Natural S./Genetic drift - chap 9.2.3.4.5

HBY

Features of natural selection

Natural selection depends on the environment

Acts on existing heritable variation

Heritable variation comes from random mutations 

Natural selection essay format

1. Overproduction
   - species is able to give birth to more offspring than what can be supported by the environment E.g give birth to multiple babies at a time

2. Genetic variation
   - there is genetic variation due to random and spontaneous mutation in DNA in germline cell

3. Selection pressure
   - exists in the form of

4. Survival of the fittest
   - Organisms with have a higher chance of SRP

5. Higher frequency of advantageous allele in the next generation of offspring 

What is variation 

Differences between different individuals within a species

• based on genotype/physical appearance and behaviour

Phenotypical variations due to environmental factors only (NOT INHERITABLE)

Genetic variation: variation due to genes can be passed down

Sources of variation

1. Random and spontaneous mutation in the DNA of germline cell

2. Crossing over of chromatids during meiosis

3. Random fertilisation

4. Random mating

Crossing over of chromatids during meiosis

crossing over in prophase 1 of homologous chromosomes

Random fertilisation

a random sperm will fertilize a random egg

Random mating

a random male has sex with different and random female

Inuit and Africans

Inuit

• long bodies short limbs

• smaller surface area in relation to body volume

• lose less heat in very cold environments

Africans

• long limbs and shirt bodies

• larger ratio of surface area to body volume

• lose more heat in very hot climates

What serious diseases persist in populations 

Black african population

• sickle cell anemia 

Ashkenazi jewish 

• Tay sachs 

Medetarranian coast 

• thalassemia 

Sickle cell anemia

• prevelant in black african population

• mutation in the HBB gene, responsible for beta globin protein in haemoglobin production

• Red blood cells have a sickle/cresent shape

• reduces oxygen carrying ability

• sickled red blood cells also die early

Symptoms

• fatigue and shortness of breath

In the black african population

• Homozygous recessive: die early due to SCA

• Homozygous dominant: susceptible to malaria

• Heterozygous: carriers, resistance to malaria

Malaria acts a more significant selection pressure

Tay sachs disease

• prevelant in the ashkenazi jewish population 

• mutation to the HEXA gene that codes for beta hexosaminidase 

• accumulation of lipid in the nervous system 

• inherited 2 recessive alleles from each parent 

• tuberculosis is the selective agent

Possible reasons why the recessive allele is common in ashkenazi jewish population

a)Genetic Drift

• population small and reproductively isolated —> increases the chance of genetic drift

b)Natural selection

Genetic drift

change in allele frequency by chance (non-directional)

• more likely to occur in small populations

Types of genetic drift 

Founders effect

• small group(founding population) moves away from the original population to a new location and establishes a new population 

• cause new population to have different allele frequency from the original population and decrease genetic variation 

Bottleneck effect 

• occurs when natural events reduce a population size significantly 

• less genetic variability 

• no longer represent the original gene pool  

Speciation structure 

1. Variation 
   In a single interbreeding population, genetic variation exists due to random genetic mutation 

2. Isolation 
   Species are reproductively isolated without gene flow due to a geographical/sociocultural barrier, no interbreeding between 2 populations 

3. Selection 
   each population now encounters different selection pressures (speciation takes longer if populations are closer to each other as smaller distance = similar selection pressure) 

4. Speciation
   allele frequency in the gene pool of each population become significantly different and the 2 populations are no longer able to interbreed and produce fertile offspring. Speciation has occured