11 lab exercise - citrus · pdf filenatural selection and evolution ... such a mechanism...

Exercise 11Exercise 11Natural Selection and Evolution

Perhaps one of the most controversial topics in science is the discussion of evolutionevolution, primarily because it raises the issue of origins especially those of the human kindorigins---especially those of the human kind.

Such controversies have surrounded this topic for as long as humanshave discussed the problemhave discussed the problem.

This laboratory will offer some historical background for the i ifi h i i d i h l i d dscientific theories associated with evolution and demonstrate some

of the principles which have been accepted as part of thescientific foundation for evolution.


Some scientists view evolution as a process of change where minorgenetic changes occur in populations through random mutations whichcan lead to a change in gene frequencycan lead to a change in gene frequency.

-This is known as microevolutionmicroevolution.

Microevolution can produce enough genetic variation to createMicroevolution can produce enough genetic variation to createa new species of organism (macroevolutionmacroevolution), one that is distinct fromthe original “parent” form.

Such a mechanism involves small continual changes over manygenerations and is known as gradualismgradualism.

Gradualism suggests that small changes accumulate in populationsgradually over time and eventually lead to the evolution of a new species.


Although gradualism is generally accepted by many scientists a newmechanism for evolutionary change has recently been proposed.

Punctuated EquilibriumPunctuated Equilibrium suggests that evolutionary change (speciationspeciation) occurs rapidly followed by long periods of time of no change.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 speciesfollowed by sudden periods of intense change

In other words punctuated equilibrium suggests that evolutionaryIn other words, punctuated equilibrium suggests that evolutionarychange is not gradual, but consists of stable periods of timefollowed by rapid speciation.


While the debate between gradualism and punctuated equilibriumWhile the debate between gradualism and punctuated equilibriumcontinues, we must understand that they are at two opposite ends of a spectrum. Proponents of both agree that evolutionary changeover time caused by Natural SelectionNatural Selection occurs in nature, but they disagree as to the nature of the change.

Whether the change is gradual over time or rapid followed byWhether the change is gradual over time or rapid followed byperiods of very little change has yet to be determined. It is quite likely that new evolutionary mechanisms will be establishedthat use portions of each of these concepts to explain evolutionary change over time. This is what makes science soimportant. There is no absolute rule; the theories change withThere is no absolute rule; the theories change withp ; g; gthe discovery of new informationthe discovery of new information.


NATURAL SELECTION

In 1831, the HMS HMS BeagleBeagle sailed from England bound for a voyage, gg g y garound the world. Aboard the ship was Charles DarwinCharles Darwin, the ship’snaturalist.

Darwin’s duties were to catalog all the natural events encounteredon the trip. Darwin did not appreciate ocean travel and was quiteoften ill due to seasickness. As a result, he was quick to volunteeroften ill due to seasickness. As a result, he was quick to volunteerfor shore duty and spent months at a time away from the ship, traveling and collecting specimens and data.


Darwin made many notable discoveries during this trip but none wereas significant as those made on the Galapagos IslandsGalapagos Islands, a cluster ofvolcanic islands situated near the equator off the coast of Ecuadorvolcanic islands situated near the equator off the coast of Ecuador.

Although the Galapagos Islands were never connected to the mainland, they contained plants and animals similar to those on themainland. Darwin noticed that the plant life was not as diverse as onthe mainland and that each island seemed to have its own uniqueplant communitiesplant communities. Darwin also noticed unusual animals on the islands, such as hugetortoises found nowhere else in the world. But, of all the terrestrialanimals Darwin found, a group of fairly common mainland birds (finches) caught his eye.


Th bi d h i b k D i ’ Fi h dThese birds have since become known as Darwin’s Finches and wereextremely influential on Darwin.On the Galapagos Islands, the roughly 13 species of finches thatp g , g y pDarwin identified displayed very different body sizes, behaviors,bill structures, and even feeding preferences. In fact, some fincheswere feeding on insects or fruit unlike their relatives on the mainlandwere feeding on insects or 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 wasDarwin began to wonder why such variation in finch biology wasobserved on this truly unique island chain.


When Darwin returned home to England in 1836, he set out to constructa 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.his observed conditions.

