OCR A Level Biology Revision Notes, Study notes of Biology

Download OCR A Level Biology Revision Notes and more Study notes Biology in PDF only on Docsity! ⬇ CONTENTS 4.3.1 Classification of Species 4.3.2 Binomial System 4.3.3 Classification of the Three Domains 4.3.4 Classification of the Five Kingdoms 4.3.5 Classification & Phylogeny 4.3.6 Evidence of Evolution 4.3.7 Types of Variation 4.3.8 Standard Deviation 4.3.9 Variation: t-test Method 4.3.10 Variation: t-test Worked Example 4.3.11 Spearman's Rank Correlation 4.3.12 Adaptation 4.3.13 Natural Selection 4.3.14 Evolution of Resistance 4.3.15 Consequences of Resistance OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 1 OCR A Level Biology Revision Notes 4.3 Classification & Evolution YOUR NOTES 4.3.1 CLASSIFICATION OF SPECIES Classification of Species Taxonomy is the practice of biological classification The biological classification system enables us to arrange species into groups based on their evolutionary origins and relationships In this hierarchical system there is no overlap between groups and each group is called a taxon (plural taxa) By grouping organisms into taxa it can make them easier to understand and remember There are several different ranks or levels within the hierarchical classification system used in biology Multiple smaller taxa can be put into one larger taxon ⬇ OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 2 4.3 Classification & Evolution YOUR NOTES Hierarchical classification The hierarchical classification system of organisms in biology is used to organise and group similar organisms together so that they can be more easily understood There are several taxonomic ranks that exist All taxonomic ranks or ‘taxa’ make up a ‘taxonomic hierarchy’ Species is the lowest taxonomic rank in the system Similar species can be grouped in a genus (plural genera) Similar genera can be grouped in a family Similar families can be grouped into an order Similar orders can be grouped into a class Similar classes can be grouped into a phylum (plural phyla) Similar phyla can be grouped into a kingdom Similar kingdoms can be grouped into a domain Domains are the highest taxonomic rank in the system There are a few different rhymes that exist to help you remember the different ranks in the taxonomic classification system. A couple of examples are given below but you could always make up your own! The first letters of all the different ranks below the domains can be remembered as: Kings Play Chess On Fancy Gold Squares Kittens Pounce Clumsily On Furry Green Spiders Kingdom Phylum Class Order Family Genus Species The hierarchical classification system – The higher ranks contain more organisms with ⬇ Exam Tip The name of a species always consists of two words: the genus and species. This means when provided with the Latin name of a species you are automatically provided with information about the last two taxonomic ranks that the organism belongs to. Remember this when being asked to show or explain the classification of an organism in the exam. OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 5 4.3 Classification & Evolution YOUR NOTES The Hibiscus rosa-sinensis (a colourful flowering plant) is another example of an organism in the eukarya domain It belongs to the following taxonomic groups: Domain: Eukarya Kingdom: Plantae Phylum: Angiospermae Class: Dicotyledonae Order: Malvales Family: Malvaceae Genus: Hibiscus Species: Hibiscus rosa-sinensis A Classification Table ⬇ OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 6 4.3 Classification & Evolution YOUR NOTES ⬇ Exam Tip The binomial for a species is always typed in italics or underlined when handwritten. The genus name should have a capital letter but the species name should not. OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 7 4.3 Classification & Evolution YOUR NOTES 4.3.2 BINOMIAL SYSTEM Binomial System A species is a group of organisms that are able to reproduce to produce fertile offspring Binomials are the scientific names given to individuals species Binomials consist of the organism’s genus and species name in modern Latin For example, the binomial for humans is Homo sapiens and the binomial for dogs is Canis familiaris Binomials are extremely useful for scientists as they allow for species to be universally identified – the binomial for a species is the same across the entire globe Naming species Species are often given common names, but these common names are often differ between countries and do not always translate directly between different languages In order to avoid confusion about what group of organisms scientists are talking about, all species are given a two-part scientific name using the binomial system This naming convention was developed and established by the Swedish scientist Carl Linnaeus in the 18th Century The binomial name is always italicized in writing (or underlined if it is not possible to italicise) For example: The most commonly known yeast is Saccharomyces cerevisiae It is common to abbreviate the genus name (e.g. S. cerevisiae) Saccharomyces paradoxus is another species of that is a member of the same genus as cerevisiae ⬇ OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 