module 2

• Speciation


  The process by which new biological species arise
• Steps for Speciation

  1. Reproductive Isolation

  2. Time - Divergence
• Reproductive Isolation


  Creates two distinct populations - no exchange of genes between gene pools
• Divergence


  Accumulation of random genetic variation over time
• Modes of Speciation


  • The key to speciation is the evolution of genetic differences between populations
  • Reduction to gene flow plays a critical role in speciation
• Allopatric Speciation


  1. Something extrinsic (not part of something) to the organism prevents two or more groups from mating with each other, eventually causing that lineage to speciate

  2. Isolation might occur because of great distances or a physical barrier, such as a desert or a river

  3. Gene flow between soon-to-be species must be greatly reduced- but it doesn't have to be reduced completely to zero
• Allopatric Speciation models

  • Dispersal model
  • Vicariance model
• peripatric enetic drift that occurs when a new population is established by a very small number of individuals from a larger population

  A genetic drift that occurs when a new population is established by a very small number of individuals from a larger populationgenetic drift that occurs when a new population is established by a very small number of individuals from a larger population
• Peripatric Speciation


  A special version of the allopatric speciation that happens when one of the isolated populations has very few individuals
• Peripatric Speciation


  1. Few survivors just by chance carry some genes that are rare in the mainland population

  2. One of these rare genes happens to cause slight variation

  3. These small differences drift to fixation in the small population over a course of generation

  4. Unique reproductive features on the island result in a cascade of changes by sexual selection

  5. Flies experience natural selection that favors individuals better suited to the climate and food of the island

  
• In peripatric speciation, small population size would make a full-blown speciation- a result of geographic isolation + genetic drift
• Genetic drift acts more quickly in small populations
• Genetic drift and selective pressures would cause rapid genetic change in small population
• This genetic change could lead to speciation
• Essential characteristic of peripatric speciation is that genetic drift plays a role in speciation
• If there is a drastic change (disease or natural catastrophe) in the survival of a large population, there is little change in the frequency of that population's alleles. In a smaller population, drastic change can result in loss of an entire allele
• Parapatric Speciation


  There is no extrinsic barrier to gene flow, new space open up for the population, individuals are more likely to mate with their geographic neighbors than with individuals in a different part of the population's range
• Parapatric Speciation example

  • Grass species, Anthoxanthum odaratum
• The plants around the mines have experienced natural selection for genotypes that are tolerant of heavy metals, while neighboring plants that don't live in polluted soil have not undergone such selection
• The two types of plants are close enough that tolerant & nontolerant individuals could potentially fertilize each other (requirement of parapatric ; continuous population)
• The two types of plants have evolved differently through time, which could be the first step in cutting off gene flow entirely between the two groups
• Sympatric Speciation

  Does not require large scale geographic distance to reduce gene flow between parts of population, requires development of reproductive isolation without geographic barriers, ultimately requires development of non-random mating within a population
• Sympatric Speciation example

  • Ancestors of apple maggot flies laid their eggs only on hawthorns, but today, these flies lay eggs on hawthorns (native to America) and domestic apples (introduced to America)

  
• Females generally choose to lay their eggs on the type of fruit they grew up in, and males tend to look for mates on the type of fruit they grew up in, so hawthorn flies generally end up mating with other hawthorn flies & apple flies mates with other apple flies also

  
• As time progress, some genetic differences between these two groups of flies have evolved
• Cospeciation


  If the association between two species is very close, they may speciate in parallel
• Cospeciation example


  • Rodent switching allows genes to flow through louse species, when the rodents get together to mate, lice get an opportunity to switch rodents & perhaps mate with lice on another rodents
• Rarely do scientists find hosts and parasites with exactly matching phylogenies, but sometimes the phylogenies indicate that cospeciation did happen along with host-switching
• Speciation by hybridization


  Several species had been formed by fertilizations between other species, often hybrid offspring of such fertilization are sterile, occasionally they are fertile and reproductively isolated from their parent species, in this case, a new species is formed
• Vegetative propagation


  A type of asexual reproduction in plants where the offspring is genetically identical to the parent plant, new plant grow from parts of the parent plant
• allo


  other
• patric 


  place
• peri


  near
• para


  beside
• sym


  same
• allopatric 


  geographically isolated populations
• peripatric 


  small population isolated at the edge of a larger population
• parapatric


  continously distributed places
• sympatric


  within the range of the ancestral population
• extrinsic 


  not part of something