18 – Variation and selection 18.1 Variation Variation – differences between individuals of the same species - Continuous variation – 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 – there can be many ‘in-between’ groups o Results in a range of phenotypes between two extremes (syllabus) o Examples: body length, body mass - o Discontinuous variation – when there are distinct differences for a characteristic o Results in a limited number of phenotypes with no intermediates o Examples: ABO blood groups, seed shape in peas and seed colour in peas o Discontinuous is usually caused by genes only and continuous is caused by both genes and the environment - Genetic variation: o Blood group o Eye colour o Gender o Ability to roll tongue o Whether ear lobes are free or fixed - Environmental variation: o Accident that may lead to scarring in the body o Eating too much and not exercising will cause weight gain o Being raised in a certain country will cause you to speak a certain language with a certain accent o A plant in the shade of a big tree will grow taller to reach more light - Continuous features – genetic and environmental: o Tall parents will pass genes to their children for height o Their children have the genetic potential to be tall o However, if their diet is poor, they will not grow very well o Although genes decide what characteristics we inherit, the surrounding environment will affect how these inherited characteristics develop\ Mutation – genetic changes - It is the way in which new alleles are formed - The new alleles can give an organism a survival advantage or harmful changes - Happens spontaneously and continuously, but ionizing radiation and some chemicals increase rate of mutation o Increased rate of mutation can cause cells to become cancerous Gene mutation – random change in the base sequence of DNA Sources of genetic variation in populations: o Mutation o Meiosis o Random mating o Random fertilization 18.2 Adaptive features Adaptive feature – inherited feature that helps and organism to survive and reproduce in its environment - Increases an organism’s fitness - Fitness is the probability of an organism surviving and reproducing in the environment in which it is found Hydrophytes: - Live in extremely wet conditions - Adaptations: o Large air spaces in their leaves to keep them close to the surface of the water where there is more light for photosynthesis o Small roots as they can also extract nutrients from the surrounding water through their tissues o Stomata usually open all the time and mainly found on upper epidermis of the lead where they can exchange gases much more easily with air Xerophytes: - Live in extremely dry conditions - Adaptations: o Thick waxy cuticle to cut down water loss by acting as a barrier to evaporation and by reflecting heat due to their shiny surface, thus lowering the temperature o Sunken stomata in pits in the epidermis – moist air trapped here lengthens diffusion pathway and reduces the evaporation rate o Leaf rolled with stomata inside and an inner surface covered in hairs – traps moist air and prevents air movement across stomata which reduces transpiration o Small, needle-shaped leaves which reduce the surface area and therefore the evaporating surface o Extensive shallow roots to allow for the quick absorption of large quantities of water when it rains o Thickened leaves of stems which contain cells that store water 18.3 Selection Steps of natural selection: 1. There is variation in the inherited characteristics shown by individuals in a population. 2. Populations tend to produce many offspring, more than can be supported in the habitat where they live. 3. There is competition between individuals for limited resources such as food, mates or space 4. There is a struggle for survival in which some individuals are more likely to survive than others. 5. Individuals that are better adapted to their environment are more likely to survive and reproduce. 6. The offspring of the better adapted individuals are more likely to inherit alleles that make them better adapted to the environment. They are more likely to survive to reproduce successfully than other individuals in the population. Antibiotic-resistant bacteria: - When a person is infected with bacteria, the bacteria reproduce very quickly in the body As the bacteria reproduce, mutations may occur that allow them to be resistant to antibiotics If the infection is treated using antibiotics, these resistant bacteria will have an advantage over the non-resistant bacteria They will survive the antibiotic treatment and reproduce They will pass the allele for antibiotic resistance to the next generation Soon, the population will only contain bacteria that are resistant to the antibiotic - - Adaptation – the process, resulting from natural selection, by which populations become more suited to their environment over many generations Selective breeding: - Selection by humans of individuals with desirable features Crossing these individuals to produce the next generation Selection of offspring showing the desirable characteristic Crossing of these offspring Repeat for many generations until all individuals show the desirable feature Selective breeding in agriculture: - Selective breeding by artificial selection is carried out over many generations to improve crop plants and domesticated animals Artificial vs natural selection:
