MODULE 6

• Population


  Total number of organisms belonging to the same group or species living in a particular geographical area and is capable of interbreeding
• Genetics


  Genes, genetic variation, heredity - strong link with information systems
• Population Genetics


  Study of genetic variation within and among populations and how these changes across generations

  
• The study of populations is intimately related to the study of evolution
• It is the population, not the individual that evolved
• Evolution can be defined in terms of what happens to the genetic structure of a population over time

  
• Populations typically undergo changes in

  • Size (number of individuals)
  • Composition (extent of phenotypic variation)
  • Behavior (mating behavior)
• Charles Darwin saw evolution in these terms
• Charles Darwin agreed with Malthus who proposed that populations could in theory increase in size exponentially
• Thomas Malthus


  British philosopher and economist, best known for his theories about population growth
• But not in practice because of resource limitations: food and space
• Natural Selection


  Organisms that are more adapted to their environment are more likely to survive and pass on the genes that aided their success
• Natural selection causes species to change and diverge over time
• Darwin proposed that in most situations in nature there will be competition to survive and reproduce and that this was governed by a process - natural selection
• This implies a better chance of reproducing – leaving offspring
• If the variation which increased the chances of survival were to be inherited, then the offspring would also have a better chance of survival and reproduction
• Over time any gene variant that contributed to a selective advantage would increase in frequency in the population
• Population


  A group of individuals that interbreed freely and randomly
• In general a population will consist of members of a species between which breeding can occur
• Gene Pool


  The sum of all alleles at all gene loci in the population
• Allele


  Alternative form or versions of a gene
• Autosome


  One of the numbered chromosomes

  
• Humans have 22 pairs of autosomes and one pair of sex chromosomes (XX or XY)

  
• Dominant gene


  One that is expressed in an organism's phenotype, or physical appearance, when present
• Recessive gene


  One that is only expressed in an organism's phenotype when two copies of the recessive allele are present
• Dominant genes mask the expression of recessive genes
• If there are only two alleles for a gene, one dominant and one recessive, then 100% of the alleles are either dominant (p) or recessive (q)

  
• • What can allele frequencies tell us about a population?
  • 1. Whether the gene pool is stable or undergoing change
  • 2. We can estimate the rate of change
  • 3. If we know the rate of change, we can make predictions about likely future trends
  • 4. This has important applications in conservation of wild populations and in captive breeding programs</b>

  
• Hardy-Weinberg Theorem or Equation


  A fundamental concept in Population Genetics that states the frequencies of alleles and genotypes in a population's gene pool will remain constant from generation to generation unless acted upon by factors other than sexual recombination

  
• Conditions for Hardy-Weinberg equilibrium


  • The population is infinitely large or at least large enough so that no sampling errors occur
  • Within the population mating occurs at random
  • There is no selective advantage for any genotype (all gametes are equally viable and fertile)
  • There is no mutation
  • There is no migration

  
• These parameters describe a non-evolving population
• Hardy-Weinberg equation


  p2 + 2pq + q2 = 1
• A non-evolving population is the exception rather than the rule
• If a population is evolving, then allele frequencies will change over time i.e. the composition of the gene pool will change from one generation to the next

  
• Microevolution


  Evolution on a small scale, referring to changes in a population's gene pool even if the frequency of alleles are changing for only a single locus

  
• 5 causes of microevolution

  • Genetic drift
  • Gene flow
  • Mutation
  • Non-random mating
  • Natural selection

  
• Genetic drift


  Random changes in allele frequencies because of chance fluctuations in population size

  
• Genetic drift in a population can lead to the elimination of an allele from a population by chance

  
• Any natural disaster that causes a population crash will have a high probability of changing allele frequencies in the next generation
• Bottleneck effect


  Certain alleles will be over-represented in the survivor population and other alleles will be under-represented