Name:___________________________________ Date:_________________ Period:______
This reading guide covers text pages 418 – 449. It is aligned with California state Biology/Life Science
standards 7a-7d and 8a-8e. After reading the above pages and completing this reading guide you should
be familiar with the following vocabulary:
Adaptation
Adaptive radiation
Allopatric speciation
Allelic frequency
Analogous structure
Artificial selection
Bottleneck effect
Camouflage
Convergent evolution
Directional selection
Disruptive selection
Divergent evolution
Embryo
Founder Effect
Gene pool
Genetic drift
Genetic equilibrium
Geographic isolation
Gradualism
Homologous structure
Mimicry
Natural selection
Polyploidy
Pre-zygotic barriers
Post-zygotic barriers
Punctuated equilibrium
Reproductive isolation
Species
Speciation
Stabilizing selection
Sympatric speciation
Vestigial structure
The following assignments will be collected in one week.
1. The book gives five evidences support evolution and a description of artificial selection. Make a
chart listing these six and give a description of each.
2. Write one paragraph explaining the differences between artificial selection and natural
selection. It will be easier if you include specific examples. Use complete sentences.
3. Given that variation within a species increases the likelihood that at least some members of a
species will survive under changed environmental conditions, in one paragraph explain how
variation can improve survival.
a. Discuss how variation is produced via mutation or genetic recombination
b. Discuss how some traits enable organisms to adapt more successfully to their
environment with examples.
c. Define survival in terms of capacity to adapt, avoid predation and produce offspring.
4. How are the two types of adaptations cited in the book beneficial to increasing fitness?
5. The Hardy-Weinberg principle states that allele frequencies in a population should be constant.
Work through the example of earlobe attachment in the textbook. Next cite the conditions
that must be present if Hardy-Weinberg principle is to hold true.
6. Describe what genetic drift is in general and two extreme examples: founder effect and
bottleneck effect; give examples as to how both extreme examples occur.
7. Make a chart listing and describing the three different types of natural selection (stabilizing,
directional, and disruptive) that act on variation. In your chart, include a graph and an example
of each type.
8. What are three factors that can cause speciation?
9. Different patterns of evolution occur throughout the world in different natural environments.
These patterns support the idea that natural selection is an important agent for evolution.
Describe the three patterns that are discussed in the book and give an example of each.
10. What is the usual rate of speciation? Describe the two hypotheses proposed that deal with this
question.
Industrial Melanism: the peppered moth example of natural selection1
The peppered moth (Biston betularia) comes in two forms and the difference between the two forms of moth is
controlled by a pair of alleles at a single chromosome locus.
The light-colored form of the peppered moth, known as typica, was the predominant form in England prior to the
beginning of the industrial revolution. Shown at left, the typica moth's speckled wings are easy to spot against a
dark background, but would be difficult to pick out against the light-colored bark of many trees common in
England. Around the middle of the 19th century, however, a new form of the moth began to appear. The first
report of a dark-colored peppered moth was made in 1848. By 1895, the frequency in Manchester had reached a
reported level of 98% of the moths.
This dark-colored form of the peppered moth is known as carbonaria, and (as shown at right), it is easiest to see
against a light background. As you can well imagine, carbonaria would be almost invisible against a dark
background, just as typica would be difficult to see against a light background. The increase in carbonaria moths
was so dramatic that many naturalists made the immediate suggestion that it had to be the result of the effects
of industrial activity on the local landscape. Coal burned during the early decades of the industrial revolution
produced soot that blanketed the countryside of the industrial areas of England between London and Manchester.
Several naturalists noted that the typica form was more common in the countryside, while the carbonaria moth
prevailed in the sooty regions. Not surprisingly, many jumped to the conclusion that the darker moths had some
sort of survival advantage in the newly-darkened landscape.
In recent years, the burning of cleaner fuels and the
advent of Clean Air laws has changed the countryside
even in industrial areas, and the sootiness that prevailed
during the 19th century is all but gone from urban
England. Coincidentally, the prevalence of the carbonaria
form has declined dramatically. In fact, some biologists
suggest that the dark forms will be all but extinct within
a few decades.
Draw a picture showing how differential predation by birds was the selective pressure involved in this natural
selection example.
1
http://www.millerandlevine.com/km/evol/Moths/moths.html and http://www.smccd.net/accounts/bucher/indumeli.pdf
Genetic Drift2
Genetic drift—along with natural selection, mutation, and migration—is one of the basic mechanisms of evolution.
In each generation, some individuals may, just by chance, leave behind a few more descendents (and genes, of
course!) than other individuals. The genes of the next generation will be the
genes of the “lucky” individuals, not necessarily the healthier or “better”
individuals. That, in a nutshell, is genetic drift. It happens to ALL populations—
there’s no avoiding the random results of chance. Genetic drift affects the
genetic makeup of the population but, unlike natural selection, through an
entirely random process. So although genetic drift is a mechanism of evolution,
it doesn’t work to produce adaptations.
