B11.4 – Variation and Selection

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Define variation as differences between individuals of the same species

Variation is the differences between individuals of the same species.

Distinguish between phenotypic variation and genetic variation

Genetic differences or variation can lead to differences in an individual’s phenotype.

For example, imagine the allele T coded for tall plants and t coded for dwarf plants. There is genetic variation between Tt and TT, but no phenotypic variation. However, there would be both genetic and phenotypic variation between TT and tt or between Tt and tt.

An individual’s phenotype can be influenced by both their genotype on their environment, e.g. height in humans, whereas genotype is only influenced by mutation and inheritance.

State that phenotypic variation is caused by both genetic and environmental factors

Phenotypic variation is caused by both genetic and environmental factors. For example, imagine T coded for tall plants and t coded for dwarf plants.

A plant with genotypes TT or Tt would be taller than a plant with genotype tt given the same conditions for growth. However, factors like available nutrients, water, sunlight, temperature, etc. can also affect the growth of the plant, meaning plants with the same genotype can have different heights because of environmental factors.

State that continuous variation results in a range of phenotypes between two extremes, e.g. height in humans

Continuous variation results in a range of phenotypes between two extremes, e.g. height in humans. The variation is continuous because there is an unlimited number of phenotypes between two extremes.

State that discontinuous variation is mostly caused by genes alone, e.g. A, B, AB and O blood groups in humans

Discontinuous variation is mostly caused by genes alone, e.g. A, B, AB and O blood groups in humans. The variation is discontinuous because there are a limited number of discrete phenotypes.

State that discontinuous variation results in a limited number of phenotypes with no intermediates, e.g. tongue rolling

Discontinuous variation results in a limited number of phenotypes with no intermediates, e.g. tongue rolling – you can either roll the sides of your tongue up, or you can’t. There are no in-betweens.

Record and present the results of investigations into continuous and discontinuous variation

One possible investigation of continuous variation would be to measure and record the heights of all of your classmates.
Usually, in continuous variation, when you plot a graph of frequency on the y axis and phenotype on the x axis, you get a smooth bell curve:

The bigger your sample size (the more people you measure), the smoother the curve.

Discontinuous variation, however, gives a step-like shape.
For example, if you recorded the frequency of different blood groups in your class and plotted a graph, it may look something like this:

Similarly, you can investigate many different types of continuous and discontinuous variation – e.g. people who can roll their tongues vs people who can’t, eye colour, gender, whether ear lobes are free (lobed) or fixed (lobeless), weight, etc.

Define mutation as a change in a gene or chromosome

A mutation is a change in a gene or chromosome.

State that ionising radiation and some chemicals increase the rate of mutation

Ionising radiation and some chemicals increase the rate of mutation.

Mutations can occur randomly and spontaneously, however, ionising radiation can interact with DNA particles, causing electrons to become knocked off and the atoms that make up DNA to become ionised. This can cause unwanted chemical reactions in DNA, often changing the base sequence of the DNA. Chemicals may also interact with the DNA, causing unwanted reactions and resulting in a change in the base sequence of the DNA. This is mutation.

It should be noted that mutations happen all the time in people, but in most cases, your body can fix the faulty genes before any serious consequences arise.

Describe natural selection with reference to:
- Variation within populations
- Production of many offspring
- Competition for resources
- Struggle for survival
- Reproduction by individuals better adapted to the environment than others
- Passing on of their alleles to the next generation

In a given population of a species, there will be variation.

Most species produce many offspring – more than is needed to maintain the size of the population.

Often, there are only limited resources available to the population, e.g. food, mates, territory, etc.

This means that members of the population must compete for these resources – this results in a struggle for survival.

Due to variation, some individuals will possess certain characteristics that make them better adapted to their environment than others. Examples could include longer necks in giraffes, greater speed in cheetahs, strength in stags, etc. Individuals that are better adapted to survive are more likely to survive – this is called ‘survival of the fittest’. It is these individuals that will live long enough to find a mate and reproduce.

As a result, the alleles of the individuals that are better adapted to survive will be passed on to the next generation. In this way, advantageous alleles are ‘selected’.

This whole process, in which organisms that are better adapted to their environment tend to survive and produce more offspring, is known as natural selection.

Describe evolution as the change in adaptive features of a population over time as the result of natural selection

Evolution is the change in adaptive features of a population over time as a result of natural selection.

Natural selection means that advantageous adaptive features are selected with each passing generation, increasing the frequency of these features over time. As the newer generations change to adapt to their environments (remember, environments can change over time as well), the species as a whole can change. This is evolution.

Define the process of adaptation as the process, resulting from natural selection, by which populations become more suited to their environment over many generations

Adaptation is the process, resulting from natural selection, by which populations become more suited to their environment over many generations.

Describe the development of strains of antibiotic resistant bacteria as an example of evolution by natural selection

Due to mutation, one or two bacteria in a population of bacteria may have developed genes that grant them immunity against a certain antibiotic, or sometimes, against multiple antibiotics. Therefore, if an antibiotic they are resistant to is used to wipe out their population, these resistant bacteria will survive, and will be able to reproduce asexually to pass on their genes to the next generation. Bacteria multiply very quickly, so entire colonies of antibiotic resistant bacteria can form in a couple of hours. In this fashion, new strains of resistant bacteria can evolve as a result of natural selection.

This is why it’s important to always complete the antibiotic course you’re on – whether you feel better before it’s over or not. Completing the course will ensure the death of all the bacteria, reducing the chance that some will survive and mutate to gain the resistance gene (Mutations in bacteria usually occur during reproduction). It is also a good idea to take a combination of antibiotics instead of relying on just one, in case you have bacteria that are resistant to one of them. However, you should never take antibiotics without consulting a doctor first.

Medicines are, after all, drugs, and if not administered properly, can be seriously harmful.

Describe selective breeding with reference to:
- Selection by humans of individuals with desirable features
- Crossing these individuals to produce the next generation
- Selection of offspring showing the desirable features

Similar to natural selection, humans can bring about changes in living organisms by selecting certain individuals for breeding.

We can choose individuals with characteristics that we find desirable, and cross these individuals to produce the next generation. Often we do not allow the other individuals to breed.

The offspring that arise will tend to show a higher frequency of the desirable feature.

Example of features that we artificially select include:

Docility (so they are easier to control), faster growth rates and higher milk yields in domesticated cattle.
Friendlier, more sociable pets such as domestic cats and dogs.
Breeds of animals that are bred for their beauty, e.g. Persian cats
Higher yield and faster growth rates in crops
Crops have been bred for resistance to different diseases, e.g. resistance to a fungal infection called head blight

State the differences between natural and artificial selection

Natural selection is the process through which organisms better adapted to their environment tend to survive and produce more offspring.

Artificial selection is the process by which animals and plants are chosen by the breeder to produce desirable and inheritable characteristics in successive generations.

Some differences include:

natural selection vs artificial selection table