Journal Number 113
August 2009

EDITORIAL

Darwin and Self-Fertilisation
By Ian St George

	I have always had difficulty with Darwin's contention, in The fertilisation of orchids (1862) that "nature abhors perpetual self-fertilisation". After all, the majority of New Zealand native orchids are predominantly self-fertilised. Thus I was happy recently to discover at least a partial retraction of his statement, made 16 years later in his The Effects of Cross & Self Fertilisation in the Vegetable Kingdom (1878).

"In 1862 I summed up my observations on Orchids by saying that nature 'abhors
perpetual self-fertilisation.' If the word perpetual had been omitted, the aphorism
would have been false.

As it stands, I believe that it is true, though perhaps rather too strongly expressed; and
I should have added the self-evident proposition that the propagation of the species,
whether by self-fertilisation or by cross-fertilisation, or asexually by buds, stolons, etc.
is of paramount importance."

In other words, survival of the species is what really matters, no matter how it is done.

Darwin's observations led him to conclude further,

"It is an extraordinary fact that with many species, flowers fertilised with their own
pollen are either absolutely or in some degree sterile; if fertilised with pollen from
another flower on the same plant, they are sometimes, though rarely, a little more
fertile; if fertilised with pollen from another individual or variety of the same species,
they are fully fertile; but if with pollen from a distinct species, they are sterile in all
possible degrees, until utter sterility is reached.

We thus have a long series with absolute sterility at the two ends; - at one end due to the
sexual elements not having been sufficiently differentiated, and at the other end to their
having been differentiated in too great a degree, or in some peculiar manner."

We know that the New Zealand orchids that self-fertilise set copious seed. While cross-fertilisation
allows greater vigour of the offspring, self-fertilisation appears to provide greater numbers of them,
thus increasing the chances, in turn, of cross-fertilisation.

Darwin deduced this. He wrote,

"Why the descendents of plants which were originally dioecious, and which profited by
always intercrossing with another individual, should have been converted into
hermaphrodites, may perhaps be explained by the risk which they ran, especially as
long as they were anemophyllous, of not being always fertilised, and consequently of
not having offspring.

This latter evil, the greatest of all to any organism, would have been much lessened by
their becoming hermaphrodites, though with the contingent disadvantage of frequent
self-fertilisation.... Dioecious plants, however fertilised, have a great advantage over
other plants in their cross-fertilisation being assured.

But this advantage is gained, in the case of anemophyllous species at the expence of
the production of an enormous quantity of pollen, with some risk to them and to
entomophyllous species of their fertilisation occasionally failing.

Half the individuals, moreover, namely, the males, produce no seed, and this might
possibly be a disadvantage."

One would have to maintain, in defence of New Zealand orchids and in defiance of Darwin, that an
hermaphroditic plant that can be cross-fertilised, but that will always revert to self-fertilisation if
disappointed, has the best possible shot at success - in numbers and in vigour.