Genetic drift and natural selection rarely occur in isolation of each other; both forces are always at play in a
population. However, the degree to which alleles are affected by drift and selection varies according to
circumstance.
In a large population, where genetic drift occurs very slowly, even weak selection on an allele will push its
frequency upwards or downwards (depending on whether the allele is beneficial or harmful). However, if the
population is very small, drift will predominate. In this case, weak selective effects may not be seen at all as the
small changes in frequency they would produce are overshadowed by drift.
Diagram examples of genetic drift in a small population and a large population. Remember to give some numbers
for gene frequencies in your examples.
2
http://evolution.berkeley.edu/evosite/evo101/IIID2Genesdrift.shtml
What Causes Speciation?3
Speciation, or the evolution of reproductive isolation, occurs as a by-product of genetic changes that
accumulate between two previously interbreeding populations of the same species. For example, let us start
with two populations of the same species that do not differ genetically. Initially, an individual from population A
is able to successfully breed with an individual from population B. As these populations evolve, they each
gradually accumulate genetic changes that are different from the other populations' genetic changes. In other
words, the two populations genetically diverge from each other. These changes can be due to different selection
pressures because of different environments, or because of genetic drift/founder events.
How does Speciation Occur?
There are several different ways in which the evolution of reproductive isolation is thought to occur. These can,
however, be generalized into a series of events, or steps.
The "Steps" in a speciation event:
Step 1: gene flow between two populations is interrupted
(populations become genetically isolated from each other)
Step 2: genetic differences gradually accumulate between the two populations
(populations diverge genetically)
Step 3: reproductive isolation evolves as a consequence of this divergence
(a reproductive isolating mechanism evolves)
The main difference between the different models of speciation is in the first step, or how the populations
become genetically isolated from each other.
Reproductive Isolation
At some point in this process, some of these genetic changes cause the two populations to become reproductively
isolated from each other. In other words, these genetic changes no longer allow an individual from population A
to successfully breed with an individual from population B. They prevent gene flow between populations. These
specific genetic differences that confer reproductive isolation are called reproductive isolating mechanisms.
There are several different types of reproductive isolating mechanisms, which are classified according to when in
the life cycle of the organism isolation occurs. Isolation can occur before fertilization (prezygotic barriers) or
after fertilization (postzygotic barriers).
Prezygotic isolation can occur either before mating occurs (premating barriers) or after mating occurs
(postmating barriers). One type of premating prezygotic isolation occurs when potential mates from the two
populations do not meet, either because they are separated in time (temporal isolation) or in space (habitat
isolation). Temporal isolation can occur if individuals in two different populations mate at different times of the
day or in different seasons, or even years (e.g. species of periodical cicadas mate either every 7 years or every
13 years). Habitat isolation occurs, for example, when herbivorous insects from two populations feed and mate on
different host plants. Another type of premating prezygotic isolation occurs when individuals from two
populations meet, but they do not mate (behavioral or sexual isolation). This occurs, for example, when
courtship behaviors differ between individuals of two populations (e.g. songs in birds, pheromones in moths, light
displays in fireflies, etc.).
3
http://evoled.dbs.umt.edu/lessons/speciation.htm#causes
One type of postmating prezygotic isolation occurs when mating actually takes place, but male gametes are not
actually transferred to the female (mechanical isolation). This happens when there is an anatomical
incompatibility between individuals from two populations. For example, the floral anatomy of some plant species
prevents some pollinators that visit the plant from actually transferring pollen. In this case, mating (pollination)
occurs, but the male gametes (pollen) are not able to reach the eggs. A second type of postmating prezygotic
isolation occurs after mating taking places, male gametes are actually transferred, but the egg is not fertilized
(gametic isolation). This can be an important isolating mechanism in externally reproducing species that send out
their gametes en masse. Sea urchins, for example, release their gametes into the water column. In
reproductively isolated species, male and female gametes actually meet, but the sperm does not fertilize the egg.
Another example of this would be when pollen from a plant of one species lands on the stigma of a plant from
another species, and a pollen tube is not completely formed. In both of these cases, a genetic mismatch between
the gametes prevents successful fertilization.
There are three types of postzygotic isolating mechanisms. In the first type, mating occurs, a zygote is formed,
but the hybrid has reduced viability (hybrid inviability). In other words, hybrids do not survive long enough to
reproduce. The other type of postzygotic isolation occurs when hybrids are viable, but they have reduced
fertility (hybrid sterility). A classic example is the mule, which is the result of a cross between a donkey and a
horse. Mules are viable, healthy animals, but they are always sterile (i.e. they are unable to successfully
reproduce). The third type of postzygotic isolation occurs when hybrids are viable and fertile, but the offspring
of the hybrids are inviable or sterile (hybrid breakdown). In all of these postzygotic examples, individuals from
the two populations will mate with each other, and the gametes fuse, but the genetic material in each of the
gametes differs enough that the combinations of alleles are not compatible.
http://videos.howstuffworks.com/wgbh-nova/13619-evolution-in-action-video.htm