The Evolution Unit in a Nut Shell
Prior to Darwin, other people tried to explain changes in populations over time. One of
those people was Jean Baptiste Lamarck. Lamarck suggested that characteristics acquired over
the course of an animal’s life could be passed on to its offspring. For example, he felt that a
giraffe’s neck could stretch over its lifetime while it reached for taller and taller leaves. He
suggested that the giraffe’s offspring would be born with a longer neck. Lamarck was, of course,
WRONG! Acquired characteristics cannot be passed on to offspring. If you burn yourself and
get a scar, your child will not be born with a scar.
Several scientists conducted experiments to disprove Lamarck. August Weisman cuts the
tails off of mice for hundreds of generations to see if the offspring were born without tails.
Despite how many mouse tails he cut off, the offspring were always born with tails.
Question 1: If a scientist pulled the wings off of fruit flies for several generations, would the
subsequent generations of flies be born without wings? Why/why not?
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Question 2: Describe the difference between an inherited trait and an acquired trait.
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While in the Galapagos, Darwin observed that organisms seemed to be suited to the
environment in which they lived. He called the variations that made organisms well-adapted to
their environments “favorable variations”. Using his observations, he spent the rest of his life
developing his ideas on natural selection. The process of natural selection can be described by
the following 5 statements.
1. There is variation with in a population.
2. Some variations are more favorable than others because they make the individuals
who have them better adapted to the environment in which they live.
3. More young are produced than can survive.
4. The individuals that have the favorable variations are more likely to survive and
reproduce.
5. Over time, the frequency of the favorable traits in the population increases
populations can change.
To Darwin, the success and failure of an organism wasn’t simply dependent upon whether it
lived or died, but if it was able to pass on its genes to subsequent generations.
Question 3: According to Darwin, which organisms are most likely to survive?
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Question 4: Within a decade of introducing a new pesticide, nearly all of the descendants of the
targeted pests are immune to the pesticide. Describe what has happened in terms of natural
selection.
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Question 5: What is a failure in Darwinian terms?
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Question 6: What is a success in Darwinian terms?
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It is important to note that natural selection acts upon phenotypes (the physical
expression of a trait), not genotypes (the genetic makeup of an organism). In other words, the
success of an organism depends on what traits it has, not whether the organism is homozygous or
heterozygous for that trait.
Natural selection in a population can be seen in 3 main types. Directional selection
occurs when the population shifts to one extreme or another. Disruptive selection occurs when
the population moves toward both extremes. Stabilizing selection occurs when the population
moves more toward the average. In the example below of bird body weights, the original
population is shown by the dotted line. It shows a bell curve distribution with few very small
and few very large birds, and the majority of birds being around the medium weight.
In directional selection, over time the population will shift to most birds being either
very small or very large. In disruptive selection, the population will shift to having many very
small birds and many very large birds, but very few medium size birds. In stabilizing selection,
there will be more and more average or medium size birds, but fewer and fewer very small and
very large birds.
Question 7: Label each graph below as directional, disruptive, or stabilizing selection.
Question 8: Does natural selection work upon genotypes or phenotypes? __________________
Evolution is defined as a change in gene frequencies over time. For example, if a
population of rabbits is composed of 20% white rabbits and 80% brown rabbits, and twenty years
later the population is 40% white rabbits and 60% brown rabbits, the gene frequency of the
population has changed and the population has evolved.
Different genes exist in a populations because of mutations. Mutations are changes in the
DNA nucleotide sequence of a gene. In the rabbit example, at some point the entire population
of rabbits was brown. A mutation caused a rabbit’s fur to be white instead of brown. The
chance of a mutation being significant and increasing in frequency in a population depends
entirely on if that mutation produces a favorable variation. If being white was a disadvantage to
the rabbits, the white rabbits would not live long enough to reproduce and pass on that gene and
more white rabbits would not be produced. However, if being a white rabbit is a favorable
variation in their niche, white rabbits will survive and reproduce and the population will have
more and more white rabbits. Remember that a niche is an organism’s environment and way of
life, so it includes things like predators, food supply, light, shelter, and weather.
Question 9: How could you determine if a population is evolving?
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Question 10: What is a mutation? What determines whether a trait caused by a mutation will
increase in frequency in a population?
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A species is defined as an interbreeding population that can produce viable (healthy and
fertile) offspring. For example, a golden retriever and a German shepherd can produce healthy,
fertile puppies, so they are of the same species. However, if a horse mates with a donkey to
produce a mule, the mule is sterile (cannot produce offspring). Therefore, horses and donkeys
are two different species.
New species can arise from common ancestors. In other words, one species can give rise
to many others. This is called the principle of common descent. For example, on the island of
Madagascar, there are over 100 distinct species of lemurs. They have many similar
characteristics and are clearly related and must have arisen from a common ancestor. This is an
example of adaptive radiation – when a large array of species arise from one.
Another example of a common ancestor giving rise new species is divergent evolution.
This occurs when a population is separated into 2 populations and diverge to become 2 new
species. In other words, two species arise from one.
Question 11: How can you determine if two organisms are members of the same species?
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Question 12: What is the principle of common descent?
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Question 13: What is the difference between adaptive radiation and divergent evolution?
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Speciation (the formation of new species) requires isolation. Ultimately, in order to be
different species, populations must be reproductively isolated (cannot produce viable offspring).
Three other types of isolation are precursors that lead to reproductive isolation. Geographic
isolation occurs when populations are separated by physical barriers such as mountains, valleys,
or large bodies of water. Behavioral isolation occurs when two populations have different
mating behaviors, such as dances or songs. Temporal isolation occurs when populations have
different mating seasons. Because these forms of isolation prevent interbreeding between the
different populations, differences between the gene pools (total collection of genes in a
population) of the two populations increase until they become reproductively isolated.
Question 14: In your own words, describe the different types of isolation.
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Relationships and relatedness between species are studied to better understand common
descent. One way to does this is biogeography. Scientists study the distribution of species and
their ancestors throughout the world. There are several ways to determine relatedness, but
comparative anatomy is one of the most common. There are 3 types of structures that are
commonly used.
Homologous structures are structures that are inherited from a common ancestor so are
very similar in structure, but are used for a different function. For example, human hand bones
and bird wing bones are very similar in structure, but humans use their hands to grab while birds
use their wings to fly. The more homologous structures two species have in common, the more
closely related they are. For example, a species is more closely related to another species that is
has 10 homologous structures in common than a species that is has 4 homologous structures in
common.
Analogous structures can show that species are not related. They are structures that are
used for the same function, but do not have similar structures and are, therefore, not inherited
from a common ancestor. An example of this is bird wings and insect wings. They are both
used to fly, but have completely different structures.
The last is vestigial structures. These are structures that are inherited from a
common ancestor, but no longer serve a purpose. Examples of this are your appendix and a
python’s hip bones.
Question 15: What is biogeography and how is it used?
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Question 16: What are homologous structures and how do they help determine relatedness?
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Question 17: What is a vestigial structure? Give an example not given on this worksheet.
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Question 18: What is an analogous structure?
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