3.4.4 genetic diversity + adaptation

What further increases genetic variation within a species during fertilisation?

The random fusion of haploid gametes

Mutations in the number of chromosomes can arise spontaneously by what?

By chromosome non-disjunction

• this occurs when a homologous pair of chromosomes don’t separate during the first meiotic division

• both members of a homologous pair move to the same side / pole of the cell

What does non-disjunction of a single homologous pair result in?

Gametes with one extra chromosome than normal + gametes with one less

How can non-disjunction chromosome pair 21 in humans result in Down’s Syndrome?

Gametes with 24 chromosomes (n+1) fuse with a gamete with 23 chromosomes producing an individual with 47 chromosomes (3x no. 21 chromosome)

Define genetic diversity

Number of different alleles of genes in a population

Genetic diversity is an important factor in enabling what to occur?

Natural selection

What are 2 major factors in the evolution of organisms?

Adaption + selection

Explain what selection means

Changes in the environment selects those organisms in a population which are best adapted to survive + breed

What is adaption?

The process by which organisms adjust to suit the changing environment in which they live

What are the principles of natural selection in the evolution of organisms?

• Variation will already be present in a population due to random mutations which can result in new alleles of a gene

• Many mutations are harmful but in certain environments, the new allele of a gene may benefit an organism leading to increased reproductive success

• A change in the environment results in organisms with the advantageous allele for a particular phenotype being selected for as they’re better adapted

• Organisms with the allele for this selected phenotype survive + reproduce, passing on this beneficial allele while those without don’t → differential reproductive success

• Over many generations, the frequency of the advantageous allele increases as it’s inherited by members of the next generation

Natural selection results in species that are better adapted to their environment → these adaptations may be what 3 things?

• Anatomical - the way we look

• Physiological - the way we function

• Behavioural - the way we act

What is directional selection?

This occurs when there’s a change in the environment → the change selects for those organisms with alleles for a phenotype towards oe extreme of a range

*distribution curve moves direction

Give an example of directional selection

Antibiotic resistance

• Due to random mutations, a few bacteria in a population possess an allele, providing resistance against a particular antibiotic - antibiotic against this bacterium species will provide a different environmental condition / selection pressure

• Bacteria with the antibiotic resistance allele will survive + reproduce by binary fission to pass on the allele to next generation

• Most non-resistant bacteria will die + fewer reproduce → frequency in population decreases over generations

• Frequency of antibiotic resistant bacteria + frequency of allele for antibiotic resistance would significantly increase in the population over generations

What is stabilising selection?

This is where selection acts against the extreme phenotypes in a population + occurs when the environment remains stable → selection occurs towards organism with phenotype closest to the mean

*distribution curve becomes narrower

Give an example of stabilising selection

Birth weight in humans when selection occurs for an optimal range of birth mass

• Natural selection favours those organisms with alleles for characteristics towards the middle of the range - birth mass

• These organisms survive to breed + pass on their alleles to the next generation

• Frequency of alleles for optimal birth mass tends to remain stable over generations

• Babies with alleles for the extremes of this range have a higher mortality rate → fewer survive + breed, passing on their alleles to the next generation