Download Exercise 11 Natural Selection and Evolution

Exercise 11
Natural Selection and Evolution
Perhaps one of the most controversial topics in science is the
discussion of evolution
evolution, primarily because it raises the issue of
origins especially those of the human kind.
origins---especially
kind
Such controversies have surrounded this topic for as long as humans
have discussed the problem
problem.
This laboratory will offer some historical background for the
scientific
i ifi theories
h i associated
i d with
i h evolution
l i andd demonstrate
d
some
of the principles which have been accepted as part of the
scientific foundation for evolution.
Exercise 11
Natural Selection and Evolution
Some scientists view evolution as a process of change where minor
genetic changes occur in populations through random mutations which
can lead to a change in gene frequency.
frequency
-This is known as microevolution
microevolution.
Microevolution can produce enough genetic variation to create
a new species of organism (macroevolution
macroevolution), one that is distinct from
the original “parent” form.
Such a mechanism involves small continual changes over many
generations and is known as gradualism
gradualism.
Gradualism suggests that small changes accumulate in populations
gradually over time and eventually lead to the evolution of a new
species.
Exercise 11
Natural Selection and Evolution
Although gradualism is generally accepted by many scientists a new
mechanism for evolutionary change has recently been proposed.
Punctuated Equilibrium suggests that evolutionary change
(speciation
speciation) occurs rapidly followed by long periods of time of no
change.
Evidence for this theory is supplied by the fossil record. By
studying the fossil record scientists have found what appears to
be long periods of time with relatively little change in species
followed by sudden periods of intense change
In other words,
words punctuated equilibrium suggests that evolutionary
change is not gradual, but consists of stable periods of time
followed by rapid speciation.
Exercise 11
Natural Selection and Evolution
While the debate between gradualism and punctuated equilibrium
continues, we must understand that they are at two opposite ends
of a spectrum. Proponents of both agree that evolutionary change
over time caused by Natural Selection occurs in nature, but they
disagree as to the nature of the change.
Whether the change is gradual over time or rapid followed by
periods of very little change has yet to be determined. It is
quite likely that new evolutionary mechanisms will be established
that use portions of each of these concepts to explain
evolutionary change over time. This is what makes science so
p
There is no absolute rule;; the theories change
g with
important.
the discovery of new information.
information
Exercise 11
Natural Selection and Evolution
NATURAL SELECTION
In 1831,, the HMS Beagle
g sailed from England
g
bound for a voyage
y g
around the world. Aboard the ship was Charles Darwin,
Darwin the ship’s
naturalist.
Darwin’s duties were to catalog all the natural events encountered
on the trip. Darwin did not appreciate ocean travel and was quite
often ill due to seasickness. As a result, he was quick to volunteer
for shore duty and spent months at a time away from the ship,
traveling and collecting specimens and data.
Exercise 11
Natural Selection and Evolution
Darwin made many notable discoveries during this trip but none were
as significant as those made on the Galapagos Islands,
Islands a cluster of
volcanic islands situated near the equator off the coast of Ecuador.
Ecuador
Although the Galapagos Islands were never connected to the
mainland, they contained plants and animals similar to those on the
mainland. Darwin noticed that the plant life was not as diverse as on
the mainland and that each island seemed to have its own unique
plant communities
communities.
Darwin also noticed unusual animals on the islands, such as huge
tortoises found nowhere else in the world. But, of all the terrestrial
animals Darwin found, a group of fairly common mainland birds
(finches) caught his eye.
Exercise 11
Natural Selection and Evolution
These birds
Th
bi d have
h
since
i
become
b
known
k
as Darwin’s
D
i ’ Finches
Fi h andd were
extremely influential on Darwin.
On the Galapagos
p g Islands,, the roughly
g y 13 species
p
of finches that
Darwin identified displayed very different body sizes, behaviors,
bill structures, and even feeding preferences. In fact, some finches
were feeding on insects or fruit
fruit, unlike their relatives on the mainland
(refer to Figure 11.1 in the lab manual).
Darwin began to wonder why such variation in finch biology was
observed on this truly unique island chain.
Exercise 11
Natural Selection and Evolution
When Darwin returned home to England in 1836, he set out to construct
a logical explanation for the unique organisms he found on his journey.
He began to write volumes of information that would take several
years to organize into a clear statement of the source (or origin) of
his observed conditions.
In the 1840’s he began to refine his ideas and was encouraged by
other scientists to ppublish his works before someone else did. In 1858
a naturalist working in the East Indies, Alfred Wallace,
Wallace sent Darwin
a manuscript in which he established a theory that species evolve in
response to changes in their environment.
environment This was the same basic
belief that Darwin had come to while evaluating the data from his
expedition.
