Evolution


Evolution is the cumulative change in the heritable characteristics of a population

  • It will be driven by random processes (genetic variation) and differentially selective processes (natural selection)
  • There are two main types of evolution:  microevolution and macroevolution


Microevolution

Microevolution describes evolutionary changes that occur over a relatively short period of geologic time (such as between generations) and results in diversification within a species

  • A population is a group of organisms of the same species who live in the same area at the same time
  • A gene pool refers to the totality of genes (and their particular alleles) within a population


Diversification within a species can result from four main processes: 

  • Mutation:  A random change in the genetic composition of an organism due to changes in the DNA base sequence
  • Random mating:  Sex can introduce new gene combinations into a population
  • Gene flow:  The movement of alleles into, or out of, a population as a result of immigration and emigration (also called gene migration)
  • Genetic drift:  The change in the composition of a gene pool as a result of a random or chance event
    • Genetic drift occurs faster and is more significant in small populations, where chance events have a bigger impact on the gene pool
    • Larger populations are less affected by isolated random events and retain more stable allele frequencies with low genetic drift
    • Two main types of genetic drift can occur when large populations are reduced to small populations


Founder Effect

  • The founder effect occurs when a small group breaks away from a larger population to colonise a new territory
  • This population subset does not have the genetic diversity of the main population and hence will be subject to a higher level of genetic drift
  • As the small population increases in numbers the new population will no longer be representative of the original gene pool


Population Bottleneck

  • Population bottlenecks are a type of genetic drift that occurs when natural events (e.g. forest fires, floods) or man-made events (e.g. deforestation, overhunting) reduce a population size by an order of magnitude  (>50%)
  • The surviving population contains less genetic variability than before (hence a smaller gene pool)
  • As the surviving members of the species begin to repopulate, the new population will no longer represent the original gene pool


The Founder Effect or Population Bottleneck


Macroevolution

Macroevolution describes evolutionary relationships that occur over a relatively long period of geologic time (such as epochs) and results in diversification between species (e.g. speciation)

  • Macroevolution describes changes in large populations and often entails significant environmental change
  • Not all populations undergo constant evolutionary change, some populations remain relatively unchanged, exhibiting stasis (e.g. sharks)
  • Diversification between species can follow one of four main patterns of evolution:


Divergent Evolution

  • When two or more species evolve from a common ancestor
  • Example:  Adaptive radiation of Darwin's finches of the Galopagos Islands to suit distinct environmental niches


Convergent Evolution

  • When two or more unrelated species adopt similar adaptations in response to common environmental conditions 
  • Example:  Sharks and dolphins are similar in appearance but are not related


Parallel Evolution

  • Two or more species continue to evolve similar characteristics even after their divergence from a common ancestor
  • Example:  Large ears seen in a number of species of Australian microbat, used for echolocation


Coevolution

  • The evolution of one species in response to another, usually occurring in a predator-prey relationship
  • Example:  Flowers evolve bright colours to attract birds, birds evolve longer beaks to gather pollen


Adaptive radiation is the divergence of many lineages from a single ancestral species (e.g. Darwin's finches)


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