migration

• gene flow - movement of genetic material from one population to another
• Barriers to gene flow:

  (inhibition of interbreeding between population due to barriers, leading to the formation of gene pools)

  • geographical barriers (oceans, mountain, lake systems, desert, ice sheets)
  • sociocultural barriers (economic status, educational background, social position)
• • natural selection process by which a species become better adapted to its environment
• Observations of Darwin’s theory of natural selection:

  • variations - all members of a species vary; they were passed on from one generation to the next
  • birth rate - all living organisms reproduce at a rate far greater at which their food supply and other resources increase
  • nature’s balance - despite high birth rate, each species’ numbers tended to remain at a relatively constant level
• Principle of evolution through natural selection:

  • variation of characteristics within a species
  • more offspring of a species are prod. than can they possibly survive to maturity
  • there is a struggle for existence or competition for survivial (excessive birth rate and limited resources)
  • individuals with characteristics best suited to the environment have higher chances of survival and reproduction
  • favourable characteristics are passed on to the next generation
  • in gene pool, proportion of alleles that prod. favourable characteristics gradually increases
• Examples of natural selection:

  • body stature
  • sickle-cell anaemia
  • Tay-Sachs disease
  • Thalassemia
• Sickle-cell anaemia:

  (disease where erythrocytes are crescent-shaped, reducing flexibility which causes blockages)

  • homozygous recessive - point mutation of HBB gene
  • HBB codes for one of the beta-globulin proteins making up haemoglobin
  • amino acid valine added instead of glutamic acid → alters haemoglobin prod. → shape distortion
  • heterozygotes show no ill effects unless O2 in short supply
  • homozygous dominant have blood with no signs of sickling
• Genetic drift:

  (random, non-directional change in allele frequency between generations)

  • also known as random genetic drift or the Sewall Wright effect
  • occurs in all populations but unlikely to have a significant effect in large populations
  • can play important role in evolution in small populations
• Founder effect:

  (occurs when a small group moves away from its homeland to a totally new area and establishes a population)

  • chance can cause new groups to have different allele frequency or decreased genetic variation
• Bottleneck effect:

  (when an event severely reduces the size of the population and the allele frequency may be different after event)

  • chance of survival is purely by chance not due to a specific trait
• Speciation:

  (process of new species developing)

  1. variation - between individuals of a species
  2. isolation - population of same species isolated without gene flow
  3. selection - each population is subjected to different selective agents
  4. speciation - allele frequency changes until they become so different that interbreeding is no longer possible