I have often thought that it is easier to believe in the creation
of organisms than in their evolution. All that is required is a belief in a
Creator, but if one does not have that, one is left to pondering the many steps
that must have occurred to produce the extraordinary adaptations of, and
associations between, living organisms that we see around us. We
cannot comprehend the time scales over which these changes have taken place, so
all we are left with are our speculations.
In the past few weeks, I have been taking a WEA course on
the influence of Darwin’s On the Origin
of Species on 19th Century thought. It is led by Paul Ranford, the excellent
historian of science, who introduced us to other works by Darwin, including his
book The various contrivances by which
orchids are fertilised by insects, orchids becoming a consuming passion of
the great man while he recovered from the effort of producing the “Origin”. It set me to reading what
Darwin had to say about the bee orchid (Ophrys
apifera – see below, with an illustration from Darwin’s book).
These are some
extracts from Darwin's book [1]:
The Bee Ophrys differs widely from
the great majority of Orchids in being excellently constructed for fertilising
itself..
..When a pollen-mass is placed on
the stigma and then withdrawn, the elastic threads by which the packets are
tied together break, and leave several packets on the viscid surface. In all
other Orchids the meaning of these several contrivances is unmistakeably clear –
namely, the downward movement of the lip of the rostellum when gently pushed –
the viscidity of the disc – the depression of the caudicle as soon as the disc
is exposed to the air – the rupturing of the elastic threads – and the
conspicuousness of the flower. Are we to believe that these adaptations for
cross-fertilisation in the Bee Ophrys are absolutely purposeless, as would
certainly be the case if this species has always been and will always be
self-fertilised? It is, however, just possible that insects, although they may
have never been seen to visit the flowers, may at rare intervals transport the
pollinia from plant to plant..
..The whole case is perplexing in
an unparalleled degree, for we have in the same flower elaborate contrivances
for directly opposed objects..
..As it can hardly be doubted that
O. apifera was at first constructed
so as to be regularly cross-fertilised, it may be asked will it ever revert to
its former state: and if it does not so revert, will it become extinct?
The question is a valid one and Darwin involved his correspondents
in finding out more about the fertilisation of bee orchids. One of his regular
correspondents was Philip Henry Gosse, the avid creationist, who was busy in
1863 “examining bee orchis for Darwin at Petit Tor” [2].
The bee orchis (orchid) gets its common name from its
appearance, said to resemble a solitary bee and we know that male bees are
essential for the fertilisation of some orchids. We do not know whether the
flower looks like a bee to bees but we do know that the floral pigments give
signatures under untraviolet light that may act as attractants. Since Darwin’s
time, we recognise that another important mechanism is involved in attracting pollinators
and this is of much greater significance than the appearance of the flowers,
that so fascinates humans. Orchids in the genus Ophrys secrete chemicals that mimic sex pheromones produced by
female bees and these vary from species to species, thus attracting specific pollinators,
although accidental fertilisation by a range of insects may also be a
possibility. Ophrys apifera is fertilised
by a solitary bee in Mediterranean regions but, as Darwin discovered,
self-fertilisation occurs in the northern part of its range.
There is a lively contemporary debate on the significance of
the various factors involved in the fertilisation of Ophrys [3,4,5] and, fittingly, this exchange of views took place in a journal of the Linnean Society, the
society that was instrumental in introducing Darwin’s ideas on evolution. The three papers (and there
are many others on the topic) show clearly just how complex the evolution of
the orchids has been. Mention must also be made of why male bees are often the
agents of fertilisation of the orchids, by transferring pollen from one flower
to another. The females of solitary bees mate soon after emergence from the
pupa [6] and it is probable that there is a surplus of males or, if mating is a
once only event, there are males constantly looking for mates and, by
deception, being attracted to orchids.
The whole arrangement is a remarkable example of
co-evolution and one wonders about the timing of the steps involved in the
association and whether they were gradual or rapid (over geological time). What
came first? Was it the mating biology of bees, the selection of colour patterns
in Ophrys flowers that became attractive to insects, the production of a series
of chemicals that act as attractants, differences across the range of the
plants, or what?
[1] Charles Darwin (1862) The various contrivances by which orchids are fertilised by insects.
London, John Murray.
[2] Edmund Gosse (1896) The
naturalist of the sea-shore: the life of Philip Henry Gosse. London,
William Heinemann.
[3] E.Bradshaw, P.J.Rudall, D.S.Devey, M.M.Thomas, B.J.Glover
and R.M.Bateman (2010) Comparative labellum micromorphology of the asexually
deceptive temperate orchid genus Ophrys:
diverse epidermal cell types and multiple origins of structural colour. Botanical Journal of the Linnean Society
162: 504-540.
[4] N.J.Vereecken, M.Streinzer, M.Assaye, J.Spaethe,
H.F.Paulus, J. Stöckl, P.Cortis
and F.P Schiestl (2011) Integrating past and present studies on Ophrys pollination – a comment on
Bradshaw et al. Botanical Journal of the Linnean Society 165: 329-335.
[5] R.M Bateman, E. Bradshaw, D.S.Devey, B.J.Glover, S.
Malmgren, G.Sramkó, M.M.Thomas
and P.J.Rudall (2011) Species arguments: clarifying competing concepts of
species delimitation in the pseudo-copulatory orchid genus Ophrys. Botanical Journal of
the Linnean Society 165: 336-347.
I would like to thank Paul Ranford and my WEA classmates for
their stimulating discussions. It is great to leave a course with many more
questions than answers – after all, that’s the fundamental nature of science.
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