Life Sciences Term 3

Lamarckism and Darwinism

Lamarck's theory

Use and disuse of organs:

• Changes in the environment create new needs that cause organisms to modify their existing organs to meet the need
• Repeated use of the organ would cause it to enlarge and become more efficient
  
• Disuse of an organ would cause it to degenerate

Inheritance of acquired characteristics:

• The modification an organism acquired during its lifetime could be passed on to its offspring

Guiding questions when Lamarck's theory is applied to a new situation:

• What was the original characteristic?
• What was the challenge?
• What did the organism do / what characteristic was then acquited?
• What was the result?
• What happened to this acquired characteristic?
• What was the result of this?

Rejection of Lamarck's theory

• Organisms do not evolve because they were determined to change but changes took place randomly due to mutations
  
• Acquired characteristics cannot be inherited i.e. the phenotype cannot affect the genotype as discovered later by Mendel

Darwin's theory

Darwin’s book published in 1859 called “On the Origin of Species by Means of Natural Selection” put forward two main ideas:

1. Species were not created in their present form but evolved from ancestral species
2. Proposed a mechanism for evolution - NATURAL SELECTION

Darwin's theory of evolution by natural selection:

• Organisms produce a large number of offspring
• There is a great deal of variation amongst the offspring
• Some have favourable characteristics, and some do not
• When there is a change in the environmental conditions or if there is competition then organisms with characteristics which make them more suited, survive whilst organisms with unfavourable characteristics, which make them less suited, die
• The organisms that survive, reproduce and thus, pass on the allele for the favourable characteristic to their offspring
• The next generation will therefore have a higher proportion of individuals with the favourable characteristic
• In this way, the characteristics of a population gradually change over a long period of time.