CHAPTER 17
VOCAB:
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Acrosome: the structure at the forward tip of an animal sperm which is the first to
fuse with the egg membrane and enter the egg cell
Allopatric speciation: formation of 2 species from one when reproductive isolation
occurs because of interposition of a physical geographic barrier
Allopolyploid: possession of 2+ chromosome sets that are derived from more than
one species
Autopolyploid: possession of 2+ entire chromosome sets that are derived from a
single species
Biological species concept: definition of a species as a group of actually or
potentially interbreeding natural populations that are reproductively isolated from
other such groups
Centromere: region where sister chromatids join; belt in the middle
Hybrid zone: region of overlap in the ranges of two closely related species where the
species may hybridize
Lineage species concept: definition of a species as a branch on the tree of life which
has a history that starts at a speciation event and ends either at extinction or at
another speciation event
Morphological species concept: definition of a species as a group of individuals that
look alike
Polyploidy: possession of 2+ entire sets of chromosomes
Postzygotic isolating mechanisms: barriers to the reproductive process that occur
after the union of the nuclei of 2 gametes
Prezygotic isolating mechanisms: barriers to the reproductive process that occur
before the union of the nuclei of 2 gametes
Reinforcement: evolution of enhanced reproductive isolation between populations
due to natural selection for greater isolation
Reproductive isolation: condition in which two divergent populations are no longer
exchanging genes; can lead to speciation
Speciation: the process of splitting one species into two pops that are reproductively
isolated from one another
Sympatric speciation: due to reproductive isolation without any physical separation
of the subpopulation
QUESTIONS:
1. The definition of species has changed as technology developed. We now can analyze
the genomes of species and identify errors in taxonomy by IDing homoplasies.
2. Isolation is important to speciation because if two populations are divided, they
evolve separately and soon have enough new adaptations and changes that they are
no longer considered to be the same species.
3. Speciation on the Galapagos is allopatric speciation because the finches were
separated from the mainland population by hundred of miles of water. This is a
physical barrier.
4. Polyploidy speciation can occur quickly when there is a nondisjunction of a set of
chromosomes in a gamete. This event only needs to occur once, not gradually over
time.
5. Obstacles associated with sympatric isolation include pursuit of separate niches,
different behavior as part of sexual selection, polyploidy event, and duplication of
single chromosomes.
6. Sympatric isolation can occur when two parts of a population pursue separate
niches. If the two prefer certain habitats for mating or rearing young, then there is a
decreased chance that they would mate because they are not in their preferred
environment. If this persists, then isolation occurs.
7. Even if conception has already occurred, prezygotic mechanisms are still in effect
for the other members of the species so they don’t create hybrids and remain
isolated.
8. Not all prezygotic mechanisms occur at once. Only one is needed to isolate two
populations.
9. A hybrid zone could increase in width if more hybrids can be created, if the species
are becoming more in common and less isolated from one another.
10. Evidence for interbreeding between Neanderthals and humans is found in modern
human DNA. Some genes that were known to belong to Neanderthals have been
found in human genomes today. Therefore the ancestors of those humans must have
interbred. One possibility for the disappearance of the Neanderthal species is that
they interbred with humans and any ancestors after that were considered human.
PRACTICE/APPLY THE CONCEPT 1:
Galapagos Finches Over Time
20
# of Islands and Species
18
16
14
12
10
# of Islands
8
# of Finch Species
6
4
2
0
0.25
MYA
0.50
MYA
0.75
MYA
1.00
MYA
2.00
MYA
3.00
MYA
4.00
MYA
2. Yes, this hypothesis is accurate because the number of islands as compared to the
number of finch species follow the same trend over time.
3. Speciation will continue even if no more islands are created because allopatric
speciation is only one aspect of the forming of new species. As long as some sort of isolation
exists (whether gametic, behavioral, etc.) speciation will occur. You could collect data from
populations of other species on another set of islands to see if they trend the same way.
APPLY THE CONCEPT 2:
Recently Hatched Tadpoles (Spring Yr 1)
R. berlandieri
R. spenocephala
F1 Hybrids
Late-Stage Tadpoles (Summer Yr 1)
R. berlandeire
R. spenocephala
F1 Hybrids
Newly Metamorphosed Froglets (Fall Yr
1)
R. berllandieri
R. sphenocephala
F1 Hybrids
Adult Frogs (Yr 2)
R. berlandiere
R. Sphenocephala
F1 Hybrids
2. Hybrid zygote viability and development become significantly lower as the years
progress. The hybrids also have decreased fitness and are much less successful than the
other species.
3. Over time, although more hybrids may be created between R. berlandieri and R.
sphenocephala, the F1 hybrids will become unviable and unable to reproduce, let alone
produce viable offspring of their own. The breeding seasons would slowly become closer
until they occurred around the same season each year. If this occurred, future pie charts
would have many more hybrids.