6. Discuss how genes and gene variation can provide us with a genetic definition of how populations evolve.

Transcription

1 OBJECTIVE SHEET EVOLUTION 1. Describe the contributions of Hutton, Lyell, Malthus, and Lamarck toward the evolutionary thought of Charles Darwin. 2. Explain Natural Selection as a mechanism to describe how evolution occurs. 3. Discuss the various clues that nature provides as evidence for evolution. 4. Define the following: gene, allele, segregation, independent assortment, mutation, chromosome, phenotype, genotype, dominant allele, recessive allele, incomplete/codominance, polygenic traits, diploid, haploid, homozygous, heterozygous 5. Solve single (hybrid) and double (dihybrid) factor crosses using Punnett squares. 6. Discuss how genes and gene variation can provide us with a genetic definition of how populations evolve. 7. Discuss how natural selection can affect the distribution of phenotypes in any of three ways: directional, stabilizing or disruptive selection. 8. Identify how Genetic Drift and Gene Flow can also provide a source of evolutionary change. Provide an example of the founder effect as an example of genetic drift. 9. Explain how the following Isolating Mechanisms help bring about Speciation: Allopatric and Sympatric Isolation. Include a discussion on geographical, temporal, ecological, behavioural, mechanical, and gametic isolation. 10. Explain the role of extinction in macroevolution. Differentiate among and give examples of Adaptive radiation, convergent evolution and co-evolution. 11. Differentiate the pace of evolution as either gradualism or punctuated equilibrium. 1

2 There is grandeur in this view of life from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved. Charles Darwin It has been called by some the greatest idea that any human has ever had. Charles Darwin was able to unite the two most disparate features of our universe, the world of purposeless, meaningless, matter in motion and the world of meaning, purpose, and life. 2

3 A new era of biology began on November 24, 1859, the day Charles Darwin published On the Origin of Species by Means of Natural Selection. Darwin s book drew a cohesive picture of life by connecting the dots among what had once seemed a bewildering array of unrelated observations. The Origin of Species focused biologists attention on the great diversity of organisms their origins and relationships, their similarities and differences, their geographic distribution, and their adaptations to surrounding environments. Darwin made two major points in The Origin of Species. First, he presented evidence that the many species of organisms presently inhabiting Earth are descendants of ancestral species that were different from the modern species. Second, he proposed a mechanism for this evolutionary process, which he termed natural selection. Evolution is such a fundamental concept that its study illuminates biology at every level from molecules to ecosystems, and it continues to transform medicine, agriculture, biotechnology, conservation biology and many others. 3

4 The H.M.S. Beagle Sets Sail Ref There is an astounding diversity of life on our planet. Evolutionary theory accounts for this diversity. It is a collection of scientific facts, observations, and hypotheses tested over the last 150 years. It has explained the differences and similarities observed with all life forms both extinct and living today. A scientific theory is a well-supported testable explanation of naturally occurring phenomena in the world. Darwin set sail aboard the H.M.S. Beagle on a trip around the world in No one knew that this voyage was one of the most important voyages in the history of science. Starting from the British Isles, follow the route taken by Darwin to South America, the Galapagos Islands and around the southern tips of Australia and Africa, only to return to South America again before heading home to England. If there was one thing that Darwin excelled at, it was his powers of observation. His curiosity and analytical nature were ultimately the keys to his success as a scientist. During his travels, Darwin came to view every new finding as a piece in an extraordinary puzzle: a scientific explanation for the diversity of life on this planet. Darwin made some keen observations on his travels. He was puzzled by the distribution of life forms in the variety of habitats that he encountered along the way. The patterns of diversity posed a challenge to Darwin. Besides the living organisms encountered on his voyage, Darwin collected a multitude of fossils aboard the ship and brought them back to England for further study. It did not escape him that the fossils below ground resembled the living organisms above them with some modification. 4

5 Of all the Beagle s ports of call, the one that influenced Darwin the most was a group of small islands located 1000 km west of Ecuador. These islands are called the Galapagos Islands. Although they were close together, the islands had unique climates and geology. Darwin was most intrigued with the three different species of land tortoises. Darwin also noticed the strange variety of finches on the island. The different islands produced birds with a multitude of beak shapes. What interested Darwin was that a particular beak shape allowed a bird to forage for the type of seed found only on the island inhabited by that species of bird. Darwin began to hypothesize that these separate species of bird would have evolved from a single original South American ancestor species after becoming isolated from one another. This was truly a revolutionary idea that would shock the world. Ideas That Shaped Darwin s Thinking James Hutton Known as the father of modern geology, Charles Darwin enthusiastically read all of Hutton s books explaining for the first time how the Earth s features were formed by natural processes over deep time. He estimated the Earth to be many millions of years old and not just a few thousand years old. He established geology as a legitimate science through observation and carefully reasoned geological arguments. 5

