Performance Benchmark L.12.D.6 Students know the concepts of natural and artificial selection. E/S The idea of evolution (organisms change over time) was not a new theory even in Darwin’s time. What was lacking was an underlying mechanism for biological evolution. The idea of natural selection, first introduced by Darwin in the seminal work “On the Origin of Species” has grown and been modified overtime with the increase in understanding of biological concepts. Such discoveries include the discovery of DNA, increased understanding of genetics and the role DNA, RNA and proteins play in the expression of inherited traits. Charles Darwin was a naturalist, meaning he was a trained observer of nature and natural history. From 1831 to 1836 he was employed as the naturalist aboard the H.M.S. Beagle for a British science exploration voyage. During his time aboard, Darwin made and recorded many observations of the variations he saw within and between species. It was these observed differences that lead him to the idea that natural selection was the mechanism behind evolution. Figure 1. Charles Darwin (from http://www.lucidcafe.com/library/96feb/darwin.html) The premise behind Darwin’s original work was the observed morphological similarities between some animals, and the obvious differences between others. This, aided with his understanding of selective breeding, gave him the idea that organisms could change over time without the influence of human interference. Figure 2. Components of Natural Selection (from http://evolution.berkeley.edu/evosite/evo101/IIIE6Nonrandom.shtml) The most well known example of natural selection is the malanism of the peppered moth during the industrial revolution in England. We will utilize this example throughout our discussion to describe the key factors in Darwin’s theory of natural selection. Before the industrial revolution, the dominant form of the peppered moth was a lighter cream color with black speckles. This coloration was an adaptation to the lighter colored trees they inhabited. There was an alternate form, the “carbonaria” form. This form is a darker color with a few white speckles. This alternate form was more susceptible to predation on the lighter colored trees. With the industrial revolution came large coal burning that caused the trees to turn to a darker color due to the build-up of soot on their trunks. When this happened the lighter colored moths were more exposed, and more susceptible to predation. With this increase in predation came a decline in the dominant cream colored form, but an increase in individuals with the darker coloring. Thus the lighter members of the population declined over time. Natural selection has several tenets or underlying concepts that allow for it to be a mechanism for evolution. Figure 3. Peppered Moths (from http://www.bbc.co.uk/schools/gcsebitesize/science/images/bi06001.jpg) 1. There is variability in a population. In other words there are differences in phenotypes or appearances of the individuals within a population. In the peppered moth example the variation in the body color of the moths was variability in the population. 2. Not all individuals in the population reproduce. a. Not all individuals have the same fitness. Some groups die before they reproduce. Death can be from competition or unfavorable environmental conditions. In our peppered moth example the moths with the light coloring originally were better suited for their environment and so therefore had greater fitness as they survived to reproduce and pass their genetic traits to their offspring. However, once the industrial revolution caused the tree color to change to black, the lighter colored moths no longer had greater fitness than the darker colored moths and so therefore fell to predation. Figure 4. The Process of Natural Selection (from: http://www.globalchange.umich.edu/globalchange1/current/lectures/selection/selection.html) b. Some traits (genotypes) confer an advantage of some sort to the individual, making better suited to their environment. (It is important to note that what may be better in one environment may not be in another environment, so the fitness of a genotype is dependent upon what environment the population lives in.) 3. Organisms produce more offspring than can survive to reproduce. Take frogs for example. Frogs lay thousands of eggs. Not all of those eggs hatch as some are eaten, some dry out etc… Of the eggs that do hatch some survive to become adult tadpoles (again predation and environmental factors can limit the number that live to reproductive age). The tadpoles undergo metamorphosis and form adults. Of the adults only some will survive to reproductive age (they have a higher fitness). 