8. Natural Selection

Success Criteria

Your learning has been successful if you can do the following:

I can discuss why individuals in a population may have different probabilities of survival based on their phenotype.
I can explain the importance of variation within a population during a change in their environment.

Vocabulary

Learn these so you can communicate this concept well.

Adaptation: Specific traits that enable individuals to survive in a particular environment.
Genetic variation: Differences in DNA / genes / alleles.
Mutation: Change in the base sequence of DNA / a gene, which may result in a new allele.
Natural selection: Process where selection pressures favour one phenotype over another.
Phenotype: Physical representation of the genotype.

Hei Mahi (Do Now)

Do Now in your OneNote/Notebook:

Write the definition for BENEFICIAL MUTATION and HARMFUL MUTATION.

Hei Mahi (Do Now)

Do Now in your OneNote/Notebook:

Past Exam Question (2020 Q3)

Avian malaria is a parasitic disease affecting hoiho (yellow-eyed penguin), which can lead to death.

a) Describe genetic variation in hoiho.

Exit Task

In your Learning Journal:

Re-write this interpreting question so it is asking about Natural Selection:

How does it relate to...?

Then, write an answer for it.

Genetic Variation in Populations

GENETIC VARIATION in a population refers to the different alleles (and combinations) existing amongst individuals in relation to particular traits. Genetic variation results in a variety of PHENOTYPES within the population.

Genetic variation in a population is an advantage as it may help some individuals survive when there is a significant change in the environment. E.g. extreme drought, arrival of new predator, introduction of new disease.

Some phenotypes may be better suited to the new environment. Individuals with these phenotypes will live longer and produce more offspring. The surviving individuals pass on the favourable allele to their offspring. Over generations, the frequency of the favourable allele (and phenotype) increases, and the unfavourable allele decreases within the population.

Genetic variation increases the chances of a population surviving adverse environmental conditions. It may prevent a species from becoming extinct.

Click on this link to watch a video on the phenotypic variation of Rock Pocket Mice.

For example, a plant population has different alleles that give individuals different root lengths.

A drought occurs in a location occupied by a population of plants.

The individuals with an allele that gives them deep roots are more likely to survive the drought.

The surviving plants will breed with other survivors, producing more plants with deep roots.

For example, a rabbit population has different alleles that give individuals different levels of immunity to a disease.

A deadly new viral disease arises in a population of rabbits.

Some individuals have an allele that gives them immunity to this viral disease and they survive.

The surviving rabbits will breed with other survivors, producing more rabbits with immunity to the disease.

Adaptations & Natural Selection

ADAPTATIONS are specific traits (aspects of the phenotype) that enable individuals to survive in a particular environment. Environmental factors like extreme droughts, arrival of a new predator or introduction of a new disease act as selection pressures. They 'weed out' individuals not well suited for current environmental conditions.

Selection pressures over many generations result in favourable PHENOTYPES surviving and reproducing. Other, less favourable, phenotypes die off.

Click on this link to watch a video on Adaptations.

For example, some adaptations of cacti to survive areas of drought.

In this case, 'drought' is the selection pressure. Cacti with these adaptations will survive in reproduce, increasing the number of individuals with these adaptations in the population.

For example, some adaptations of arctic hares to avoid predators.

In this case, 'predators' are the selection pressures. Rabbits with these adaptations will survive and reproduce, increasing the number of individuals with these adaptations in the population

New phenotypes appear within a population as MUTATIONS and new combinations of alleles in sexual reproduction. Selection pressures will again act on these new phenotypes.

This ongoing process, where different selection pressures favour one phenotype over another, is called NATURAL SELECTION. This is the process by which a new population becomes adapted to its environment.

Click on this link to watch a video on the Recipe for Natural Selection.

In summary...

Individuals that are better suited to the conditions, will survive better. These better suited individuals are more likely to produce more offspring and pass on their successful alleles / DNA / genes. The next generation will therefore be better suited than the previous generation. Over time, the population will contain more individuals with that favourable trait. This will lead to an increase in number of individuals able to survive. If there was no variation, all individuals in a population could die if the environment changes.

Over time, the POPULATION can adapt. If all individuals in the population were all identical, they may all die from the same disease or the same set of unfavourable environmental conditions.