3rd Quarter>
LS3.A Inheritance of Traits
Chromosomes consist of a single long DNA molecule
1. Students should be able to give a basic description of the way in which DNA is packaged in eukaryotic chromosomes, the role of histone proteins, and the formation of chromatin. They should understand that it is the histones that organize the DNA into the 'beads on a string' configuration and indeed provide the energy for this process. The organization not only provides a means to pack the DNA into the nucleus, but (as students will see later) enables the transcription of genes to be tightly regulated. (HS-LS3-1)
2. Students should recognize the gene as the basic unit of heredity. While genes do code for specific proteins, students should appreciate (and will learn later) that genes may also code for functional RNA products involved in gene regulation. (HS-LS1-1, HS-LS3-1)
Genes expressed by the cell can be regulated in different ways
3. Students should recognize the two stages of gene expression: transcription into the mRNA and translation of the mRNA code into proteins by ribosomes. The assessment does not include the biochemistry of protein synthesis. (HS-LS3-1)
4. The one gene-one polypeptide hypothesis followed the original one gene-one enzyme hypothesis that was first proposed following experiments on Neurospora crassa. Although still applicable to prokaryotes, the hypothesis is deficient in explaining gene expression in eukaryotes. The basic tenets of the revised view of gene expression include:
• Not all exon RNA is translated into protein.
• The non-protein-coding exonic RNA may contribute to microRNAs or have a function on its own.
• Many introns are processed into regulatory microRNAs which are involved in regulating development. (HS-LS3-1)
5. Cells regulate gene expression to control when functional gene products are produced and in what quantities. Regulating gene expression enables the cell to produce specific gene products, in the right amounts, at the right time, and it is important in the differentiation of cells into specialized types with specific functions. Moreover, cells produce protein products in accordance with their own specialized role, e.g. B lymphocytes produce large amounts of immunoglobulins. Control of expression allows the cell to maintain itself and respond to a variable environment. Good illustrations of where the control of gene expression is important include:
• Barr bodies and X chromosome inactivation in female mammals to prevent double-doses of some genes.
• Expression of cyclin levels to regulate the cell cycle.
• Blood glucose regulation and control of insulin production (the right amounts at the right time).
• Control of development, as gene regulation is the basis for cellular differentiation and morphogenesis. e.g. formation of digits in mammals.
Students should appreciate that gene expression can be regulated at any of one or more steps in the gene expression pathway:
• Before transcription (condensation or decondensation of chromatin through methylation or acetylayion)
• After transcription but before translation, e.g. multiple mRNA versions (achieved through exon splicing and other mechanisms), prolonged or rapid mRNA activity (achieved through poy-A tails and 5' capping).
• Post translation: Changing the nature or longevity/ stability of the polypeptide product. Thus can include post-translational cleavage or addition of sugar, phosphate, or lipid groups.
Differences in gene expression also account for variable penetrance and expressivity of some mutations, e.g. retinoblastoma and many developmental disorders such as cleft palate and Van der Woude syndrome. (HS-LS3-1)
Crosscutting concepts
1. CE: Empirical evidence clarified the role of chromosomes and DNA in coding for heritable traits. (HS-LS3-1)
Science and engineering practices
1. Ask and evaluate questions about how the structure of DNA and chromosomes can encode the instructions for heritable traits.
SEP: Asking questions and defining problems (HS-LS3-1)