10 4.3 Classification & Evolution YOUR NOTES Eukarya Organisms that have eukaryotic cells with nuclei and membrane-bound organelles are placed in this domain They vary massively in size from single-celled organisms that are only several micrometres across, to large multicellular organisms many-metres in size, such as blue whales Eukaryotic cells divide by mitosis Eukaryotes can reproduce sexually or asexually Example: Canis lupus, also known as wolves Key differences between archaea & bacteria Initially, all organisms within the Archaea domain were classified as Bacteria Then several unique features possessed by Archaea were discovered that separated them from both Bacteria and Eukarya The main differences between Archaea and Bacteria are seen in: Membrane lipids Ribosomal RNA Cell wall composition Membrane lipids The membrane lipids found in the cells of Archaea organisms are completely unique They are not found in any bacterial or eukaryotic cells The membrane lipids of Archaea consist of branched hydrocarbon chains bonded to glycerol by ether linkages The membrane lipids of Bacteria consist of unbranched hydrocarbon chains bonded to glycerol by ester linkages Ribosomal RNA Both Archaea and Bacteria possess 70S ribosomes The 70S ribosomes in Archaea possess a smaller subunit that is more similar to the subunit found in Eukaryotic ribosomes than subunits in Bacterial ribosomes The base sequences of ribosomal RNA in Archaea show more similarity to the rRNA of Eukarya than Bacteria The primary structure of ribosome proteins in Archaea show more similarity to the ribosome proteins in Eukarya than Bacteria ⬇ OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 11 4.3 Classification & Evolution YOUR NOTES Composition of cell walls Organisms from the Bacteria domain have cells that always possess cell walls with peptidoglycan Organisms from the Archaea domain also have cells that always possess cell walls, however these do not contain peptidoglycan Characteristics & features of the three domains table ⬇ OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 12 4.3 Classification & Evolution YOUR NOTES 4.3.4 CLASSIFICATION OF THE FIVE KINGDOMS Classification of the Five Kingdoms Before the three domains of life (Bacteria, Archaea and Eukarya) were introduced and then widely accepted as the highest rank in the classification of life on Earth, most people thought there to be five kingdoms at the top of the classification hierarchy These five kingdoms include: Prokaryo ta Protoctis ta Fungi Plantae Animalia Prokaryota This kingdom includes bacteria and blue-green bacteria The main features of all organisms within Prokaryota include: Most are unicellular (some can be found as filaments of cells or groupings of similar cells known as colonies) Their cells have cell walls (not made of cellulose) and cytoplasm but no nucleus or mitochondria They vary in size over a wide range: the smallest are bigger than the largest known viruses and the largest are smaller than the smallest known single-celled eukaryotes Their cells divide by binary fission Blue-green bacteria and some bacteria are autotrophic (they are photosynthetic) Many bacteria are heterotrophic (feeding by decomposing living or dead organic materials) ⬇ OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 15 4.3 Classification & Evolution YOUR NOTES Kingdom Fungi Two examples of protoctist cells All fungi are eukaryotic cells The cells of fungi: Possess non-cellulose cell walls (often made of the polysaccharide chitin) Don’t have cilia Fungi are heterotrophs: They use organic compounds made by other organisms as their source of energy and molecules for metabolism They obtain this energy and carbon by digesting dead/decaying matter extracellularly or from being parasites on living organisms Fungi reproduce using spores that disperse onto the ground nearby Fungi have a simple body form: They can be unicellular (like the common baker’s yeast Saccharomyces cerevisiae Some consist of long threads called hyphae that grow from the main fungus body and form a network of filaments called the mycelium Larger fungi possess fruiting bodies that release large numbers of spores (this is how many fungi reproduce) The mould found on bread is actually a fungus: Rhizopus nigricans ⬇ OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 16 4.3 Classification & Evolution YOUR NOTES A typical fungal cell ⬇ OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 17 4.3 Classification & Evolution YOUR NOTES The structure of a fungus with its hyphae, mycelium and fruiting bodies Kingdom Plantae Plants are multicellular eukaryotic organisms Plant cells: All have cell walls composed of cellulose Possess large (and usually permanent) vacuoles that provide structural support Are able to differentiate into specialized cells to form tissues and organs Possess chloroplasts that enable photosynthesis (not all plant cells have chloroplasts) Can sometimes have flagella They are autotrophs This means they can synthesize their organic compounds and molecules for energy use and building biomass from inorganic compounds Plants have complex body forms They have branching systems above