Transcript Spontaneous Generation

Spontaneous Generation
Darwin
Natural and Artificial Selection
Galapagos
The first scientist who demonstrated with a experiment the falsity of the
theory of the spontaneous generation was the Italian Francisco Redi
(1626-1698)
In his book "Experiences around the generation of the bugs", Francisco
Redi wrote:
" I put in four flasks with wide mouths one sneak, some fish of river,
four small eels of Arno river and a piece of calf and I locked very
well the mouths of the flasks with paper and string. Afterward n I
placed in other four flasks the same things and left the mouths of
flasks open.
Short time later the meat and the fishes inside the open flasks
became verminous, and after three weeks I saw many flies around
these flasks, but in the locked ones I never seen a worm ".
Natural selection
• Natural selection is a process that occurs over
successive generations. The following is a
summary of Darwin's line of reasoning for how it
works (see Figure 2).
• If all the offspring that organisms can produce
were to survive and reproduce, they would soon
overrun the earth. Darwin illustrated this point by
a calculation using elephants. He wrote:
Natural Selection Requires...
For natural selection to occur, two requirements
are essential:
• There must be heritable variation for some trait.
Examples: beak size, color pattern, thickness of
skin, fleetness.
• There must be differential survival and
reproduction associated with the possession of
that trait.
• Unless both these requirements are met,
adaptation by natural selection cannot occur.
Some examples:
• If some plants grow taller than others and so are
better able to avoid shading by others, they will
produce more offspring. However, if the reason
they grow tall is because of the soil in which their
seeds happened to land, and not because they
have the genes to grow tall, than no evolution
will occur.
• If some individuals are fleeter than others
because of differences in their genes, but the
predator is so much faster that it does not
matter, then no evolution will occur (e.g. if
cheetahs ate snails).
• In addition, natural selection can only
choose among existing varieties in a
population. It might be very useful for polar
bears to have white noses, and then they
wouldn't have to cover their noses with
their paws when they stalk their prey. The
panda could have a much nicer thumb
than the clumsy device that it does have.
An Illustration
• The following are examples that illustrate the
adaptation of populations to local conditions.
• The rat snake, Elaphe obsoleta, has
recognizably different populations in different
locales of eastern North America (see Figure 4).
Whether these should be called geographic
"races" or subspecies is debatable. These
populations all comprise one species, because
mating can occur between adjacent populations,
causing the species to share a common gene
pool (see the previous lecture on speciation).
• Artificial selection is the intentional breeding for certain
traits, or combinations of traits, over others. It was
originally defined by Charles Darwin in contrast to the
process of natural selection, in which the differential
reproduction of organisms with certain traits is attributed
to improved survival and reproductive ability in the
natural habitat of the organism. Artificial selection that
produces an undesirable outcome from a human
perspective is sometimes called negative selection (but
note that this term has a better-established meaning as a
type of natural selection; see negative selection).
Artificial selection can also be unintentional; it is thought
that domestication of crops by early humans was largely
unintentional.[1]
A variety of vegetables of the
Brassica oleracea species
Artificial Selection
….more examples
Thoroughbred racehorses are one example of
artificial selection of animals. The meats we eat
are the result of the careful selective breeding of
cows, pigs, sheep, and chickens. Our pets are a
far cry from their “wild” ancestors. Cats and
dogs, which were originally domesticated for
pest control, hunting, or shepherding, eventually
were bred to become companion animals. A
glance at a group of dogs — all of the species
Canis familiaris — reveals an astounding
diversity of body type, size, and coloration.
Problems with Artificial Selection
• There can be a down side to artificial selection.
Because this process essentially removes
variation in a population, selectively bred
organisms can be especially susceptible to
diseases or changes in the environment that
would not be a problem for a natural population.
Inbreeding — the mating of closely related
individuals — is also a problem. In dogs, this has
resulted in breeds that have health issues
ranging from decreased life span to hip
dysplasia.
A glance at a group of
dogs — all of the
species Canis
familiaris — reveals
an astounding
diversity of body type,
size, and coloration.
Darwin’s voyage to the
Galapagos Islands
During his five weeks in the Galapagos, Darwin
found the giant tortoises that differed from one
another so greatly that anybody with half an eye
could immediately say which island they came
from.
Land vs. marine iguana
Two forms of iguanas lived in the islands. Each
type had affinities with the common South
American green iguana, yet they had adapted so
profoundly to different ecologic niches in the
islands that they had evolved into separate
genera. Conolophus, adept at living on the arid
islands and feeding on the sharp-spined Opuntia
cactus became the land iguana, while
Amblyrhynchus, with its flattened tail for
swimming, its strong claws for hauling itself out
on the water, and its blunt, shortened snout for
scraping algae off of rocks, became the marine
iguana.
Old forms of life, new forms
Moreover, many islands developed their
own races of these unusual lizards. Many
of the birds that Darwin found, especially
the land birds, were endemic species
found nowhere else on earth. Here were
thirteen different types of finches whose
beeks were modified to different subenvironments on the islands.
San Francisco fossils
Examples of fossils from the
San Francisco Bay region.
Mesozoic fossils are all
marine, such as (J) Jurassic
"clams" (mollusks, genus
Buchia), (K) a Cretaceous
ammonite, and (L) a
Cretaceous ichthyosaur
similar to the one pictured
here. Land-animal fossils are
all Cenozoic in age, such as
(M) part of the Miocene
beaver skull shown in this
drawing, (N) a Pleistocene
saber-toothed cat, and (O) a
Pleistocene mammoth
similar to the one in this
drawing.