2. Use models to explain how many proteins can be produced from just one gene.
SEP: Developing and using models (Not aligned to a performance expectation)
LS3.B Variation of Traits
Variation can result from genetic processes
1. Students should appreciate that a trait is a variant of a phenotypic characteristic, e.g. brown hair color (as opposed to just hair color). Traits are encoded by genes. Different version of genes (alleles) differ very slightly in their base sequences and confer different traits, e.g. long coat vs short coat in guinea pigs. (HS-LS3-1)
2. Students should recall the adaptive value of producing genetically variable offspring. Environmental factors, including temperature, diet/nutrients, wind exposure, and interactions with other species (e.g. gender ratios, competition, predation), may influence the expression of the genotype. Plants and invertebrates offer many of the best examples of phenotypic plasticity. (HS-LS3-2)
3. Alleles are different versions that a gene may have in a population (there may be more than two).Students should appreciate that homologs can carry the same or different alleles for the same genes. The location of the gene on the chromosome is its locus. Genes are represented by letters, with dominant and recessive alleles represented by capital and lower case letters respectively. (HS-LS3-2)
4. Students should know that pairing of homologs at synapsis is followed by crossing over and the random exchange of alleles between maternal and paternal chromosomes at regions of homology (recombination). Chromosome mutations are commonly the result of errors during crossing over. Recombinants will have different allele combinations to those of the parents. When two genes are close together on the same chromosome, they do not assort independently and are said to be linked (recombination frequency that is less than 50%). Students can demonstrate using dihybrid crosses how recombination increases variation. (HS-LS3-2)
5. Spontaneous mutations arise as natural errors in DNA replication. The mutation rate in prokaryotes is higher than in eukaryotes. The highest mutation rates are found in viruses, which can have either RNA or DNA genomes. Mutagens are physical or chemical agents that alter the genetic material of an organism and increase the frequency of mutations above the natural background level. Students should understand that, in sexually reproducing eukaryotes, somatic mutations will not be inherited, but mutations to the gametic (germline cells) may be passed on to offspring. The evolutionary view of mutations is to classify them by their effect on fitness. Students should understand that the classification of a mutation as beneficial, harmful, or neutral is always within the context of the prevailing environment. Mutations that are neutral may be carried (not subject to selection pressure) and be beneficial or harmful at some later stage. Silent (or synonymous) mutations are those in which there is no change in the amino acid sequence. Such mutations were once thought to be neutral (and the two terms were used interchangeably), but evidence is now indicating that alterations to the triplet code do affect translation efficiency and protein folding. (HS-LS3-2)
6. In completing Punnett squares, students should recall that alleles are simply different versions that a gene may have in a population (there may be more than two). The genes are represented by letters, with dominant alleles represented by capital letters and recessive alleles by lower case letters. Simple monohybrid and dihybrid crosses (for unlinked genes) are given in the workbook and problems are provided to solve. Students should understand that in predicting the outcomes of crosses, the F1 generation refers specifically to the first generation cross of true breeding parents. (HS-LS3-3)
Variation can result from environmental influences
7. Genotype provides the basis for variation, but the environment limits how the genotype is expressed. Environmental variation may produce a range of phenotypes. Examples include the effect of temperature and presence of other organisms. A burgeoning field now is epigenetics: the study of the heritable variation arising from changes in gene activity, rather than changes to the DNA itself. It is helping us to understand how environment operates to alter phenotype, even in organisms of identical genotype. (HS-LS3-2)
8. The effects of environment may be inter-generational and may persist for several generations. One example is the Dutch famine (hungerwinter) of November 1944-May 1945. The hungerwinter birth cohort study showed that maternal under-nutrition had a life-long impact on children born, making them more susceptible to various diseases. The effects persisted through to adulthood but the effect was contingent on timing. Children conceived during the famine were affected but those conceived before the famine, even if carried during that period, were not affected. This material is extension to an extent but useful in that it ties in with gene regulation (LS3.A) and shows the mechanisms by which environmental influences may operate (i.e. through methylation and other changes to DNA regulation at critical times during development). (HS-LS3-2)
Crosscutting concepts
1. CE: Empirical evidence enables us to distinguish between cause and correlation and support claims about the causes of genetic variation. (HS-LS3-2)
2. SPQ: Algebraic thinking is used to explain the variation and distribution of expressed traits in a population. (HS-LS3-2)
3. SPQ: Algebraic thinking is used to examine scientific data and predict the outcome of genetic crosses and explain the expression of traits. (Not aligned to a performance expectation)
Science and engineering practices
1. Use a model based on evidence to explain that heritable genetic variation arises through meiosis and mutation.
SEP: Developing and using models (Not aligned to a performance expectation)
2. Argue from evidence that the environment can affect the phenotype of subsequent generations.
SEP: Engaging in argument from evidence (HS-LS3-2)
3. Apply concepts of statistics and probability to explain the variation and expression of traits in a population.
SEP: Analyzing and interpreting data (HS-LS3-3)
4. Use evidence to build and defend the claim that variation in a population is essential for its survival and evolution.