In the 1840’s he began to refine his ideas and was encouraged byother scientists to publish his works before someone else did. In 1858pa naturalist working in the East Indies, Alfred WallaceAlfred Wallace, sent Darwina manuscript in which he established a theory that species evolve inresponse to changes in their environment This was the same basicresponse to changes in their environment. This was the same basicbelief that Darwin had come to while evaluating the data from hisexpedition.


While Wallace had a general concept in mind, Darwin had volumesof data to substantiate his beliefs and even though he acknowledgedWallace’s contributions, Darwin’s writings on “descent withWallace s contributions, Darwin s writings on descent withmodification” have been accepted as the basis of the Theory of Theory of Natural SelectionNatural Selection.

The Theory of Natural Selection is briefly outlined on the followingslide.


DARWIN’S MAIN IDEASDARWIN’S MAIN IDEAS

Facts Based on Observations of Nature:1) O i i d d i ti i h it d Withi i1) Organisms are varied, and some variations are inherited. Within a species, 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 to3) 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

“ i l f h fi ” l i di id l i h d i i li l h“survival of the fittest”, only individuals with adaptive traits live long enough totransmit traits beneficial in that environment. Over time, natural selection canchange the characteristics of populations, even molding new species.


In short, Darwin’s theory held that changes occur in all naturalpopulations. Today, we know that these changes --mutations-- occurat random in populations and contribute to a change in the geneticat 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 traitChanges in the environment can favor or select against a given traitand either increase or decrease its frequency in a population. This“struggle for existence”, in which nature determines the “goodness”

“b d ” f t it h t b ith Ch lor “badness” of a trait, has come to be synonymous with CharlesDarwin.

In this laboratory exercise we will conduct some simpleIn this laboratory exercise we will conduct some simpleIn this laboratory exercise, we will conduct some simple In this laboratory exercise, we will conduct some simple experiments to establish how the theory of Natural Selection experiments to establish how the theory of Natural Selection might operate.might operate.


PROCEDUREPROCEDURE

To demonstrate the principles of Natural Selection, you and the other people at your lab table will select samples of colored popcornother people at your lab table will select samples of colored popcornand milo maize seeds in the following proportions:

100 f h l dd llll dd blbl100 of each color - redred, , greengreen, , yellowyellow, , orangeorange, and , and blueblue100 of the milo seeds


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.


Since the variation in seed color/size represents variation in a naturalSince the variation in seed color/size represents variation in a naturalplant community, youyou 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.capture the seeds.

The beaks will include spoons, knives, forks, forceps, spatulas, etc.

You and your partners will take the seeds outdoors to an areadetermined by your instructor. Here you will toss the seeds into any y yarea of grass that will not overlap that of the other groups in your class.


i bi d i l i d h i i i i i d dSince birds in natural settings do not have infinite time to find food,(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. Onceagain sort and record the number color and type of seeds recoveredagain, sort and record the number, color and type of seeds recovered.

After the “second recapture” travel to a third location and repeat theexperiment one last time.

After the third recapture you will return to lab and classroom data will betabulated on the board and discussed.

The pictures below depict students catching seeds with theirThe pictures below depict students catching seeds with their“beaks”.“beaks”.

The following movie clip demonstrates students picking up seeds with their “beak”with their beak .


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 th i t i t bl d d t h ti h t dIf the environment is stable and does not change over time, what seed phenotype will become the most common in the environment?If the environment experienced a drought, what seed phenotype would be atthe advantage?the advantage?What beak morphology(s) is(are) at an advantage in this environment?Which beak morphologies would become the most common in this environment over time if the environment is stable and does not change?g


NATURAL SELECTION AND DARWIN’S FINCHESNATURAL SELECTION AND DARWIN’S FINCHES

As you learned in the previous section, some individuals within populationsleave more offspring than others. When some organisms survive and leavemore offspring than others because they have advantageous traits (size, color)natural selectionnatural 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 Islands, each having evolved from a common ancestor.p g , g

Each of the finch species is similar in bodily proportions and coloration, butdiffer from one another in beak shape. The variation in beak shape reflects The variation in beak shape reflects diff i di b bi d idiff i di b bi d i ( Fi 11 1 i l b l)differences in diet between bird speciesdifferences in diet between bird species (see Figure 11.1 in lab manual).


Alteration of beak morphology is associated with climatic fluctuationson the Galapagos. In some years the islands are parched by drought and in otheryears the islands are drenched by El Nino rains.y y

In 1973, Peter and Rosemary Grant from Princeton University began studyingDarwin’s Finches on Daphne MajorDaphne Major, an islet about 100 acres large.

The Grants and their graduate students successfully banded every individualbird from two species living on the island--the cactus finch, Geospiza scandens, and the medium ground finch, Geospiza fortis.g , p f

The birds were studied exhaustively and measurements of bodybody and beak sizebeak sizewere made year after year.


h j i d d h hi h i d ilIn 1977, Daphne Major experienced a drought which continued until January of 1978. The researchers found that, although birds of all sizesdecreased in number during the drought, the smallersmaller ones were reduced the most Birds with larger beaks tended to survive better than birds withthe most. Birds with larger beaks tended to survive better than birds withsmaller beak sizes.

The same pattern was repeated in 1982; when there was very little rain, the p p ; y ,small birds perished in a much greater frequency than birds with large beaks.

However, during wet years the small beaked birds did very well, survivingand reproducing more than the large beaked birdsand reproducing more than the large beaked birds.

The researchers concluded that the average phenotype in the populationthe average phenotype in the populationchanges over time, large beaked birds being selected for during periods of changes over time, large beaked birds being selected for during periods of drought and small beaked birds being selected for during wet years.drought and small beaked birds being selected for during wet years.


What you should ask yourself at this point is why small beaks might beWhat you should ask yourself at this point is why small beaks might beselected for during wet years and large beaks in dry years.

Think about the importance of a beak to a bird--what is it used for?Think about the importance of a beak to a bird what is it used for?

FOODFOOD

So, how might the amount of precipitation affect the finch’s food supply?, g p p pp y


The major environmental consequence of drought is the decline in food supply.For the ground finch, this means seedsseeds.

During normal wet seasons, an abundance of small seeds are produced by During normal wet seasons, an abundance of small seeds are produced by grasses and herbs. During drought, the number of small seeds decrease andgrasses and herbs. During drought, the number of small seeds decrease andlarger, harder to crack seeds predominatelarger, harder to crack seeds predominate.g , pg , p

You should be able to use this information to explain why average phenotypicbeak size oscillates over time.

Why should you see an increase in beak size during Why should you see an increase in beak size during periods of drought?periods of drought?Why should you see a decrease in average beak size Why should you see a decrease in average beak size during wet years?during wet years?


In lab, a hypothetical data set is given to you. You are expected touse the data and graph beak size vs. time on a sheet of graph paper.

Once the graph has been constructed, use the information given inthe previous slides (as well as in the lab manual) to predict wet and dry yearsdry years.


Beak Diversity Within Communities

The previous exercise demonstrated the importance of beak morphology to theThe previous exercise demonstrated the importance of beak morphology to thesurvival of the individual.

The beak is of obvious importance to the bird--it allows the bird to consumepthe appropriate food source. Since different bird species specialize on differentfood 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 morphologyThe following slides demonstrate variation in beak morphologyThe following slides demonstrate variation in beak morphologyThe following slides demonstrate variation in beak morphologyamong different bird species.among different bird species.


Bird “a” Bird “b”Bird a Bird b


Bird “c” Bird “d”


Bird “e” Bird “f”

Bi d “ ”Bird “g”


Which of the birds depicted in the previous slides would bebest adapted for each of the following?

Consuming large seeds (hard to crush)catching fish (slippery)shoveling through wet sandy soil for small food sourcesg g ycatching insects hiding in the bark of treestearing flesh from a rodentconsuming small soft seedsprodding for food so rces in the m dprodding for food sources in the mud

Match the bird with the most probable food source.


CONCLUSION

This lab has introduced you to the concept of evolution andThis lab has introduced you to the concept of evolution andThis lab has introduced you to the concept of evolution andThis lab has introduced you to the concept of evolution andnatural selection. It has demonstrated how average phenotypesnatural selection. It has demonstrated how average phenotypescan change in a population over time and why diversity is can change in a population over time and why diversity is f d ithi itf d ithi itfound within a communityfound within a community.

11 lab exercise - citrus · pdf filenatural selection and evolution ... such a mechanism...

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