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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