Charles Darwin & the Tree of Life
Charles Darwin made several major approaches to the Tree of Life concept in his evolutionary theory.
A one-time theology student in training to become a minister of religion, albeit one with a passionate interest in
natural history field studies, Darwin was informally recruited as a geological advisor to accompany british
naval Captain Fitzroy on a surveying voyage to South America and the Pacific Ocean.
This voyage, on board HMS Beagle, lasted some five years from 1831 to 1836 and helped to transform
the would-be country parson into a theologically skeptical man of science.
Darwin's Tree of Life sketch of 1837
(Some explanatory notes are presented further down this page with the aim of providing for an easier understanding of Darwin's concepts and for a fuller explanation of the terminolgy used.)
This sketch shows Charles Darwin's early theoretical
insight of how a genus of related species might
originate by divergence from a starting point (1).
The text annotations read:-
Case must be that one generation then should be as many living as now. To do this & to have many
species in same genus (as is) requires extinction.
Thus between A & B immense gap of relation. C & B the finest gradation, B & D rather greater
distinction. Thus genera would be formed. - bearing relation to ancient types with several extinct forms...
From Darwin's Notebook B now stored in Cambridge University library.
The number of surviving individuals would be limited by the food supply. Individuals drawn from an "original" species could be out-competed in terms of overall survivability by
slightly variant individuals. Such variance would thereby be re-inforced and tend towards the emergence of one or more separate species. Where dramatically out-competed the original
ancestor species could even be forced into extinction.
A related group of well-adapted variant species could, in time, constitute a new Genus of emergent species. The term Genera refers to a plurality of Genus(es).
Thus new Genera might "form" each composed of adapted specimens which had out-competed unadapted descendants of their common ancestors resulting in the original ancestor
species coming under survival pressures and possibly even becoming extinct.
In 1837 it was very much the case that most people in the westernised world would have thought that each species existed as a result of its being Created by God.
Whilst those involved in Natural History studies had initiated systems of classification where individual species which bore strong similarities to each other were "grouped"
and regarded as being members of the same Genus, such grouping was based on observable close, and presumably "God ordained," similarities themselves rather than on an explicit acceptance
of "biological" relatedness.
Darwin had, by 1837 and 1838, thought of the possible existence of processes leading to the extinction, and to the "formation", of species.
He only seems to have arrived at further insights that tended to give validity to such possible "species extinction", and such possible "species formation", many months after
entering his early "Tree of Life sketch" into his notebooks.
"In October 1838, that is, fifteen months after I had begun my systematic enquiry, I happened to read for amusement Malthus on Population, and being well prepared to appreciate the
struggle for existence which everywhere goes on from long-continued observation of the habits of animals and plants, it at once struck me that under these circumstances favourable
variations would tend to be, preserved, and unfavourable ones to be destroyed. The result of this would be the formation of new species. Here, then, I had at last got a theory by
which to work; but I was so anxious to avoid prejudice, that I determined not for some time to write even the briefest sketch of it."
Such "struggle for existence" related theorisings were later explicitly expressed in Darwin's 'Origin of Species' of 1859.
"Owing to this struggle for life, any variation, however slight and from whatever cause proceeding, if it be in any degree profitable to an individual of any species, in its infinitely complex relations to other organic beings and to external nature, will tend to the preservation of that individual, and will generally be inherited by its offspring. The offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive. I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection, in order to mark its relation to man's power of selection."
- Charles Darwin, The Origin of Species
The full title of Charles Darwin's book actually reads as :-
On the Origin of Species by Means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life.