SEP: Engaging in argument from evidence (HS-LS3-2)
Nature of Science
Technological advances have influenced the progress of science and science has influenced advances in technology. (HS-LS3-3)
LS4.A Evidence for Evolution
Evidence of common ancestry and diversity
1. Students should appreciate that overwhelming evidence supports the fact that life has existed for billions of years and has changed over time. The history of living things is documented through multiple lines of evidence. Students should understand that the scientific debate around evolution is centered on the mechanisms by which it occurs and the pace of change, and not the fact of evolution itself. They should also appreciate that evolution is not something confined to past events. Modern examples abound and, in taxa with short generation times (e.g. insects, bacteria), changes in allele frequencies can be observed within observable time scales. Most obvious examples include the evolution of antibiotic resistance in bacteria, antigenic shifts in viruses, pesticide resistance in insects, and speciation in Hawaiian Drosophila. (HS-LS4-1)
2. A phylogenetic tree is simply a hypothesis for the evolutionary history of an organisms or taxon. It can therefore be modified in the light of new evidence as it arises. Phylogenies are increasingly established on the basis of molecular studies consider in conjunction with existing morphological data. Sometimes the data agree and sometimes they lead to reconstructions of established phylogenies. (HS-LS4-1)
3. In considering fossil evidence, students should understand the difference between absolute (chronometric) and relative age. Relative dating is based on the principle of superposition, i.e. as layers accumulate through time, older layers are buried beneath younger layers. Fossils therefore document the order of appearance of taxa, and the physical characteristics of rock formations and their fossils can be used to correlate one stratigraphic column with another. The discovery of means for absolute dating in the early 1900s was a huge advance in geology and archaeology, although relative dating should not be regarded as the inferior method. Absolute dating methods are based on radioactive decay and different radiometric methods are used for fossils of different ages. Both relative and chronometric dating may contribute to a more complete picture of past events. Students should choose appropriate examples to illustrate how fossils can document the evolution of a particular group. Horses, birds, whales, elephants, and ceratopsians all provide excellent, easily understood chronologies. (HS-LS4-1)
4. Transitional fossils can be identified by their retention of certain primitive (plesiomorphic) traits in comparison with their more derived relatives. A transitional fossil will represent an organism near the point where individual lineages (clades) diverge. Despite the limited (biased) nature of the fossil record, there are good examples, e.g. cetaceans, horses, Archaeopteryx, and Tiktaalik. Although there are many recently described examples of transitional fossils, the student book focuses on examples where the fossils show a mix of features that are simple for students with a limited knowledge of anatomy to understand. (HS-LS4-1)
5. Students should be able to define common ancestor (individual from which all organisms in the taxon are directly descended) and distinguish homology from analogy (and understand the significance of the difference). Students should be able to describe various examples of homology, from molecular homologies (e.g. DNA and amino acid sequences, homeobox genes) to morphological homologies (e.g. the pentadactyl limb, vestigial structures). Note that vestigial structures are often homologous to structures functioning normally in other species. They may be of some limited function but still degenerate over time; the important point is that they do not confer a significant enough advantage in terms of fitness to be retained. Students should appreciate that the sophisticated analytical tools now available are providing powerful evidence of phylogenetic relationships. In particular, the study of the genetic sequences that control developmental processes shows that even small modifications of developmental processes can lead to profound changes in morphology. (HS-LS4-1)
Crosscutting concepts
1. P: Patterns can be observed at each of the scales a system is studied (e.g. biochemical, anatomical) and can provide evidence for causality in explanations of phenomena, e.g. common ancestry and biological evolution. (HS-LS4-1)
Science and engineering practices
1. Evaluate and communicate the evidence for the common ancestry of life on Earth.
SEP: Obtaining, evaluating, and communicating information. (HS-LS4-1)
2. Use a model based on evidence to show how evolutionary relationships can be constructed from biological evidence.
SEP: Developing and using models (Not aligned to a performance expectation)
3. Argue from evidence to defend the use of the fossil record to provide a relative sequence of events, including the appearance and extinction of organisms.
SEP: Engaging in argument from evidence (Not aligned to a performance expectation)
4. Evaluate and communicate information about the use of DNA homology and protein sequence similarities to determine evolutionary relationships.
SEP: Obtaining, evaluating, and communicating information. (HS-LS4-1)
5. Evaluate and communicate information about the use of limb homology and similarities in embryological development as evidence of relatedness.
