Cambridge IGCSE Biology Variation & Natural Selection PDF

Head to www.savemyexams.com for more awesome resources Cambridge (CIE) IGCSE Biology Your notes Variation & Natural Selection Contents Variation in Biology Adaptive Features Adaptive Features Continued Natural Selecti...

Head to www.savemyexams.com for more awesome resources Cambridge (CIE) IGCSE Biology Your notes Variation & Natural Selection Contents Variation in Biology Adaptive Features Adaptive Features Continued Natural Selection Artificial Selection Page 1 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Variation in Biology Your notes Types of Variation What is Variation in Biology? Variation is defined as differences between individuals of the same species Phenotypic variation is the difference in features between individuals of the same species Some of these differences are caused by differences in genes, which is genetic variation Phenotypic variation can be divided into two types depending on how you are able to group the measurements: Continuous Variation is when there are very many small degrees of difference for a particular characteristic between individuals and they are arranged in order and can usually be measured on a scale Examples include height, mass, finger length etc. where there can be many ‘inbetween’ groups Discontinuous Variation is when there are distinct differences for a characteristic For example, people are either blood group A, B, AB or O; are either male or female; can either roll their tongue or not - there are no ‘inbetweens’ When graphs of these data are plotted, continuous variation gives smooth bell curves (a result of all the small degrees of difference), whereas discontinuous gives a ‘step – like’ shape Page 2 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Height is an example of continuous variation which gives rise to a smooth bell-shaped curve when plotted as a graph Page 3 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Blood group is an example of discontinuous variation which gives rise to a step-shaped graph Phenotypic Variation Phenotypic variation can be caused in two main ways: It can be genetic - controlled entirely by genes Or it can be environmental - caused entirely by the environment in which the organism lives Genetic Variation Examples of genetic variation in humans include: blood group eye colour Page 4 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources gender ability to roll tongue Your notes whether ear lobes are free or fixed Whether earlobes are attached (lobeless) or free (lobed) is an example of genetic variation Environmental Variation Characteristics of all species can be affected by environmental factors such as climate, diet, accidents, culture and lifestyle In this instance ‘environmental’ simply means ‘outside of the organism’ and so can include factors like climate, diet, culture, lifestyle and accidents during lifetime Examples include: An accident may lead to scarring on the body Eating too much and not leading an active lifestyle will cause weight gain Being raised in a certain country will cause you to speak a certain language with a certain accent Page 5 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources A plant in the shade of a big tree will grow taller to reach more light Genetic and Environmental Causes Your notes Discontinuous variation is usually caused by genetic variation alone Continuous features often vary because of a combination of genetic and environmental causes, for example: tall parents will pass genes to their children for height their children have the genetic potential to also be tall however if their diet is poor then they will not grow very well therefore their environment also has an impact on their height Another way of looking at this is that although genes decide what characteristics we inherit, the surrounding environment will affect how these inherited characteristics develop Mutation Mutations are genetic changes Most mutations have no effect on the phenotype as the protein that a mutated gene produces may work just as well as the protein from the non - mutated gene Rarely, mutations lead to the development of new alleles and so new phenotypes and if they do, most have a small effect on the organism Occasionally, the new allele gives the individual a survival advantage over other members of the species For example: A bird develops a mutation leading to a change in feather colours This makes it more attractive to birds of the opposite sex Which causes the bird to breed more frequently and have more chances of passing on the mutated phenotype to the next generation Mutations can also lead to harmful changes that can have dramatic effects on the organism - for example, sickle cell anaemia in humans Mutations happen spontaneously and continuously but their frequency can be increased by exposure to the following: Ionising radiation (e.g. gamma rays and X - rays) - which can damage bonds and cause changes in base sequences Page 6 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Some non-ionising radiation (e.g. ultra-violet) - can also damage bonds and cause changes in base sequences Your notes Certain types of chemicals - for example chemicals such as tar in tobacco Increased rates of mutation can cause cells to become cancerous, which is why the above are linked to increased incidence of different types of cancer Mutation: Extended Genetic variation in populations can be caused by a variety of sources: Mutations Mutations are random genetic changes to the base sequence of DNA New alleles form through these random changes to DNA Meiosis New allele combinations form through segregation Random mating Which partnerships form for sexual reproduction Random fertilisation Which sperm and egg combinations occur during sexual reproduction Page 7 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Adaptive Features Your notes Adaptations & Fitness Adaptive features are the inherited functional features of an organism that increase its fitness Fitness is the probability of an organism surviving and reproducing in the environment in which it is found You should be able to interpret images or other information about a species in order to describe its adaptive features, for example: A typical question here might be to explain how the leaf area and distribution and density of stomata help different species of plant survive in their different habitats Page 8 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Adaptive Features Continued Your notes Hydrophytes & Xerophytes: Extended Extended Tier Only Hydrophytes Plants adapted to live in extremely wet conditions Common adaptations include: Large air spaces in their leaves for flotation, to keep the leaves close to the surface of the water where there is more light for photosynthesis Small roots as they can also extract nutrients from the surrounding water through their tissues Stomata usually open all the time and mainly found on the upper epidermis of the leaf where they can exchange gases much more easily with the air Page 9 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Hydrophytes