4So why the controversy?Aristotle viewed species as fixed. The scala naturae…The Old Testament: species were individually designed by God…The earth was thought to be “young” a few thousand years old…Scientific evidence did not support this!Fossil evidence, geologic evidence of the earth’s age, Lamarck attempted to explain the changes incorrectly with use and disuse…
5Current evolution theory Explains the adaptations of organisms and the unity of life as well as the diversity of life!Just a theory? video
6Evolving Ideas: Why Does Evolution Matter Now? Evolution continues to have an impact on our lives -- the food we eat, our environment, and our health. Through the story of a multi-drug resistant strain of tuberculosis in the prisons of Russia, we see evolution in action today. This video illustrates how evolution is important to our understanding of disease prevention and treatment in Russia and around the WGBH Educational Foundation and Clear Blue Sky Productions, Inc. All rights reserved.
8Enduring understanding 1.A: Change in the genetic makeup of a population over time is evolution.Essential knowledge 1.A.1: Natural selection is a major mechanism of evolution.
93. EvolutionCHANGE!Biologists use it to mean cumulative changes in organisms through time.Like the bacteria that as a population becomes resistant to the antibiotics….
10artificial selectionThe process of directed breeding to produce offspring with desired traits.
11Natural selection.Nature selects the traits best suited for survival based on the following criteria:1. organisms in a population are naturally different from each other.2. the differences are inherited from their parents and can be passed on to their offspring.3. Populations tend to overproduce, there is not enough food or resources for the number produced leading to competition4. Some of the variations provide an advantage to the ability to survive and reproduce more offspring compared to the others with out that variation.
12Natural selection is a way in which evolution can take place. It is an attempt to explain how the changes Darwin observed might have happened.Wallace and Darwin both wrote essays with almost the exact same conclusion.
13The Theory of Evolution by Natural Selection ● Based on years of observations during a trip Darwin wrote a book On the Origin of Species.
20Evidence of Evolution● Evidence of orderly change can be seen when fossils are arranged according to their age.● ● Similarities of structures in different vertebrates provide evidence that all vertebrates share a common ancestor.
21In some cases widely divergent organisms possess a common structure, adapted to their individual needs over countless generations yet reflective of a shared ancestor. A fascinating example of this is the pentadactyl limb, a five-digit appendage common to mammals and found, in modified form, among birds. The cat's paw, the dolphin's flipper, the bat's wing, and the human hand are all versions of the same original, an indication of a common four-footed ancestor that likewise had limbs with five digits at the end.
224. Homologous structureAnatomically similar structures inherited from a common ancestor. They may not perform the same function but have a common origin or structure internally..
245. Vestigial structureStructures that are reduced in size and form, but are similar in structure to functioning structures in other organisms.Examples:
25Though their ancestors ceased to walk on four legs many millions of years ago, snakes still possess vestigial hind limbs as well as reduced hip and thigh bones.
266. Analogous structure convergent evolution Superficially similar features.Such as wings of an insect and of a bird…no relationship in the structure exists!Just similar environments!
27camouflageThe adaptation that allows an organism to blend in with their environment.
288. MimicryOne species evolves to look like another.
299. FitnessA measure of the relative contribution that an individual trait makes to the next generation.This is usually the number of reproductively viable offspring an organism makes in the next generation.Better fit means better able to survive and reproduce
30The embryonic forms of animals also reflect common traits and shared evolutionary forebears. This is why most mammals look remarkably similar in early stages of development. In some cases animals in fetal form will manifest vestigial features reflective of what were once functional traits of their ancestors.
31Differences in amino acid sequences and DNA sequences are greater between species that are more distantly related than between species that are more closely related.
32Comparing DNA sequences is one way Biologists determine how closely related organisms are.
35The fossil recordFossils that help demonstrate the transition from life on land to life in the water…. Whales and the loss of the hind legs.Fossil evidence shows differences in organisms living today from those living in the past.