The Tree of Life sketch from
This diagram presented below, which is a significant updating of Charles Darwin's original Tree of
Life sketch of 1837, is the only illustration in the Origin of Species of 1859 and is referred extensively in Chapter IV., entitled Natural Selection, as an assistance
to the development of several key aspects of Darwin's theorising:-
The Origin of Species of 1859
… After the foregoing discussion, which ought to have been
much amplified, we may, I think, assume that the modified descendants of
any one species will succeed by so much the better as they become more
diversified in structure, and are thus enabled to encroach on places
occupied by other beings. Now let us see how this principle of great
benefit being derived from divergence of character, combined with the
principles of natural selection and of extinction, will tend to act. …
The Tree of Life diagram from Charles Darwin's seminal work~ The Origin of Species (1859) :-
The text of Chapter IV includes these excerpts:-
The accompanying diagram will aid us in understanding this
rather perplexing subject. Let A to L represent the species of a genus
large in its own country; these species are supposed to resemble each
other in unequal degrees, as is so generally the case in nature, and as is
represented in the diagram by the letters standing at unequal distances. …
… The intervals between the horizontal lines in the diagram,
may represent each a thousand generations; but it would have been better
if each had represented ten thousand generations. After a thousand
generations, species (A) is supposed to have produced two fairly
well-marked varieties, namely a1 and m1.
…After ten thousand generations, species (A) is supposed to
have produced three forms, a10, f10, and m10, which, from having diverged
in character during the successive generations, will have come to differ
largely, but perhaps unequally, from each other and from their common
parent. If we suppose the amount of change between each horizontal line in
our diagram to be excessively small, these three forms may still be only
well-marked varieties; or they may have arrived at the doubtful category
of sub-species; but we have only to suppose the steps in the process of
modification to be more numerous or greater in amount, to convert these
three forms into well-defined species: thus the diagram illustrates the
steps by which the small differences distinguishing varieties are
increased into the larger differences distinguishing species.
…If then our diagram be assumed to represent a considerable
amount of modification, species (A) and all the earlier varieties will
have become extinct, having been replaced by eight new species (a14 to
m14); and (I) will have been replaced by six (n14 to z14) new species.
Charles Darwin's explicit summary to
The Origin of Species, Chapter IV., "Natural Selection"
Chapter IV. is ended with an explicitly indicated chapter summary by Charles Darwin himself.
The final paragraph of this chapter summary reads:-
The affinities of all the beings of the
same class have sometimes been represented by a great tree. I believe this
simile largely speaks the truth. The green and budding twigs may represent
existing species; and those produced during former years may represent the
long succession of extinct species. At each period of growth all the
growing twigs have tried to branch out on all sides, and to overtop and
kill the surrounding twigs and branches, in the same manner as species and
groups of species have at all times overmastered other species in the
great battle for life. The limbs divided into great branches, and these
into lesser and lesser branches, were themselves once, when the tree was
young, budding twigs; and this connexion of the former and present buds by
ramifying branches may well represent the classification of all extinct
and living species in groups subordinate to groups. Of the many twigs
which flourished when the tree was a mere bush, only two or three, now
grown into great branches, yet survive and bear the other branches; so
with the species which lived during long-past geological periods, very few
have left living and modified descendants. From the first growth of the
tree, many a limb and branch has decayed and dropped off; and these fallen
branches of various sizes may represent those whole orders, families, and
genera which have now no living representatives, and which are known to us
only in a fossil state. As we here and there see a thin, straggling branch
springing from a fork low down in a tree, and which by some chance has
been favoured and is still alive on its summit, so we occasionally see an
animal like the Ornithorhynchus or Lepidosiren, which in some small degree
connects by its affinities two large branches of life, and which has
apparently been saved from fatal competition by having inhabited a
protected station. As buds give rise by growth to fresh buds, and these,
if vigorous, branch out and overtop on all sides many a feebler branch, so
by generation I believe it has been with the great Tree of Life, which
fills with its dead and broken branches the crust of the earth, and covers
the surface with its ever-branching and beautiful ramifications.
Another Sketch about Life?
If Charles Darwin were alive today we at Age-of-the-Sage would be urgently seeking to interest him
in our discovery of the fact that there is close agreement between several major World Faiths, Plato,
Socrates, Pythagoras and Shakespeare in suggesting that Human Wisdom / Spirituality is relative to
Human Desire / Materialism and to Human Wrath / Ethnicity.