Chapter 18: Variation and selection

variation

difference between individuals of the same species

causes and result of continuous variation

influenced by genes and environment, resulting in a range of phenotypes between two extremes

e.g. height and body mass

investigation of continuous variation

causes and result of discontinuous variation

caused by genes alone and results in a limited number of distinct phenotypes

e.g. ABO blood groups, seed shape and colour in peas

investigation of discontinuous variation

mutation

genetic change, the way new alleles are formed

gene mutation

random change in the base sequence in DNA

what increases the rate of mutation

ionising radiation and some chemicals

sources of genetic variation in populations

mutation, meiosis, random mating, and random fertilisation

adaptive features

an inherited feature that helps an organism to survive and reproduce in its environment

xerophytes

live in deserts where water is scarce and evaporation is rapid, or in windy habitats

adaptive features: xerophytes

deep roots to reach water far underground

leaves reduced spines with minimum surface area for transpiration

shallow spreading roots to collect occasional rain

rolled leaves, leaf hairs and stomata sunk in pits to trap moist air

waxy leaf cuticle, impermeable water

stomata opening at night and closed at midday when evaporation is highest

hydrophytes

live in fully or partly submerged in water

adaptive features: hydrophytes

natural selection

the individuals that have the best adaptive features are the ones most likely to survive and reproduce

individuals show a range of variation caused by differences in genes

they reproduce more offspring than the environment is able to support

leading 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

meaning that in the next generation, there will be a greater number of individuals with the better adapted variations in characteristics

theory of natural selection was put forward by Charles Darwin and became known as ‘survival of the fittest’

example of natural selection

adaptation

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

selective breeding

to select individuals with desirable characteristics and breed them together

process doesn’t stop there though because it’s likely that not all of the offspring will show the characteristics wanted so offspring that do show the desired characteristics are selected and bred together

This process has to be repeated for many successive generations before a ‘new breed’ is said to be made which will reliably show those selected characteristics in all offspring

e.g. selective breeding is dog breeders who select which dogs can mate together to increase the likelihood of puppies displaying desirable characteristics eg. coat colour

differences between natural selection and artifical selection