10 - Variation and Evolution

10 - Variation and Evolution

1. Variation

   • Continuous
     • e.g. height
     • Normal distribution curve
     • Polygenic
     • Range of phenotypes
     • Environmental factors have big influence
   • Discontinuous
     • E.g. blood group
     • Monogenic
     • No intermediates
     • Environmental factors have little influence
     • Bar chart
     • Characteristics fit into distinct groups
   • Causes
     • Gene (point) mutations
     • Crossing over in Prophase I
     • Independent assortment in Metaphase I/II
     • Random mating
     • Random fusion of gametes
     • Epigenetic modifications from environmental factors
     • Non-heritable variation from environmental factors

2. Competition

   • Intraspecific = members of the same species compete for the same resource
   • Interspecific = members of different species compete for the same resource

3. Selection pressure

   • = an environmental factor that can alter the frequency of alleles in a population, when it is limiting
   • Natural selection = the increased chance of survival and reproduction of organisms with phenotypes suited to their environment, enhancing the transfer of favourable alleles from one generation to the next
   • Value of an allele depends on the environment
   • e.g. availability of nesting sites, day length, overcrowding, predation, temperature, human impact

4. Population genetics

   • Gene pool = All the alleles present in a population at a given time
   • Allele frequency = The frequency of an allele as a proportion of all the alleles of that gene in a gene pool
   • Hardy-Weinberg Equation
     • p+q=1
     • p^2+2pq+q^2=1

5. Evolution and selection

   • Evolution = A change in the average phenotype of a population
   • Founder effect and genetic drift
     • Genetic drift = Chance variations in allele frequencies in a population
     • Founder effect = The loss of genetic variation in a new population established by a very small number of individuals from a larger population
   • Natural selection
     • Stabilising selection = the average phenotype provide greater advantage than either extreme
       • Normal distribution curve
     • Directional selection = an extreme phenotype becomes advantageous
     • Disruptive selection = average phenotype does not provide an advantage and is selected against, a lower and higher value are selected

6. Isolation and speciation

   • Speciation = formation of a new species
   • Species = A group of phenotypically similar organisms that can interbreed to produce fertile offspring
   • Reproductive isolation = The presentation of reproduction and gene flow between breeding groups within a species
   • Deme = Sub-group within a population that breeds more often
   • Pre-zygotic isolation
     • Geographical
       • Allopatric speciation = the evolution of new species from demes isolated in different geographical locations
     • = gametes are prevented from fusing, so a zygote never forms
     • Behavioural
       • Mating rituals and courtship displays
       • Sympatric speciation = the evolution of a new species from demes sharing a geographical location
     • Morphological
       • Sympatric speciation
       • Compatible/incompatible genitalia
     • Gametic
       • e.g. germination/pollination
     • Seasonal
   • Post-zygotic isolation
     • = gametes fuse and zygote forms, is sterile
     • Hybrid sterility
       • hybrid = the offspring of a cross between members of different species
       • embryo develops but offspring is sterile, because homologous pair cannot form at prophase I
       • Hybrid no. chromosomes between that of each parent
     • Hybrid inviability
       • fertilisation occurs but embryo does not form

7. Darwinian evolution

   • 1. Organisms overproduce offspring, so large genetic variation of genotypes in population
     • 2. Environmental changes bring new selection pressures
       • 3. Only individuals with beneficial alleles have a selective advantage, increasing their chance of survival
         • 4. These individuals reproduce more successfully than those without beneficial alleles
           • 5. Offspring likely to inherit beneficial alleles
             • 6. Beneficial allele frequency increases within the gene pool

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