282 ❯ STEP 5. Build Your Test-Taking Confidence
- Mentioning the Galapagos finches as an
example of geographic barriers leading to
reproductive isolation and divergent evo-
lution. (1 ⁄ 2 point) - Mentioning that if after a long period of
time, these divergent species come back
together and are unable to reproduce, they
have become a new species. (1 point)
B. Polyploidy (maximum 4 points) - Definition of polyploidy as a condition in
which an individual has more than the
normal number of sets of chromosomes.
(1 point) - Description of how polyploidy initially
occurs—an accident during cell division
could double the chromosome number in
the offspring, producing a tetraploid (4n)
organism. (1 point) - Alternate description of how polyploidy
could initially occur—the breeding of
two individuals from different species
leads to a hybrid that is usually sterile and
contains chromosomes that are not able
to pair up during meiosis because they are
not homologous. (1 point) - Definition of an autopolyploid—organism
with more than two chromosome sets all
from the same species. (1 ⁄ 2 point) - Definition of an allopolyploid—organism
with more than two chromosome sets
that come from more than one species.
(1 ⁄ 2 point) - Mentioning that although an individual
may be healthy, it cannot reproduce with
nonpolyploidic members of its species.
(1 point) - Mentioning that polyploidic individuals are
able to mate only with other individuals
who have the same polyploidic chromo-
somal makeup. (1 point)
C. Balanced polymorphism (maximum 3 points) - Definition of balanced polymorphism—
some characters have two or more pheno-
typic variants, such as tulip color. (1 point) - Mention of the fact that if one phenotypic
variant leads to increased reproductive
success, directional selection will eventu-
ally eliminate all other varieties because
only those who have the particular pheno-
typic variant of choice will survive to be
able to reproduce, and thus only their
genes will be passed along. (1 point)- Mentioning that this requirement for a
particular variant of the trait in order to
survive reproductively isolates individuals
of the same species from each other,
opening the door for sympatric specia-
tion. (1 point) - Mentioning that if the balanced polymor-
phism causes the two variants to diverge
enough to no longer be able to interbreed,
speciation has occurred. (1 point) - Citing an example of balanced polymor-
phism. (1 point)
D. Reproductive isolation (maximum 4 points) - Mentioning that any barrier that prevents
two species from producing offspring can
be categorized as reproductive isolation.
(1 ⁄ 2 point) - Definition of prezygotic barriers as repro-
ductive barriers that make the fertilization
of the female ovum impossible. (1 point) - Mentioning, as an example of prezygotic
barriers, any of the following (1 ⁄ 2 point
each, up to 1 point total for prezygotic bar-
rier examples): (a) habitat isolation—two
species live in different habitats (they just
don’t see each other, so they cannot repro-
duce); (b) temporal isolation—two species
mate at either different times of the year or
different times of the day (either way, they
are isolated from each other because they
do not mate at the same time); (c) behavioral
isolation—two species have different mating
behaviors that do not mix well (members
of the other species do not understand the
actions of the other as mating signals—
a simple communication breakdown ☺);
(d)mechanical isolation—mating may actu-
ally be attempted, but the physical sexual
structures do not function together prop-
erly (they are incompatible). - Definition of postzygotic barriers as
reproductive barriers that prevent a prop-
erly formed hybrid between two species
from reproducing themselves. (1 point)