomenclature used in this paper follows Hedberg (1957) to which publication the
reader is referred for authors' names and full references.
EVOLUTION
AND SPECIATION
IN MOUNTAIN
FLORA
137
been found down to 1740 m on Ruwenzori, c. 3500 m on Elgon, c. 2900 m on M t
Kenya, and c. 3600 m on Kilimanjaro (cf. Gregory, 1894; Mackinder, 1900; Meyer,
1900; Nilsson, 1932; Hedberg, 1951; Baker in Coe, 1967; Livingstone, 1967). The
retreat of the glaciers on Ruwenzori has been shown by radiocarbon dating to be
contemporaneous with deglaciation in Europe (Livingstone, 1962, 1967) and hence
with glacial retreat also in North America (Flint & Deevey, 1951), Patagonia (Auer,
1958), and New Zealand (Gage, 1961). According to Osmaston (1967) the lowering of
the firn-line on Ruwenzori during the glacial maximum could be accounted for by a
decrease of about 4" C below the present mean temperature, whereas on Kilimanjaro
the corresponding decrease was estimated at about 6". Additional evidence for worldwide changes of climate has been supplied by oceanographic studies from the floor of
the Atlantic, suggesting that the temperature of its surface water was about 5" lower
during the last glaciation than at present (Emiliani, 1958). Other estimates of reduced
mean temperature during the last Glacial are quoted in Moreau (1963 : 399) ; see also
van Zinderen Bakker (1967b).
Studies of ancient shorelines along some of the large east African lakes have indicated
a much higher water level during some earlier 'pluvial' epochs (cf. Nilsson, 1932, 1940,
1949). These Pluvials were at first believed to have been contemporaneous with the
glaciations on the mountains, but recent investigations have shown them to have been
at least partly out of phase-some of them were furthermore evidently caused by
tectonic movements rather than by climatic shifts. The last major lake phase of Lake
Victoria seems to have occurred between 13,000and 3000years ago (Kendall in van Zinderen Bakker, 1 9 6 7 ~98)-at
:
about the same time as a Pluvial prevailed in central
Sahara, allowing the southwards extension of the Mediterranean flora into the mountains of this area (QuCzel, 1963).
The method of pollen analysis, which has been so helpful in elucidating the vegetation history of Boreal countries, has been only recently attemptedineast Africa (Hedberg,
1955), and even today only a few pollen diagrams are available from this part of the
world (Osmaston, 1958;van Zinderen Bakker, 1964; Coetzee, 1964,1967 ;Livingstone,
1967; Morrison, 1968, etc.). These diagrams certainly indicate the occurrence of
important vegetational changes during and after the last glaciation, but their interpretation is by no means easy (cf. Morrison, 1966; Livingstone, 1967).
Both van Zinderen Bakker (1964) and Coetzee (1964,1967) interpret their diagrams as
indicating a considerable downwards shift, amounting to some 1000-1100 m, of the
vegetation belts of Mt Kenya and Cherangani during the maximum of the last glaciation (the 'Mt Kenya hypothermal'), which was calculated by Coetzee (1967: 88) to
imply a decrease in annual mean temperatures by some 5*1"-8*8"C. Similarly Morrison (1968) concluded that near the site investigated by him at Muchoa in south-west
Uganda the vegetation at the same time must have approached that at present occurring
in the ericaceous belt, from which he calculated for the relevant epoch an annual mean
temperature 5" C lower than today. In my opinion, however, the evidence for these conclusions is insufficient, Occurrence of afroalpine vegetation has been deduced mainly
by high pollen percentages of grasses and Compositae, and of ericaceous belt vegetation largely by grasses plus Artemisiu, Stoebe and Cltzortia together with Ericaceae.
