Chapter 24 (Origin of Species)
1.) What is the difference between microevolution and macroevolution?
Microevolution refers to changes in the gene pool of a population over time
which result in relatively small changes to the organisms in the population —
changes which would not result in the newer organisms being considered as
different species. Examples of such microevolutionary changes would include
a change in a species’ coloring or size and overall adaptations to its
environment.
Macroevolution, in contrast, refers to changes in organisms which are
significant enough that, over time, the newer organisms would be considered
an entirely new species. In other words, the new organisms would be unable
to mate with their ancestors, assuming we were able to bring them together.
2.) What is the relationship between the biological species concept and
reproduction? What are some other ways to define a species?
In the biological species concept, a species is defined as a group of individuals
who are able to breed with one another and produce viable (living), fertile
(able to reproduce themselves) offspring. This implies a continuation of a
genome from one generation to the next. For reproduction to be successful,
many conditions must be met. There are other ways to define species other
than by just reproductive success, important in deducing species information
for fossilized organisms or asexually producing organisms such as the
prokaryotes. Some other methods include the morphological concept which
relies on anatomical structures and appearances to group related organisms,
the ecological concept which involves the study of where and how organisms
live and how they interact with the environment and the phylogenetic concept
which used the evolutionary tree and common ancestry to separate species.
3.) Describe some prezygotic and postzygotic barriers to successful reproduction.
One prezygotic barrier to reproduction is habitat isolation in which different
species occupy different locations in the habitat such as some being aquatic
organisms and some being terrestrial. Temporal isolation refers to organisms
that live in the same area but reproduce during different times of the year so
that they avoid reproducing together. Behavioral isolation can mean different
species have evolved elaborate mating rituals and behaviors that vary so
much that different species are not attracted to each other. Mechanical
isolation refers to physical differences that impede successful implantation of
sperm between species. Finally gametic isolation involves differences in
reproductive tracts which make it difficult for gametes to successfully meet.
Postzygotic barriers typically involve the reduced viability and/or survival of
hybrids. Some hybrids themselves are viable but have reduced fertility or are
sterile due to chromosomal differences typically. Some hybrids are able to
reproduce but their offspring are less robust and fail to thrive or reproduce
due to too many genetic differences between the parental species.
4.) Compare and contract allopatric and sympatric speciation.
Both allopatric and sympatric speciation will lead to the reproductive
isolation of individuals due to separation of gene pools that will eventually
cause the creation of a new species. In allopatric speciation, geographic
isolation will prevent populations from mixing their gene pools and once
separate, each population is subject to potential differences in mutations,
genetic drift and possibly different selection pressures that will cause
microevolution in each population. After enough microevolution, the 2
populations could be reproductively isolated enough to speciate. In sympatric
isolation, you don’t need complete geographic isolation but changes in
chromosome numbers due to mistakes in meiosis may create polyploidy
which can lead to new species even in the same geographic locations. Some
other factors can cause sympatric speciation such as exploiting different zones
within the same geographic area (habitat differentiation) or sexual selection
(variety of behaviors or mate choice factors that can isolate some members of
a population). Whatever the cause of the separation, eventual speciation can
occur.
5.) What is a hybrid zone and what possible long term outcomes of a hybrid zone?
A hybrid zone is an area between the geographic distributions of two species
that are similar enough to allow reproduction (reproductive barriers are not
complete) so that offspring are created that are a mixture of each parental
species. Depending on factors such as hybrid viability, different outcomes can
occur. If the hybrids are ultimately less fit than either parent species,
reinforcement will occur which will select against hybrids so, over time, there
are fewer hybrids. If there continues to be the creation of new hybrids and
their fitness levels are similar to the parental species’, fusion may occur where
the two species become one single species. Finally, stability may occur if
enough hybrids survive compared to the parental populations as to
overwhelm any potential selection against the hybrids. In this case, the
hybrids will still continue to form in the hybrid zone area.
6.) How does punctuated equilibrium contrast with a more gradual speciation
model?
Sometimes the fossil record shows the existence of species that seem to
suddenly appear, remain unchanged for a period of time and then just as
suddenly disappear. This contrasts with a fossil record that shows a slow
change in certain species with many small differences noted along the way.
The important point in punctuated equilibrium is that what appears to be
“sudden” changes can still be a considerably long time. A time period of
50,000 years is relatively short in geologic time so that a species forming in a
shorter time span (due to increased speciation events such as geographic
isolation or intense environmental selection), might appear to just come “out
of nowhere” as opposed to those species that took longer to evolve.