SEP: Obtaining, evaluating, and communicating information. (HS-LS4-1)
Nature of Science
Scientific knowledge is based on the assumption that natural laws operate today as they did in the past and they will continue to do so in the future. (HS-LS4-1)
A scientific theory is a substantiated explanation of some aspect of the natural world, based on a body of facts that have been repeatedly confirmed through observation and experiment. If new evidence is discovered that the theory does not accommodate, the theory is generally modified in light of the new evidence. (HS-LS4-1)
LS4.B LS4.C Natural Selection and Adaptation
Natural selection: differential survival of favorable phenotypes
1. Natural selection is responsible for (most) evolutionary change by selectively reducing and changing genetic variation and establishing adaptive genotypes. Fitness describes the capability of an individual of certain genotype to reproduce; it usually is equal to the proportion of the individual's genes in all the genes of the next generation. Students can use the snail Gene Pool Exercise to model different gene pool scenarios and simulate evolutionary change. Students should appreciate that mathematical models provide a useful way to clarify the features of what is a very complex process, although exploring these is beyond the scope of the standards. The Hardy-Weinberg Equilibrium model is one example. Analysis of genotype frequencies generation to generation can indicate whether or not evolution has occurred and in what direction and rate for a selected trait. However, students should understand that the Hardy-Weinberg equation cannot determine which of the various possible selection pressures were responsible for the changes in frequencies. Artificial selection provides another model that helps us understand natural selection. (HS-LS4-2, HS-LS4-3)
2. Students should recall the definition of a trait and the role of mutation in producing new alleles (crosscutting to HS-LS3-2). The examples of the apolipoprotein mutation, the mutation for lactose tolerance, and the sickle cell mutation conferring malarial resistance (Activity #201) provide examples of how a new trait becomes relatively more common in a population as a result of increased fitness. In the case of malarial resistance, the mutation is maintained in the population because of heterozygote advantage. (HS-LS4-3)
Natural selection leads to adaptation
3. Understanding of Darwinian principles should include:
• The tendency of populations to overproduce.
• The fact that overproduction leads to competition.
• The fact that members of a species show variation, that sex promotes variation, and that variation is (usually) heritable.
• The differential survival and reproduction of individuals with favorable, heritable variations.
Change in allele frequencies (evolution) is an inevitable consequence of the fact that the conditions for genetic equilibrium in a gene pool are rarely, if ever, met. Conditions for genetic equilibrium are: a large population size, little or no gene flow, no natural selection on the locus or trait under consideration, random mating. Students might discuss the reasons for this. (HS-LS4-2)
4. Adaptation is an often misused term and students should understand that it is something that happens to populations over time, not to individuals themselves in their lifetime. (HS-LS4-3, HS-LS4-4)
5. A change in the allele frequencies in a population (i.e. evolution) will create a shift in phenotypic norm. This shift will reflect the selective environment. (HS-LS4-4)
6. Changes in the environment create selective pressure for survival of those phenotypes (and genotypes) best suited to the prevailing conditions. This is a dynamic process: there may be both phenotypic shifts and shifts in the range of occupation (contraction or expansion of range) associated with survival and fitness. Physical displacement or large scale changes in environment (e.g. isolation across a newly created geographical barrier) can cause a sudden change in selective environment such that the pace of evolutionary change quickens. (HS-LS4-5, HS-LS4-6)
7. Students should understand that extinction is a natural process and that mass extinction events have been associated with large scale climate shifts or other catastrophic events such as asteroid impacts and widespread volcanism. Extinction provides opportunities for the emergence of new species by creating new selective environments. The current phase of higher extinction rates is associated with human activity. (HS-LS4-5)
Crosscutting concepts
1. CE: Empirical evidence enables us to support claims about how evolution occurs, how new species arise, and how species become extinct. (HS-LS4-4, HS-LS4-5)
2. P: Observed patterns can provide evidence for causes of evolutionary change. (HS-LS4-3)
Science and engineering practices
1. Evaluate the evidence for claims that change in environment can lead to an increase in population numbers, emergence of new species, or extinction.
SEP: Engaging in argument from evidence. (HS-LS4-5)
2. Use concepts of statistics and probability to support explanations for how organisms with advantageous traits become proportionally more numerous.
SEP: Analyzing and interpreting data (Not aligned to a performance expectation)
3. Construct an evidence-based explanation for how biological evolution occurs and how natural selection leads to adaptation.