are adapted to live in wet conditions such as ponds Xerophytes Plant adapted to live in extremely dry conditions Common adaptations include: Thick waxy cuticle - the cuticle cuts down water loss in two ways: it acts as a barrier to evaporation and also the shiny surface reflects heat and so lowers the temperature Sunken stomata: stomata may be sunk in pits in the epidermis; moist air trapped here lengthens the diffusion pathway and reduces the evaporation rate Leaf rolled with stomata inside and an inner surface covered in hairs - traps moist air and prevents air movement across stomata which reduces transpiration Page 10 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Small leaves: many xerophytic plants have small, needle-shaped leaves which reduce the surface area and therefore the evaporating surface Your notes Extensive shallow roots allow for the quick absorption of large quantities of water when it rains Thickened leaves or stems which contain cells that store water Xerophytes are adapted to live in extremely dry conditions such as deserts Page 11 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Natural Selection Your notes Natural Selection In any environment, the individuals that have the best adaptive features are the ones most likely to survive and reproduce This results in natural selection: Individuals in a species show a range of variation caused by differences in genes When organisms reproduce, they produce more offspring than the environment is able to support This leads to competition for food and other resources which results in a ‘struggle for survival’ Individuals with characteristics most suited to the environment have a higher chance of survival and more chances to reproduce Therefore the alleles resulting in these characteristics are passed to their offspring at a higher rate than those with characteristics less suited to survival This means that in the next generation, there will be a greater number of individuals with the better adapted variations in characteristics This theory of natural selection was put forward by Charles Darwin and became known as ‘survival of the fittest’ An example of natural selection Page 12 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Natural selection illustrated by snail shell colour Within the population of snails there is variation in shell colour Normal varieties of shell colours in this snail species is black or grey (as evidenced by the first picture) Chance mutations lead to a small number of snails / one snail having a white shell This ‘small number’ is shown in the second diagram where there are less white shelled snails than black or grey shelled snails The white shelled snail(s) survive longer This is the ‘survival of the fittest’, a term used to explain why some organisms succeed in the competitive struggle for survival against other members of their population The reason the white shelled snail(s) survive longer is because they are better camouflaged Page 13 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources This means that they are less likely to be seen by predators and eaten As they survive longer they get more opportunities to reproduce Your notes And so the allele for white shells is passed onto offspring more frequently than the alleles for black or grey shells Over generations, this is repeated until the majority of snails in the population have white shells Another good example of natural selection is the evolution of the peppered moths Page 14 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Examiner Tips and Tricks Your notes There are hundreds of thousands of examples of natural selection and you cannot possibly be familiar with all of them, however, they ALL follow the same sequence described above: Based on the idea that within a species there is always variation and chance mutations, some individuals will develop a phenotype (characteristic) that gives them a survival advantage and therefore will: live longer breed more and be more likely to pass their genes on Repeated over generations, the ‘mutated’ phenotype will become the norm Remember, it is the concept you have to understand, not the specific example. Adaptation by Natural Selection: Extended If the environment does not change, selection does not change This will favour individuals with the same characteristics as their parents If the environment changes, or a chance mutation produces a new allele, selection might now favour individuals with different characteristics or with the new allele So the individuals that survive and reproduce will have a different set of alleles that they pass on to their offspring Over time, this will bring about a change in the characteristics of the species - it will produce evolution Evolution is defined as the change in adaptive features of a population over time as a result of natural selection Natural selection results in a process of adaptation, which means that, over generations, those features that are better adapted to the environment become more common This is how evolution occurs This means populations of organisms become better suited to their environment A good example of this is the development of antibiotic resistance by bacteria Page 15 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Page 16 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Artificial Selection Your notes Selective Breeding Selective breeding means to select individuals with desirable characteristics and breed them together The process doesn’t stop there though because it’s likely that not all of the offspring will show the characteristics you want so offspring that do show the desired characteristics are selected and bred together This process has to be repeated for many successive generations before you can definitely say you have a ‘new breed’ which will reliably show those selected characteristics in all offspring Selective breeding in domesticated animals An example of selective breeding is dog breeders who select which dogs can mate together to increase the likelihood of puppies displaying desirable characteristics eg. coat colour This has given rise to the many recognisable dog breeds of today, all the same species (Canis familiaris) and all descended from one breed Selective breeding in crop plants Plants are selectively bred by humans for the development of many characteristics, including: Page 17 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources disease resistance in food crops increased crop yield Your notes hardiness to weather conditions (e.g. drought tolerance) better tasting fruits large or unusual flowers An example of a plant that has been selectively bred in multiple ways is wild brassica, which has given rise to cauliflower, cabbage, broccoli, Brussels sprouts, kale and kohlrabi: Page 18 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Selective breeding in crop plants Page 19 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Natural vs Artificial Selection: Extended Your notes Page 20 of 20 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers