Evolution

• Microevolution: change in allele frequency within a population or species over time
• Macroevolution: changes in allele frequencies over many thousands of years, eventually gives rise to a new species
• Five agents of evolutionary change:

  1. natural selection
  2. Disruption of gene flow
  3. genetic drift
  4. non-random mating
  5. mutations
• Natural selection:

  • Phenotypic variation must exist in population for natural selection to act on
  • Individuals with favourable traits and thus selective advantage will be selected for, can survive and reproduce to pass down favourable alleles to the next generation
  • changes allele frequency
• Directional (natural) selection:

  • phenotype at one extreme is repeatedly selected for
  • favours initially rare individuals
  • due to a specific selection pressure
• Disruptive (natural) selection:

  • intermediate phenotypes selected against
  • favours individuals of both extreme ends
  • can result in polymorphisms (2 or more forms found in one species)
• Stabilising (natural) selection:

  • extreme phenotypes selected against
  • favours more common intermediate variants
  • maintains phenotypic stability over time
• Gene flow: transfer of alleles from one population to another through movement of fertile individuals or their gametes

  • can reduce differences in allele frequencies between neighbouring populations
• Disruption in gene flow

  • result in differences in allele frequency over time
  • needed for speciation to occur
  • can be geographical disruption (geographical isolation) or change in mobility of individuals (physiological/ecological isolation)
• Genetic drift: change in allele frequency due to chance events

  • tends to reduce genetic variation in populations through loss of alleles
  • the smaller the original population, the greater the impact of genetic drift
  • founder effect and bottleneck effect
• founder's effect:

  • a few random individuals of a larger population became pioneers of a newly isolated population
  • continuous breeding, rare alleles may become more common
• bottleneck effect: catastrophic event leading to a drastic reduction in allele frequencies, even elimination of alleles

  • few, random surviving individuals constitute the genetic sample of the original pre-catastrophe population
  • certain alleles may be over-represented or under-represented among survivors
• non-random mating:

  • individuals preferentially choose mates
• Mutation: a random change in an organism's DNA, a source of new alleles

  • mutation must occur in gametes to have an impact on evolution
  • mutation rates are low, it is the accumulation of mutations that can be significant on evolution
• Preservation of genetic mutation:

  • diploidy/heterozygote protection
  • balancing selection
• diploids/heterozygote protection:

  • recessive alleles hidden from selection can persist as dominant allele masks effect of the recessive allele
  • maintains a huge pool of alleles that could bring new benefits when environment changes
• balancing selection:

  • when natural selection maintains two or more alleles at a locus
  • arise due to heterozygotes having a selective advantage
• balancing selection (heterozygote advantage):

  • heterozygotes have greater fitness than both kinds of homozygotes
  • eg sickle cell anaemia
• balancing selection (frequency dependent):

  • fitness of phenotype depends on how common it is
  • eg scale eating fish
• biological species concept: a species is a group of organisms capable of interbreeding and producing fertile, viable offspring, offspring of the same species are reproductively isolated from those of a different species (A LEVEL)

  • can be used to determine if organisms are the same species
  • cannot be used for asexually reproducing organisms and extinct species whose breeding behaviour cannot be observed in nature
  • different species can mate and give rise to viable offspring in plants due to polyploidy
• genetic species concept: a species is a group of genetically compatible interbreeding organisms in a natural population that is genetically isolated from other groups, organisms in a species have sufficient similarity in their DNA sequences and share same number of chromosomes

  • uses genetic data
  • tech needed to study dna sequences is expensive, not accessible to everyone
• ecological species concept: a species is a group of organisms sharing the same ecological niche

  • every organism has a niche
  • cannot be applied to unrelated species that occupy similar niche
  • determining niches can be time-consuming and difficult
• niche: both the place where an organism lives and its interactions with the environment (role it plays in its habitat)
• morphological species concept: a species is a group of organisms sharing similar body shape and other structural features