SEP: Constructing explanations and designing solutions (HS-LS4-2, HS-LS4-4)
4. Use models to illustrate natural selection and explain patterns of evolution.
SEP: Developing and using models (Not aligned to a performance expectation)
Nature of Science
Scientific knowledge is based on the assumption that natural laws operate today as they did in the past and they will continue to do so in the future. (HS-LS4-1)
LS2.C ETS1.B The Dynamic Ecosystem
Ecosystems are dynamic, open systems
1. The nested relationship between population (one species, usually in a geographically connected region), community (the biotic component of an ecosystem), and ecosystem (the community and its abiotic environment) needs to be understood so that these terms are used appropriately. The specific characteristics of particular ecosystems therefore arise as a result of the interactions between biotic and abiotic components, although abiotic factors ultimately determine productivity, which in turn determines carrying capacity and many community characteristics. (HS-LS2-2)
2. An ecosystem's 'steady' state (actually a dynamic equilibrium) refers to the maintenance of relatively constant conditions by negative feedback systems operating within the ecosystem. Stable ecosystems usually use sunlight as their energy source of energy, recycle all elements, maintain sustainable consumer levels, and maintain biodiversity. (HS-LS2-6)
3. Keystone species are seen as important because they occupy key functional positions in ecosystems. These roles may be different in each case but include species with roles in nutrient recycling (termites), as predators (sea otters, mountain lions, sea stars), in shaping the characteristics of the environment (elephants, prairie dogs), and even as pollinators (hummingbirds in the Sonoran desert). (HS-LS2-2, HS-LS2-6)
4. Ecosystem resilience refers to the ability of an ecosystem to regain its normal structure and function after a disturbance (as opposed to its resistance to disturbance). Species diversity is often the key to both ecosystem resistance and resilience, as diverse systems have many biotic interactions, which buffer them against small internal fluctuations (such as a decline in the population of a single component species). (HS-LS2-2, HS-LS2-6)
5. Students could cite examples of where population 'explosions' result in extreme changes in ecosystem structure and function. Any of the keystone species examples provided (activity #158 and also crosscutting to LS4.D) would be appropriate. Extreme fluctuations can destroy normal negative feedback (self-limiting) regulation and lead to destructive positive feedback loops and irreversible change. One example of this is the ice-albedo feedback loop, where melting snow exposes more dark ground or ocean surface, which in turn, absorbs heat and causes more snow to melt and so on. A good discussion point (ETS1.B) is the positive feedback loop between growth in the human population and rapidly advancing agricultural developments. What will be the outcome? (HS-LS2-2, HS-LS2-6)
6. Ecosystems with a high biodiversity are regarded as having higher ecosystem stability because there are more biological interactions (and more functional redundancy) to buffer against species loss. Having many different species may stabilize ecosystem processes, such as nutrient cycling, because different species will respond to environmental changes in different ways. (HS-LS2-2)
Anthropogenic changes can threaten species survival
7. Students should appreciate that biodiversity and ecosystem resilience are not academic notions; humans depend on the resilience of terrestrial and aquatic ecosystems to provide essential ecosystem services (such as nutrient cycling and provision of clean water). Human-induced loss of biodiversity can reduce the resilience of natural ecosystems and make them more vulnerable to long-term irreversible change. (HS-LS2-7)
8. Students should recognize and describe human activities that threaten biodiversity. Human activities can reduce biodiversity directly or indirectly through species loss (e.g. deforestation or overfishing) or by creating environments that are less favorable to species survival or cause shifts in species distribution, timing of breeding, or migration (e.g. climate change). (HS-LS2-7)
9. Students can appreciate through (individual or group) activities #161, 165, and 167 that there are almost always conflicts involved in making any conservation decision. The best decision will be one that achieves its aims with the east cost and most favorable outcomes for all concerned parties (HS-ETS1-3, HS-ETS1-4)
Crosscutting concepts
1. SPQ: The concept of orders of magnitude can be used to understand how a model of the factors affecting ecosystems can operate at different scales. (HS-LS2-2)
2. SC: Many aspects of science, including ecology, involve explaining how things (e.g. ecosystems) change and how they remain stable. (Not aligned to a performance expectation)
Science and engineering practices
1. Analyze examples of ecosystem resilience using second-hand data.
SEP: Using mathematics and computational thinking (Not aligned to a performance expectation)
2. Use mathematical representations to support explanations about the factors affecting biodiversity and populations in ecosystems.
SEP: Using mathematics and computational thinking (HS-LS2-2)
3. Evaluate the evidence for the role of complex interactions in the stability of ecosystems and the role of changing conditions in ecosystem change.
SEP: Engaging in argument from evidence (HS-LS2-6)
4. Develop and evaluate solutions for sustaining biodiversity while allowing essential human use of resources.
SEP: Constructing explanations and designing solutions (HS-LS2-7)
Nature of Science
Scientific argument is a mode of logical discourse used to clarify the strength of relationships between ides and evidence that may result in revision of an explanation. (HS-LS2-6)
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