4B Diversity and selection

• What is disruptive selection?

  When individuals with alleles for either extreme side of characteristics are more likely to survive e.g. squirrels with short and long tails.
• What is stabilising selection?

  Where individuals with alleles for middle ground characteristics are more likely to survive when presented with no environmental change. E.g. small babies struggle to maintain body temperature due to high surface area to volume ratio and large babies can cause complications when moving through the pelvis but middle sized babies have less complications.
• What is directional selection?

  Individuals with alleles for extreme characteristics are more likely to survive and reproduce when presented with environmental change. E.g. bacteria with antibiotic resistance survive when exposed to antibiotics, passing on the allele.
• What are the three types of selection?

  directional, stabilising, disruptive
• What are anatomical adaptations?

  Structural features of the body that increase survival chances. E.g. whales have a thick layer of blubber to hep stay warm in the cold sea.
• What are physiological adaptations?

  Processes inside the body that increase survival chances e.g. brown bears hibernate to lower metabolism rate and conserve energy over winder so they don't need to use energy to hunt for food in scarce months.
• What is a behavioural adaptation?

  Ways an organisms acts to increase survival chances e.g. possums play dead to escape attack.
• What are the three types of adaptations?

  behavioral, anatomical, physiological
• What is evolution?

  Gradual change in species over time.
• Give an example of natural selection.

  In 1810 a heard of caribou had the average fur length of 3.5, but when they were moved from the artic to a warmer climate in 1960, the average fur length was 2.1.
• What is the process of natural selection?

  Some mutations produce alleles that are beneficial when faced with selectional pressures e.g. disease or predators.

  
  

  This produces differential reproductive success in populations as those with alleles that increase survival chance are more likely to survive, reproduce, and pass on the gene.

  
  

  This means a greater proportion of the next generation inherit this allele.

  
  

  And over generations evolution occurs and the allele becomes more common where the original one phases out.
• What is natural selection?

  the process where organisms better adapted to their environment tend to survive and produce more offspring so the proportion of advantageous alleles increase through generations
• Why does the founder effect occur? Give an example

  Migration due to geographical seperation or religion e.g. the Amish population descended from a small number of Swiss who migrated. They have little genetic diversity as they are isolated due to religion.
• What is the founder effect?

  the reduced genetic diversity that results when a few organisms from a population start a new colony and reproduce.
• What are genetic bottlenecks?

  A reduction in allele frequency of a population following a dramatic reduction in the size of a population
• What does genetic diversity allow?

  natural selection to occur and therefore evolution.
• How do we increase genetic diversity?

  Mutations in the amino acid sequence of DNA causing new alleles.

  
  

  Gene flow, when individuals migrate and reproduce.
• What are the problems of low genetic diversity?

  Organisms may not be able to adapt to change.

  
  

  Large number of the species could be wiped out by a single external change.
• What is genetic diversity?

  The total number of different alleles of genes in a population.
• Give an example of a chromosome mutation and its effects.

  Chromosome non-disjunction is when the chromosomes fail to separate. When in chromosome 21 it causes Down syndrome.
• What do chromosome mutations cause?

  Inherited conditions as the errors are present in heridatory gametes.
• What are chromosome mutations?

  When the structure or number of a part of a chromosome varies due to an error in meiosis.
• What are mutagenic agents, give examples.

  A factor that increases the probability of mutations occurring. E.g. UV and some chemicals and viruses.
• How and when do mutations occur?

  When dna is misread during translation.
• What is a deletion mutation?

  One or more amino acid base is removed.
• What is a substitution mutation?


  One amino acid base is swapped for another.
• What are the two ways mutations occur?

  Substitution or deletion.
• What are mutations?

  Changes in the base sequence of chromosomes.
• What is the difference between the outcomes of meiosis and mitosis?

  Mitosis

  - produces diploid cells.

  - daughter cells are genetically identical to parents and eachother.

  - 2 daughter cells are produced.

  
  

  Meiosis

  - produces haploid cells.

  - daughter cells are genetically different to parents and eachother.

  - 4 daughter cells are produced.
• What is independent segregation?

  The random division of homologous chromosomes into separate daughter cells during meiosis.
• When does independent segregation occur?

  Metaphase 1
• What is crossing over?

  The chromatids of homologous pairs twist around eachother and exchange small parts of DNA so they contain the same gene but slightly different alleles.
• When does crossing over occur?

  Prophase 1
• What are the two ways that chromosomes become genetically different in meiosis?

  Crossing over and independent segregation.
• Describe the process of meiosis?

  DNA unravels and replicates to form two copies of each chromosome called chromatids.

  DNA condenses to form double armed chromosomes consisting of 2 sister chromatids joined by a centromere.

  Chromosomes arranged themselves into homologous pairs and are separated by the spindle during meiosis 1.

  The pairs of sister chromatids are further separated by the spindle at the centromere in meiosis 11.

  This produces four genetically different haploid granddaughter gametes.
• What does fertilisation produce?

  Zygotes with different combinations of chromosomes which increases genetic diversity.
• What happens during sexual reproduction?

  Two gametes join at fertilisation to form a zygote which contains the normal diploid chromosome number (half maternal and half paternal) this then divides and develops into a new organism.
• Are gametes diploid or haploid cells?

  Haploid, they only contain one copy of each chromosome.
• What is a gamete?

  reproductive cells, sperm and egg.
• How many chromosomes do humans have?

  46 (23 homologous pairs)