and below the ground ⬇ OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 20 4.3 Classification & Evolution YOUR NOTES Five Kingdoms Comparison Table ⬇ OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 21 4.3 Classification & Evolution YOUR NOTES 4.3.5 CLASSIFICATION & PHYLOGENY Classification & Phylogeny Traditional biological classification systems grouped organisms based on the features that they shared This is known as homology (features are homologous if they are shared by organisms that have evolved from a common ancestor) In the past, scientists have encountered many difficulties when trying to determine the evolutionary relationships of species based on this method Using the physical features of species (such as colour/shape/size) has many limitations and can often lead to the wrong classification of species Advances in DNA, RNA and protein sequencing, as well as immunology, has allowed scientists to further investigate the evolutionary relationships between species This has allowed us to understand the true phylogeny of taxa and to correctly group them together and show how they are evolutionarily related to one another Phylogeny is the term used to describe the evolutionary history of organisms Phylogenetic trees are diagrams that show the evolutionary relationships between different taxa ⬇ OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 22 4.3 Classification & Evolution YOUR NOTES Using molecular evidence in classification Three types of sequence data are used to investigate evolutionary relationships DNA mRN A Amino acids (of a protein) Sequencing technology can determine the order of DNA bases, mRNA bases and amino acids within an organism’s genome This technology is especially useful for comparison with an extinct species (using ancient DNA) or when distinguishing between species that are very physically similar Scientists will choose specific proteins or sections of the genome for comparison between organisms Looking at multiple proteins or multiple regions of the genome will allow for a more accurate estimate of evolutionary relatedness Note the protein used needs to be present in a wide range of organisms and show su cientffi variation between species Cytochrome c is often used as it is an integral protein to respiration (in the electron transport chain) which is used by all eukaryotic organisms For all types of sequence data it can be said that the more similar the sequences, the more closely related the species are Two groups of organisms with very similar sequences will have separated into separate species more recently than two groups with less similarity in their sequences Species that have been separated for longer have had a greater amount of time to accumulate mutations and changes to their DNA,mRNA and amino acid sequences Sequence analysis and comparison can be used to create phylogenetic trees that show the evolutionary relationships between species ⬇ OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 25 4.3 Classification & Evolution YOUR NOTES ⬇ Exam Tip You may be wondering why you would use amino acids when you could look at DNA or mRNA. This is because it is often easier to find and isolate proteins from cells and as a result protein sequencing was the method traditionally used. In some cases, however, amino acid sequences may be exactly the same between different species even if there are differences in the corresponding DNA sequences. This is because genes for the same protein may have slightly different base sequences in different species due to differences in their introns which are not translated into differences in the protein molecules. In addition, the genetic code is a degenerate code, meaning that more than one codon may code for the same amino acid. As a result, DNA sequencing has largely replaced protein sequencing in taxonomy and the creation of phylogenetic trees. OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 26 4.3 Classification & Evolution YOUR NOTES An image showing the use of albumin and antibody production in comparing the relationship between different species ⬇ OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 27 4.3 Classification & Evolution YOUR NOTES 4.3.6 EVIDENCE OF EVOLUTION Evidence of Evolution Regarding evolution, it is important to distinguish between two key ideas: Firstly, the term ‘evolution’ can refer to the general theory of evolution. This refers to the way in which species have changed over time and continue to change Secondly, the term ‘evolution’ may be used as a shorthand way of referring to the theory of evolution by natural selection (i.e. the specific process by which evolution occurs) Although the general idea of evolution was acknowledged at the time, two biologists, named Chares Darwin and Alfred Russel Wallace, contributed greatly to developing the specific theory of evolution by natural selection Wallace spent many years travelling in South America and South-East Asia and collecting specimens from these places Darwin took part in a voyage around the world and collected specimens and information about many species These experiences and the notes and samples they collected along the way led to both men publishing a joint scientific paper proposing their theory of evolution by natural selection in 1858 Darwin’s observations Darwin made several key observations that helped him to develop the theory of evolution by natural selection, including: All organisms produce more offspring than could ever actually survive (i.e. not all offspring survive) Populations of organisms do fluctuate (change over time) but not significantly (i.e. their numbers usually stay fairly constant) Populations of the same species of organisms show variation in characteristics between individuals (known as intraspecific variation) Offspring inherit characteristics from their parents The theory of evolution by natural selection is now widely accepted Two key sources of evidence for this theory include: Fossil evidence Molecular evidence (our understanding of genetics has made clear the mechanism by which natural selection can occur and it has been shown that characteristics are passed on to offspring in genes) ⬇ OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 30 4.3 Classification & Evolution YOUR NOTES Discontinuous variation Qualitative differences in the phenotypes of individuals within a population give rise to discontinuous variation Qualitative differences fall into discrete and distinguishable categories, usually with no intermediates (a feature can’t fall in between categories) For example, there are four possible ABO blood groups in humans; a person can only have one of them It is easy to identify discontinuous variation when it is present in a table or graph due to the distinct categories that exist when data is plotted for particular characteristics Graph showing population variation in blood types: an example of discontinuous variation with qualitative differences Continuous variation Continuous variation occurs when there are quantitative differences in the phenotypes of individuals within a population for particular characteristics Quantitative differences do not fall into discrete categories like in discontinuous variation Instead for these features, a range of values exist between two extremes within which the phenotype will fall For example, the mass or height of a human is an example of continuous variation The lack of categories and the presence of a range of values can be used to identify continuous variation when it is presented in a table or graph ⬇ OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 31 4.3 Classification & Evolution YOUR NOTES Graph showing population variation in height: an example of continuous variation with quantitative differences Causes of variation Variation can be explained by genetic factors, environmental factors or a combination of the two Causes of discontinuous variation This type of variation occurs solely due to genetic factors The environment has no direct effect Phenotype = genotype At the genetic level: Different genes have different effects on the phenotype Different alleles at a single gene locus have a large effect on the phenotype Remember diploid organisms will inherit two alleles of each gene, these alleles can be the same or different A good example of this is the F8 gene that codes for the blood-clotting protein Factor VIII The different alleles at the F8 gene locus dictate whether or not normal Factor VIII is produced and whether the individual has the condition haemophilia ⬇ OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 32 4.3 Classification & Evolution YOUR NOTES Whether earlobes are attached or free is an example of discontinuous variation that is solely due to genetic factors Causes of continuous variation This type of variation is caused by an interaction between genetics and the environment Phenotype = genotype + environment At the genetic level: Different alleles at a single locus have a small effect on the phenotype Different genes can have the same effect on the phenotype and these add together to have an additive effect If a large number of genes have a combined effect on the phenotype they are known as polygenes ⬇ 15 rats were timed how long it took them to reach the end of a maze puzzle. Their times, in seconds, are given below. Find the mean time. 12, 10, 15, 14, 17, 11, 12, 13, 9, 21, 14, 20, 19, 16, 23 OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 35 4.3 Classification & Evolution YOUR NOTES Step 1: Calculate the mean 12 + 10 + 15 + 14 + 17 + 11 + 12 + 13 + 9 + 21 + 14 + 20 + 19 + 16 + 23 = 226 226 ÷ 15 = 15.067 Step 2: Round to 3 significant figures Mean (X ) = 15.1 seconds ⬇ The ear lengths of a population of rabbits were measured. Ear lengths (mm): 62, 60, 59, 61, 60, 58, 59, 60, 57, 56, 59, 58, 60, 59, 57 Calculate the mean and standard deviation. Worked Example OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 36 4.3 Classification & Evolution YOUR NOTES Step 1: Calculate the mean Mean = 885 ÷ 15 = 59 mm Step 2: Find the difference between each value and the mean Subtract the mean from each value to find the difference Example: 62 – 59 = 3 Step 3: Square each difference Square the difference for each value Example: 32 = 9 Step 4: Total the differences ⬇ OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 37 4.3 Classification & Evolution YOUR