Chapter 18: Variation and selection Flashcards
Define ‘variation’
Differences between individuals of the same species
What does continuous variation result in?
A range of phenotypes between two extremes, e.g. body length and body mass
What does discontinuous variation result in?
A limited number of phenotypes with no intermediates, e.g. ABO blood groups, seed shape in peas and seed colour in peas
What causes continuous and discontinuous variation?
Discontinuous variation is usually caused by genes only, and continuous variation is caused by both genes and the environment
How do you investigate continuous variation?
Measure the heights of 30 people, divide the heights into suitable categories, record the numbers in each category, and construct a histogram
Define ‘mutation’
Genetic change, how new alleles are formed
Define ‘gene mutation’
A random change in the base sequence of DNA
What are the sources of genetic variation in populations?
Mutation, meiosis, random mating and random fertilisation
What increases the rate of mutation?
Ionising radiation and some chemicals
Define ‘adaptive feature’
An inherited feature that helps an organism to survive and reproduce in its environment
What are the adaptive features of xerophytes? (5)
Deep and shallow spreading roots that reach the water far underground and collect occasional rain; leaves so small they are just spines to reduce surface area for reduced transpiration; leaves rolled with stomata inside and an inner surface covered in hairs to trap moist air and prevent air movement across stomata, which reduces transpiration; thick waxy cuticle acts as a barrier to evaporation and also the shiny surface reflects heat and lowers the temperature; sunken stomata traps moist air and prevents air movement across stomata which reduces transpiration
What are the adaptive features of hydrophytes? (4)
Leaves are highly divided to create a large surface area for absorption and photosynthesis; lack of xylem tubes, stomata on the upper epidermis where they can exchange gases much more easily with the air; thick waxy layer on stomata to repel water and keep stomata open and clear; small roots and absent root hair as they can extract nutrients from the surrounding water through their tissues
What is natural selection?
The greater chance of passing on genes by the best-adapted organisms
What is the process of natural selection?
Individuals in a species show a range of variation caused by differences in genes, genetic variation. 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
Define ‘adaptation’
The process, resulting from natural selection, by which populations become more suited to their environment over many generations
What is an example of natural selection?
The development of strains of antibiotic-resistant bacteria
Define ‘artificial selection/selective breeding’
Breeding organisms with valued characteristics together to produce offspring that share those valuable characteristics
What is the process of selective breeding?
Selecting individuals with desirable features, crossing these individuals to produce the next generation, and selecting offspring that show the desirable features and breeding them together. This process has to be repeated for many successive generations
What are the differences between natural and artificial selection? (5)
Natural selection is the environment that ‘selects’ which individuals are able to survive and breed; there is random mating, instead of humans deciding.
Adaptive features that are selected from natural selection increase the chances of an individual surviving and reproducing in its natural environment, but in selective breeding, the features that are selected are features that the breeder wants, which may make it less likely that the organism could survive in the wild.
Selective advantages from natural selection can be quite small, giving only a slightly better chance of some individuals surviving than others, and even individuals that don’t have the best adaptive features still have a chance of reproducing. In artificial selection, only the very ‘best’ organisms are chosen to reproduce.
Relatively weak selection pressures in natural selection lead to greater genetic variation than in populations produced by artificial selection.
The speed of change in natural selection is quite slow. In artificial selection, large changes in features can happen over short periods of time