4. The variability in individuals is heritable. The traits of the surviving individuals in the population can be inherited by their offspring. The traits that allow for survival become more numerous in the population, and eventually that characteristic will become the more prevalent form. Understand that Darwin lived long before the discovery of DNA (See L.12.A.1, & A.2) so had no genetic basis for his ideas of natural selection, they were derived from his observations of species throughout his travels and other life experiences. Although Mendel and Darwin lived at the same time, Darwin had no idea of the research Mendel was doing (See L.12.A.5). The ideas of mutation as a mechanism for introducing variation within a species is a relatively new idea, that has occurred in the past few decades. Scientists now understand that random mutations within the genome (the genes that make up a species DNA) of an organism can be beneficial and are the source of new traits. It is important to understand that the mutations are RANDOM, and can occur in different regions of the DNA sequence. If the mutation occurs in a non-coding portion of the DNA, it will often not have a detrimental effect on the organism. If the mutation occurs in a coding region of the DNA, it can be lethal or detrimental mutation or it can introduce a new beneficial gene. In other words, not all genetic mutations are beneficial. In fact a majority of mutations do not allow for the survival of the individual to reproductive age. A great modern example of natural selection is the apparent immunity of some individuals to HIV. Essentially individuals that had genetic mutations in a particular gene, called CCR5delta32, have immunity upon repeated exposure to HIV. It is now believed that these individuals with the mutant gene are survivors of a bottlenecking event… the bubonic plague. Further detail about mutations can be found in the TIPS L12D5 performance benchmark. For more information on immunity of some individuals to HIV, go to http://anthro.palomar.edu/synthetic/synth_4.htm Remember that a bottlenecking event is an event that limits the number of individuals that reproduce (or are able to reproduce) and so therefore only the traits those individuals possess are found within the gene pool. These events can be caused by an isolation of some individuals from the original population, or the events can be caused by some catastrophe where the original gene pool is drastically reduced by massive death. Figure 5. Genetic Drift – Bottleneck Effect. (from http://biology.unm.edu/ccouncil/Biology_203/Summaries/PopGen.htm) In contrast to natural selection, artificial selection is driven by humans rather than nature. Artificial selection occurs when humans breed populations together to produce offspring with desired characteristics. The classic example of this is the differential breeding of wolves to produce a very large variety of canine subspecies. If you look at the over 400 different breeds of domestic dogs, it is obvious that they have similar ancestry. They are all obviously “dogs,” but each subspecies has characteristics that make it unique. These variations, rather than selected for naturally, were selected for by man. Humans have selected certain characteristics within one or a few individuals and bread them together in the hope that the offspring will possess the desired traits. The problem with selective breeding is that it does not allow for other genes or traits to be introduced into the population. Thus the only genes within the gene pool are those that were within the original breeding population. If mutation occurs, they have the potential to be handed down to the next generation and all further generations. Thus problems like hip dysplasia can become common in certain subspecies of dogs. Performance Benchmark L.12.D.6 Students know the concepts of natural and artificial selection. E/S Common misconceptions associated with this benchmark: Much of the following information is taken from an article that was published in the Journal of Research in Science Teaching in 1990. The authors discuss common misconception student have about natural selection and the role it plays in evolution. Bishop, B.A. & Anderson, C.W. (1990). Student conceptions of natural selection and its role in evolution. Journal of Research in Science Teaching 27(5) 415-427. 1. Students mistakenly believe that nature “needs” a change to occur. One of the main misconceptions present in students is the idea that the environment creates a “need” for changes in the organism’s characteristics. Nature does not “need”. While genetic mutation is random, the idea of natural selection is that those individuals with beneficial traits or a higher fitness for the given environment are the ones that survive. The organism does not develop a characteristic based on the need for a characteristic within the population; it is based on genetic variation and fitness over several generations. Figure 6. Evolution Cartoon (from: http://evolution.berkeley.edu/evosite/evo101/IIIE6Nonrandom.shtml) 2. Species Adaptations are Heritable: Students misunderstand the scientific concepts of adaptations and why something is heritable. Another misconception that students hold is that an organism can adapt to their environment and hand that adaptation down to their offspring. It is important for students to understand that only variations that are heritable will be handed down from one generation to the next. If the organism develops a characteristic in it’s lifetime, it is not going to be heritable if it is not handed down in the gametes… it has to be part of the organisms genome to be passed on to the next generation. For