There is a simple grandeur in the view of life with its powers of growth, assimilation and reproduction, being originally breathed into matter under
one or a few forms, and that whilst this our planet has gone circling on according to fixed laws, and land and water, in a cycle of change, have gone on
replacing each other, that from so simple an origin, through the process of gradual selection of infinitesimal changes, endless forms most beautiful and
most wonderful have been evolved, Charles Darwin, 1842 sketch, later used in 1859 (with some rewording) as the concluding lines of
The Origin of Species.
"These complex affinities and the rules for classification, receive a rational explanation on the theory of descent, combined with the principle of
natural selection, which entails divergence of character and the extinction of intermediate forms. How inexplicable is the similar pattern of the hand of
a man, the foot of a dog, the wing of a bat, the flipper of a seal, on the doctrine of independent acts of creation! how simply explained on the principle
of the natural selection of successive slight variations in the diverging descendants from a single progenitor!" -- Charles Darwin, 1868,
Variation of Animals and Plants Under Domestication, Introduction
"A plausible argument can be made that evolution is the control of development by ecology." -- Leigh Van Valen, 1973
BIG QUESTION:What is evolution?
What is "Evolution?"
Literally "unfolding" or "unravelling"
Pre-1860s, term used for development of an embryo
Generally used for "change through time":
Sometimes for predetermined set of changes, such as stellar evolution or evolution of
Also for the general process of change, as in "evolution of the automobile"
More specifically, organic evolution, or the change of groups of living things
Often summed up in terms of genetics: "changes of gene frequency through time" (literally
true, if a bit boring...)
Darwin himself used the phrase "DESCENT WITH MODIFICATION" rather than "evolution"
In other words, evolution in the broadest sense is no more than the observation that
"none of us looks exactly like our parents."
Species and Life's Diversity
Were species fixed: that is, unchanging? Many people thought so:
"Species have a real existence in nature, and a transition from one to another does not exist" -- History of the Inductive Sciences (1837), Rev. William Whewell (incidentally, the man who coined the word "scientist")
However, several sets of observations showed that this was not so:
Geographic variations ("varieties" or "subspecies"): were the end members of the variation the same species or different ones?
Geologic variations: that is, fossils were often not the same as modern species
Natural hybrids: offspring of two distinct species that (despite the common definition otherwise) are sometimes fertile themselves
Fixity vs. Transmutation
Traditionally, people accepted the fixity of species just as they accepted that the world today is pretty much the same now as in the past.
Theological argument for fixity under the Biblical concept of the Plenum ("fullness"):
Ecclesiastes 1:9 and 3:14-15, if you want to look it up
"Nothing new under the sun": nothing has been taken from Creation, nor added to it
Many early naturalists accepted the Plenum, but evidence of extinction (man-made, as in the dodo, and natural, as in fossils) showed that things could be removed from Creation. What about adding to it?
The discoveries of the early (18th and 19th Century) geologists put paid to the idea that the surface of the Earth was unchanging:
"These facts, unknown to the vulgar, but well known to all who observe nature, force the physical scientist to recognize that all the surface of our globe has changed; that it has had other seas, other continents, another geography." --Nicolas Boulanger (1722-1759)
"Life, therefore, has been often disturbed on this earth by terrible events - calamities which, at their commencement, have perhaps moved and overturned to a great depth the entire outer crust of the globe, but which, since these first commotions, have uniformly acted at a less depth and less generally. Numberless living beings have been the victims of these catastrophes; some have been destroyed by sudden inundations, others have been laid dry in consequence of the bottom of the seas being instantaneously elevated. Their races even have become extinct, and have left no memorial of them except some small fragments which the naturalist can scarcely recognize." --'Preliminary discourse', to Recherches sur les Ossemens Fossiles (1812), trans. R. Kerr Essay on the Theory of the Earth (1813),
Baron Georges Leopold Chretien Frederic Dagobert Cuvier
While some thinkers once thought that life as we see it now is the way it has always been, the discovery of the fossil
record showed that strange creatures once roamed the Earth that are no longer there. Naturalist John Herschel (in an 1836 letter to Charles Lyell)
"I allude to that mystery of mysteries, the replacement of extinct species by others. Many will doubtless think your speculations too bold, but it is as well to face the difficulty at once. For my own part, I cannot but think it an inadequate conception of the Creator, to assume it as granted that his combinations are exhausted upon any one of the theatres of their former exercise, though in this, as in all his other works, we are led, by all analogy, to suppose that he operates through a series of intermediate causes, and that in consequence the origination of fresh species, could it ever come under our cognizance, would be found to be a natural in contradistinction to a miraculous process -- although we perceive no indications of any process actually in progress which is likely to issue in such a result."
How to explain these observations? Two main possibilities:
The successive appearance and disappearance of different forms through time, without genetic connection (as
supported by Owen, Cuvier, Herschel, and others)
Transmutationism: direct lineal relationships between ancestor and descendant species. So living species are descendants of earlier distinct species, which themselves were the descendants of even earlier ones. "Transmutationism" became known as "evolution" after the work of Darwin and Wallace.
Transmutationism, a set of early evolutionary models, accepted by several prominent scientists by the late 1700s. Among them were Jean Baptiste Pierre Antoine de Monet, Chevalier de Lamarck (normally known as Jean Baptiste de Lamarck) and Erasmus Darwin (doctor, scientist, surgeon, abolitionist, and INCREDIBLY rich).
Fossils demonstrated that the living component of the Earth changed through time; shared homologies showed connections between groups; adaptations showed organisms "fit" to their environment. Already accepted the central tenets of Evolutionary Theory:
The Diversity of Living Things is the Product of Descent with Modification
New species are the modified descendants of previously existing species
But what caused the modifications?
Spontaneous generation of new lineages of organisms throughout time; thus, many living things represent separate origins at different points in Earth History
Within each lineage, "driving forces" impel organisms towards improvement (i.e., simple forms become complex) down predetermined pathways
Inheritance is from use and disuses: characters acquired during the lifetime of an individual are passed onto descendants
Problems with these ideas, however:
Spontaneous generation doesn't work
"Driving forces" never identified, and are more metaphysical than naturalistic
Continuity of lineages through long periods of Earth history, rather than appearance, transformation, and reappearance:
Also, fossils documented linkages between groups rather than separation
Inheritance doesn't happen by use & disuse; transformations to adult are not passed onto offspring
Darwin did not discover evolution, nor did its study stop with his work. At least some of the evidence for evolution was long known before his time (although we've added a LOT, even to these lines!)
Initial Evidence for Evolution
Homologies: the same anatomical structures ("body parts") are repeated in different organisms. This allows us to recognize how they differ from each other, and how they resemble each
Living things can be grouped using a nested hierarchy based on shared presence of homologous structures of similar form
System of classification codified by Carolus Linnaeus (18th Century Swedish botanist)
Many of his principles, such as Latin names for organisms, and the use of genus and species still used today
However, species are not fixed entities. They vary across their range, and they can often hybridize with closely related forms
Adaptations: any structure or behavior which allows an organism to interact with its environment in certain specific ways
Analogous structures: non-homologous structures found two or more organisms that are adapted for the same function
Vestigial structures: anatomical features which have some significant adaptive function in some forms, but are reduced and non-functional (or nearly non-functional) in a related form
Transitional fossils: extinct species intermediate in morphology between now-distinct groups
Both studied natural history, including geology, in the UK
Thus, both were familiar with fossil organisms and with the (then-new) ideas of geologic time
Both traveled to distant lands (Darwin to South America, the Galápagos Islands, and various other localities in the Pacific Ocean; Wallace to Amazonia and Indonesia)
Both made collections of organisms, and so had direct experience with the varieties of nature
The two made the same sets of important observations independently, and independently came up with the same mechanism
to explain evolution. Darwin (older than Wallace) had developed his ideas earlier, but kept them secret. In 1858 when Wallace asked Darwin for advice about his ideas, Darwin went to other scientists to present both his and Wallace's ideas at the same time, so that they both got credit for their independent discovery. (However, Darwin's book On the Origin of Species by Means of Natural Selection sold extremely well, so more people then and now know Darwin's name.)
Their model was called Natural Selection, and was analogous to "artificial selection" (e.g., domestication). Darwin and Wallace's observations:
Variability: There is variation in all populations.
No two members of a population are totally identical.
Some sources of variation include age and sexual differences; the results of factors that happened during the lifetime (differences of nutrition, disease, accident, etc.); individual difference in inherited traits; etc.
The idea that individual variation was significant was a blow to previous models of Nature. Most earlier natural historians believed in perfect types, and thought variation was degeneration from those types. Darwin and Wallace documented that the variation is the reality, and the "perfect types" were just myths.
Heritability: Some (but not all) variation is inherited.
Causal mechanism of inheritance unknown in Darwin's time.
Discovery by Gregor Mendel of genetics came later, and discovery of DNA came later still
Heritable traits are coded in DNA and passed on to descendants
Note that DNA is NOT a "blueprint" as commonly thought: it is a set of instructions for putting bodies together and maintaining them after they've been built
Each little instruction is called a gene: a piece of code that helps the cell to build a protein
Most genes have slightly different versions called alleles that produce different end products
It is these alleles (one copy for each gene per parent) that is passed on to offspring
Different combinations of alleles result in different traits being expressed (that is, different phenotypes). Depending on the particular combination of alleles an offspring gets, they might have the same trait as their mother, their father, or something different than either.
This was the major source of individual variation that Darwin & Wallace never knew about!
Mutations are new variations in heritable traits, caused by miscopied DNA (duplication of parts of genes; miswritten code; etc.)
Some mutations may be deleterious (they result in harm to the organism)
Many mutations may be neutral (they don't benefit the organism in an obvious way, nor hurt it)
A small number of mutations may wind up being beneficial (the variation they produce allow it to do better somehow
in the world)
Superfecundity: Organisms produced far more offspring than can possibly survive
Application of demographer Thomas Malthus' reproductive excess concept to Nature
Violated another previously-held belief: that Nature was perfect and everything had its place
Thus, IF some variation gives the individual a slight advantage (bigger, stronger, smaller, smarter, less tasty, whatever) at surviving; and IF that variation is heritable; THEN there is a somewhat better than average chance that organisms with that variation will survive to bear the next generation. Over the long expanse of geologic time, the
accumulation of these variations will change the population from one form to another: the origin of species.
"As many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form." -- From "Introduction", The Origin of Species, Charles Darwin (1859)
Natural Selection is the differential survival and reproduction of variants in a population resulting in a net change in phenotype of the descendants.
(Short form: "Natural selection is the differential survival and reproduction of variants in a population.")
(Even shorter form with a 20th Century slant: MUTATION PROPOSES, SELECTION DISPOSES)
(Or, a different emphasis, from a 1973 book review by paleontologist Leigh Van Valen: Evolution is the Control of Ecology on Development)
If Evolution can be summarized as "no one is identical to their parents", then Natural Selection can be summarized as
"no one is identical to their siblings, either; plus, life's hard!"
Key points of Natural Selection:
Does NOT happen to individuals, only to populations (lineages)
Analogous to "artificial selection" (domestication), but operates:
On all traits rather than a few (humans can keep alive crops, farm animals, or pets that might otherwise die in the wild; obviously, wild plants and animals don't have that help!)
Over vast amounts of geologic time, rather than just a few generations
Does NOT require simple things evolving into complex: sometimes a simplified mutation of a structure might be advantageous
than the ancestral complex one (hence, vestigial organs)
Cannot evolve towards something with a goal in mind; only favors variations that are advantageous at the time of selection
"Survival of the Fittest"?: Not as such. Phrase not in the earlier editions of the Origin, nor was it coined by Darwin. Comes from economist/philosopher Herbert Spencer:
Unlike popular idea, evolutionary fitness is NOT being the biggest, strongest, fastest, etc.
So a great grandmother with dozens of children, grandchildren, and great grandchildren is far more "fit" (in evolutionary terms) than all the childless Nobel prize winners and Olympic athletes put together!
From Darwin and Wallace, we get the beginnings of modern evolutionary theory. It has five major components:
Evolution is descent with modification: that is, the anatomical traits and other features of populations change over
time from generation to generation
These modifications occur relatively slowly on average: small incremental changes added up over many generations
Populations may diverge into two or more distinct lineages (which may or may not produce their own descendant branches)
All species share a common ancestry: thus, the shape of the history of lineages can be seen as a Tree of Life
Much (although not all) evolutionary change is due to natural selection, which is the sole process for producing adaptations
Some of the things that come out of Darwin's work:
The Importance of Time: "No one but a practiced geologist can really comprehend how old the world is, as the measurements refer not to the revolutions of the sun & our lives." -- Notebook E, late 1838 to 1839
The Importance of Geologic Change & Isolation: "Change of external conditions, and isolation either by chance landing of a form on an island, or subsidence dividing a continent, or great chain of mountains, and the number of individuals not being numerous will best favour variation and selection... Barrier would further act in preventing species formed in one part migrating to another part." -- Sketch, 1842
Some things Evolution and Natural Selection are NOT:
An argument for atheism per se (at least any more than geologic time, the germ theory of disease,
Newtonian/Einsteinian physics, etc., are)
Although clearly, like geologic time, it required a world that operated on a different time scale than a literal reading of Genesis!
An account of the origin of Life (that's abiogenesis: a different field of research)
A prescription for social behavior or policy (after all, Natural Selection only operates by the mass death of many individuals: hardly a good society!)
Here is Carl Sagan's summary of Darwinian evolution and the Tree of Life, from the TV series Cosmos:
And here is another summary of evolution and how it works (and how it ISN'T like the parody-version of evolution which Creationists claim scientists believe):