Exercise 11
Natural Selection and Evolution
While Wallace had a general concept in mind, Darwin had volumes
of data to substantiate his beliefs and even though he acknowledged
Wallace’ss contributions, Darwin
Wallace
Darwin’ss writings on “descent
descent with
modification” have been accepted as the basis of the Theory of
Natural Selection.
Selection
The Theory of Natural Selection is briefly outlined on the following
slide.
Exercise 11
Natural Selection and Evolution
DARWIN’S MAIN IDEAS
Facts Based on Observations of Nature:
1) O
Organisms
i
are varied,
i d and
d some variations
i ti
are iinherited.
h it d Within
Withi a species,
i no
two individuals (except identical siblings) are exactly alike.
2) More individuals are born than survive to reproduce.
3) Individuals compete with one another for the resources that enable them to
survive.
Inferences from Observations:
4) Within populations, the characteristics of some individuals make them more able
to survive and reproduce in the face of certain environmental conditions.
5) As a result of the environment’s selection against nonadaptive traits, or
“
“survival
i l off the
h fittest”,
fi
” only
l individuals
i di id l with
i h adaptive
d i traits
i live
li long
l
enoughh to
transmit traits beneficial in that environment. Over time, natural selection can
change the characteristics of populations, even molding new species.
Exercise 11
Natural Selection and Evolution
In short, Darwin’s theory held that changes occur in all natural
populations. Today, we know that these changes --mutations-- occur
at random in populations and contribute to a change in the genetic
makeup of the individuals within a population.
Changes in the environment can favor or select against a given trait
and either increase or decrease its frequency in a population. This
“struggle for existence”, in which nature determines the “goodness”
or “badness”
“b d
” off a ttrait,
it has
h come to
t be
b synonymous with
ith Charles
Ch l
Darwin.
In this laboratory exercise,
exercise we will conduct some simple
experiments to establish how the theory of Natural Selection
might operate.
Exercise 11
Natural Selection and Evolution
PROCEDURE
To demonstrate the principles of Natural Selection, you and the
other people at your lab table will select samples of colored popcorn
and milo maize seeds in the following proportions:
100 off each
h color
l - red
red,
d, green
green,, yellow
yellow,
ll , orange
orange,, andd blue
bl
100 of the milo seeds
Exercise 11
Natural Selection and Evolution
This group of seeds should be mixed together in a tray and
resemble the photo below:
The seeds represent variation in coloration within a population.
Imagine that each color/size is determined by a different allele.
Exercise 11
Natural Selection and Evolution
Since the variation in seed color/size represents variation in a natural
plant community, you will represent the predator that feeds on the
seeds --birds.
Since birds have variation in beak size/shape in natural communities,
each group of students will be given a different beak with which to
capture the seeds.
The beaks will include spoons, knives, forks, forceps, spatulas,
etc.
You and your partners will take the seeds outdoors to an area
determined byy yyour instructor. Here yyou will toss the seeds into an
area of grass that will not overlap that of the other groups in your class.
Exercise 11
Natural Selection and Evolution
Since
i
birds
bi d in
i naturall settings
i
do
d not have
h
infinite
i i i time
i to find
i d food,
d
(due to natural predators), you (the bird) will be given one minute to
recover all the seeds you can with your beak.
Place the captured seeds back in the tray, sort and record the number,
color and types of seeds recovered (note that these represent “dead”
seeds).
)
Once the seeds have been recaptured your group will travel to a
different area (devoid of seeds) and repeat the experiment. Once
again sort and record the number,
again,
number color and type of seeds recovered.
recovered
After the “second recapture” travel to a third location and repeat the
experiment one last time.
After the third recapture you will return to lab and classroom data will be
tabulated on the board and discussed.
The pictures below depict students catching seeds with their
“beaks”.
The following movie clip demonstrates students picking up seeds
with their “beak”
beak .
Exercise 11
Natural Selection and Evolution
QUESTIONS TO THINK ABOUT
What seeds were recaptured in the highest frequency?
What seeds had the highest survival rate?
What beak morphology had the highest overall capture rate?
What seed phenotype is at an advantage in this environment?
If the
th environment
i
t is
i stable
t bl andd does
d
nott change
h
over time,
ti
what
h t seedd
phenotype will become the most common in the environment?
If the environment experienced a drought, what seed phenotype would be at
the advantage?
What beak morphology(s) is(are) at an advantage in this environment?
Which beak morphologies would become the most common in this
g
environment over time if the environment is stable and does not change?