36Web quest Evidence for Evolution Assignment What Is the Evidence for Evolution?Evidence for Evolution WebQuestBackground info: Theodosius Dobzhansky, a geneticist whose work influenced 20th century research on evolutionary theory, said, "Nothing in biology makes sense, except in light of evolution." This quote emphasizes the role of evolution as the most important unifying principle in biology. Living things might, at first, seem very diverse, but closer inspection reveals a surprising unity. This unity, or common ancestry, can be explained by evolutionary theory. With such an important theory at stake, it is essential to understand the evidence upon which it is based.
37Your assignment:In this Evolution WebQuest you will investigate a variety of types of evidence for evolution. Your team will be responsible for learning about fossil evidence, structural evidence, and genetic evidence for evolution and presenting this information to the class. You will make a chart of the areas of interest (Anatomy, Molecular Biology and Paleontology), the evidence and their significance and present it to the class.
38Process: 1. You will be assigned a special area of interest. 2. Specialists include: anatomy and physiology, paleontology, and molecular biology. Anatomists study the structure of organisms, physiologists study the function of organisms, molecular biologists study genetics, and paleontologists study fossils.3. Review the sites that apply to your specialty.4. Find four to five examples of evidence for evolution. Try to find specific examples, so that when you present to the class you will all have different examples to share. If one has already been included try to find something else. Also, try to find the date on which the evidence was discovered.5. Use the recommended sites or find some of your own, but you must site your sources. Take hand written notes as you complete the assignment.6. Do the individual research at home and then add your information to the class power point on the students on chsms2 site/ classes /period 4.7. Present your section to the class. So that you learn about more than the one section that you researched, you will be asked to take notes on all sections during the presentation. I will provide a chart.8. Turn in hand written notes on your section along with the sites used to obtain the information.
39Special Areas of Interest Evidence(descriptions or drawings)SignificanceAnatomyMolecular BiologyPaleontology
40AnatomistsSources:• Evidence Supporting Biological Evolution (see "Common Structures") • How Scientists Study Evolution (search for "Evolution," click on "How Scientists Study Evolution," and look for "Anatomical Similarities") • What Is Morphology and Why Is It Important? • Fossils Can Show How Certain Features Evolved • It's a Bird, It's a Dinosaur? • Dunking the Trunk (search for "Dunking the Trunk")
41Molecular Biologists• Evolution Makes Sense of Homologies • Axing the Family Tree • Evidence of Evolution • Chemical Clues to Darwin's Abominable Mystery • Salvaged DNA Leads to Neanderthals' Mystique
42Paleontologists• Fossils Can Show How Certain Features Evolved • Transitional Vertebrate Fossils FAQ • Mother of All Mammals (search for "Andre Wyss" and "Andrew Knoll") • Shaking the Family Tree Evolution Makes Sense of Homologies • The Nature of Fossils • Dating Fossils • Getting into the Fossil Record
43Essential knowledge 1.A.1: Natural selection is a major mechanism of evolution. You must know these!Natural selection basics video
44a. According to Darwin’s theory of natural selection, competition for limited resources results in differential survival. Individuals with more favorable phenotypes are more likely to survive and produce more offspring, thus passing traits to subsequent generations.b. Evolutionary fitness is measured by reproductive success.c. Genetic variation and mutation play roles in natural selection. A diverse gene pool is important for the survival of a species in a changing environment.d. Environments can be more or less stable or fluctuating, and this affects evolutionary rate and direction; different genetic variations can be selected in each generation.e. An adaptation is a genetic variation that is favored by selection and is manifested as a trait that provides an advantage to an organism in a particular environment.Re write each of these in your own words!