6 Charles Lyell A geologist, paleontologist, and friend of Darwin, he strongly influenced Darwin s thinking about evolution as a slow process in which small changes gradually accumulate over immense spans of time. He emphasized natural law as an explanation for geological formations contrary to biblical flood stories. Darwin saw natural law as a way to explain the change and diversity of organisms on Earth. Lyell assisted Darwin with fossil interpretations. Thomas Malthus An economist who wrote an essay about human population growth strongly influenced Darwin s thinking about life. Malthus predicted that the human population will grow faster than the space and food supplies needed to sustain it. Darwin made comparisons of this idea to the struggle for existence among organisms. Jean-Baptiste Lamarck Lamarck s ideas were found to be incorrect however he was one of the first to propose that life forms could change and evolve over time. His ideas involved the concept of Use and Disuse. He believed that through selective use or disuse of organs, organisms acquired or lost certain traits during a lifetime. He incorrectly thought that these traits could then be passed on to their offspring. 6

7 Lamarck s ideas about species change contrasts with Darwin s ideas in interesting ways: Lamarck s view The original short Keeps stretching and continues finally, a longnecked ancestor neck to reach stretching until neck descendant leaves higher up neck becomes after very many on the tree. progressively generations longer Darwin s view Original group has variation in neck length. Natural selection favors longer necks The favored characteristic gets passed on to next generation in greater proportion than the shorter neck. After many, many generations, the group is still variable but showing a general increase in neck length. For many years, biologists hypothesized the origins of the giraffe s long neck. Initially the explanation by early biologists rested on the assumption that the giraffe had the advantage of eating leaves higher on the trees where other animals could not reach. Careful observation through the years indicates that most of their grazing occurs at lower levels. The latest evidence indicates that the giraffe s neck evolved due to sexual selection as male giraffes battle each other for reproductive rights by swinging their necks and hitting each other with their ossicones (antler-like protuberances) with tremendous leverage. Eventually one of the giraffes would back-down and the victor would become the dominant male with reproductive rights. What s interesting to note is that the giraffe has the same number of vertebrae as you and I. 7

8 Evolution By Natural Selection The simplicity of Darwin s theory of evolution can be beautifully summarized as follows: Individual organisms differ, and some of this variation is heritable. Organisms produce more offspring than can survive, and many that do survive do not reproduce. Because more organisms are produced than can survive, they compete for limited resources. Each unique organism has different advantages and disadvantages in the struggle for existence. Individuals best suited to their environment survive and reproduce most successfully. These organisms pass their heritable traits to their offspring. Other organisms die or leave fewer organisms. This process of natural selection causes species to change over time. Species alive today are descended with modification from ancestral species that lived in the distant past. This process by which diverse species evolved from common ancestors unites all organisms on Earth into a single tree of life. 8

9 The Mountain of Evidence That Is Evolution The RNA World All life on Earth shares the self-replicating DNA molecule. DNA is a large, complex molecule whose origin may be linked to RNA. RNA is a smaller, simpler molecule than DNA. Scientists were surprised to discover that RNA can actually act like an enzyme to help it make copies of itself and forge some very simple proteins. It has been hypothesized that RNA might have existed before DNA and that this simple molecule evolved into the more complex, stable, and efficient molecule of DNA that all life forms share today. Homologous Structures Ref Homologous structures suggest a common ancestor. The limbs above appear to be different and they even have different functions, but they all have similar bone structure and are developed from the same embryonic genes. Evolution lies exposed in the imperfections that record a history of common descent. Why should a rat run, a bat fly, a porpoise swim, and I type this essay with structures built of the same bones unless we all inherited them from a common ancestor? - Stephen Jay Gould, It has not escaped biologists that even the genes and indeed the DNA that produces these structures are also homologous. 9

10 Vestigial Structures Ever wonder why whales and snakes have hip bones? Do chickens have teeth? Of course not, but why do they have the gene for the production of tooth dentin? Why do we have wisdom teeth (third molars) when their only purpose seems to be to crowd out our other teeth? Why do males have nipples? Why do we all have muscles attached to our ears? These and many more are vestigial structures which are remnants of our evolutionary heritage from the past. Evolution doesn t re-invent structures and organs from scratch, it takes those same inherited structures and tinkers with them to see how successful any changes would be in a new untested environment. 10

11 The Fossil Record Fossils and the order in which they appear in layers of rock provides some of the strongest evidence of evolution. Fossils found deeper are older than the fossil layers found on top. The fossil record shows that animals and plants have appeared in a historical sequence. Moreover, the sequence of fossils is consistent with what is known from other lines of evidence. For example, evidence from biochemistry, geology, molecular biology and cell biology places prokaryotes as the ancestor of all life. And, indeed, the oldest known fossils dating from about 3.5 bya, are prokaryotes. Another example is the chronological appearance of the different classes of vertebrates (animals with backbones) in the fossil record. Fishlike fossils are the oldest fossils in the fossil record; amphibians are next, followed by reptiles, then mammals, and finally extending from the reptiles are the birds. This sequence is consistent with the history of vertebrate descent as revealed by many other types of evidence. Hundreds of years of fossil collection have produced a vast library, or catalogue, of the ages of the Earth and the life on it. We can now identify general time periods when major changes occurred. Interested in the origin of mammals? Go to rocks from the period called the Early Mesozoic; geochemistry tells us that these rocks are likely about 210 million years old. Interested in the origin of primates? Go higher in the rock column, to the cretaceous period, where rocks are about 80 million years old. A trilobite fossil minus the rock around it. This extinct species of Arthropod existed around 300 mya. Many fossils of trilobite species have been found in the Kootenay region within the last few decades. The fossil record even provides us with amazing predictive power about where to look for particular transitional forms. Paleontologists look for rocks of the right age, rocks of the right type to preserve fossils, and rocks that are exposed at the surface. A classic example of this allowed Paleontologist Neil Shubin to find the most complete transitional fossil from sea to land in 375 million year old rock in northern Canada. 11

12 Neil Shubin knew that in rocks dated around mya that he would find diverse kinds of fossilized animals that we would all recognize as amphibians and reptiles. With their necks, their ears, and their four legs, they do not look like fish. This rock layer throughout the world consistently produced the same variety of extinct species. But in rocks that are 385 million years old, we find whole fish that look like, well, fish. They have fins, conical heads with eyes on top, and scales; and they have no necks so the head moves with the body. It was no surprise that Shubin focused on rocks in the 375 mya range in Canada to search for fossils showing the transition from sea to land. In the fall of 2004, his intuition was rewarded with the discovery of Tiktaalik roseae. This extinct creature broke down the distinction between two different kinds of animal. Like a fish, it has scales on its back and fins with fin webbing. But, like early land-living animals, it has a flat head and a neck. And when we look inside the fin, we see bones that correspond to the upper arm, the forearm, even parts of the wrist. The joints are there too; this is a fish with shoulder, elbow, and wrist joints. All inside a fin with webbing Tiktaalik roseae 12

13 Embryology Embryologists study the development of embryos from different species. There is striking similarity in the development of similar species. We understand that this similarity is due to these organisms having similar genes. Their differences result from the timing that these genes turn on and off, as well as even how long these genes remain turned on. This similarity in genes is due to the fact that we share a Common ancestor. This provides further evidence to explain how evolution occurs. This is a human embryo at a stage of its development called a blastocyst. There is no differentiation or specialization of cells at this point of development. Similar genes in similar organisms cause certain cells to develop in predictable ways. The timing of these genes can determine the shape and make-up of these structures. This is a tubal (ectopic) pregnancy of a human embryo at about 6 weeks of development. You can see limb-buds, eyes, and the beginnings of a backbone. 13

14 Life s Origins The question about how life on Earth started is not within the scope of evolutionary theory. Life from non-life remains one of the greatest mysteries in life science research. Again, biologists see very compelling clues and evidence in nature today that provide us with insight into how life may have started. Under certain conditions, large organic molecules can form tiny bubbles similar to cell membranes. They contain higher than normal concentrations of the molecules of life. Theses tiny microspheres can contain extremely high concentrations of organic material (which is the building blocks of life). The spheres even contain lipid membranes similar to cell membranes The Miller-Urey Experiment Stanley Miller and Harold Urey were able to re-create the conditions of the early Earth and produced amino acids, some of the building blocks of protein by passing sparks through a mixture of H 2, CH 4, NH 3, and water. Although they did NOT create life they did show that experimentally under the right abiotic conditions, the building blocks of all living things could be made. This became known as the Miller-Urey experiment in the early 1950 s. 14

15 The Endosymbiont Hypothesis Given the evidence that life presents us we can visualize how life may have started. An even greater challenge occurs when we began to wonder how simple prokaryotic cells could become the complex eukaryotic cells complete with organelles. Convincing evidence today shows us that about 2 billion years ago, prokaryotic cells began to evolve internal cell membranes. Other prokaryotic organisms entered into these cells and began living as they were surrounded by membrane. Over time, a mutualistic relationship developed. This became known as the Endosymbiont Hypothesis. Eukaryotic cells formed from a symbiosis among several different prokaryotic organisms living together. One prokaryotic organism could generate ATP and evolved into a mitochondrion while another prokaryote cell that carried out limited photosynthesis evolved into the chloroplasts found in plant cells today. Label the following diagram (pg.427) to explain the ENDOSYMBIONT HYPOTHESIS It is interesting to note that the internal structure of chloroplasts and mitochondria look very similar to these types of prokaryotic bacteria. Chloroplasts and mitochondria continue to have their own separate and unique DNA from the rest of the DNA in your cell s nucleus. 15

16 Molecular Genetics It s fascinating that all life forms share the same molecule, the DNA molecule, to transfer traits to future generations. The sequence (or order) of base pairs (the building blocks of DNA) can be read by biologists to determine genetic relatedness among species. Of the 3 billion base pairs found in humans and chimps, over 98% of the sequence is exactly the same base for base, producing over a 96% match in genes. We did not evolve from chimpanzees, rather they are our evolutionary cousins as we both separated from a common ancestor about 3 mya. We even share sequences and genes with yeast, mice, corn, and long ago extinct species. Of the five great apes, Homo sapiens are the only primate with 23 pairs of chromosomes in our cells. The other four great apes, including Gorillas, Chimps, Bonobos, and Orangutans all have 24 pairs of chromosomes. How could this happen? The loss of an entire chromosome would be lethal to any developing embryo. The evidence is found on human chromosome number 2. A distinct fusion of two ancestral chromosomes 12 and 13 from our shared common ancestor occurred. We still contain all the same genes. Molecular genetics studies have confirmed in fantastic detail the evolutionary relationships among all living things on Earth in ways that would have astounded Charles Darwin. 16

17 Developmental Genes and Body Plans Reference text pg. 440 Biologists have long suspected that changes in genes during embryological development could produce drastic transformations in body plans of organisms. By turning on and off the master control genes called hox genes we have seen various changes occur. If one gene called wingless is turned on in an insect body segment, that segment grows no wings. This is interesting because some ancient insects had wing-like structures on all body segments. Modern insects today have wings on only one or two segments. Small changes in the timing of cell differentiation and gene expression can make the difference between chimpanzee brains and human brains! 17

18 Natural Selection and the Beak of the Finch Introduction There are 13 different species of finch on the Galapagos Islands off the coast of Ecuador. On one of the islands, Daphne Major, biologists Peter and Rosemary Grant have devoted many years to studying four of these bird species. The Grants have studied the effects of drought and periods of plenty on the finches, and the results of their experiments have had an enormous impact on evolutionary science. For this challenge, you will first analyze the characteristics of the 13 species of finch found on the Galapagos Islands. Then you will watch a short film about the research conducted by the Grants. Based on the information presented in the film (The Origin of Species: The Beak of the Finch, at and your own observations, you will conduct an argument and make predictions about the role of natural selection on the evolution of finch populations. Materials Finch cards (13 cards per student team) Computer access Camera (optional) Sticky notes Graph paper Notebook paper for writing Different colored pens (option) Procedure Break into your team as instructed and find your team s location marked in the room. Each team will receive 13 cards of different finch species. Follow the instructions below, recording observations and answers to questions in your Biology notebook or on a sheet of paper. PART 1: What Do You Already Know? 1. Working with others on your team, examine the cards of the Galapagos finches and arrange the species into groups on your counter space based on their characteristics. Grouping species according to shared characteristics can provide clues to how they have evolved. 2. Give each group an informative name and write that name down next to each group. On sticky notes, list the justification or reason why you created each group. 3. Pause for a gallery walk. Walk around the class and examine the displays by the other teams, paying attention to the following: How were other teams groupings similar to your team s? How were they different? What evidence did your classmates use to justify their groupings? How does the evidence they provided support their groupings? On your own, write two questions about each team s presentation on a sticky notes. Put your initials on your question. 18

19 4. Based on what you observed during your gallery walk, does your team want to make any changes to its own groupings? What additional evidence would you need to better justify your team s groupings? Make your changes and write down in your notebook or on a sheet of paper your rationale for revising (or not revising) your groupings. Write additional justification on your sticky notes or replace them all together. PART 2: Sorting Finch Groups 5. Watch the first segment of the film The Origin of Species: Beak of the Finch at from the beginning to 5:36 minutes. As you watch listen for answers to the following questions: What do the different beaks tell us about the different finch species? What evidence did scientists use to determine that the 13 species of finches on the Galapagos arose from a single common ancestor? What was an alternative explanation, and how did the scientists discount it? Why was the scientific conclusion of common ancestry important for understanding the effects of natural selection on these bird species? 6. Your teacher will pause for a short discussion on these answers. 7. Return to your finch groupings at your counter from Part 1. Work with others on your team and, if necessary, rearrange the bird groupings based on what you heard in the film. Then use the information from the film to revise the names and your justifications on your sticky notes. Answer the following questions in your notebook or on a sheet of paper. a. What did you change? b. What evidence from the film convinced you to make the change? c. What do the different groups of finches that you created represent? PART 3: Examining Finch Beaks 8. Watch the second segment of the video, from time stamp 5:36 minutes to 9:00. As you watch, listen carefully for evidence to help you answer the following questions. a. Describe beak sizes of the medium ground finch population (species 12 finch cards) b. How did the population of medium ground finches on the island of Daphne Major change as a result of environmental changes? 9. Make a prediction. After watching the segment, create a bar graph on your own that shows the beak sizes of the population of medium ground finches before and after the drought. Your graph should indicated the number of medium ground finches with each of four different beak sizes (from smallest to largest) before and after the drought. (Hint: You will create two bars for each category of beak size, one representing the populations before the drought, and one representing the 19

20 populations after the drought.) Include the following categories of beak sizes in your graph: Medium ground finches with much smaller beaks Medium ground finches with smaller beaks Medium ground finches with larger beaks Medium ground finches with much larger beaks 10. Share your graph with others on your team and provide feedback by asking your team members two or more questions about their graphs. Be ready to explain your own graph. 11. Watch the third segment of the video, from time stamp 9:00 minutes to 11:12 minutes. After watching the film, answer the following questions. a. How did your graph compare to the graph in the film? Did anyone on your team have a graph that was similar? b. If no one on your team had a graph that was similar, what evidence did you not consider? c. If your graph was close to the one in the film, what part of your thinking was the same as that of the scientists in the film? d. Why did the drought have such an impact on the medium ground finch population? 12. Class prediction: What was the response of Peter and Rosemary Grant to the dramatic change in the distribution of beak sizes in just one generation of birds? If the drought had continued longer, what would you expect your beak graph to look like? PART 4: Understanding Speciation (the making of new species) 13. Watch the final segment of the video from time stamp 11:12 minutes to 15:45 minutes. As you watch the film, listen carefully for an answer to the following: How did one ancestral finch population give rise to 13 species, each with different characteristics? 14. With your team, create a graphic representation of the process that led to 13 different finch species. You may use the cards with the finches to construct your graphic. Prepare your presentation like a museum exhibit, so it will stand alone without your needing to explain it. However, you can include a written caption, as museum exhibits do. You will get a chance to look around at other exhibits and then you may possibly want to revise yours. Offer suggestions or feedback on other exhibits as you see fit. Make sure your representation can stand up to peer review after you finalize it. 20

21 Genetics, The Study of Heredity It is very interesting to note that as Charles Darwin was deriving his monumental ideas on Evolution and Natural Selection, he did not know of a reason for inherited traits. Gregor Mendel (the father of modern genetics) was devising his principles of genetics at the same time. 21

22 Characteristics appear to be repeated from generation to generation. The passing of traits from parents to offspring is called heredity. Your biological traits are controlled by genes located on chromosomes that are found in every cell of your body. You inherited half of your 46 chromosomes from your mother and the other 23 from your father. Essential Terminology 1. Chromosomes, Chromatids, Centromere and Homologous chromosomes A chromosome is a long DNA molecule containing many genes. In eukaryotes, it is associated with protein. During prophase, the chromosomal material has duplicated itself and the two new halves are called sister chromatids. They are joined at the centromere and the centromere is attached to the spindle. A homologous chromosome is the matching chromosome inherited from the second parent. It is important to note that if the top paternal and maternal strands are both called individual chromosomes, then when they duplicate below to form sister chromatids, each of the new duplicated paternal and maternal double strands are now considered to be only a single chromosome. Homologous chromosomes contain the same genes. For example, the gene for eye colour would be found on both the paternal and the maternal chromosome. That s what homologous means there are two chromosomes that have the same genes, one from mother and the other from father. An important point to remember is that even though there are two copies of a single gene, they may be different versions of that gene. We call these different versions of the same gene alleles. There might be the allele for green eyes, blue eyes or brown eyes present on a chromosome but it s still the gene for eye colour. Which allele an offspring inherits is completely random. 22

23 2. Genes Genes are units of nucleotide base pairs located on the chromosome that provide instructions to a cell to produce a specific trait. You have about 20,00 genes (i.e. the gene for hair colour). Many genes can interact with other genes to produce various effects. 3. Alleles Alleles are two or more alternate forms of a gene. (i.e. an allele for black hair or blonde hair are forms of the gene for hair colour). 4. Dominant genes (alleles These genes (or alleles) determine the expression (the way you look) of the genetic trait. We represent dominant alleles with capital letters. 5. Recessive genes (alleles) These alleles are masked by the dominant alleles and are not expressed (do not show up). We represent recessive alleles with lower case letters. 6. Genotype Genotype refers to the genes (represented by letter combinations) of a trait for an organism. eg. BB or Bb 7. Phenotype Phenotype refers to the physical appearance of the traits in an organism. It tells us how the genotype actually looks like. i.e. Blue eyes, green eyes, curly hair. 8. Homozygous Homozygous refers to a genotype in which both genes of a pair are identical (i.e. RR, aa) 9. Heterozygous Heterozygous refers to a genotype in which the gene pairs are different (i.e. Rr, Aa) 10. Incomplete dominance When two alleles interact and are equally dominant, they produce a new phenotype which is a blending of the two traits. (i.e. a red flower makes a pink flower when crossed with a white flower.) 11. Codominance Both genes are expressed at the same time producing a mottled effect. (eg. Red and white flowers make flowers with red and white specks) 23

