In the 18th century about 8000 species of organisms were recognized by Europeans. Many more were being found on voyages of exploration around the world. Carolus Linneaus (1758) standardized the biological classification system. He grouped things according to morphology (form). Kingdom, Phylum, Class, Order, Genus, Family, Species.
The Fossil Succession
Around the end of the eighteenth century and early 19th century
Smith and Cuvier knew there were differences in the species found
from one strata to the next. They used this fossil succession to
develop their geologic maps, correlate strata from place to place,
and tell relative geologic time. But, what caused the fossil
succession?
What Caused the Fossil Succession?
Cuvier knew that some species were lost and others followed in a
stratigraphic sequence. He had proved the extinction of a species
(archaic elephants in the Paris basin). These were counter to
Biblical notions of Perfect Creation.
Cuvier knew there were several unconformities in the Paris basin stratigraphic sequence. He proposed that extinctions were caused by violent oscillations of the sea (Catastrophism).
D'Obrigny (~1859) postulated that new species were created following catastrophes (Special Creation). With each catastrophe life moved toward the present "perfection."
This view of Catastrophism and Special Creation saw no ancestral connection between fossil organisms of successive strata.
Evolution of Species
An alternative view that was also being discussed in the late
18th and early 19th century was that succeeding species in the fossil
record had evolved from previous species. Species were related by
ancestry. The hypothesis for the evolution of species was very much
counter to the Biblical view in which God created all things and
those who proposed evolution faced bitter attacks.
Buffon (1749) made some important contributions to the discussion of evolution. He defined species as a separate (non-interbreeding) biological entity. He knew that inheritance is from the parents; characteristics are passed down from parents to offspring. He wondered if theenvironment somehow caused variations in offspring [cold weather could cause offspring to have extra fur???].
Erasmus Darwin (early 1800s) also was aware of the importance of environment in heredity. He recognized that one important way in which species adapted to their environment was by their method of getting food.
Lamarck (1744-1829) proposed that characteristics acquired by an animal in its life could be inherited by its offspring. For example, generations of primitive, short-necked giraffes that stretched their necks to reach high branches all their lives could pass down this character that they had developed for slightly longer necks. This ultimately resulted in today's long-necked giraffes - according to Lamrack.
But experiments in the 1890's (cutting off 20
generations of lab rat tails) showed that changes in the body
couldn't change inheritance. Also, shepherds have docked the tails of
lambs for thousands of year. Nevertheless, lambs are born every year
with tails intact.
Charles Darwin (1809-1882) sailed on the global scientific voyage of HMS Beagle (1831-1836) as ship's naturalist. He collected much observational evidence that later allowed him to support his theory of evolution. While in the Galapagos Islands off South America, Darwin saw a great variety of finches with unique bills and body shapes, unlike any other finches he knew of. Darwin surmised that these finches evolved from a single species of finch that had come from South America; perhaps a flock had been blown out to sea in a storm and had been stranded on some logs or a mat of seaweed until washing up on the largely uninhabited young volcanic islands. These birds had then evolved over a few million years time into various forms by independent lines of adaptation to the available food sources. This is known as evolutionary radiation, the evolution of one species into many; each making a living from the environment in different ways.
I) Darwin's Evidence
Darwin used a number of lines of evidence to support the notion
that species evolved, including:
1) fossil succession shows that
species have changed over geologic time; at least some species have
been replaced by others.
2) branching organization of life: suggests ancestral
relationships like a family tree.
3) homology: organs with different functions built from
the same basic structure (eg. scales, feathers, and hair, or fins,
wings, hooves, and hands).
4) vestigial structures: subtle remnants of now useless
organs (eg. hind-limbs in whales).
5) embryonic history: all vertebrate (fish, amphibians,
reptiles, birds, mammals) embryos start out with gill slits and a
long tail.
6) biogeography: unique species assemblages found on
different continents and islands appear to have evolved independently
through geographic isolation.
II) Darwin's Mechanism for Evolution
Darwin was impressed by a book of Thomas Malthus
(1798) that portrayed life in nature as a continuous struggle for
food. Darwin knew of the importance of stresses on animal populations
such as predator/prey relations, need for food, for warmth, to
reproduce, etc. Darwin also knew there was considerable variability
within a species; no two individuals are exactly alike. Each
individual has its own strengths and weaknesses; some are taller,
some faster, some have thicker fur, etc. His theory of evolution by
natural selection, published in 1859, is based on these two
ideas.
Because of the stresses in the environment, not every individual in a breeding population would survive to produce offspring. The individuals that were best adapted to the particular conditions would most likely survive to pass on its characteristics. So Nature (the environmental conditions) selects the traits best suited to the conditions.
III) The Rate and Style of Evolution
Darwin believed that species evolved steadily and gradually, ever
moving toward a more perfect adaptation to conditions.
Eldridge and Gould (1972) proposed a model of evolution called Punctuated Equilibrium which states that species evolve in short spurts of adaptation, then remain unchanged for long periods of time. This is counter to the long-standing view of Darwinian Gradualism. This debate over the style and rate of evolution continues. However, there is no debate within the scientific community whether evolution actually occurred.