45Examples of Evolution● Individuals that have traits that enable them to survive in a given environment can reproduce and pass those traits to their offspring.●The moth example is directional selection
47Industrial Melanism and the Pepper Moth Both natural selection and mutation play a role in industrial melanism, a phenomenon whereby the processes of evolution can be witnessed within the scale of a human lifetime.Industrial melanism is the high level of occurrence of dark, or melanic, individuals from a particular species (usually insects) within a geographic region noted for its high levels of dark-colored industrial pollution.
48With so much pollution in the air, trees tend to be darkened, and thus a dark moth stands a much greater chance of surviving, because predators will be less able to see it.At the same time, there is a mutation that produces dark-colored moths, and in this particular situation, these melanic varieties are selected naturally.On the other hand, in a relatively unpolluted region, the lighter-colored individuals of the same species tend to have the advantage, and therefore natural selection does not favor the mutation.
49The best-known example of industrial melanism occurred in a species known as the pepper moth, or Biston betularia, which usually lives on trees covered with lichen.Prior to the beginnings of the Industrial Revolution in England during the late eighteenth century, the proportion of light-colored pepper moths was much higher than that of dark-colored ones, both of which were members of the same species differentiated only by appearance
51As the Industrial Revolution got into full swing during the 1800s, factory smokestacks put so much soot into the air in some parts of England that it killed the lichen on the trees, and by the 1950s, most pepper moths were dark-colored.It was at that point that Bernard Kettlewell ( ), a British geneticist and entomologist (a scientist who studies insects), formed the hypothesis that the pepper moths' coloration protected them from predators, namely birds.
52Kettlewell therefore reasoned that, before pollution appeared in mass quantities, light-colored moths had been the ones best equipped to protect themselves because they were camouflaged against the lichen on the trees.After the beginnings of the Industrial Revolution, however, the presence of soot on the trees meant that light-colored moths would stand out, and therefore it was best for a moth to be dark in color.This in turn meant that natural selection had favored the dark moths
53In making his hypothesis, Kettlewell predicted that he would find more dark moths than light moths in polluted areas, and more light than dark ones in places that were unpolluted by factory soot.As it turned out, dark moths outnumbered light moths two-to-one in industrialized areas, while the ratios were reversed in unpolluted regions, confirming his predictions.
55To further test his hypothesis, Kettlewell set up hidden cameras pointed at trees in both polluted and unpolluted areas.The resulting films showed birds preying on light moths in the polluted region, and dark moths in the unpolluted one—again, fitting Kettlewell's predictions
59Breeding Bunnies LabIntro: In this activity, you will examine natural selection in a small population of wild rabbits. Evolution, on a genetic level, is a change in the frequency of alleles in a population over a period of time. Breeders of rabbits have long been familiar with a variety of genetic traits that affect the survivability of rabbits in the wild, as well as in breeding populations.
60One such trait is the trait for furless rabbits (naked bunnies) One such trait is the trait for furless rabbits (naked bunnies). This trait was first discovered in England by W.E. Castle in The furless rabbit is rarely found in the wild because the cold English winters are a definite selective force against it.
61Note: In this lab, the dominant allele for normal fur is represented by F and the recessive allele for no fur is represented by f. Bunnies that inherit two F alleles or one F and one f allele have fur, while bunnies that inherit two fs have no fur.
62Procedures:1. Use the gene frequency data and discussion questions and write a hypothesis and specific predictions based on your hypothesis.2. You will be working in assigned groups. Everyone should take turns doing each part of the lab. Gather all materials Breeding Bunnies: Gene Frequency Data , Breeding Bunnies: Discussion Questions, 50 red beans, 50 white beans, 1 paper bag or deep bowl, 3 dishes or containers, labels and pens.3. The red beans represent the allele for fur, and the white beans represent the allele for no fur. The container represents the English countryside, where the rabbits randomly mate.
63Breeding Bunnies: Gene Frequency Data How does natural selection affect gene frequency over several generations?1. Clearly state your hypothesis (a tentative explanation or solution to the problem).__________________________________________________________________________________________State what you would predict (if your hypothesis is true) about the frequency of F alleles and f alleles in the population of rabbits after 10 generations, where ff bunnies are selected against (do not survive).
