The person would be expected to serve as naturalist (at least in part) for this
Henslow recommend that Darwin read the Principles of Geology, with the warning
that he "on no account accept the views therein advocated"
Beagle left Plymouth on Dec. 27, 1831
Darwin immediately fell seasick...
At Cape Verde Islands, noted a shell bed 45 feet above sealevel, not quite parallel
with sea level. Supported Lyellian view of uplift.
Beagle mapped South America
After 3 crewmen died, the ship's surgeon left. (This may also be because HE was
supposed to have been the naturalist).
Darwin collected many living plants and animals, but also rocks and fossils
Noted that the fossil forms were more similar to modern South American animals, though often of much larger size, than to
comparably-aged fossils from Europe
Found evidence of terrestrial-marine-terrestrial transitions in the rock record, even for relatively recent rocks
Witnessed an earthquake, noting the rise of land in the region and seeing it as supporting Lyell's uniformitarian views
Sept. 15, 1835: first sighting of Galápagos
Sept. 16-Oct. 20, 1835: Exploration of Galápagos
Main aspects that interested Darwin
Islands were all volcanic, so very new
All living things on it had their closest relations in South America
The South American forms might be more commonly associated with very different
(more humid environments)
Why not animal and plant species from similar arid environments, but from other parts
of the world?
Each island had slightly different variations of same species or closely related
Yet these species were not found elsewhere, even if some (e.g., marine iguanas) might
do very well in other parts of the world
Some closely related species on the islands had anatomies or behaviors more like many
distantly related forms in other parts of the world (esp. the finches)
Animals on these islands were essentially without fear of humans, and records show
them being more fearless in the past
Similar records show similar traits in other islands (Falklands), although they were more cautious by the time that Darwin arrived
Traveled on to Tahiti, New Zealand, Australia, the Cocos Islands, Mauritius, Cape Town
Colony, St. Helena and Ascension Islands, back to eastern South America for a few
corrections in measurements, then the Azores and finally home to England (Oct. 2, 1836)
Darwin's Professional Life
Brought information and data to colleagues, and was introduced to other
naturalists (including Sir Richard Owen
and Charles Lyell, both on October 29, 1836)
Put forth as a Fellow of the Royal Geological Society on Nov. 1
Gave his first talk on Jan. 4: about slow geological change in South America, and
how its inhabitants adapted to those changes. Differed from Lyellian views on changing faunas,
where old species simply disappeared and new ones appeared to replace them as environments changed.
By 1837 began to keep several different notebooks on aspects of transmutation
In his "Notebook B", he wrote "Cuvier objects to transpropogation of species, by saying, why not have some intermediate forms been
discovered between palaeotherium, megalonyx, mastodon, and the species now living—Now according to my view, in S. America parent of all
armadillos might be brother to Megatherium—uncle now dead"
But he recognized that he needed to find a mechanism to produce those changes: from the Introduction to the 1st Edition of the Origin:
"In considering the Origin of Species, it is quite conceivable that a naturalist, reflecting on the mutual affinities of organic beings, on
their embryological relations, their geographical distribution, geological succession, and other such facts, might come to the conclusion that
each species had not been independently created, but had descended, like varieties, from other species. Nevertheless, such a conclusion, even
if well founded, would be unsatisfactory, until it could be shown how the innumerable species in habiting this world have been modified, so as
to acquire that perfection of structure and coadaptation that most justly excites our admiration"
Married well (his first cousin, Emma Wedgewood): their combined wealth and family
stipends meant that they were better off than most lawyers or physicians, if less than a
big-time merchant, banker, or aristocrat
Allowed Darwin to devote a lot of time to thinking and researching without needing to
get revenue from publishing or teaching
Darwin began to publish (on the origin of coral reefs and atolls; on the voyage of the
Beagle; on the biology, anatomy, and taxonomy of barnacles)
Became increasingly sickly during the late 1830s onward; almost never traveled far
from Down House: even missed his own father's funeral
Nearly all copies sold out on first day, and Darwin immediately start's editing
a new version
The Origin is just an abstract of the proposed much longer book Natural Selection,
which is never finished (although parts were published separately)
The Origin had a far bigger audience than the 1858 papers, and soon discussions
of Darwin, Darwinism (term coined by T. H. Huxley in April 1860), and the implications of
evolution throughout the educated world
Huxley helped bring ideas about evolution to the working classes, too
Through all this, Darwin was largely a home-body
Spring 1871: 6th Edition
of The Origin is published: first version with the word "evolution" in it!
Darwin continued to write throughout his life, although slowly. Focused mainly on
plants and garden-related topics late in life.
So, What Did Darwin & Wallace Discover?
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.
Or, to use Darwin's own words (from the Introduction to the first edition of the Origin: "As many more individuals of each species are
born than can possibly survive; and as, consequently, there is a frequently occurring 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"
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:
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
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!)
Scientific Reactions to Darwin and the Origin
In the late 19th Century, many parts of Darwin's work were accepted:
Evolution in general (special creation no longer a serious scientific possibility)
Common ancestry (separate origins for different lineages no longer considered reasonable)
On the other hand, while some (Thomas Huxley, Joseph Hooker, O.C. Marsh, etc.) accepted Natural
Selection, many others did not. In part they thought it could not create entirely new morphologies, but
mostly because they accepted Lord Kelvin's incredible short (and inaccurate) time scale. With (for example)
only 3 million years for all the Cenozoic Era, it would be hard for Natural Selection to produce
the vast diversity of modern mammals.
Darwin died without knowing the mechanism by which variation was generated and passed on: genetics.
(Trivia time: he actually had a copy of Mendel's work at his desk, but he had never gotten around to opening it!).
Gregor Mendel's genetic work only became well-known and studied in the 20th Century. By the
mid-20th Century, new discoveries in genetics, paleontology, ecology, and statistics led to the
New Synthesis: a model demonstrating that Natural Selection is indeed a major force
However, there is more to it than that. Stay tuned...
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 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.
Sexual Selection: Traits which might be deleterious to the survival of the individual might propogate if their presence results in
an increased overall reproductive success of the bearer.
Correlated Progression: [Not a phrase used until recently, but the concept dates to Darwin.] Ancestor and descendants form a
lineage (historical line). Sometimes a particular life habit favors the slight increase in multiple different traits (e.g., longer and
longer legs, more compact body, more effcient heart and respiration for fast running; longer and longer necks, longer legs, better cropping
teeth and/or grasping tongue for browsing in trees; more and more streamlined body profile, more paddle like legs, more dorsal nostrils, etc.
in swimmers; etc.) Traits that go against the general trend will be selected against; traits that go with the general trend will be selected
for. (For many people, this series of trends in adaptations represents the totality of evolution)
Closely related species are close because their common ancestor diverged relatively recently
in Earth history
Other species are more distantly related because of divergences of THEIR common ancestors
even farther back in time
No separate origins for differnt groups; instead, patterns of common ancestry and diverging
Divergence from common ancestors
Two (or more) distinct variations in an ancestral population convey their own
advantage against the rest of the population
Over time, these two (or more) variations will become more distinct from each other
If they diverge enough, they will no longer be able to mate with each other: will be
"We may look at Megatherium, armadillos & sloths as all offsprings of some still older type
some of the branches dying out." -- Notebook B, mid 1837-February 1838
Divergence can also occur (perhaps more commonly!) if an ancestral population is divided
into two or more by changes in geography: because natural selection works by chance
survivals, it is unlikely that exactly the same variations of the ancestral population
will survival in the two or more separated populations. Over time, if the populations
meet again, the accumulation of variations may be significant enough that they are
Combining Common Ancestry & Divergence, the most important pattern of all: the Tree of Life.
The basic pattern of the history of living things is a
Tree of Life, where the trunk and stems are lineages of ancestors,
the branching points representing divergences between lineages, and the tips of the branches living species (or extinct species that died
"The tree of life should perhaps be called the coral of life, base of branches dead" – Notebook B, mid 1837-February 1838
Adaptive Radiation: If a population evolves some significant new adaptation, or colonizes a region without competitors,
or is present when competitors die off, many different variations from that common ancestral population might survive
(fill new or unoccupied "niches" (ways of life) in environment). Over a geologically short period time, a common ancestor can
radiate into many very different descendant lineages.
"Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from
an original paucity of birds in this archipelago, one species had been taken and modified for different ends." – Chap. 17, Voyage of the
Beagle, 2nd edition, 1844
Niche Partitioning: during an adaptive radiation, the early members of the divergence will (naturally) still be
relatively similar to each other (and to their common ancestor) in terms of size, shape, behavior, etc. Over time, those
variations in each lineage that are least like their relatives will more likely survive, because they will have less
competition. Consequently, the different species will "partition" (divide up) the niches and the resources.
Convergence: Some adaptations are mechanically advantageous and easy to produce developmentally. Different
lineages of organisms can independently develop some of the same features, even though ancestors were quite different
(i.e., streamlining in sharks, tunas, ichthyosaurs & dolphins).
Co-evolution: Selection of one species due to activity of an interactor leads to counter-selection in response of the first
For example, plant species develop traits (shapes, colors, tastes of nectar) that favor a select few number of pollinators, thereby promoting
greater chance of getting their own pollen rather than some other plant's
Or, in the Galápagos: drier islands have fewer small plants, so tortoises preferentially feed on Opuntia cacti. Cacti on these
islands have evolved taller woody trunks, and in response the tortoises have evolved a "saddle-backed" shell that allows them to
reach higher than dome-backed ancestors.
Living Fossils: Species do not have a fixed duration, but will persist until the evolve into something else
and/or go extinct. In some cases, species (or genera) may persist for extremely long periods of time with no major
Exaptation: Formerly called "preadaptation", the co-option of a structure that previously had some
entirely different function for a new use. Seems to be the more common pattern of evolution than the appearance of
entirely novel structures. For example, the wings of birds and bats were initially arms and hands; the mouthparts of
various arthropods were legs; etc.
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):