Natural Selection

• Individuals with certain heritable traits survive and reproduce at a higher rate than other individuals

• Natural selection increases the match between organisms and their environment over time

• If an environment changes over time, natural selection may result in adaptation to these new conditions and may give rise to new species

  • Note that individuals do not evolve; populations evolve over time

• Natural selection can only increase or decrease heritable traits that vary in a population

• Adaptations vary with different environments

• Why natural selection can’t fashion perfect organisms:

  • Selection can act only on existing variations

  • Evolution is limited by historical constraints

  • Adaptations are often compromises

  • Chance, natural selection, and the environment interact

Homology

• Homology: similarity resulting from common ancestry and is the result of divergent evolution.

• Homologous structures: anatomical resemblances that represent variations on a structural theme present in a common ancestor

• Convergent evolution: the evolution of similar, or analogous, features in distantly related groups

  • Convergent evolution does not provide information about ancestry

• Analogous traits arise when groups independently adapt to similar environments in similar ways

• Homologous genes: two genes derived from the same ancestral gene

  • Orthologs occur in separate species

• Molecular processes reveals molecular details of evolutionary change

• Two sequences may be similar, but not identical due to the independent accumulation of different random mutations

Darwin’s Observations of Nature

• There’s variability in traits within a population

• Traits are inherited from parent to child.

• No two of the same species are exactly alike even if they have the same parents

• All species are capable of reproducing beyond the support of the environment

• Many offspring do not survive due to limitation of the environment.