Genetic Diversity and Natural Selection

What is genetic diverisity

• total number of different alleles of genes in a population

• more genetic diversity within species - more phenotypes so more able to adapt to environmental change and evolve

• A new allele for a gene -

  • The gene has mutated → new base sequence = new allele

  • The allele produces a different amino acid sequence

  • This could change the protein’s shape or function

  • That could lead to a new phenotype

What is evolution

• the change in allele frequency over many generations in a population

Natural Selection

• theory created by Charles Darwin

• can only occur if there is genetic diversity

• results in species becoming better adapted for their environment e.g. antibiotic resistance in bacteria

What kind of adaptations can occur

• Anatomical (structural features)

• Physiological (chemical reactions in the body)

• Behavioural

Explain the process of natural selection

• population of species there is a gene pool - contain wide variety of alleles

• new alleles of a gene are created by random mutations - results in new allele of gene

• Survival Advantage - new allele increases the chances (provides an advantage) of survival in the environment - more likely to survive and reproduce

• Reproduction those who reproduce successfully will pass on their alleles to next generation - new individuals have the new advantages allele and more likely to survive and reproduce

• Change in Allele Frequency - Over many generations those with advantageous allele increase at the expense of the individuals with less advantages alleles

• over time - new allele increases in frequency in population

What are the two types of natural selection

• Stabilizing selection

• Directional selection

Directional selection

• Directional Selection - one of the extreme traits are more likely to survive and reproduce (phenotype at one end of a range of variation is selected - population changes in direction of favoured characteristic)

• over time leads to a shift in the mean of the populations trait

  • occurs when there is a change in the environment

  • can be a new allele that has appeared and it is advantageous

• Before selection:

  • Normal distribution (bell-shaped curve)

  • Mean trait value is in the middle

  After selection:

  • The curve shifts towards one extreme phenotype

  • Mean trait value changes

  • Selection is against the other extreme and the average

Give an example of directional selection

• A few bacteria have alleles for antibiotic resistance (extreme phenotype)

• After antibiotic exposure, resistant bacteria survive

• They reproduce and pass on the resistance allele

• Over time, the population shifts towards greater resistance

🎓 Exam Tip:

They may ask you to interpret a graph OR describe the process using an example. Make sure to:

• Mention survival and reproduction

• Talk about alleles being passed on

• Explain that the mean shifts over time

Stabilising Selection

• individuals with alleles for the average phenotype are more likely to survive and reproduce. (modal phenotype is best adapted and selected for)

  • reduces the range of phenotypes and maintains the status

  • occurs when there is no change in the environment

  • over generations - more individuals will have the modal trait

Example of stabilizing selection

• Very low birth weight: Higher risk of health issues/death

• Very high birth weight: Complicated birth, also higher risk

• Average birth weight: More likely to survive

• So, alleles for average birth weight are passed on

Compare directional selection to stabilizing selection

What is genetic variation

• there is small difference in DNA base sequence between individual organism

• genetic variation is passed down - leads to genetic diversity

What causes genetic variation

mutations - these alleles can be advantages, disadvantages or no effect on the phenotype

sexual reproduction - during meiosis and fertilization

What are selection pressures

• environmental factors that affect chance of survival e.g. predators

• increases the chance of survival with those with the specific phenotype compared to others

What is fitness

• Those who have the ability to survive and pass on its alleles e.g. those who have the desired phenotype

Variation can be

• Interspecific - between different species

• Intraspecific - within one species (could be due to genetic factors, environmental factors or combination of both)

Definition of species

a group of organisms capable of interbreeding to produce fertile offspring

What are the key stages in natural selection

• Mutation

• Variation

• Adaptation

• Survival of the fittest

• Reproduction

What does the advantages allele depend on

the environmental conditions at any one time

You need to say certain traits and selected FOR and AGAINST depending on the selection pressures