Bio1.J. Linn. SOC.,
1 , p p . 219-222
April 1969
Evolutionary mechanisms in tropical ferns
I. MANTON, F.R.S., F.L.S.
Department of Botany, University of Leeds
Cytotaxonomic work on tropical fern floras has now reached the stage at which it is possible
to compare different geographical regions meaningfully. It is also possible, although the
evidence is on a more restricted scale, to compare the taxonomic and geographical incidence
of different evolutionary mechanisms of the kind which can be investigated by the experimental
techniques of biosystematics. Autopolyploidy, allopolyploidy, genetical speciation without
visible chromosome changes, and speciation involving visible structural chromosome changes,
are all now known to have taken place in tropical examples some of which will be illustrated.
Certain types of change are restricted to certain fern genera being apparently absent from
others. Such restrictions do not explain the marked regional peculiarities exhibited by Africa
versus India, or by Malaya when compared with Ceylon or Jamaica. Interpretation of the
results requires attention to be directed to factors in the past history of the various areas which
might not otherwise be recognized.
It is not practicable at this time to compile a formal statement in support of the verbal
communication because this would imply a definitive summary of 20 years' work, much
of it unpublished, which I hope eventually to complete but which cannot be completed
at short notice. T h e following notes on some of the points raised may however be
helpful.
Attention was drawn to a recent paper in which cytological information, from the
literature on several temperate and tropical floras, was summarized (see Table 1
after Manton & Vida, 1968) to provide a background for the interpretation of new data
Table 1. Cytological composition of selected island and continental floras
(from Manton & Vida. 1968)
Flora
studied
No. taxa
studied*
Diploid
Polyploid
(",)
(%)
(%)
Britain
Hungary
Madeira
W. Tropical Africa
Ceylon
100
100
91
c. 50
60-70
47
37
38
94
132
47
46
58
40
40
53
54
42
Jamaica
Tristan da Cunha
50-60
80
270
20
40
35
* These figures include both species and cytotypes.
60
60
60
65
Max.
grade
6-ploid
6-ploid
12-ploid
12-ploid
12-ploid
16-ploid
12-ploid
(Manton, 1950)
(Vida, 1966)
(Manton, 1950)
(Manton, 19596)
(Manton, 1953;
(Manton & Sledge,
1954)
(Walker, 1966)
(Manton & Vida,
1968)
220
I. MANTON
from the remote South Atlantic island of Tristan da Cunha. The extraordinary resemblance shown by the tropical floras of Ceylon, Jamaica and continental west Africa,
when treated in this way with respect to certain statistics of fern cytology was an
unexpected but significant fact.
This resemblance does not extend to all tropical floras of the world. We know, for
example, that mainland India differs substantially from Africa (for two recent discussions see Manton, Roy & Jarrett, 1966 with respect to the Cheilanthesfarinosa complex
and Manton, Ghatak & Sinha, 1967 with respect to the Adiantum caudatum (incisurn)
complex) in that several unrelated complexes when studied experimentally by biosystematic methods have shown predominantly diploid representatives in India but
predominantly, or indeed exclusively, polyploid representatives in Africa. Moreover
we also know from a large body of work done by Indian colleagues that there is a cline
in degree of polyploidy as one passes northwards up the Indian subcontinent, South
India, as represented by the work of Abraham, Ninan & Mathew (1962) and Bir (1965)
showing about 54 % polyploidy in a sample of over 120 taxa, compared with 60 yoin Ceylon (Manton & Sledge, 1954) and substantially less than 50% in the Himalayas.* The
latter cannot usefully be explained away by treating the Himalayan flora as temperate in
the sense used for Europe or North America as has sometimes been suggested (Mehra &
Bir, 1960a), and indeed the overwhelmingly tropical affinities of Himalayan ferns are
clearly expressed in the floristic lists published by these authors in 1964 with respect
to the Pteridophyte flora of Darjeeling and the Sikkim Himalayas. Moreover we have
independent evidence from Malaya that a strictly equatorial flora can sometimes
show an equally low percentage of polyploids (Manton, 1954, 1959). In my statement
to the Botanical Congress at Montreal (Manton, 1959) I assessed the proportion of
polyploids in the Malay peninsula as 39%, on the basis of over 100 taxa for which
facts were at that time available. There are others still to add from my unpublished
files but, even if these in the end shift these figures a little, the statistical sample is
already sufficient to make it highly unlikely that they will differ very much from those
compiled for northern India by Indian workers. These facts suggest, if they do not yet
prove, that part of the land mass represented by Malaya, India, perhaps some of the
islands in the Indian Ocean together with the Mediterranean basin which is also rich
in diploids, some of which extend to Madeira, may represent an ancient Tertiary flora
which except for local variants such as Ceylon has been less disturbed by the climatic
vicissitudes which elsewhere seem to have stimulated speciation by polyploidy in
both temperate and tropical regions.