NOTES Step 5: Divide the total by (n-1) to get value A 36 ÷ (15 – 1) = 36 ÷ 14 = 2.571 Step 6: Get the square root of value A Standard Deviation = 1.60 ⬇ OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 40 4.3 Classification & Evolution YOUR NOTES Step 2: Calculate the standard deviation for each set of data, s1 = standard deviation of sample 1 and s2 = standard deviation of sample 2 Step 3: Square the standard deviation and divide by n (the number of observations) in each sample, for both samples: Step 4: Add the values from step 3 together and take the square root: Step 5: Divide the difference between the two means (see step 1) with the value calculated in step 4 to get the t value: ⬇ OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 41 4.3 Classification & Evolution YOUR NOTES Step 6: Calculate the degrees of freedom (v) for the whole data set (remember the formulae for this will not be given in the exam): v = (n1 – 1) + (n2 – 1) Step 7: Look at a table that relates t values to the probability that the differences between data sets is due to chance to find where the t value for the degrees of freedom (v) calculated lies ⬇ OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 42 4.3 Classification & Evolution YOUR NOTES T values table ⬇ OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 45 4.3 Classification & Evolution YOUR NOTES ⬇ OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 46 4.3 Classification & Evolution YOUR NOTES To find the standard deviations divide the sum of each square by n – 1 for each data set, and take the square root of each value Step 3: Square the standard deviation and divide by n (the number of observations) in each sample, for both samples: Step 4: Add the values from step 3 together and find the square root Step 5: Divide the difference between the two means by the value from step 4 ⬇ OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 47 4.3 Classification & Evolution YOUR NOTES Step 6: Calculate the degrees of freedom (v) for all the data: v = (n1 – 1) + (n2 – 1) = 14 + 14 = 28 Step 7: Look at a table that relates t values to the probability that the differences between data sets is due to chance to find where the t value of 1.91 for 28 degrees of freedom (v) calculated lies ⬇ A student conducted an experiment using quadrats to measure the abundance of different plant species in a neglected allotment. They wanted to see if there was correlation between the abundance of species C and D. When they looked at their data and plotted a scatter graph they saw some correlation. Investigate the possible correlation using Spearman’s rank correlation coe cient.ffi Scatter graph showing the correlation between the abundance of species C and species D OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 50 4.3 Classification & Evolution YOUR NOTES ⬇ OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 51 4.3 Classification & Evolution YOUR NOTES As the data was not normally distributed they decided to use Spearman’s rank correlation coe cient.ffi Null hypothesis: there is no correlation between the abundance of species A and species B. n = 10 as there are 10 quadrat samples ⬇ Exam Tip You will be provided with the formula for Spearman’s rank correlation in the exam. You need to be able to carry out the calculation to test for correlation, as you could be asked to do this in the exam. You should understand when it is appropriate to use the different statistical tests that crop up in this topic, and the conditions in which each is valid. Correlation does not always mean causation. Just because there is a correlation between the abundance of species A and species B it does not mean that the presence of species A causes the presence of species B. OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 52 4.3 Classification & Evolution YOUR NOTES Step 1: Rank each set of data (rank 1 being the smallest data figure) Step 2: Find the difference in rank between the two species, D Step 3: Square the difference in rank, D2 (= 6) Step 4: Substitute the appropriate numbers into the equation (remember n = 10) Step 5: Refer to a table that relates values of rs to probability. Look for the 0.05 probability level with n = 10 As Rs = 0.964, it is greater than the critical value of 0.65. The null hypothesis can be rejected, there is a genuine positive correlation between the abundance of species A and B ⬇ OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 55 4.3 Classification & Evolution YOUR NOTES Types of adaptations Adaptations enable organisms to survive in the conditions in which they normally live An adaptation can be anatomical, physiological or behavioural Anatomical adaptations Structural or physical features Example: The white fur of a polar bear provides camouflage in the snow so it has less chance of being detected by prey Physiological adaptations Biological processes within the organism Example: Mosquitos produce chemicals that stop the animal’s blood clotting when they bite, so that they can feed more easily Behavioural adaptations The way an organism behaves Example: Cold-blooded reptiles bask in the sun to absorb heat Types of Adaptations Table ⬇ Exam Tip You may be asked