Exercise 11
Natural Selection and Evolution
NATURAL SELECTION AND DARWIN’S FINCHES
As you learned in the previous section, some individuals within populations
leave more offspring than others. When some organisms survive and leave
more offspring than others because they have advantageous traits (size, color)
natural selection is said to have occurred.
Such a situation has been demonstrated in finch populations on the
Galapagos Islands. Thirteen species of Darwin’s Finches live on the
Galapagos
p g Islands,, each having
g evolved from a common ancestor.
Each of the finch species is similar in bodily proportions and coloration, but
differ from one another in beak shape. The variation in beak shape reflects
diff
differences
in
i diet
di between
b
bird
bi d species
i (see
(
Fi
Figure 11
11.11 in
i lab
l b manual).
l)
Exercise 11
Natural Selection and Evolution
Alteration of beak morphology is associated with climatic fluctuations
on the Galapagos. In some years the islands are parched by drought and in other
years the islands are drenched by
y
y El Nino rains.
In 1973, Peter and Rosemary Grant from Princeton University began studying
Darwin’s Finches on Daphne Major,
Major an islet about 100 acres large.
The Grants and their graduate students successfully banded every individual
bird from two species living on the island--the cactus finch, Geospiza scandens,
and the medium g
ground finch,, Geospiza
p ffortis.
The birds were studied exhaustively and measurements of body and beak size
were made year after year.
Exercise 11
Natural Selection and Evolution
In 1977, Daphne
h Major
j experienced
i
d a drought
d
h which
hi h continued
i d until
il
January of 1978. The researchers found that, although birds of all sizes
decreased in number during the drought, the smaller ones were reduced
the most
most. Birds with larger beaks tended to survive better than birds with
smaller beak sizes.
The same ppattern was repeated
p
in 1982;; when there was veryy little rain,, the
small birds perished in a much greater frequency than birds with large beaks.
However, during wet years the small beaked birds did very well, surviving
and reproducing more than the large beaked birds.
birds
The researchers concluded that the average phenotype in the population
changes over time, large beaked birds being selected for during periods of
drought and small beaked birds being selected for during wet years.
Exercise 11
Natural Selection and Evolution
What you should ask yourself at this point is why small beaks might be
selected for during wet years and large beaks in dry years.
Think about the importance of a beak to a bird
bird--what
what is it used for?
FOOD
So,, how might
g the amount of pprecipitation
p
affect the finch’s food supply?
pp y
Exercise 11
Natural Selection and Evolution
The major environmental consequence of drought is the decline in food supply.
For the ground finch, this means seeds.
During normal wet seasons, an abundance of small seeds are produced by
grasses and herbs. During drought, the number of small seeds decrease and
larger,
g , harder to crack seeds predominate.
predominate
p
You should be able to use this information to explain why average phenotypic
beak size oscillates over time.
Why should you see an increase in beak size during
periods of drought?
Why should you see a decrease in average beak size
during wet years?
Exercise 11
Natural Selection and Evolution
In lab, a hypothetical data set is given to you. You are expected to
use the data and graph beak size vs. time on a sheet of graph paper.
Once the graph has been constructed, use the information given in
the previous slides (as well as in the lab manual) to predict wet and
dry years.
years
Exercise 11
Natural Selection and Evolution
Beak Diversity Within Communities
The previous exercise demonstrated the importance of beak morphology to the
survival of the individual.
The beak is of obvious importance
p
to the bird--it allows the bird to consume
the appropriate food source. Since different bird species specialize on different
food sources, not all birds will have the same type of beak. For example, the
beak of a fish catching bird should look different from the beak of an insect
catching bird. Therefore, beak morphologies differ between birds of the
same community.
The following slides demonstrate variation in beak morphology
among different bird species.
Exercise 11
Natural Selection and Evolution
Bird “a”
a
Bird “b”
b
Exercise 11
Natural Selection and Evolution
Bird “c”
Bird “d”
Exercise 11
Natural Selection and Evolution
Bird “e”
Bird “f”
Bi d “g”
Bird
“ ”
Exercise 11
Natural Selection and Evolution
Which of the birds depicted in the previous slides would be
best adapted for each of the following?
Consuming large seeds (hard to crush)
catching fish (slippery)
shovelingg through
g wet sandyy soil for small food sources
catching insects hiding in the bark of trees
tearing flesh from a rodent
consuming small soft seeds
prodding for food so
sources
rces in the m
mud
d
Match the bird with the most probable food source.
Exercise 11
Natural Selection and Evolution
CONCLUSION
This lab has introduced you to the concept of evolution and
natural selection. It has demonstrated how average phenotypes
can change in a population over time and why diversity is
f
found
d within
ithi a community.
community
it