  • can be easily studied with a specimen
  • difficult to determine degree of difference required to indicate separate species and what structural features should be used to distinguish differences
  • some organisms are superficially similar but have different evolutionary origins
• Phylogenetic species concept: a species is the smallest group of organisms that share a most recent common ancestor and can be distinguished from other such groups

  • phylogenetic history can be obtained by comparing homologous morphological structures and/or homologous molecular sequences
  • can avoid mistakenly classifying organisms based on superficial morphological similarities
  • provides accurate historical info
  • difficult to come up with accurate phylogenetic tree as multiple sources of evidence need to be gathered and processed
• Allopatric speciation:

  • sub-populations are divided and geographically isolated due to physical barrier
  • gene flow is disrupted
  • accumulation of mutations, genetic drift and natural selection occur independently of the other sub population due to different selection pressures, become reproductively isolated
  • over time new species form
• Sympatric speciation:

  • population either physiologically or behaviourally isolated
  • gene flow disrupted
  • accumulation of mutations, genetic drift and natural selection occur independently within each sub-population due to different selection pressure, leads to reproductive isolation
  • over time new species form
• physiological isolation: mating between individuals of different sub-populations existing in the same geographical area is not possible due to unique physiology or behavioural differences, blocking gene flow
• Linnaean classification:

  • hierarchical
  • use of binomial name where genus name is followed by the species name
  • advantages: species can be easily categories and named, provides an accurate identity for the species
  • disadvantages: cannot infer evolutionary relationships between members of each category, cannot tell how distantly related one species is to another
• phylogeny: organisation of species to show their evolutionary relationship, presented in a phylogenetic tree
• crafting phylogenetic trees:

  • use of homologous characters to group species (phenotypic and genetic similarities due to shared ancestry)
  • use of morphological, molecular, anatomy and fossil records
• convergent evolution: different species that do not share a recent common ancestor can independently evolve similar traits as a result of having to adapt to similar environments or ecological niches

  • traits will be known as analogous structures
  • eg flipper of dolphin, fins of a fish
• descent with modification: environmental conditions select for or against the modifications in the descendants which came about as a result of genetic variation in the population

  • derived from common ancestor, serve different functions but retain common underlying similarity
  • pentadactyl limb of tetrapods (bats, humans, cats, whales)
• shared ancestral characters: character that originated in an ancestor and is shared by all its descendants
• shared derived character: a unique character of the group but not found in ancestors or the ancestors’ other descendants
• Use of molecular methods:

  • evolutionary changes captures in genetic sequences, the more closely related the species, the more similar nucleotide sequences there will be
• advantages of molecular methods:

  • can be used to compare species that are morphologically indistinguishable due to convergent evolution or are very closely related
  • remotely related organisms can be compared due to common proteins
  • objective, molecular character states are unambiguous, A T C G are easily recognisable compared to morphological characters that could be similar
  • quantitative, molecular data easily converted to numerical form, can be used for statistical analysis and hence computation
  • mutations occur regularly, and accumulate over time, can then estimate time of speciation
• Usage of mitochondrial dna:

  • no rearrangement of alleles in mtDNA from parent to offspring (not used in meiosis to form gametes), no recombination or crossing over
  • changes in mtDNA sequence is solely due to accumulation of mutations over time
  • faster rate of mutations compared to nuclear DNA, can be used in comparing species that are closely related
• Vestigial structures:

  • features that resemble structures of organism’s presumed ancestors
  • typically degenerate, atrophied or rudimentary, have lost some or all of functional roles
  • detrimental to organism
• Give rise to genetic variation:

  • crossing over between non-sister chromatids of a pair of homologous chromosomes during prophase I of meiosis, new combinations of alleles
  • independent assortment of chromosomes during metaphase I of meiosis, diff combinations of paternal and maternal chromosomes in gametes
  • random fusion of gametes during fertilisation leading to diff combinations of alleles
  • gene mutation