to identify whether an adaptation is anatomical, physiological or behavioural so make sure you have a good grasp of the difference between these types of adaptations! Learning an example for each can sometimes help you, as you then have them for comparison in an exam. OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 56 4.3 Classification & Evolution YOUR NOTES Convergent evolution Adaptation and selection contribute to the large diversity of living organisms and are major factors in the process of evolution Evolution is the change in adaptive features of a population over time as a result of natural selection In some cases, however, species that do not share a recent common ancestor and live in completely different parts of the world can show very high levels of similarity in terms of the adaptations they possess This occurs when the two habitats, in which the two species have evolved and to which they have become adapted, are very similar This process is known as convergent evolution For example, marsupial moles that live in the Australian desert (whose ancestors migrated to Australasia 50 million years ago) and the European mole (a placental mammal) have evolved separately and in isolation from each other but show convergent evolution – their adaptations have converged (become more and more similar) Both species burrow through soil and very rarely emerge in daylight. They are both adapted to their similar niches in the following ways: Both have short and powerful limbs, with large, strong front claws for shovelling soil out of the way Both have no external ears (these are not necessary underground) and have simple ear canal openings in their fur Both have limited eyesight (again, this is not needed underground) – the loss of an unnecessary feature is still an adaptation! Both have very similar, highly silky and fine fur that allows them to move e cientlyffi underground You would have to travel back many millions of years to find the common ancestor of these species, which would have looked nothing like a mole and would not have possessed any of the adaptations described above Despite this, these two completely separate species are now incredibly similar in terms of the adaptations they possess ⬇ OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 57 4.3 Classification & Evolution YOUR NOTES 4.3.13 NATURAL SELECTION Effects of Natural Selection Genetic variation Organisms of the same species have very similar genomes, but two individuals (even twins) will have differences between their DNA base sequences These differences in DNA base sequences between individual organisms within a species population are called genetic variation Genetic variation is transferred from one generation to the next and results in genetic diversity within a species Effects of genetic variation There needs to be some level of genetic diversity within a population for natural selection to occur Differences in the alleles possessed by individuals within a population result in differences in phenotypes Some phenotypes may be advantageous, disadvantageous or neutral, compared to other phenotypes Selection pressures increase the chance of individuals with a specific (more advantageous) phenotype surviving and reproducing over others The individuals with the favoured phenotypes are described as having a higher fitness The fitness of an organism is defined as its ability to survive and pass on its alleles to offspring Organisms with higher fitness possess adaptations that make them better suited to their environment A population with a large gene pool or high genetic diversity has a strong ability to adapt to change If a population has a small gene pool or very low genetic diversity then they are much less able to adapt to changes in the environment and so can become vulnerable to extinction Cheetahs are an example of a species with a small gene pool They experienced a very large decline in numbers approximately 10,000 years ago This left small, fragmented populations of individuals remaining There was no mixing between populations and large amounts of inbreeding occurred This is problematic for conservation as low genetic variation means the species are less likely to be able to respond (survive) in the event of any environmental changes ⬇ Exam Tip Exams often ask questions about how the process of natural selection occurs for a certain species. The principles of natural selection described above are always the same! You just need to edit the details to make them specific to the question: Within a species, there is always variation and chance mutation Some individuals will develop a phenotype (characteristic) that gives them a survival advantage and this allows them to: ⚬ live longer ⚬ breed more ⚬ be more likely to pass their genes on Repeated over generations, the ‘mutated’ phenotype will become the norm If genetic differences accumulate and the population is isolated then a new species may evolve Remember, it is the concept you have to understand, not a specific example. You will be expected to use unfamiliar information to explain how selection produces changes within a population of a species and interpret data relating to the effect of selection in producing change within populations. OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 60 4.3 Classification & Evolution YOUR NOTES Selective pressures acting on a rabbit population for one generation. Predation by foxes causes the frequency of brown fur alleles in rabbits to increase and the frequency of white fur alleles in rabbits to decrease. ⬇ OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 61 4.3 Classification & Evolution YOUR NOTES ⬇ OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 62 4.3 Classification & Evolution YOUR NOTES 4.3.14 EVOLUTION OF RESISTANCE Evolution of Resistance Antibiotics When humans experience a pathogenic bacterial infection they are often prescribed drugs known as antibiotics by a healthcare professional Antibiotics are chemical substances that inhibit or kill bacterial cells with little or no harm to human tissue Antibiotics are derived from naturally occurring substances that are harmful to prokaryotic cells (structurally or physiologically) but usually do not affect eukaryotic cells The aim of antibiotic use is to aid the body’s immune system in fighting a bacterial infection Penicillin is a well-known example; it was the first antibiotic to be discovered in 1928 by Sir Alexander Fleming Drug resistance in microorganisms Within a bacterial population, there is variation caused by mutations (as occurs in populations of all species) A chance mutation might cause some bacteria to become resistant to an antibiotic (eg. penicillin) When the population is treated with this antibiotic, the resistant bacteria do not die For example, a mutation may change an existing gene within the bacterial genome, causing it to give rise to a nucleotide sequence that codes for a slightly different protein that is not affected by the antibiotic being used This means the resistant bacteria can continue to reproduce with less competition from the non-resistant bacteria, which are now dead Therefore the genes for antibiotic resistance are passed on with a much greater frequency to the next generation As bacteria only have one copy of each gene, a mutant gene will have an immediate effect on any bacterium possessing it Over time, the whole population of bacteria becomes antibiotic-resistant because the antibiotic-resistant bacteria are best suited to their environment This is an example of evolution by natural selection Some pathogenic bacteria have become resistant to penicillin as they have acquired genes that code for the production of the enzyme β- lactamase (also known as penicillinase), which breaks down penicillin ⬇ OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 65 4.3 Classification & Evolution YOUR NOTES Antibiotic resistance in bacteria can spread by vertical or horizontal transmission ⬇ OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 66 4.3 Classification & Evolution YOUR NOTES Antibiotic resistance in bacteria is an example of natural selection that humans have helped to develop. This is due to the overuse of antibiotics in situations where they were not really necessary or the incorrect use of antibiotics, for example: For treatment of non-serious infections Routine treatment of animals in agriculture Failure to finish the prescribed course of antibiotics A variety of steps can be taken to reduce cases of antibiotic resistance, including: Only prescribing antibiotics when absolutely necessary Ensure patients complete courses of antibiotics Rotate which antibiotics are used so that one type is not continuously used in the treatment of a specific disease Hold back some antibiotics from being used at all so they are available as a ‘last resort’ More investment in research into new antibiotics Pesticide resistance in insects Pesticides are chemicals that kill pests of any kind, including insect pests, pathogenic organisms or weeds There are various types of pesticides, including: Insecticides (kill insect pests) Herbicides (kill plant pests) Fungicides (kill fungal pests) Molluscicides (kill slug and snail pests) Rodenticides (kill rodent pests) A major global use of pesticides is in the control of insect pests that consume or otherwise damage human food crops (e.g. Colorado beetles that eat potato crops) or insects that are vectors of disease (e.g. Anopheles mosquitos that transmit malaria) In a similar way to antibiotic resistance in bacteria, insecticides that are sprayed on crops act as selective agents A selective agent is any environmental factor that influences the survival of a particular species and so drives natural selection in that species For example, any insect that has a mutation making them resistant to the insecticide will survive and reproduce, passing on the resistant gene ⬇ Exam Tip Remember, vertical transmission is only responsible for passing on antibiotic resistance within a single bacterial population, whereas horizontal transmission can spread antibiotic resistance within a single bacterial population, between two populations of the same species of bacteria, or occasionally between populations of different species of bacteria. OCR A Level Biology Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 67 4.3 Classification & Evolution YOUR NOTES