Diversity & selection

• What is meiosis?

  Cell division in reproductive organs. Cells that divide by meiosis are diploid to begin with-cells formed are haploid (chromosome number halves). Produces gametes.
• What happens before meiosis?

  DNA unravels & replicates, so there's 2 copies of each chromosome, called chromatids ( 2 x 2n). DNA condenses to form double-armed chromosomes, each made from 2 sister chromatids. Sister chromatids joined in middle by centromere.
• What happens during meiosis 1?

  First division- chromosomes arrange themselves into homologous pairs. These homologous pairs are then separated, halving the chromosome number.
• What happens during meiosis 2?


  Second division- the pairs of sister chromatids that make up each chromosome are separated (centromere is divided). 4 haploid cells that are genetically different from eachother are produced.
• What is random fertilisation?

  Any sperm can fertilise any egg. Produces zygotes with different combinations of chromosomes to both parents. Mixing genetic material increases genetic diversity within a species.
• What 2 events lead to genetic variation?

  Crossing over of chromatids & independent segregation of chromosomes.
• Crossing over of chromatids:

  During meiosis 1, homologous chromosomes come together & pair up. Chromatids twist around eachother & bits of chromatids swap over. Chromatids still contain same genes but now have different combination of alleles. Means each of 4 daughter cells formed from meiosis 2 contain chromatids with different alleles.
• Independent segregation of chromosomes:

  Each homologous pair of chromosomes in cell is made from 1 chromosome from mum & 1 from dad. When homologous pairs separated in meiosis 1, it's completely random which chromosome from each pair ends up in which daughter cell. 4 daughter cells produced have different combinations of the maternal & paternal chromosomes.
• What are chromosome mutations?

  Cells produced contain variations in number of whole chromosomes or parts of chromosomes. Lead to inherited conditions as errors present in gametes (hereditary cells). E.g, chromosome non-disjunction mutation (failure of chromosomes to separate properly).
• What is genetic diversity?


  The number of different alleles of genes in a species or population.
• What are genetic bottlenecks?

  Events that causes a big reduction in a population. E.g, when large number of organisms within a population die before reproducing. Reduces number of different alleles in gene pool & so reduces genetic diversity.
• 2 ways genetic diversity within a population can be increased:

  Mutations in DNA form new alleles (some advantageous, some not) & different alleles introduced via migration (gene flow). Allows natural selection to occur.
• What is the founder effect?

  Few organisms from a population start a new colony, & there's only small number of different alleles in initial gene pool. Can occur as result of migration or another separation reason e.g, religion. Frequency of allele that was rare in OG population may be more common in new colony- can lead to higher incidence of genetic disease.
• What is evolution?

  Gradual change in species over time. Led to huge diversity of organisms on Earth. Adaptation & selection both key factors.
• What is natural selection?

  Increases proportion of advantageous alleles within a population. Leads to evolution as advantageous alleles become more common.
• What are adaptations?


  Features that help organisms to survive in their environment. 3 types: behavioural, anatomical (structural) & physiological.
• What are behavioural adaptations?

  Ways an organism acts that increases its chance of survival, e.g, playing dead.
• What are physiological adaptations?

  Processes inside an organism's body that increase its chance of survival. E.g, hibernation conserves energy by lowering metabolism rate. Some bacteria produce antibiotics.
• What are anatomical (structural) adaptations?

  Structural features of an organism's body that increase its chance of survival. E.g, otters have streamlined shape to glide through water & catch prey & escape predators. Whales have thick layer of blubber to keep warm.
• What is directional selection?

  Individuals with alleles for characteristics of an extreme type are more likely to survive & reproduce. Could be in response to an environmental change. E.g, antibiotic resistance.
• What is stabilising selection?

  Where individuals with alleles for characteristics towards the middle of the range are more likely to survive & reproduce. Occurs when the environment isn't changing & it reduces the range of possible characteristics. E.g, human birth weights.
• Why shouldn't you seal the petri dish completely?

  It'll prevent oxygen entering dish- could encourage growth of anaerobic disease-causing bacteria.
• Required practical 6 (investigating selection using antibiotics):

  Different antibiotics' effect on bacterial growth using aseptic techniques. Various concentrations of antibiotics used.
• What is a liquid broth?

  Bacteria grown in mix of distilled water, bacterial culture & nutrients.