spontaneous generation darwin natural and artificial selection galapagos
TRANSCRIPT
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 theBrassica 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 sub-environments 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.