more information on this see: http://www.zi.biologie.uni-muenchen.de/evol/Evobio/Evo4-Summary.pdf This is a very Lamarckian idea that organisms adapt to need rather than those that are more “fit” for their environment surviving because they have a selective advantage over other organisms that possess traits that are NOT fit for the environment. In other words only those organisms that survive can contribute to the gene pool of future generations. Part of the reason for this misconception is the scientific understanding of the words adapt and adaptation, and the everyday usage of those terms. Often, they are not synonymous. The scientific definition of the word adaptation means either the process by which a population becomes more suited to its environment or the characteristics the population has inherited as a result of the process. Adapt, in contrast is a tenuous or temporary response to the environment. If I enter into an air conditioned room from a hot environment, it will take me time to adapt. This is not a heritable condition it is your body’s temporary response to the environmental conditions. See also: http://evolution.berkeley.edu/evolibrary/article/0_0_0/misconcep_06 3. Use and Disuse of Organs: Students mistakenly assume that the disuse or use of a body part will determine its disappearance in future generations. Students often incorrectly believe that use or disuse of an organ can lead to species changes (evolution). The driving force behind the loss of an organ is not due to a need it is due to the proportion of individuals with or without the trait surviving to the next generation. Because the trait may not be needed, it doesn’t confer an evolutionary advantage to have one so the disappearance of the trait is not detrimental to the species. Performance Benchmark L.12.D.6 Students know the concepts of natural and artificial selection. E/S Sample Multiple Choice Questions 1. The ability of an individual organism to survive and give birth to healthy offspring in its natural environment is called a. natural selection b. evolution c. adaptation d. fitness 2. Which of the following is(are) important concept(s) in Darwin’s theory of natural selection? I. Species do not change over time. II. There is variability of characteristics within a population of organisms III. A single organism can acquire traits over their lifetimes that are then passed to its offspring. a. I only b. II only c. I and II only d. II and III only 3. A farmer’s use of the best livestock for breeding is an example of a. natural selection b. artificial selection c. extinction d. fitness 4. What natural process produced the different types of beaks shown in the following figure? a. artificial selection b. natural selection c. geographical distribution d. inheritance of acquired traits e. disuse of the beak (Figure reference: http://www.vanderbilt.edu/AnS/english/Clayton/Galapago_finches.gif) 5. Differences among individuals of the same species are referred to as _____. a. variation b. adaptation c. fitness d. natural selection Performance Benchmark L.12.D.6 Students know the concepts of natural and artificial selection. E/S Answers to Sample Test Questions 1. (d) 2. (b) 3. (b) 4. (b) 5. (a) Performance Benchmark L.12.D.6 Students know the concepts of natural and artificial selection. E/S Intervention Strategies and Resources 1. Evolution and Natural Selection This website is devoted to aiding students in the understanding of how natural selection plays a role in the overall bigger concept of biological evolution. http://www.globalchange.umich.edu/globalchange1/current/lectures/selection/selection.html 2. Understanding Evolution for Teachers This website, created by UC Berkeley, is a great resource for the teaching of evolution in general, but also for understanding the concepts of natural selection and artificial selection as they are such large concepts in the understanding of evolution. http://evolution.berkeley.edu/evosite/evohome.html http://evolution.berkeley.edu/evolibrary/home.php 3. Natural Selection This website is designed to explore the genetic basis for natural selection and evolution. It starts with an explanation of Darwin’s idea of natural selection and then continues with the genetic basis for selection. http://anthro.palomar.edu/synthetic/synth_4.htm 4. Teaching about Evolution and the Nature of Science This is a book published by National Academy Press specifically for teachers to address concerns and misconceptions about evolution. It gives teachers activities to do with their students, as well as aiding teachers with the understanding of key concepts in evolution. It can be found online at: http://books.nap.edu/html/evolution98/front.html 5. List of primary literature Nehm, R.H. & Reilly, L. (2007). Biology majors’ knowledge and misconceptions of natural selection. BioScience 57(3) 263 – 272. Udovic, D., Morris, D., Dickman, A., Postlethwait, J, & Wetherwax, P. (2002). Workshop Biology: Demonstrating the effectiveness of active learning in an introductory biology course. BioScience 52(3) 272 – 281. Palmer (1996). Students’ application of a biological concept: Factors affecting consistency. Research in Science Education 26(4), 409-419.