Evolutionary Theories

Modern evolutionary theories began emerging in the 1800's as new geological and biological discoveries reformed existing knowledge

Previously, the dominant paradigm described the 'fixity' of species - immutable and unable to change across generations (man had always been man)

Development of Evolutionary Theories


  • Jean Baptiste Lamarck, a French scientist, propose that species change because of the habitual use or disuse of features ('use it or lose it' concept)
  • Excessive use of a feature would cause it to develop, while a lack of use would result in atrophy (similar to muscle growth)
  • Lamarck proposed that these modified features could then be passed on to successive generations, changing the species over time
  • His theory that individually acquired traits could be inherited was wrong - cutting of a rat's tail does not result in tailless offspring

Lamarck's Theory:  Giraffe's inherited long necks from short necked ancestors who continually stretched their necks to reach food

Malthusian Dilemma

  • The Malthusian Dilemma was proposed by English clergyman, Thomas Malthus, who stated that populations multiply geometrically, while food resources only increase arithmetically
  • If left to follow course, a stable population (A) would inevitably outgrow it's resource base (B), leading to mass starvation and resource wars
  • This phenomena - that more offspring are born than will survive to reproduce - became central to Darwin's understanding of competition for survival
  • The idea that certain traits may be beneficial in such a struggle for survival lead to the concept of evolution via natural selection

Charles Darwin

  • Darwin's first theory of evolution, descent with modification, was based on a combination of Lamarckian ideas and recent fossil discoveries
  • He theorised that organisms living today had been changed over time and stemmed from a single (or perhaps few) ancestral organisms
  • Darwin expanded on this idea with his second theory, modification by natural selection, which built on the ideas of Thomas Malthus
  • He noted that although populations have the ability to grow uncontrollably, they will not do so for long due to natural limiting factors
  • Individuals who possess traits more desirable within a given environment would have an adaptive advantage and reproduce more effectively
  • Similar ideas were also proposed at about the same time by another man, Alfred Wallace (he corresponded with Darwin but published separately) 

Darwin's Theory:  Long necked and short necked giraffes once existed together, but the long-necked giraffe had an adaptive advantage


  • Neo-Darwinism is the synthesis of Darwinian theory and modern genetics
  • Darwin knew very little about the mechanism of variation or the biological basis for inheritance - he merely recognised that, whatever it's source, variation was necessary for natural selection to operate
  • Neo-Darwinism combines the theory of natural selection with other relevant and associated discoveries, including:
    • The works of Gregor Mendel in describing how features are inherited (Mendelian inheritance)
    • The works of James Watson and Francis Crick in determining the genetic basis of inheritance (DNA structure)

Pace of Evolution

Wile it is generally accepted within scientific communities that evolution within a species (microevolution) is gradual and continuous, debate exists as to whether this model is true when applied across the species barrier (macroevolution)

Two opposing theories regarding the pace of evolution leading to speciation exist:

Phyletic Gradualism

  • According to this model, speciation generally occurs uniformally and by the steady and gradual transformation of whole lineages
  • In this view, evolution is generally seen as a smooth and continuous process

Punctuated Equilibrium

  • According to this model, most sexually reproducing populations experience little change for most of their geological history
  • When phenotypic evolution does occur, it is localised in rare and rapid events of branching speciation (called cladogenesis)

Phyletic Gradualism versus Punctuated Equilibrium

  • While the relative lack of transitional fossils in the fossil record would seem to support the theory of punctuated equilibrium, such an absence could also be explained due to the relatively irregular and rare conditions required for fossilisation