24 All of your body cells contain a full complement of 46 chromosomes. (23 pairs) These cells are referred to as somatic cells. The male sperm cell or the female ova (egg) are called gamete cells and are the only cells to contain only ½ of this number. A gene for a particular trait can be found on one of your chromosomes. The alleles come in pairs on your chromosomes (one from mom and one from dad). During the formation of gametes (sperm or egg) the alleles on a chromosome will segregate (separate). This ensures that each gamete cell has only one copy of the alleles for that trait. Because of this independent assortment, a male s sperm cell may contain the allele for blue eyes while another of his sperm may carry the allele for brown eyes. Both gametes carry the gene for eye colour and both gametes have only one allele (form) of the gene. When fertilization is complete, the first somatic cell is created. This cell is called a zygote. The zygote has a full 46 chromosomes in 23 pairs. Monohybrid Crosses Go and observe the Chart called GENETICS II at the front of the class. The Law of Segregation is crucial to understanding how organisms can inherit alleles. The Law of Segregation states that This law ensures that alleles received from the haploid sperm and egg to the offspring occur at random, but they form the first diploid cell in the offspring. Genetic crosses that involve the study of only one trait, such as seed colour, are called one-factor crosses because we are working out the possible genetics for only one factor. Use the letter from the chart below for the expression of the dominant and recessive alleles for the genes controlling these traits. TRAIT DOMINANT ALLELE RECESSIVE ALLELE Pod shape Smooth (N) Constricted (n) Pod Colour Green (G) Yellow (g) Flower Position Axial (A) Terminal (a) Plant height Tall (T) Short (t) 24

25 For each of the following problems, use the Punnett square and the chart on page 29 to answer the following Example: A plant that is heterozygous for green pods is crossed with a plant that has yellow pods. What are the probably genotypic and phenotypic ratios in the offspring? Answer: draw a Punnett square and use the letter combinations in the above chart: G g g Gg gg g Gg gg With the Punnett square complete, you can now determine the phenotypes of the offspring: Since green (G) is dominant over yellow (g), plants that have G in their genotypes have green pods. Only plants with genotype gg have yellow pods. In this example, 1 out of 2 of the offspring have green pods and 1 out of 2 have yellow pods. In this example, the genotypic ratio is 2 Gg: 2 gg or simply reduced to 1:1. The phenotypic ratio is 2 green: 2 yellow or simply again 1:1. Always reduce the ratios to their simplest form. For each of the following problems draw a Punnett square and check your work up at the front desk: 1. Nn x NN Genotypic ratio: Phenotypic ratio: 2. Aa x aa Genotypic ratio: Phenotypic ratio: 25

26 3. Tt x Tt Genotypic ratio: Phenotypic ratio: 4. Show a cross for two plants that are heterozygous for green pods. Genotypic ratio: Phenotypic ratio: 5. Cross a plant that is heterozygous for axial flowers with a plant that has terminal flowers. Genotypic ratio: Phenotypic ratio: 6. Cross a homozygous tall plant with a short plant. Genotypic ratio: Phenotypic ratio: 26

27 7. Cross a plant that is heterozygous for smooth pods with a plant that has constricted pods. Genotypic ratio: Phenotypic ratio: 8. When a tall plant is crossed with a short plant, some of the offspring are short. What are the genotypes of the parents and the F1 generation? What is the phenotypic ratio of the F1? 9. ¾ of the plants produced by a cross between two unknown pea plants have axial flowers and ¼ have terminal flowers. What are the parent plant genotypes? 10. What cross would result in ½ of the offspring having green pods and ½ of the offspring having yellow pods? 27

28 Dihybrid Crosses Crosses that involve two traits, such as pod colour and pod shape, are called dihybrid crosses or sometimes (two-factor crosses). Predicting the outcome of these crosses requires basically the same procedure as that for single factor crosses. Keep in mind that in dihybrid crosses, the genes controlling the two different traits are located on nonhomologous chromosomes. During meiosis, nonhomologous chromosomes assort independently. This means that each of the chromosomes of any pair of homologous chromosomes has an equal probability of ending up in a gamete with either chromosome from any other pair of homologous chromosomes. The genes that are located on nonhomologous chromosomes also assort independently as you can see below: GgNn x GgNn Because of independent assortment, a plant that is heterozygous for two traits (genotype AaBb) will produce equal numbers of four types of gametes AB, Ab,aB, and ab. In the example that follows, we will predict the results of a cross between two plants that are heterozygous for both pod colour and pod shape. Example: What are the genotypic and phenotypic ratios in the F1 resulting from a cross between two pea plants that are heterozygous for pod colour and pod shape? What is the phenotype of the parents in this cross? Answer: choose letters to represent the alleles in the cross and write the letters of the parent genotypes. GgNn x GgNn Determine the possible gametes that the parents can produce. Independently assorted, there would be four possible types GN, Gn, gn, and gn. Enter the possible gametes at the top and side of a Punnett square and complete the F1 generation. 28

29 Because of independent assortment, a plant that is heterozygous for two traits (genotype AaBb) will produce equal numbers of four types of gametes AB, Ab,aB, and ab. In the example that follows, we will predict the results of a cross between two plants that are heterozygous for both pod colour and pod shape. Example: What are the genotypic and phenotypic ratios in the F1 resulting from a cross between two pea plants that are heterozygous for pod colour and pod shape? What is the phenotype of the parents in this cross? Answer: Choose letters to represent the alleles in the cross and write the letters of the parent genotypes. GgNn x GgNn Determine the possible gametes that the parents can produce. Independently assorted, there would be four possible types GN, Gn, gn, and gn. Enter the possible gametes at the top and side of a Punnett square and complete the F1 generation. GgNn x GgNn GN Gn gn gn GN GGNN GGNn GgNN GgNn Gn GGNn GGnn GgNn Ggnn gn GgNN GgNn ggnn ggnn Gn GgNn Ggnn ggnn ggnn The phenotypic ratio is 9:3:3:1 There are 9 green smooth to 3 green constricted to 3 yellow smooth to 1 yellow constricted. 29

30 In mice, the ability to run normally is a dominant trait. Mice with this trait are called running mice (R). The recessive trait causes mice to run in circles only. Mice with this trait are called waltzing mice (r). Hair colour is also inherited in mice. Black hair (B) is dominant over brown hair (b). For each of the following problems, draw a Punnett square to answer the following: 1. Cross a heterozygous running, heterozygous black mouse with a homozygous running, homozygous black mouse. Identify the phenotypic ratio. 2. Cross a homozygous running, homozygous black mouse with a heterozygous running, brown mouse. Identify the phenotypic ratio. 30

31 3. Cross a waltzing brown mouse with a waltzing brown mouse. Identify the phenotypic ratio. 4. Cross a homozygous running, heterozygous black mouse with a waltzing brown mouse. Identify the phenotypic ratio. 31

32 5. Cross a heterozygous running, brown mouse with a heterozygous running, homozygous black mouse. Identify the phenotypic ratio. 32

33 The Evolution of Populations Genetic variation is studied in populations. It is important to note that individuals do not evolve populations evolve. A population is a group of individuals of the same species that interbreed to produce fertile offspring under natural conditions. Because members of a population interbreed, they share a common group of all genes called a gene pool. A gene pool consists of all the genes, including all the different alleles that are present in a population. The relative frequency of an allele is the number of times that the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur. Gene pools are important to evolutionary theory because evolution involves changes in population over time. In genetic terms: ** Evolution is any change in the relative frequency of an allele in a population. ** Changes in the allele frequency of a population can occur in numerous ways. All of the changes are still subject to sorting by natural selection. If the change in allele frequency aids in the survival of the population in their environment, we would expect to see an increase in the relative frequency of that favorable allele. The biological fitness of the species would increase. Fitness refers to the ability to reproduce and produce offspring. The opposite would be true if the allele change hinders the survival of a population in an environment. The fitness of the population would decrease. As environments change, populations constantly evolve, and those with the best-suited adaptations are more likely to reproduce and leave offspring who have inherited these characteristics. 33

34 Five Different Ways To Cause Evolutionary Change In A Population The Agents of Evolutionary Change 1. Natural Selection When biologists study how a population of organisms have evolved, they choose a particular trait, characteristic, or adaptation found in that population and make comparisons of that trait to earlier times. It is very useful to plot this information on a graph. When studying population characteristics, a Normal Distribution Graph can show the relative numbers of individuals that currently have a particular characteristic or trait. The Normal Distribution Graph This graph shows the various heights for a population of people. 5 8 is the average height for this population and coincidently has the greatest number of individuals. We would expect less numbers of really short and really tall individuals. This is also called a typical bell-curve graph. Normal Distribution graphs can show how the distribution of genes and therefore allele frequencies are in a given population. By graphing gene frequencies as they occur over time in a population being studied; and comparing these to previous graphs, a biologist can determine if evolution is occurring. In other words, biologists can tell if there is a change in the relative frequency of an allele in a population over a given time period. 34

35 Three Kinds of Natural Selection Directional Selection Stabilizing Selection Disruptive Selection All three forms of Natural Selection are compared to the Normal Distribution Graph of an earlier time. These comparisons indicate that evolution in the population has occurred and can even give convincing evidence as to the kind of selection that has occurred in the environment. 35

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37 2. Genetic Drift Allele frequencies in a population can change by chance, especially in small populations. Genetic drift is evolution by chance. When a small sample of individuals separate from the original population, they take their genes and allele frequencies with them. They might begin their own population in a new location. These new populations are very likely to have a different allele frequency than the original population. This change in allele frequency is still considered to be evolutionary change but the population is still subject to the effects of natural selection. This is commonly referred to as the founder effect. Sample of Original Population Descendants Founding Population A Founding Population B 37

38 3. Gene Flow This is the transfer of alleles or genes as organisms randomly move in or out of a population. Due to either immigration or emigration, gene frequencies in a population can change. Natural selection still determines whether this new gene frequency is beneficial or not in the environment. This is similar to genetic drift in that as individuals come and go, so do their genes, however, genetic drift is more about establishing new populations rather than just the adding or subtracting of alleles in a population. 4. Mutations New alleles can arise by mutation, a change in the nucleotide sequence of an organism s DNA. The mutation in the DNA is unpredictable and random but the result on the population is far from random. Natural selection must still act on the mutation to determine if the change is beneficial or detrimental to survival. In most multicellular organisms, the mutation must occur within the gametes (sex cells) in order to be inherited. This is not always the case in plants and fungi since many different cell lines within the organism can produce gametes. A change of as little as a single base can have a significant impact on the physical appearance of an organism. Mutations are usually rare events; between 1 gamete in 100,000 and 1 gamete in 1 million will include a mutation. New mutations are so infrequent, in fact, that they exert little or no immediate effect on allele frequencies in most populations. But over evolutionary time scales, their numbers are significant mutations have been accumulating in biological lineages for billions of years. Because it is a mechanism through which entirely new genetic variations arise, mutation is a major source of heritable variation. 38

39 5. Non-Random Mating Nonrandom mating can change genotype or allele frequencies because the individuals in a population do not choose mates at random. Any time individuals mate preferentially with other individuals of the same genotype, homozygous genotypes will increase in frequency and heterozygous genotypes will decrease in frequency. A good example of nonrandom mating is sexual selection. Sexual selection occurs when individuals of one sex mate preferentially with particular individuals of the opposite sex rather than at random. This can produce allele frequency changes in a population over time. The desired traits appear more often in the population. The remarkable tail of the male African long-tailed widowbird actually inhibits its ability to fly. Wouldn t natural selection remove a trait such as this? Darwin first suggested that these conspicuous traits that would appear to inhibit survival, such as bright colors or elaborate courtship displays may actually increase an organism s chance for passing on its genes. Darwin argued that while natural selection typically favors traits that enhance the survival of their bearers, sexual selection is primarily about successful reproduction. An animal that survives but fails to reproduce makes no contribution to the next generation. Thus sexual selection may favor traits that enhance an individual s chances of reproduction even when these traits reduce its chances of survival. This sexual selection is a form of non-random mating. 39

40 Speciation Speciation is the formation of new species as a result of geographical, physiological, anatomical, or behavioral factors that prevent previously interbreeding populations from breeding with each other. The concept of species has over 22 different definitions in the biological realm. We generally consider a species to be: A group of interbreeding individuals that produce fertile offspring under natural conditions. However useful this definition is, remember that not all species reproduce sexually. Many distinct species today are now identified as a hybrid of several species when their DNA is tested and compared with others. Even the modern human today has been shown to contain up to 4% Neanderthal and Denisovan DNA. These two Hominid branches are extinct but partially remain in our DNA today. Reproductive Isolation Very often, there are naturally occurring obstacles to mating or fertilization if mating occurs. These obstacles help to make new species. These obstacles include: 1. Allopatric Isolation Geographical Isolation occurs when two populations (and their genes) are separated by physical barriers such rivers and canyons. 2. Sympatric Isolation This occurs when the organisms still live in the same area together but they are isolated by a variety of other means that include: Temporal isolation is a barrier that works because of how and when species mate during different times of the year or day. Two populations of plants may produce flowers in different seasons, making mating between the populations impossible. 40

41 Ecological Isolation occurs when two species occupy the same general area, but rarely come in contact with each other, even though they may not be physically barricaded. Flies who live in single habitat may exist in different areas of the habitat, such as soil as apposed to water will not mate. Behavioral Isolation works as an effective barrier because of distinct mating habits each species has. Several species of crickets are morphologically identical, but can be distinguished by the fact that females will only respond to the mating songs of males of their own species and males of other species are ignored. Mechanical Isolation also works because you can't fit a square peg in a round hole. Bush babies are divided into several species. Each species has distinctly shaped genitalia that, like locks and keys, only fit with the genitalia of its own species. Pre-Zygotic Barriers Post-Zygotic Barriers Gametic Isolation may happen when sperm from one species is biologically unable to fertilize an egg from another species. Gametic isolation is important in aquatic environments because many aquatic animals release their gametes into the water, where fertilization takes place. Chromosomes changes can cause problems 41

42 The process of Speciation occurs over long periods of time. It has been directly observed and studied by the now famous study of the Galapagos finches by Peter and Rosemary Grant. There are many other incidences of documented observable change. This is an example of Adaptive Radiation that is sometimes called Divergent Evolution Convergent Evolution occurs when species evolve to share similar characteristics,structures, and even functions because they share a similar environment. By sharing a similar environment, they share similar selective pressures to evolve and adapt in a certain way that better suits that environment. 42

43 Another classic example of Convergent Evolution can be shown in these organisms of different origin. These four ant-eaters are not the same species. Their environments are similar on their respective continent resulting in features that have converged to suit the pressures put on them by their environment. This is typically how Convergent evolution can cause a change in the appearance of two different species. Natural selection favors any mutations or changes over time that improve an organism s fitness. A D B E C F 43

44 CoEvolution This is a phenomena seen in nature when two very different species evolve together for their mutual benefit. The moth has evolved mouth structures to be one of the only insects capable of obtaining nectar from this particular species of orchid. The orchid benefits by having a dedicated moth to pollinate other orchid plants of this species. What is the difference between Punctuated Equilibrium and Gradualism? 44

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