Unit 7 Natural Selection

Evolution by Natural Selection

Overview

• Evolution: a journey into where we're from and where we're going.

Historical Context

• Doctrine of the Creation of the Animals (1550).

Fossil Record

• The fossil record shows life’s natural history with successions and extinctions through different geological eras such as Quaternary, Tertiary, Cretaceous, etc.

• Key species transitions include arthropods, chordates, and mammals.

Lamarck vs. Darwin

Lamarck's Theory

• Organisms adapt to their environments by acquiring traits during their lifetime.

• Disuse leads to loss of parts (e.g., tapeworms).

• Constant use of an organ increases its size.

Darwin's Contributions

• Charles Darwin (1809-1882): Proposed evolution by natural selection.

• Collected evidence on the HMS Beagle (1831-1836), observing biodiversity in the Galapagos Islands.

Observations on the Galapagos Islands

• Notable species include native armadillos and Glyptodont fossils.

• Darwin collected 14 species of finches which he later realized were variations of the same species.

Adaptive Radiation

• Finches adapted to different food sources, leading to varied beak sizes and shapes:

  • Large Ground Finch, Small Ground Finch, Warbler Finch, etc.

• Evidence of differential survival: finches with beneficial beak traits survived better.

Mechanisms of Evolution

Natural Selection

• Variation in traits; overproduction of offspring; competition; differential survival and reproduction.

• Traits that improve survival accumulate.

Genetic Drift

• Changes in trait frequencies due to chance, such as founder effects and bottleneck effects.

Gene Flow & Mutation

• Movement of alleles due to migration; mutations introduce new genetic variations.

Evidence of Evolution

Fossil Record

• Illustrates changes from ancestral species to modern organisms.

Homologous Structures

• Similar structures in different species indicate common ancestry (e.g., limbs of humans, cats, whales, and bats).

Vestigial Organs

• Structures that serve little to no function in modern species reflect evolutionary changes (e.g., human tailbone, pelvis in whales).

Molecular Evidence

• Molecular records through DNA comparisons show evolutionary relationships.

Speciation

• New species arise through isolation:

  • Allopatric: geographic isolation.

  • Sympatric: reproductive isolation despite shared habitats.

• Mechanisms include behavioral, temporal, and mechanical isolation, and gametic isolation.

Final Thoughts

• Agents of evolutionary change include natural selection, genetic drift, mutations, gene flow, and non-random mating.

• Evolution is a continuous process, shaping life on Earth and influencing the characteristics of species.