variation and natural selection - Coggle Diagram
variation and natural selection
causes of variation
Genetic variation exists because of changes to chromosomes or genes (DNA).
Mutations are random changes in the number of chromosomes (e.g. Down syndrome) or the structure of a gene (e.g. cystic fibrosis).
Independent assortment during meiosis increases variation because it results in genetically different gametes.
The random nature of sexual reproduction also gives rise to variation.
An individual’s environment can also cause variation.
For example, human height is genetically controlled but actual height reached will depend on the nutrition available.
There is variation among the phenotypes of individuals in a population.
Competition (for food, mates etc) causes a struggle for existence.
The best-adapted individuals survive and the less well-adapted individuals do not survive – this is differential survival.
The surviving phenotype (the fittest) is able to reproduce.
The surviving phenotype passes it’s beneficial gene to the next generat
There is variation within bacterial phenotypes. Some are resistant to antibiotics and some are not.
Treating bacteria with antibiotics provides competition.
The resistant individuals survive and the non-resistant individuals do not survive – this is differential survival.
The resistant bacteria are able to reproduce.
The resistant bacteria pass on the resistance gene to the next generation.
Resistant bacteria increase and non-resistant bacteria decrease in number.
The gene for antibiotic resistance is usually caused by a beneficial mutation in the bacterial cells.
Darwin used the theory of natural selection to explain the process of evolution.
Evolution is a continuing process of natural selection that leads to the gradual change of an organism over time.
It may result in the formation of a new species.
Fossils are the remains of living organisms that have been preserved in rock for millions of years.
They provide evidence for evolution by showing how an organism looked millions of years ago and how it has changed over time.
Species are extinct if there are no living examples left.
Extinction happens if a species fails to adapt to changes in its environment.
Fossils have provided us with information on some extinct species.
Extinct species include dinosaurs (caused by a meteor strike) and dodos (hunted by humans).
Species that are at risk of becoming extinct are known as endangered.
A species can become extinct for many reasons, including:
Hunted by humans.
Hunted by non-native animals (introduced by humans).
Loss of habitat (e.g. deforestation).
Extinction can be avoided by:
Legislation preventing the hunting of endangered species.
International agreements, including those that plan to limit climate change.
Special programmes such as creating nature reserves to protect habitats.
Education that encourages people to do their part in protecting the environment.
Natural selection can be manipulated by selecting desirable characteristics in crop plants or domesticated animals that are of use to humans.
This is known as selective breeding or artificial selection.
Humans select individual plants or animals with desirable characteristics (such as increased crop yield, disease resistance and longer shelf life). These are then bred to produce offspring.
Repeated selection and breeding over many generations results in all offspring containing the desirable characteristic.
Wheat has been bred over many years to produce the following desirable characteristics:
a uniform size that’s easier to harvest
a shorter stalk length less likely to suffer wind damage
and a larger yield.
This is a gradual change in a characteristic across a population, it could be human height, mass and shoe size.
Continuous variation is represented as a histogram.
A histogram should show normal distribution, with most individuals around the average value and a few at the extremes.
The population can be clearly divided into discrete groups.
Individuals will fit into a group – there are no intermediates.
The ability to roll your tongue, blood group (A, B, AB, O) and hand dominance (left or right handed) are all examples.
Discontinuous variation is represented as a bar chart.