644. Label one dish FF for the homozygous dominant genotype 4. Label one dish FF for the homozygous dominant genotype. Label a second dish Ff for the heterozygous condition. Label the third dish ff for those rabbits with the homozygous recessive genotype.5. Place the 50 red and 50 white beans (alleles) in the container and shake up (mate) the rabbits. (Please note that these frequencies have been chosen arbitrarily for this activity.)6. Without looking at the beans, select two at a time, and record the results on the data form next to "Generation 1." For instance, if you draw one red and one white bean, place a mark in the chart under "Number of Ff individuals." Continue drawing pairs of beans and recording the results in your chart until all beans have been selected and sorted. Place the "rabbits" into the appropriate dish: FF, Ff, or ff. (Please note that the total number of individuals will be half the total number of beans because each rabbit requires two alleles.)
657. The ff bunnies are born furless 7. The ff bunnies are born furless. The cold weather kills them before they reach reproductive age, so they can't pass on their genes. Place the beans from the ff container aside before beginning the next round.8. Count the F and f alleles (beans) that were placed in each of the "furred rabbit" dishes in the first round and record the number in the chart in the columns labeled "Number of F Alleles" and "Number of f Alleles." (This time you are really counting each bean, but don't count the alleles of the ff bunnies because they are dead.) Total the number of F alleles and f alleles for the first generation and record this number in the column labeled "Total Number of Alleles."9. Place the alleles of the surviving rabbits (which have grown, survived and reached reproductive age) back into the container and mate them again to get the next generation.
6610. Repeat steps five through nine to obtain generations two through ten. If working as a team, make sure everyone in your group has a chance to either select the beans or record the results.11. Determine the gene frequency of F and f for each generation and record them in the chart in the columns labeled "Gene Frequency F" and "Gene Frequency f." To find the gene frequency of F, divide the number of F by the total, and to find the gene frequency of f, divide the number of f by the total. Express results in decimal form. The sum of the frequency of F and f should equal one for each generation.12. Record your group's frequencies on the board so your classmates can see them.13. Graph your frequencies. Prepare a graph with the horizontal axis as the generation and the vertical axis as the frequency in decimals. Plot all frequencies on one graph. First, plot your own data. Use a solid line for F and a dashed line for f. Then plot the class totals. Use the same symbols for each group but a different color.14. Complete the Discussion Questions form with your group.
671 2 3 4 5 6 7 8 9 10 Generation Number of FF Individuals Ff Allelesof fTotalof AllelesGeneFrequency12345678910
682. State your original hypothesis 2. State your original hypothesis. Based on your lab data, do you accept or reject the original hypothesis? Explain.3. Compare the number of alleles for the dominant characteristic with the number of alleles for the recessive characteristic.4. Compare the frequencies of the dominant allele to the frequencies of the recessive allele.5. In a real rabbit habitat new animals often come into the habitat (immigrate), and others leave the area (emigrate). How might emigration and immigration affect the gene frequency of F and f in this population of rabbits? How might you simulate this effect if you were to repeat this activity?6. How do your results compare with the class data? If significantly different, why are they different?7. How are the results of this simulation an example of evolution?
69Essential knowledge 1.A.2: Natural selection acts on phenotypic variations in populations.a. Environments change and act as selective mechanism on populations.b. Phenotypic variations are not directed by the environment but occur through random changes in the DNA and through new gene combinations.c. Some phenotypic variations significantly increase or decrease fitness of the organism and the population. ( the naked bunny lab!)d. Humans impact variation in other speciesUse the peppered moth example to explain each of these statements!
