Microevolution = a change over time in the frequency of
one or more alleles in the population
Macroevolution = large-scale change over time resulting
in new species
Genotype= Genetic makeup of an individual. The term
makeup of an individual or to the alleles at a
Phenotype = the physical characteristics of an individual
Species(as defined by the Biological Species Concept) =
a group of organisms that can interbreed and
produce fertile offspring. Members of a
species are reproductively isolatedfrom
members of other species.
Species are the lowest taxonomic unit
Gene flow = the movement of genes within or among
Reproductive isolation can occur in several ways.
A member of species A will not mate with a member of species B
because that member does not look like his or her own species.
A member of species A does mate with a member of species B but a
zygote (fertilized egg) is not formed, or a viable fetus does not form.
To use the example provided in the textbook: suppose a population of finches lives
on an island with both wet and dry habitats. In the dry habitat, selection favors
deep beaks, while selection in the wet habitat favors shallow beaks. Because
interbreeding leads to extensive gene flow between populations in the two habitats,
the average island beak size will eventually become the average of the two
populations. Gene flow, therefore, REDUCESthe variation between and among
NOTE that the mean beak size for the dry and wet population are associated with
the highest fitness in their corresponding environments.
There may be problems with this example, however. The new population average
is associated with comparatively low fitness (i.e. success in producing fertile
offspring), unless the ability to exploit both wet and dry environments is associated
Suppose two populations of finches live on different islands, one wet and one
dry, with no gene flow between the two islands. Now the deep beaks are
common on the dry island, and shallow beaks are common on the wet island.
Imagine this same example on a single island where a wet environment
occurs high in the hills, while the lowlands experienced a drier environment.
These two different environments exert different selection pressures: 1) big
beaks are adaptive in the dry environment, while the shallow beaks are
adapted to a wet environment. Eventually there would strong differences in
beak sizes despite some gene flow between the two populations. This
difference could increase to the point that these two populations were
reproductively isolated from each other, thus forming two new species. This
is an example of the ECOLOGICAL SPECIES CONCEPT which
emphasizes the role of natural selection in creating and maintaining species.
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the fact that there is substantial gene flow among them. The red line
represents the amount of food available in the environment for birds with
different-size beaks. Each peak in this curve represents a different species.
This is another illustration of the ecological species concept.
Although Natural Selection is obviously an important evolutionary force,
most biologists would agree that speciation is usually associated with some
form of reproductive isolating mechanism in conjunction with other
evolutionary forces such as genetic drift (a subject for NEXT semester),
mutation (next semester) AND natural selection.