Selection

• a mechanism in which physically, physiologically, environmentally and generally adapted organisms survive and reproduce
• organisms best suited in the environment survive and reproduce to pass on their allele in the next generation.
• organisms that are not suited in the environment do not survive and reproduce hence they don’t continue to exist(extinction).
• environmental factors may result in other organism best suited for surviving and other die
• Such environmental factors exert selection pressure
• selection can be classified into:
  • Artificial Selection
  • Natural Selection

 

Explain, using named examples, how mutation can affect phenotype.

• gene) example ; (sickle cell / PKU )
• change in gene / DNA / base change ;
• different amino acid ;
• different polypeptide / different protein / non-functional protein ;
• (chromosome) example ; (Down’s, Turner’s syndromes)
• structural changes in chromosomes ;
• change in number of chromosomes ;
• change in sets of chromosomes / ref. polyploidy ;

 

Explain, using examples, how the environment may affect the phenotype of an organism.

• phenotypic variation results from interaction of genotype and environment
• environment may limit expression of gene(s) ;
• e.g. for size / mass / height ;
• because, food / nutrients / ion, missing / malnutrition ;
• named, nutrient / ion / mineral, missing ;
• environment may, trigger / switch on, gene ;
• ref. low temperature and change in animal colour ;
• ref. high temperature and, curled wing in Drosophila / gender in crocodiles ;
• ref. UV light and melanin production ;
• ref. wavelength of light and, flowering / germination / fruit colour ;
• other named trigger plus example ;
• environment effect usually greater on polygenes ;
• environment may induce mutation affecting phenotype ;

 

Natural Selection

• process in which individuals best suited in the environment survive and reproduce naturally to produce their phenotype offsprings.
• selection pressure is exerted by environmental factors eg presence of light and nutrients may affect the survival of plant organism in a given area.
• it results in competition for light and nutrients.
• the best adapted organism only survive to reproduce whilst other organisms die.

 

The role of natural selection in evolution.

• individuals in population have great reproductive potential;
• in population remain roughly constant ;
• many fail to survive / die ;
• do not reproduce ;
• due to environmental factors / named factor ;
• variation in members of population ;
• those best adapted survive ;
• reproduce / pass on alleles ; R genes
• genetic variation leads to change in phenotype ;
• ref: changes in gene pool ;
• over time produces evolutionary change ;
• new species arise from existing ones.

 

Reasons why variation is important in natural selection.

• ref. continuous / discontinuous variation ;
• genetic / inherited variation ;
• variation in phenotype / characteristics ;
• (can be due to) interaction of genotype and environment ;
• e.g. of characteristic that influences survival ;
• ref. intraspecific competition / struggle for existence ;
• those with favorable characteristics survive / AW ;
• pass on favorable characteristics to offspring ;
• those with disadvantageous characteristics die ;

 

How natural selection may bring about evolution.

• individuals in population have great reproductive potential ;
• numbers in population remain roughly constant ;
• variation in members of population ;
• environmental factors / named factor (biotic or abiotic) ;
• (cause) many, fail to survive / die / do not reproduce ;
• those best adapted survive / survival of the fittest ;
• (reproduce to) pass on alleles ; R genes
• genetic variation leads to change in phenotype ;
• ref: changes in, gene pool / allele frequency ;
• over time produces evolutionary change ;
• new species arise from existing ones / speciation ;
• directional / stabilizing, selection ;

 

Differences between natural and artificial selection

 

Artificial selection	natural selection
Selection pressure applied is by humans	Environmental selection pressure
Genetic diversity is lowered	Genetic diversity remains high
Inbreeding is common	Outbreeding is common
inbreeding depression	less inbreeding depression
Increased homozygosity	Decreased homozygosity
Usually faster	Usually slower
Not for survival/evolution	Promotes survival/evolution
No isolation mechanisms operating	Isolation mechanisms do operate

 

Artificial Selection

Its a mechanism which involves isolation of individuals which the desired characteristics are bred to produce a phenotype with the desired characteristics

 

Describe and explain, using an example, the process of artificial selection.

• humans ; must be linked to, choosing / selecting / mating etc
• parents with desirable feature ;
• e.g. organism and feature ;
• bred / crossed ;
• select offspring with desirable feature ;
• repeat over many generations ;
• increase in frequency of desired allele(s) / decrease in frequency of undesired allele(s) ;
• background genes ;
• loss of hybrid vigour / increase in homozygosity / ref. inbreeding depression ;

 

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Practice questions

• Describe and explain, using an example, the process of artificial selection.

 

Explain the role of isolating mechanisms in the evolution of new species.

• allopatric speciation ;
• geographical isolation / spatial separation ;
• e.g. of barrier ;
• e.g. of organism ; must relate to 3
• sympatric speciation ;
• example ;
• meiosis problems ;
• polyploidy ;
• behavioural / temporal / ecological / structural, isolation ;
• (isolated) populations, prevented from interbreeding / can only breed
• amongst themselves ;
• no, gene flow / gene mixing, (between populations) ;
• different selection pressures operate ;
• natural selection ;
• change in allele frequencies ;
• different gene pool ;
• over time (differences prevent interbreeding) ;
• reproductively isolated ;

Explain how meiosis and fertilization can result in genetic variation amongst offspring.

• chiasma / crossing over ;
• between non-sister chromatids ;
• of, homologous chromosomes / bivalent ;
• in prophase 1 ; linked to first point
• exchange of genetic material ;
• linkage groups broken ;
• new combination of alleles ;
• independent assortment ;
• metaphase 1 ; linked to previous point
• detail of independent assortment ;
• possible mutation ;
• random mating ;
• random fusion of gametes ;

 

Explain, using named examples, how mutation can affect phenotype.

• gene) example ; (sickle cell / PKU )
• change in gene / DNA / base change ;
• different amino acid ;
• different polypeptide / different protein / non-functional protein ;
• (chromosome) example ; (Down’s, Turner’s syndromes)
• structural changes in chromosomes ;
• change in number of chromosomes ;
• change in sets of chromosomes / ref. polyploidy ;