70All of these can be used as examples, each of you will be assigned one to research and explain Flowering time in relation to global climate changeSickle cell anemiaDDT resistance in insectsArtificial selectionLoss of genetic diversity within a crop speciesOveruse of antibiotics ( the video at the beginning of this unit explains this concept)
71Learning Objectives:LO 1.4 The student is able to evaluate data-based evidence that describes evolutionary changes in the genetic makeup of a population over time. [See SP 5.3]LO 1.5 The student is able to connect evolutionary changes in a population over time to a change in the environment. [See SP 7.1]
72The Evolution of Populations Ch 23 Genetic variation : differenced in individuals as a result of differences in their genes or DNA segments. This excludes any differences that occur due to the environment that does not change the DNA!Like working out! Or dying your hair a different color! NO DNA change has been made!Without variation evolution is impossible!
73Average Heterozygosity How much variation is in a population?Gene variability is quantified as the average heterozygosity or percent of loci that are heterozygous. This means there are two options! Usually, one dominant and one recessive. A high percent means a good opportunity for natural selection to operate!Gel electrophoresis can be used to look at the proteins produced or DNA sequences can be compared to determine differences.
74Variation between populations Geographic differences due to mountain ranges or oceans that cause isolation of populations. Chance events seem to cause these differences rather than natural selection.Clines: graded changes in temperature or light can cause changes due to natural selection
75Where do the variations come from? gene duplicationSexual reproduction is the source of new gene combinationsMutation in gametes (are random)Chromosomal rearrangements ( if not lethal)Gene duplication, transposons,.Rapid reproduction rate= more mutations, like prokaryotes and viruses
76Essential knowledge 1.A.1: Natural selection is a major mechanism of evolution.f. In addition to natural selection, chance and random events can influence the evolutionary process, especially for small populations.g. Conditions for a population or an allele to be in Hardy-Weinberg equilibrium are: (1) a large population size, (2) absence of migration, (3) no net mutations, (4) random mating and (5) absence of selection. These conditions are seldom met.h. Mathematical approaches are used to calculate changes in allele frequency, providing evidence for the occurrence of evolution in a population.
77Questions? 1. Why are small populations more affected than large ones? 2. What is the Hardy Weinberg equilibrium?3. When are populations in Hardy Weinberg?4. How can math show evolution has occurred?
78Learning Objectives:LO 1.1 The student is able to convert a data set from a table of numbers that reflect a change in the genetic makeup of a population over time and to apply mathematical methods and conceptual understandings to investigate the cause(s) and effect(s) of this change. [See SP 1.5, 2.2]Can you look at gene frequencies and see a change? Can you determine the cause? How the change will alter the population?LO 1.2 The student is able to evaluate evidence provided by data to qualitatively and quantitatively investigate the role of natural selection in evolution. [See SP 2.2, 5.3] Can you use data to explain how small changes over long periods of time lead to changes in populations and even new species forming?
79LO 1.3 The student is able to apply mathematical methods to data from a real or simulated population to predict what will happen to the population in the future. [See SP 2.2]Can you use the Hardy Weinberg equation to see if a population is in equilibrium or is in the process of having the allele frequencies change due to forces like selection…genetic drift, sexual selection, immigration or emigration.
80Essential knowledge 1.A.3: Evolutionary change is also driven by random processes.a. Genetic drift is a nonselective process occurring in small populations.b. Reduction of genetic variation within a given population can increase the differences between populations of the same species.
81Learning Objectives:LO 1.6 The student is able to use data from mathematical models based on the Hardy-Weinberg equilibrium to analyze genetic drift and effects of selection in the evolution of specific populations. [See SP 1.4, 2.1] are allele frequencies changing?LO 1.7 The student is able to justify data from mathematical models based on the Hardy-Weinberg equilibrium to analyze genetic drift and the effects of selection in the evolution of specific populations. [See SP 2.1]LO 1.8 The student is able to make predictions about the effects of genetic drift, migration and artificial selection on the genetic makeup of a population. [See SP 6.4] if p and q are = in HW, then how will these things alter them?
83SC.912.L.15.14ch Discuss mechanisms of evolutionary change other than natural selection such as genetic drift and gene flow.
84There are several ways that evolution can occur besides natural selection. Now that scientists understand genes they can explain these much better.Population genetics is one.What is a population? A group of individuals of the same species that live in the same area and interbreed, producing fertile offspring.
8510. Hardy Weinberg principle 1. Hardy Weinberg states that unless there are outside forces a population will stay in what?Genetic equilibrium
86P2 + 2pq + q2 = 1 p is the dominant allele q is the recessive allele, P2 = homozygous dominantq2= homozygous recessiveand 2pq is the heterozygous genotype.
87Gene pool all of the copies of every type of allele at every locus in all members of the population
88Hardy Weinberg requirements no mutations- the gene pool is altered if mutations alter alleles or if genes are deleted or duplicatedMating must be random- if individuals mate preferentially, like with in a group ( inbreeding) random mixing of gametes will not occur!No natural selection- differences in the survival and reproductive success of individuals can alter gene frequencies.No gene flow when genes are moved in or out of a population , it can alter allele frequencies.Extremely Large Population Size The smaller the population, the more likely it is that the allele frequencies will fluctuate by chance from one generation to the next. ( genetic drift) so NO genetic drift!
8911. Genetic driftAny change in the allelic frequencies in a population that results from chance.Types of genetic driftFounder effect: an extreme example of genetic drift when a small group moves away from a population. The group carries a small sample of the larger groups alleles, if variations are present, they may start to show up more in the small sub set.Bottleneck: when a population declines to a very small number and then rebounds the genes in the small group are what are carried on.
90Genetic driftIt will randomly alter gene pools of small populations more than large ones!!!!
91Think Amish ! Extra fingers or toes! Polydactyly -- extra fingers or sometimes toes -- is one symptom of Ellis-van Creveld syndrome. The syndrome is commonly found among the Old Order Amish of Pennsylvania, a population that experiences the "founder effect." Genetically inherited diseases like Ellis-van Creveld are more concentrated among the Amish because they marry within their own community, which prevents new genetic variation from entering the population. Children are therefore more likely to inherit two copies of the particular recessive genes that lead to genetic disease.
92Effects of genetic drift 1. it is significant in small populations2. it can cause allele frequencies to change at random3. it can lead to loss of genetic variation within a population4. it can cause harmful alleles to become fixed.
93Gene flowOnly isolated populations have no gene flow! This is very rare.
94Non random MatingMost organism mate with others near them. This can lead to inbreeding and can change the allelic frequencies.
95mutationsMost mutations are random and will cause changes in the allelic frequencies so this will keep a population out of genetic equilibrium!
96Natural selection is one mechanism of evolution. How it happens…
97Natural selectionMost organisms are not equally adapted to their environment, so natural selection will occur.There are different types of Natural selection.Stabilizing selection is the most common and removes the extremes.Directional selection selects one phenotype over another.
98Disruptive selection selects the extreme. Sexual selection is based on the ability to attract a mate.
99Types of selection other than Natural selection●
100adaptive radiationOne common ancestor gives rise to many species based on the environments. A good example would be the birds of the Galapagos Islands
101allopatric speciation In allopatric speciation, an ancestral population is geographically isolated, resulting in the evolution of separate species largely due to genetic drift. The additive effect of differences due to genetic drift can eventually result in behavioral isolation (refusal to mate) if the two groups were to meet again in the future. If they refused to interbreed, they would be considered separate species.
102Allopatric speciation A physical barrier divides one population into two or more populations
104when the Colorado River cut open the Grand Canyon, separating groups of squirrels who lived in the high-altitude pine forest. Eventually, populations ceased to interbreed, and today the Kaibab squirrel of the northern rim and the Abert squirrel of the south are separate species.
105sympatric speciationSympatric speciation involves speciation without a geographic barrier. One example of sympatric speciation is polyploidy, found more often in plants. Polyploidy occurs when a failure of meiosis increases the number of chromosome sets to 3N or more. At this point, the two "cousin" species can no longer mate with each other.
