Natural Selection

What is evolution?

Gradual change in the characteristics of a species

Who first proposed the Theory of Evolution?

Charles Darwin and Alfred Wallace in 1858

What are the three observations Darwin based the theory of evolution on?

• variation: all members of a species differes; characteristics passed from parents to offspring

• birth rate: all organisms reproduce more than food supply supports and resources, which would normally result in overcrowding

• natures balance: high birth rate but each species population remained relatively constant

What were Darwin’s interpreations from his observations?

• struggle for existence => high birth rate, low resources

• survival of the fittest => organisms with characteristics best suited for their enviroment increased chance of survival (unfavourable = death)

What is the modern interpretation of natural selection?

• natural selection => selection of those alleles in a population gives organisms a better chance of survival

• selective agent => enviromental factor acting on the population

• surviving organisms passe on favourable alleles => population becomes more suited for their enviroment

• individual organisms do not dapat; species adapt over generations

What are the principles of evolution through natural selection?

• There is variation of characteristics within a species.

• More offspring of a species are produced than can possibly survive to maturity. • Because of excessive birth rate and limited resources, there is a struggle for existence or competition for survival.

• Individuals with characteristics best suited to the environment have more chance of surviving and reproducing. This is known as survival of the fittest.

• Favourable characteristics are passed on to the next generation.

• In the gene pool, the proportion of alleles that produce favourable characteristics gradually increases

How does long bodies/short limbs favour some humans?

• smaller surface area relative to body volume

• lose less heat in cold enviroments (increased survivial advantage)

Why is sickle-cell anaemia higher in Africa than anywhere else in the world?

• humans cleared forrests in Africe for agriculture creating breeding sites for Anopheles mosquitoes, increasing the presence of malaria

• people with the sickle-cell trait have an increased resistance to malaria parasites, providing a survivial advantage (more resistant than haemoglobin)

What is sickle-cell anaemia?

• occurs when homozyhous for recessive allele

• point mutation in HBB gene

  • valine added instead of glutamic acid causing the haemoglobin to form differently and become distorted

• complications: sickle-shaped red blood cells die early (anaemia), inflexible cells can block blood vessels, fatigue, jaundice, increased blood pressure, heart failure, organ damage

What is Tay-Sachs disease?

• a recessive condition that is lethal

• detrioration of the nervous system due to absence of beta-hexosominidase

In places where tuberculosis is high, why is the Tay-Sachs allele high?

• heterozygous people have lower than usual beta-hexosaminidase which increases their protection against tuberculosis

• survivial and reproduced to pass on allele