None of these genera or families are, however, by any means restricted to the afroalpine
OLOVHEDBERG
138
and ericaceous belts; they may occur in profusion also at lower levels, provided the
ground is open and dry enough (cf. Hedberg, 1957). At least part of the vegetation
changes registered in these pollen diagrams could therefore equally well be explained
by changes in precipitation. That the ‘hypothermal’ vegetation concerned on Mt
Kenya and Cherangani must have been of a dry type was repeatedly emphasized by
Coetzee (1967:68,81,etc.). It might be worth while investigating by means of surface
samples to what extent the ‘alpine’spectra recorded by her from Mt Kenya and Cherangani might agree with spectra from, for example, the montane scrub grassland described
0
50
I00
150
200
250
300
(km)
FIGURE
1. Schematicprofile from Ruwenzori across Lake George and Kitwe to the easterngroup
of the Virunga Volcanoes (Muhavura) showing the approximate present altitudinaldistribution
of the montane forest belt (small dots), the ericaceous belt (wavy lines) and the afroalpine belt
(black). For further explanation see the text.
by Kerfoot (1964)from levels above 2450 m on the Mbeya mountains in S. Tanganyika.
Concerning his Ruwenzori diagram Livingstone (1967:50)concluded :‘Adrier climate
prior to 12500 B.P.would account for all the pollen changes. A colder climate would
not’.
Evidently much more work must be devoted to present-day vegetation and ecological
conditions on these mountains, including studies of the actual pollen spectrum of
various plant communities, and many more pollen diagrams must be constructed
before we can reach reliable conclusions concerning their Pleistocene vegetation history. Awaiting further evidence I am not prepared to believe in temperature changes
and vegetation shifts of the magnitude suggested by Coetzee.
That some downwards shift of the vegetation belts of the high mountains occurred
during the glaciation seems nevertheless inescapable-on Ruwenzori, Elgon, M t
Kenya and Kilimanjaro the uppermost belts of vegetation must have been literally
pushed down-or telescoped-by the glaciers. However, the nature and extent of
EVOLUTION
AND SPECIATION
IN MOUNTAIN
FLORA
139
this shift remains to be established. It should also be remembered that below the firnline glacier tongues keep in the main to valley bottoms, leaving the intervening ridges
ice-free up to high level (cf. Nilsson, 1932), and that dense afroalpine vegetation may
grow close to the side of a glacier (Hedberg, 1964: Fig. 9; 1968: 186). T o what extent
the montane forest belt was also lowered is hardly possible to deduce from the
palaeoclimatical and pollen-analytical evidence available. Its lower limit on most
mountains has been pushed up in historical time by exploitation for agriculture (cf.
Hedberg, 1951).
What possibilities may then have existed for contacts between the biotas of the
different mountains during the glaciations? The profile in Fig. 1 demonstrates the
f
500
-- 400
E
al
3
+.
._
2
300
200
0
100
200
400
300
500
600
(km)
FIGURE
2. Schematic profile drawn from Mt Elgon across Cherangani Hills, Mau Range and the
Aberdares to Mt Kenya, showing the approximate present altitudinal distribution of the
montane forest belt (small dots), the ericaceous belt (wavy lines) and the afroalpine belt
(black).
obstacles for such contacts between the Virunga Volcanoes and Ruwenzori. Montane
forest is at present restricted to altitudes above 2200-2500 m on the north slopes of the
Virunga Volcanoes and 1800-2000 m on the south and east slopes of Ruwenzori. On
the map of ecological zones in Pratt, Greenway & Gwynne (1966: fig. 7) there is a
corridor of ‘zone 2’, with climatic potential for forestry or intense agriculture, covering
most of the way traversed by this profile, although the amount of forest remaining
intact along this stretch is very limited (Pratt et al., 1966: fig. 8). Presumably a continuous band of forest existed here only a couple of thousand years ago. There must
still have remained a gap of some 50 km across the depression round Lake George,
however, and the establishment of direct contacts between the two montane forest
areas concerned will presumably have called for a depression of the lower limit of the
montane forest belt by some 300-500 m.
From the profile in Fig. 2 which has been drawn in a zig-zag manner between the
OLOVHEDBBRC
140
mountains concerned, and from the maps in Pratt et al. (1966), it appears that before
the destruction of so much forest by man there would presumably have been direct
contacts between the montane forests of Elgon, Cherangani and the Mau Range.
The climatic lower limit of the montane forest belt on Aberdare and Mt Kenya is
certainly a good deal lower than its actual limit, and direct contacts between these
montane forests and those of Mau, Cherangani and Elgon may well have existed here
under present climatic conditions before the intervention by agricultural man. The
obstacles to similar contacts between the montane forest ecosystems of Elgon and
Ruwenzori are considerably larger, because of a long gap of intervening lowland below
1200 m altitude. Fairly recent direct contacts between all or most of the now
isolated areas of montane forest in east Africa were nevertheless deemed necessary
by Moreau (1963) to explain the present distribution of montane birds and other
organisms.
Direct contact between the montane forests of Kenya-Uganda and Ethiopia would
call, on the other hand, not only for a more considerable depression of the lower limit
of the montane forest belt (by some 1000 m, cf. Fig. 3) but also for a large increase
0
100
500
300
700
900
(km)
FIGURE
3. Schematic profile from the Amaro Mts in Ethiopia across Mt Kulal, Mt Nyiru and
Losiolo to Mt Kenya, showing the approximate present distribution (in Kenya) of the montane
forest belt (small dots), the ericaceous belt (wavy lines), and the afroalpine belt (black). The
vegetation zonation in southern Ethiopia is insufficiently known.
in precipitation in the intervening dry area. According to Moreau (1963) there seems to
have existed in this case a potent barrier preventing recent exchange of montane forest
elements." Great obstacles must also have existed to the exchange of montane taxa
between the high mountains of west Africa and east Africa. Not all biologists would be
prepared to believe in a depression of the vegetation belts by some 1000 m, as suggested
by Moreau (1963) to account for their faunistic and floristical similarities (cf. also
Morton, 1962, and van Zinderen Bakker, 1967b). Given sufficient time most montane
It may be worth recollecting, however, that the high mountain flora of southern Ethiopia appears to
show a stronger resemblance to that of the east African mountains than to the north Ethiopian one
(Gillett, 1955; Hedberg, 19620).
EVOLUTION
AND SPECIATION
IN MOUNTAIN
FLORA
141
forest species of animals and plants should, I believe, be able to pass even a fairly wide
gap without depending upon continuous distribution of the whole ecosystem. Time
has indeed been available here on quite a different scale from that in the formerly
glaciated parts of Europe, where dispersal of plants and animals has nevertheless
been remarkably efficient (cf. Webb, 1966).
One potent vector of transport which has received little attention although it has
been available in tropical Africa is cyclones. Admittedly, cyclones are rare in east
Africa-there are only two on record, with an interval of 80 years (Sansom, 1953).
During the time of, say, one million years such a frequency would, however, by no
means be insignificant. I can find no better way of illustrating their potentialities for
the dispersal of plant diaspores, birds and insects than to quote the report on the Lindi
cyclone, which struck the coast of southern Tanganyika near Lindi on 15 April, 1952,
destroying, inter aZia, most of the township of Lindi, and uprooting forest trees and sisal
plants over large areas. Before it crossed the coastline the centre of the cyclone passed
a small ship a few miles off the coast, the captain of which stated in his report that in
the calm storm centre birds and insects were ‘falling all over the ship. Many were dead
and all were dazed’. Commenting on this the meteorological author said: ‘Their
condition might be explained by the strain of their buffeting by the storm; but it is
at least equally plausible to suggest that they had been swept up to great heights in the
inner ring of the storm and fell through the subsiding air of the central calm.’ (Sansom,
1953 : 9.) A cyclone like this will certainly have a large ‘collecting area’ for air masses
containing wind-blown organisms and diaspores, and when these are swept up to great
height in the central whirl they may evidently be transported over considerable distances before they are released. The track of the present cyclone traversed much of
southern Tanganyika (Sansom, 1953).
AGE OF ISOLATION OF AFROALPINE ENCLAVES
While direct migration of forest elements between at least some of the relevant
mountains seems plausible in the recent past the possibilities for similar dispersal of the
afroalpine flora would at first sight appear very limited. Both from phytogeographical
and ecological points of view however, this flora is rather heterogeneous (Hedberg,
1961, 1965). By definition it comprises all species known to occur in the afroalpine
belt. Even when some accidental afroalpines are left out, the altitudinal range of the
remaining species shows a very wide variation (Fig. 4). Some are entirely confined to
the afroalpine belt, others occur also in the ericaceous belt, and many occur even far
down in the montane forest belt. The possibilities for inter-mountain dispersal of these
species must be assumed to be largely dependant upon their altitudinal range-those
which regularly occur in the lower part of the montane forest belt should have found
much greater opportunities for dispersal during glacial or pluvial epochs than those
which are confined to higher levels. This assumption is strongly supported by analysis
of the percentage of endemics in various altitudinal groups (Table 1). That group of
species which do not occur below 3000 m consists mainly of endemics-only 3 % of them
occur outside the mountain areas of east Africa and Ethiopia. The lower down the
mountain the species descend, the lower becomes the percentage of endemics, especially
OLOVHEDBERC
142
Y
Species
FIGURE
4. Diagram illustrating the variability in altitudinal distribution displayed by the
afroalpine flora of vascular plants. Each ‘regular’ afroalpine species is represented by a vertical
l i e , indicating its known altitudinal range. The ranges have been sorted according to falling
values of, firstly, their lower altitudinal limit and, secondly, their upper altitudinal limit.
Data from Hedberg, 1957.
Table 1. Number of regular afroalpine vascular plants reaching their lower altitudinal limit
within each of four altitude intervals, and percentage of one-mountain endemics, one-mountah-group endemics, two-mountain-group endemics, and non-endemic species in each of
these altitude groups.
Number of regular afroalpine vascular plant species, the lower altitudinal limit of which
fall within each interval
Altitude
intervals
Total No.
of species
+No. of species restricted in East Africa to:
1 E.A. mountain
onlv
%
NO.
Above 3000 m
101
65
64%
1 group of E.A.
mountains
No.
27
2400-1 800 m
70
8
,
56
1
92
91%
11%
30
23
%
6
6%
3
3%
43%
18
26%
14
20%
20
36%
22
39%
3
13%
17
74%
I
38
54%
2%
13
23%
I
\
Below 1800m
No.
27%
,
\
3000-2400 m
%
Both groups of
E.A. mountains
No.
%
Occurring also
outside east
Africa and
Ethiopia
1
14
25%
4%
2
3
13%
~
Some of them also occur in Ethiopia.
9%
EVOLUTION
AND SPECIATION
IN MOUNTAIN
FLORA
143
of one-mountain endemics. These results indicate that the ecological islands formed by
the upper parts of the high mountains, particularly their afroalpine (and ericaceous)
belt(s) have been effectively isolated from each other much longer than their zones of
montane forest. The possibilities for earlier continuous distribution areas of these
‘qualified’ afroalpines are so slight that their migration between the mountains must be
assumed to have taken place mainly by independent long-distance dispersal (Hedberg,
1961)” The vectors of dispersal between the mountains for such afroalpine vascular
plants are evidently beyond observation or experimental study, but also in this case I
would like to suggest cyclones as a plausible agent.
Establishment of an afroalpine vascular plant species on a new mountain may
have been facilitated in many cases by the occurrence of vast lava fields and ash screes,
created by volcanic eruptions and acting as ‘agar plates’ for alien diaspores of ecologically adapted species. The patches of open soil which are common in stabilized
vegetation in the upper part of the afroalpine belt may not be so favourable in this
respect, since most seedlings on these are often eliminated by solifluction (Hedberg,
1964: 33).
DEGREE OF DIFFERENTIATION I N VARIOUS
GROUPS
Detailed morphological studies of the afroalpine flora have demonstrated that the
degree of differentiation between different mountain populations differs considerably
in different groups of plants (Hedberg 1957: 374; 1958: 187). Some species occur on
many or all of the high mountains concerned without displaying any perceptible
morphological differentiation between their isolated mountain populations, e.g.
Sagina afroalpina, Arabis alpina and Subularia monticola. In other cases different
mountain populations of one species show statistical differences but their variation
ranges overlap so much as to preclude taxonomic distinction, e.g. in Alchemilla johnstonii, Veronica glandulosa and Valeriana kilimandscharica (concerning the latter see
Kokwaro, 1968). In yet other cases different mountain populations considered to be of
common ancestry present distinctly discontinuous variation ranges, making taxonomic separation possible, as in the Philippia trimera group, the Lobelia deckenii group,
and the species pair Romulea congoensis-R. keniensis. The results of this comparison
indicate that the speed of evolutionary diversification has been comparatively low in
the afroalpine representatives of the families Juncaceae, Caryophyllaceae, Cruciferae,
Crassulaceae, Labiatae and Rubiaceae, and comparatively rapid in the genera Alchemilla, Philippia, the Giant Lobelias, and in some genera of the Compositae, notably
Helichrysum and Senecio (cf. Hedberg, 1957: 376). This generalization holds good also
is the comparison is restricted to such taxa which have not been found below 2400 m,
and which consequently would seem to have had small chances of migrating between
the mountains by other means than long-distance dispersal of diaspores.
The lower percentage of endemics among those species occurring down to 2400 m altitude may not
necessarily imply that the latter have had access to more continuous migration routes between the
mountains. The explanationmay at least partly be that such species can utilize a much larger area on each
mountain-both for the production of diaspores on one mountain and for successful reception of them
on another-than those species restricted to areas above 3000 m.
144
OLOVHEDBERC
GENETIC SYSTEMS, POLLINATION BIOLOGY, POLYPLOIDY
In the earlier part of this paper I have stressed that the afroalpineflora offers beautiful
examples of evolutionary differentiation under the influence of geographical isolation,
and I have tried to analyse the probable extent of isolation involved. It may now be
time to devote some attention to what is known of the mode of evolution in this flora.
The genetic systems of afroalpine vascular plants have been very little studied.
Sexual propagation seems to be much more common than apomixis, although apomixis
(probably facultative) occurs in at least some species of Alchemilla (Hjelmqvist, 1956 ;
Hedberg, 1957 :281). A considerable proportion of the sexually reproducing afroalpine
plants nevertheless have the capacity of reproducing also by asexual means, such as
branching of the rhizome or caudex (Carex spp., Haplosciadium,Haplocarpha, etc.),
or by basal adventitious shoots (as in most tufted, cushion-forming or tussock-forming
plants, e.g. most grasses, Sagina spp., Alchemilla spp., Valerianu kilimandscharica,
and in most Giant Lobelias (cf. Hedberg, 1957 :Plate 6A). Several sexuallyreproducing
species are probably largely autogamous, e.g. Montia fontana, Subularia monticola,
Sibthorpia europaea, and several grasses, but detailed knowledge is lacking.
The pollination biology is unknown for most afroalpines (cf. Hedberg, 1964: 38).
Ornithogamy occurs in at least the Giant Lobelias and in Proteas, both of which are
regularly visited by the sunbird Nectarinia johnstonii; the Giant Senecios also are said
to be often visited by the same sunbirds (Hedberg, 1964). Although numerous species
of insects occur in the afroalpine belt, e.g. on flowering specimens of Helichrysum and
Euryops, etc. (Salt, 1954; Hedberg, 1964; Coe, 1967) no detailed observations of insect
pollination are known to me. Wind pollination is certainly important in grasses, sedges,
Alchemilla spp., and some Ericaceae.
The role of polyploidy for speciation seems to be relatively limited in the afroalpine
flora. Differences in ploidy between different mountain populations of one species are
indicated only for Deschampsia jlexuosa, although more than one chromosome number
from one and the same species has been encountered also in a few other cases (Anthoxanthum, Pentaschistis, Poa, Anagallis and Senecio-cf. Hedberg, 1957).
MODE OF EVOLUTION
Most mountain populations of at least such ‘qualified’afroalpines which do not occur
below the 3000 m level are likely to have originated from one or a few diaspores brought
by long-distance dispersal. From the outset each population of this kind will represent
a very small random sample of the gene pool of the parent species. As long as the population was small and perhaps fluctuating in sue, evolution may be expected to have been
comparatively rapid and genetic drift may easily have occurred. A probable example of
the latter is provided by the vicarious species of the Lobelia deckenii group, which differ
mainly in some minute details of splitting of the corolla and pubescence of the bracts,
sepals, petals and anthers, for which it is difficult to visualize any selective advantage (cf.
Plate 1 :A, B, D). The main pollinator for all of them seems to be the same species of
Nectarink.
In other cases the differentiation into distinct taxa seems to have been aided by differential selection, as in the vicarious species pair Lobelia wollastonii-L. telekii. These
EVOLUTION
AND SPECIATION
IN MOUNTAIN
FLORA
145
two differ, inter alia, in the height of the stem carrying the inflorescence, in the length
and pubescence of the bracts, the size of the flowers, etc. Their differences seem to be
well adjusted to the ecological differences between the almost permanently wet afroalpine environment on Ruwenzori and the Virunga Volcanoes, where the former species
often occurs in dense Dendrosenecio forest, and the much drier afroalpine vegetation
on Elgon, Aberdare and M t Kenya, where Lobelia telekii occurs as a rule in open
Alchemilla scrub (cf. Hedberg, 1964: Figs 46-48 and Plate 1 : C here).
Beautiful examples of geographically and altitudinally vicarious taxa are furnished
by the Giant Lobelias and Giant Senecios, which also display some remarkable ecological adaptations to the inhospitable afroalpine climate (Hedberg, 1964). T h e former
are represented in the montane forest belt by a few species with lax and sometimes
branched stems, considered by Hauman (1933) to approach the ancestral forms of all
the relevant species. Their afroalpine representatives are more robust with dense
leaf rosettes and compact inflorescences, creating ameliorated microclimates of their
own(seeillustrationsinHedberg, 1964).Thesespeciesfallnaturallyintothree groups (the
Lobelia deckenii group, the L. telekii group, and the L. lanuriensis group), each of which
is represented by at most one species on each of the high mountains. This indicates,
of course, that those groups are older than the differentiation between the species
within each of them, and that hybridization between the groups is insignificant. Only
one, sterile, inter-group hybrid is known to me, viz. L. keniensis x L. telekii (see Hedberg, 1957: 188, 334).
T h e taxonomic interrelations between the Giant Senecios are more complicated.
On each mountain harbouring more than one species there is a succession of altitudinally more or less vicarious species, so that the low-level ones are thin-stemmed with
open, few-leaved leaf rosettes, thin, caducous and sparsely pubescent leaves, and small
capitula with long ray-florets, etc., whereas high-level species tend to have shorter
and more robust stems, dense leaf rosettes closing up at night and formed by thick
leaves, which often are densely pubescent below and marcescent, large capitula without
or with small ray-florets, etc. (cf. Hauman, 1935). Superimposed upon this pattern
there occurs variation in some other characters of less obvious ecological importance
such as the occurrence of revolute leaf margins, cordate or attenuate leaf bases, winged
or unwinged petioles, and a very remarkable type of procumbent stem (see Table 2).
T h e character combinations accounted for in this table would seem to indicate that,
for instance, the three species known from Kilimanjaro are more closely related to one
another than, e.g., S. johnstonii to any of the other thin-stemmed forest species (S.
eyicirosenii, S. amblyphyllus, and S. battiscombei). AS indicated in the table hybridization is known or inferred in several cases (cf. Hedberg, 1957: 227, 356), and extensive
introgression between two species seems to occur on the Virunga Volcanoes (Hedberg,
1957: 369). The variation pattern in the collective species S . adnivalis on Ruwenzori is
insufficiently known, but at least within this species the variation appears so continuous
as to suggest hybridization. Gene exchange between different species on one mountain
Seems plausible in several cases
T h e variation pattern displayed by Dendrosenecio might be explained by parallel
adaptation of (upper) montane forest taxa to afroalpine conditions, complicated by
occasional long distance dissemination of diaspores. T h e closely related species pair
10
OLOVHBDBERC
146
Table 2. Character combinations in the species of Senecio subgen. Dendrosenecio.