106directional selection, When one extreme phenotype is favored by natural selection, the distribution of the phenotype shifts in that direction.
108disruptive selectionSelection that favors the extremes of a population trait is DISRUPTIVE SELECTION. An example of this would be the size of clams. Predators find it harder to locate smaller clams and are unable to pry open larger clams. Predators favor clams of a medium size.
109genetic driftGenetic drift - random events due to small population size. Random events have little effect on large populations. Consider a population of 1 million almond trees with a frequency of r at 10%. If a severe ice storm wiped out half, leaving 500,000, it is very likely that the r allele would still be present in the population. However, suppose the initial population size of almond trees were 10 (with the same frequency of r at 10%). It is likely that the same ice storm could wipe the r allele out of the small population.
110founder effect and bottle neck Reduction in genetic variability makes these species vulnerable to disease and environmental changes!
111gradualism Gradualism is the slow change from one form to another. The opposite of gradualism is:Punctuated equilibrium which implies long periods without appreciable change and short periods of rapid evolution. Volcanic eruptions and meteor impacts affecting evolution on Earth could cause these changes.
113sexual selectionSexual selection may produce elaborate characteristics, for example the feathers of a male peacock or the ornamental plumage of male birds of paradise. Other organisms such as elephant seals fight over territories.
114stabilizing selection Stabilizing selection occurs when the intermediate, or most common, phenotype is favored. This type of selection tends to narrow the variation in the phenotype over time. This is the most common type of selection because it is associated with the adaptation of an organism to the environment.
115Speciation begins as a population adapts to its environment. ● Reproductive isolation keeps newly forming species from breeding with one another.
116Speciation:the formation of a new species by splitting of an existing species.
118Write the term or phrase that best completes each statement Write the term or phrase that best completes each statement. Use these choices: adaptive radiation, allopatric speciation, directional selection, disruptive selection, founder effect, genetic drift, gradualism, sexual selection, stabilizing selection, sympatric speciation________________________ is a change in allelic frequencies in a population that is due to chance.________________________ removed individuals with average trait values, creating two populations with extreme traits.The most common form of selection, ___________________________, removes organisms with extreme expressions of a trait.When a small sample of the main population settles in a location separated from the main populations, the ___________________________ can occur.
119In _________________________, a species evolves into a new species without any barriers that separate the populations.__________________________ will shift populations toward a beneficial but extreme trait value.In __________________________, a population is divided by a barrier, each population evolves separately, and eventually the two populations cannot successful interbreed.__________________________ is a change in the size or frequency of a trait based on competition for mates.One species will sometimes diversify in a relatively short time into a number of different species in a pattern called ________________________________.The idea that evolution occurred in small steps over millions of years in a speciation model is currently known as ________________________________.
120Write the term or phrase that best completes each statement Write the term or phrase that best completes each statement. Use these choices: adaptive radiation, allopatric speciation, directional selection, disruptive selection, founder effect, genetic drift, gradualism, sexual selection, stabilizing selection, sympatric speciationgenetic drift is a change in allelic frequencies in a population that is due to chance.disruptive selection removed individuals with average trait values, creating two populations with extreme traits.The most common form of selection, stabilizing selection, removes organisms with extreme expressions of a trait.When a small sample of the main population settles in a location separated from the main populations, the founder effect can occur.In sympatric speciation , a species evolves into a new species without any barriers that separate the populations.directional selection will shift populations toward a beneficial but extreme trait value.In allopatric speciation , a population is divided by a barrier, each population evolves separately, and eventually the two populations cannot successful interbreed.sexual selection is a change in the size or frequency of a trait based on competition for mates.One species will sometimes diversify in a relatively short time into a number of different species in a pattern called adaptive radiation.The idea that evolution occurred in small steps over millions of years in a speciation model is currently known as gradualism .