CAIE Biology IGCSE Variation and Selection Notes PDF

CAIE Biology IGCSE 18: Variation and Selection Notes (Content in bold is for Extended students only) This work by PMT Education is licensed under https://bit.ly/pmt-cc https://bit.ly/pmt-edu-cc CC BY-NC-ND 4.0...

CAIE Biology IGCSE 18: Variation and Selection Notes (Content in bold is for Extended students only) This work by PMT Education is licensed under https://bit.ly/pmt-cc https://bit.ly/pmt-edu-cc CC BY-NC-ND 4.0 https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc  Variation Variation refers to the differences between each organism in a species. Variation is beneficial to a species as it allows natural selection to occur and reduces the risk of extinction from diseases. There are two types of variation: genetic variation and phenotypic variation. Genetic variation - each organism in a species has a different set of DNA, which is due to genetic variation. Genetic variation is increased during meiosis, which produces gametes. Each gamete has a different set of alleles, which means that when the two gametes fuse an entirely new set of genes are produced. Mutation, meiosis, random mating and random fertilisation are sources of genetic variation in populations. Phenotypic variation - The phenotype of an organism refers to its observable characteristics, such as height or hair colour. Phenotypical variation can be caused by both genetic and environmental factors. For example, the potential height of an organism is decided in genes which come from the parents, although some organisms will never reach this height as they do not receive enough nutrients from their environment. Variation can be continuous and discontinuous. Continuous variation results in a range of phenotypes between two extremes, for example height or weight. Discontinuous variation, however, is limited to a discrete number of categories, such as blood group, which is limited to A, B, AB, or O in humans, seed shape and seed colour in peas. Discontinuous variation is mainly caused by genes alone while continuous variation is caused by both genes and the environment. Mutations: Mutations are genetic changes which result in a random change in the sequence of DNA bases. The rate of mutation can increase due to a variety of factors, including exposure to some chemicals and ionising radiation. If the mutation occurs at a particular allele, this allele may be altered, changing how it functions. This is how new alleles are formed. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc  Adaptive features Adaptive features are inherited functional features that help the organism by increasing its fitness, which is the ability of the organism to survive and reproduce in its environment. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc Xerophytes are plants that are adapted to live in very dry climates, such as cacti. They have a number of adaptive features that help to increase survival by reducing water loss: Fewer stomata - water vapour diffuses out of the plant via the stomata, thus less water is lost if there are fewer stomata. Stomata are also sunken in pits in the leaf, which allows bubbles of moist air to be trapped around them. This lowers the water potential gradient, so less water is lost from the leaf. Small, rolled leaves or spines - this reduces the surface area of the leaf and traps moisture to lower the water potential gradient, reducing water loss. Deep roots - this allows plants to absorb water from the soil. Roots are also adapted to absorb lots of water when it rains for storage, e.g. in monsoon seasons. Thick waxy cuticle - this provides a waterproof barrier around the leaf to prevent water loss. Hydrophytes, in contrast, are plants which are adapted to live in very wet conditions and include species such as water lilies and lotus. These plants are adapted differently to xerophytes as they do not need to minimise water loss: Leaf shape - leaves are usually large and flat to have a large surface area which promotes water loss. Stomata - positioned on the top of the leaf where the sun hits. There is also a large number of stomata, which are usually open to allow water vapour to diffuse out of the leaf. Thin/no waxy cuticle - water loss does not need to be restricted by this layer in hydrophytes. Small root system - as there is a large amount of water readily available, root systems can be shallow, and water can diffuse directly into the stem. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc  Selection Natural selection is where organisms with favourable alleles and advantageous characteristics have a higher probability of surviving and reproducing. This is due to competition within a population for resources and mates. Due to variation in the alleles of each species, each organism within a species has different traits, some positive and some negative. Those with more positive traits can adapt to the environment more effectively and are thus more likely to survive and produce many offspring, which inherit these alleles. Over time, negative characteristics are lost from the species as organisms with those characteristics are not able reproduce to pass on their alleles. This is known as evolution. Evolution allows a population to become more adapted to its environment over time, as a result of natural selection. Antibiotic resistance: Some bacterial strains become resistant to antibiotics as a result of natural selection: 1. A mutation occurs in a bacterial cell allele which makes it resistant to an antibiotic. 2. When that antibiotic is administered, this cell is not killed, whereas cells which have not become resistant are killed. 3. The resistant bacterial cell can therefore survive and reproduce, passing on the resistant allele to produce more resistant bacteria. Selective breeding: Selective breeding is where humans select animals or plants with desirable features and breed these together to make more offspring with these desirable features. This process is repeated over many generations, where only offspring showing desirable features will be chosen to be cross-bred together. As this breeding is controlled by humans, it is known as artificial selection. An example of selective breeding of animals is the German Shepherd. These dogs were originally bred as working dogs to herd sheep as they are known for their intelligence and agility. Humans selectively breed these dogs to exaggerate desirable qualities, such as their sloping backs and large ears. This involves crossing dogs which show these traits so that the alleles are passed on to their offspring. Farmers also selectively breed crops. For example, bananas are selectively bred for their size, shape and easiness to peel. This means that plants which express these characteristics are bred to produce more offspring with desirable characteristics. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc Differences between natural and artificial selection: Natural selection Artificial selection Process occurs in nature without human Controlled by humans interference Organisms with advantageous traits survive Individuals with the desirable trait are and reproduce and pass on the favourable chosen to breed together to produce allele to the offspring offspring with the trait Takes a long time to occur Takes less time https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc