Darwin (and Wallace) 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
FROM WHERE DO NEW SPECIES COME?
Historically have been two primary competing views about life:
Both ideas can be found in ancient Greek writing, and might have been even older.
- Species do not change, but are fixed.
- Life changes over time.
Traditionally, most 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"):
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?
- 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 removed from 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 recognise." --'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, sometimes linking what are now distinct groups;
shared anatomical features showed connections between groups; adaptations showed organisms "fit" to their environment; the
nested hierarchy of classification showed structure to the distribution of anatomical features among organisms; natural hybrids demonstrated
that species were not distinct inseparable kinds; embryology revealed organized similarities between distanct groups; the distribution of
species over the surface of the Earth and through time showed similar groups in close regions. Together, all this showed that life had
a history. The Transmutationists thus already accepted the central tenets of Evolutionary Theory:
But what caused the modifications?
- The Diversity of Living Things is the Product of Descent with Modification
- New species are the modified descendants of previously existing species
II. On the Origin of Species by Means of Natural Selection
The discovery of the primary mechanism of evolution was the work of two English naturalists:
These two had similar backgrounds:
The two made the same sets of important observations independantly, and independantly 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 independant 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.)
- 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
This was the independent discovery of Charles Robert Darwin
and Alfred Russel Wallace. Their model was called Natural Selection, and was analogous to "artificial selection" (e.g., domestication).
Here is Natural Selection as described in Darwin's own words:
"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." --
Introduction to The Origin, first edition, 1859.
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 descendents
- 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
- 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
- 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.
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)
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:
Evolution by natural selection explained a lot:
- Unlike popular idea, evolutionary fitness is NOT being the biggest, strongest, fastest, etc.
- Instead, Fitness = Reproductive Success
- 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
- Homologies are shared structures inherited from common ancestors
- Adaptations are produced by the net effect of differential survival of particular
variations in the populations, favoring a particular interaction with the environment
- Life is organized by nested hierarchy because it IS a tree of descendants branching
out from common ancestors
- The Principle of Fossil Succession works because Life had a single, unique,
- Vestigial organs exist because variants with reduced versions of these organs have
a better than average survival rate (perhaps because physiological resources aren't being
used up for less-used structures) than those with fully developed organs
- However, vestigial organs represent existence of ancestor with more fully developed
version of that organ: vestigial legs on whales & snakes indicate legged whale- and
- Common pattern of embryology between closely related forms because they share very
recent common ancestors; more distantly related organisms with different forms of
development because longer times between divergence
- Fossil record of groups intermediate between distinct modern groups represent common
ancestors (or relatives of the actual common ancestors) of these groups
- Because life has a history of divergence from common ancestors, regions with shared
history of particular creatures will have closely related species
- New species are the modified descendants of older species
- 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 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
- We now refer to this as allopatric speciation if the barrier entirely separates interbreeding, or peripatric speciation if
one population is on the fringes of the original with only limited gene flow.
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
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Last modified: 27 February 2014