Describe the role of natural selection in evolution. [8] #
ZIMSEC June 2014/2/12(a)
- 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
Explain how natural selection may bring about evolution. [8] #
- 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 ;
Describe why variation is important in natural selection. [6] #
- 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 favourable characteristics survive / AW ;
- pass on favourable characteristics to offspring ;
- those with disadvantageous characteristics die ;
Explain the role of isolating mechanisms in the evolution of new species. [8] #
- ref. to definition of species ;
- ref. allopatric ;
- geographical isolation ;
- >ref. to examples e.g. islands / lakes / mountain chains / idea of barrier ;
- >ref. to example organism ;
- ref. to populations prevented from interbreeding ;
- isolated populations subjected to different selection pressures / conditions ;
- over time sufficient differences to prevent interbreeding ;
- ref. sympatric ;
- ref. to reproductive isolation ;
- ref. behavioural barriers (within a population) ;
- e.g. day active / night active ;
- correct ref. to gene pool ;
- change in allele frequencies ;
Explain, using named examples, how mutation can affect phenotype. [8] #
ZIMSEC June 2014/2/12(b)
- 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. [8] #
- 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 ;
Explain how meiosis and fertilization can result in genetic variation amongst offspring. [8] #
- 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 the role of isolating mechanisms in the evolution of new species. [8] #
- 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 ;
Describe and explain, using an example, the process of artificial selection. [8] #
ZIMSEC November 2015/2/11(b)
- 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 ;
Diffences 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 |