If this is true, further evidence ought to be forthcoming and may indeed already be
in existence from entirely different sources. Major world forces can scarcely act in
isolation on single components of a flora or fauna and if the suggestion made above is
right some signs leading to similar conclusions can scarcely fail to exist among other
Informationfrom north India is being compiled by the Amritsar group of workers who will doubtless
in due course summarize their own findings. At present the information from nine publications listed
below and involving 136 taxaseems to work out as 81 diploid and 55 polyploid,aproportionof polyploids
of approximately40%. The literature cited is: Mehra & Verma (1957, 1960), Mehra & Khanna (1957),
Mehra& Bir (l957,1960a, b), Bir (l960,1962a, b). On the other hand Abraham et al. (1962: 404)quoting
figures from a presidentialaddress by Mehra 1961 which seems not to have been published in full quotes
23.8% polyploidy in 51 taxa from the western Himalayas and 36.2% polyploidy in 160 taxa from the
eastern Himalayas.
EVOLUTION
IN TROPICAL
FERNS
221
members of the biota, be they fishes, birds, insects or other types of plant. Until such
evidence has been collected for the significant areas and confirmation (or the reverse)
provided for the apparent facts mentioned above it is useless to give further thought to
possible explanations.
T h e suggestion is nevertheless obvious that we may be able to approach plant geography in a new and meaningful way by means of a further extension of the biosystematic approach outlined in my talk. Thus a comparison between east tropical Africa
and west tropical Africa (which alone is represented in the Table) might be highly
illuminating. So too would be an extension of the work to South America. We already
know that South America is on the one hand a continent in which virtually no work of
this kind has been done on ferns and on the other that it represents the largest land
surface under tropical rain forest at present in existence. At this symposium the first
speaker (Professor Mayr) drew attention to a situation involving speciation of birds
which was closely linked with the past history of the different regions in the Amazon
drainage area. This situation suggests strongly to me that here we may have the converse
of that discussed above, namely that biosystematic evidence from ferns might give
valuable (and confirmatory) supplementary evidence to that already obtained from other
sources, if intensive cytological sampling applied to the various regions could be carried
out on the scale and with the methods hitherto applied to the more accessible parts of
the New and Old World. Such a programme may be impossible to carry out for many
different reasons. A symposium such as this is nevertheless in part at least convened to
permit ideas for further work to be put forward whether they can be carried out or
not. The work which has already been done could not have been attempted by an
earlier generation without our present facilities of quick and reliable observational
methods, air transport, low temperature refrigerators and the other paraphernalia
of the post-war era, including that excellent innovation the plastic bag, for the use of
collectors. Who can say that our successors may not do easily what is impossible to
us ?
These are only a few of the topics touched upon in my talk but they perhaps indicate
sufficiently that fern cytology conducted on a large enough scale and with an experimental follow-up, can contribute insight of perhaps a novel kind to the evolutionary
problems posed by tropical floras.
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41:
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~
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222
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