Download Speciation - Mrs. Cardoza Biology

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Speciation
Speciation is the formation of a new species. Species is defined as organisms that are physically
similar and can produce fertile offspring. Changes in gene frequencies that lead to the development of a new
species are more likely to occur in small populations than in large ones. Small groups may be isolated from
the main population by a geographic barrier, such as a body of water or a mountain range, or man made
objects like roads and cities. As a result of this geographic isolation, the small population cannot interbreed
with the larger, main population. In time, the isolated population may evolve into a new species. The
following factors may be involved in the evolution of a new species: a) the gene frequencies in the isolated
population may already have been different from the gene frequencies in the main population. b) Different
mutations occur in the isolated population from the main population. c) Different environments cause natural
selection to be different for the small group compare to the original larger group.
Since there is no interbreeding between the two populations, any mutations that occur in one
population cannot be passed to the other. Over long periods of time, the two populations may become so
different that they can no longer interbreed even if direct contact is made. In such a case, two new species
have evolved from one. An example is the evolution of two populations of squirrels on the Grand Canyon:
the Kaibab and the Abert. These two squirrel populations were originally members of one species that
became separated, over time, by the formation of the canyon. As a result of natural selection, the divided
populations evolved to be the two different species that exist today.
Darwin observed the effect of geographic isolation among several finches he collected on the
Galapagos Islands. Darwin hypothesized that the different species he observed had evolved from a single
species that had originally migrated to the islands from the mainland of South America. Over time, the
different environments on the islands had gradually resulted in the evolution of new, separate species. This
process, by which several populations evolve from one original population, is known as speciation.
Geographic isolation can eventually lead to reproductive isolation. The isolated population becomes
so different from the main population that members of the two groups can no longer interbreed even if the
geographic barriers were removed. When two species can no longer breed and produce fertile offspring, they
have become two distinct species. For example, a horse and a donkey can mate and produce a mule, which is
infertile. The horse and donkey, even though closely related, are not the same species.
Questions:
1) When can two organisms be considered different species?
2) Distinguish between geographic isolation and reproductive isolation.
Practice MCAS Open Response
The illustrations below show a South American finch and some of the species of finches found on the Galápagos
Islands. The map shows the relationship of the Galápagos Islands to the west coast of South America.
There are 13 species of finches found on the Galápagos Islands. These finches have a wide variety of food sources and
beak shapes. There is one genetically similar species of finch found on the South American mainland. This finch eats
small seeds.
Use your knowledge of natural selection and speciation to explain how one finch became 13 separate species of
finches.