Evolution: Occurs when heritable characteristics of a species change over time

Four pieces of evidence for evolution:

  • Fossil record
  • Homologous structures
  • Selective breeding 
  • Melanism
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Fossil Record

Fossil: Preserved remains or traces of any organism from the remote past

  • Direct (body fossils) such as bones, teeth, shells and leaves
  • Idirect (fossil traces) such as footprints, tooth mark traks

Fossil record: Sum of all the discovered and undiscovered fossils and their relative placements in rock

This provides the dimension of time to the study of evolution. The layer of rock can be dated and used to deduce the age of the fossil

Gaps in the fossil record are due to:

  • Special circumstances are required for fossilization to occur
  • Only hard parts of an organism are preserved
  • Fossils can be damaged so that only fragments remain
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Fossil Record

The fossil record clearly shows changes in the characteristics of organisms. Fossils show a chronological sequence in which characteristics appear and develop in complexity

Transitional fossils: Shows the links between groups or species by exhibiting traits common to both both the ancesteral group/species and its derived decendant/species

Archaeopteryx is an example of a transitional fossil that provides evidence for the evolution of birds from dinosouars

  • Bird features: Feathers
  • Dinosaur features: Jaws with sharp teeth, three fingers with claws, long bony tail
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Homologous structures & Adaptive Radiation

Homologous structures: Structures that have evolved from the same part of a common ancestor

Example of homologous structure: Pendactyl limb in vertebrates. All these vertebrates share an ancestor that had pendactyl limbs

Adaptive Radiation: Different groups have evolved from a common ancestor, but use different types of locomotion, the limbs developed in a variety of different ways to suit the type of locomotion

  • Bat wing for flying
  • Horse hoof for galloping
  • Monkey hand for grasping

All the organisms share the same bones. The bones may vary in size and shape but all vertebrates have five fingered "hands" at the end of each limb


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Selective Breeding

Selective breeding: Selecting individuals with disirable traits and breeding from them

The differences in heritable characteristics of domesticated breeds gives evidence that species can evolve from artificial selection


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Melanistic: Dark varities of typically light coloured insects

Example: Peppered Moth

Melanistic variety was very rare and then industries developed allowing the melanistic variety to become more common

A higher proportion of the melanic variety survived to breed and pass on the dark wing colour causing populations to evolve from peppered to melanic

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Gradual Divergence

  • If populations (example: fruit flies) become separated (two islands) then it is very likely that the population will experience different ecological conditions
  • As a result of natural selection over many generations the shape of the flies, food prefrence and mating rituals will diverge
  • If the island fruit flies migrate back to the mainland they will not easily mate with the population that stayed on the mainland because:

- Their mating rituals are different

- Due to genetic differences the resulting offspring are infertile

Speciation: Island fruit flies have evolved into different species because the genes cannot flow between the populations 

Example: Darwin's finches

Populations evolving different characteristics can take millions of years eventually being so different that populations would not be able to interbreed and have therefore evolved into separate species.

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Gradual Divergence

Debates about whether two populations of the same species have sufficiently diverged to become two separate species

Populations will gradually diverge over time and it is natural to see continuous variation across a geographical range

Continuous variation: A series of intermediate types connects the extremes

The bigger the geographical separation and the longer the populations have been seperated the greater the divergence


  • Isolation
  • Genetic Variation
  • Natural Selection
  • Speciation
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