S. kahuzicus
S. erici-rosenii
S. erici-rosenii x S. adniv. v.
alticola
S . adnivalis v. alticola
S . adnivalis v. adnivalis
S. adnivalis v. erioneuron
S. adnivalis v. stanleyi
S . adnivalis v. petiolatus
S . friesiorum
3000-3300
2600-4000
- - -
3300-3900
3500-4250
3500-4500
3200-4500
4000-4300
3500-4250
4050-4400
(+I
S. amblyphylh
S. elgonensis
S. elgonensis x S . barbatipes
S. barbatipes
2800-3 500
3200-4250
4100
3650-4300
- -
S . cheranganiensis
s. &lei
2600-3 200
3050
- -
S. battiscombei
S. brassiciformis
S. brassica
- -
S. keniodendron
2900-3 800
3000-3800
3300-4500
4200
3800-4650
+
+
+
+
S . johnstonii
S.kilimanjari
S. cottonii
2450-3350
31OO-4000
3700-4500
- -
S. meruensis
2900-3500
- -
S.brassica x S . keniodendron
+
+
+
+
+
+
zt
+
+
+
+
+
+
+ +
+ +
+ +
+ -
+
+
+
+
+ +
+ +
The taxa have been arranged in geographical order, the first group of nine pertaining to Mt. Kahuzi (No.
l), the Virunga Volcanoes (Nos 24), and Ruwenzon (Nos 4-9); the second group of four species, to
Elgon; the following group, of two species, to the Cherangani Hills; next again to the Aberdares (Nos 1
and 2) and Mt. Kenya (Nos 1 and 3-5); the last group but one to Kilimanjaro; and the final species to
Mt. Meru.
EVOLUTION
AND SPECIATION
IN MOUNTAIN
FLORA
147
Senecio brassica and brassicijiimis must obviously represent a case of such intermountain dispersal, and S . kahuxints on M t Kahuzi is so strikingly intermediate
between S. erici-rosenii and S. adnivalis var. alticola that it might easily be visualized
as a recent introduction from the hybrid population between the latter occurring on
the Virunga Volcanoes.
In conclusion I would like to comment upon the meaning of the term speciation
as used in the title of my contribution. To a zoologist, speciation would in most cases
imply the formation of internal barriers to interbreeding between two or more morphologically distinguishable population systems. Botanists are on the whole less demanding
in their definitions-to most of us speciation means simply the formation of genetically
or spatially isolated and morphologically distinguishable population systems-or,
alas, groups of specimens. This may appear very primitive to many of you, but for the
larger part of the plant kingdom there is no way around it at present. To my knowledge
only one investigation has been made of the compatibility of different populations of an
afroalpine species, and that is my own study of Arabis alpina (Hedberg, 1962b). There
is certainly a wide scope for experimental taxonomy in the afroalpine flora, but the task
is not easy. The Giant Senecios I have just been discussing are, for instance, very
slow-growing with irregular flowering periods perhaps 10-20 years apart, and very
difficult to keep in cultivation. For some time to come we shall therefore presumably
have to be satisfied with the sort of data I have presented in this paper.
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EXPLANATION OF PLATE
PLATE
1
A. Lobelia deckenii, flowering specimen. Tanzania, Kilimanjaro, S.slope above Mweka, in the lower part
of the ericaceous belt, c. 3000 m. The flowers are barely visible between the wide ovate bracts. Photo:
0.Hedberg, Nov. 1967.
B. Lobelia keniensis, flowering specimen. Kenya, Mt. Kenya, Teleki Valley, in the afroalpine belt, C .
4200 m. Photo: 0.Hedberg, July 1948. Lobelia keniensis and L. deckenii are closely related vicarious species
(cf. text).
C. Lobelia telekii, flowering specimen. The flowers are hidden between the long, hairy bracts. Kenya,
Mt. Kenya, Teleki Valley, in the afroalpine belt, C.4200 m. Photo 0.Hedberg, July 1948.
D. Lobelia keniensis, young flowering specimen photographed early in the morning. The leaves have just
started to unfold from their tightly folded night position (cf. Hedberg, 1964). Note the snow cover on the
ground. Kenya, Mt. Kenya, Teleki Valley, in the afroalpine belt,c.4200m. Photo: 0.Hedberg, July 1948.