1. O b s e r v a t i o n a n d
experiment
Dar win was fascinated by
A.sesquipedale, which has an
impressing long spur, measuring
up to 35cm long. He thought there
should be a pollinator with a
proboscis long enough to reach
the nectar at the end of the spur.
It was only when he used a
cylinder with a diameter of 1/10
that he was able to detach the
pollinarium after retracting the
cylinder.
Angraecum sesquipedale, also known as Darwin's orchid, is an orchid
endemic to Madagascar. Darwin conducted some research and
observations on this species and made far-reaching discoveries and
hypothesis.
DARWIN'S ORCHID
Why and how did Darwin’s fascination for orchids influence modern ecology ?
Immediate influence
The publication of his book was the beginning of all the work on the evolution of extreme
specialization. Its publication led almost immediately to research by many other naturalists
into specialization and coevolution.
Specialization develops from the need for flowers to attract pollinating insects, and from the
evolution of pollinators to adapt to changes in the location of rewards such as nectar. Hermann
Müller was the first to use a combination of morphological and ecological approaches to
understand patterns in the evolution of interactions and specialization.
Continuing Darwin's work
During the 80s', several studies strengthened Darwin's work, revealing a high degree of
specialization between long-spurred orchids and their specific pollinators.²
More recently, various studies developed work on:
- Pollinator observation and characterization, behavior and specificity
- Flower morphology,
- Breeding systems,
-Reproductive strategies and reproductive success of flowering plants
Orchids are the most specialized family of flowering plants. Darwin's idea of the promotion of
cross-pollination has been accounted in 40% of orchids species, for the most important factor
of evolution.
Orchids have been a model system for studying floral adaptation and diversification ever since Darwin’s influential book on the subject was published in 1862. They provide some of the most exceptional examples of highly
specialized interactions between animals and plants. It is therefore easy to grasp why Charles Darwin was so fascinated by orchids throughout the last twenty five years of his life.
Darwin, during his numerous natural observations throughout the years, became quite interested in orchids. He started a series of studies concerning orchids from Britain and the rest of the world. They were fascinating for him and ideal in
order to study plant-insect interactions. Three years after his theory of evolution, he published his book "Fertilisation of Orchids". As the most important pioneer in biology, his book led to the development of plant-insect ecology.
On the various contrivances by which British and
foreign orchids are fertilised by insects, and on the
good effects of intercrossing.
In this book written in 1962 while he was ill,
Darwin studied the interactions between plants and
insects. He had already introduced this concept In
the origin of Species throughout concepts of
coevolution and co-adaptation. In the orchids book,
the first of a series of books on his research on
plants, he developed the orchids theme within
observations, dissections, experiments and methods.
He explained how complex ecological relationships
resulted in the coevolution of orchids and insects.
A controversial hypothesis
Darwin's prediction were controversial. The notion of a pollinator with a
35cm long proboscis was generally ridiculised and not believed. Some
believed that the complexity of this species implied that it was created by a
supernatural being. Alfred Russel Wallace replied, explaining that the moth
and the flower could have co-evolved with no other orientation than natural
selection.
DARWIN'S INFLUENCE ON POLLINATION ECOLOGY
¹Darwin C. 1862. Fertilisation of orchids, ² Nilsson LA, et. al, 1987. Angraecoid orchids and hawkmoths in central Madagascar: specialized pollination systems and generalist foragers. Biotropica 19: 310–318, ³Micheneau et. al, 2009. Orchid pollination: from
Darwin to the present day. Botanical Journal of the Linnean Society, 2009, 161, 1-19
The Bee-orchid The monkey-orchid
The Duck-orchid
DARWIN'S BOOK
Agathe HUBERT, Clio MARIDAKIS & Camille ZIEGLER
Darwin's book opened up the new study areas of pollination research and reproductive
ecology, related to his theory of evolution, supporting his view that natural selection led to a
variety of forms through the important benefits achieved by cross-fertilization.
He introduced his new concept, the process of coevolution.
"A flower and a pollinator might slowly become adapted in the most perfect manner to
each other, by the continued preservation of individuals presenting mutual and slightly
favorable deviations of structure."¹
He assumed that the pollinarium would attach to the pollinator’s proboscis
during its attempt at getting the nectar at the bottom of the spur. Thus, the
next visited orchid would be pollinated by the pollinarium.
21 years later, the specific moth was indeed
discovered and named Xanthopan morgani.
He examined differences in orchids, showing how
they had diverged and developed specialized pollen-
dispersal mechanisms. He carefully described
morphology and anatomy of each plant in order to
show how variations in similar structures of closely
related flowers led to specialized modifications of
pollinators with different ways to cross-fertilize.
His work influenced botanists and revived interest
in the neglected idea that insects played a part in
pollinating flowers.
Tuesday, December 3, 13