LECTURE NOTES:  Origin of Species and the Theory of Evolution

HMS Beagle 1831-1836
Charles Darwin as ships naturalist

Darwin’s Observations

from South America

  • Darwin observed unusual animals and their fossilized ancestors.
  • sloths, armadillos, rhea (flightless bird)

Atlantic marine life differ greatly from that found off the Pacific coast.

  • Islands off the S.A. coast (3000 miles away)
  • He observed that the only native species were bats. All other were introduced by people.
  • Still other observations came from the Galapagos Islands

From the Galapagos Islands

  • tortoise populations showed distinctive features unique to three local islands.
  • Three species of finch show most similarity to finches found on the S.A. mainland
    • short beak to crush seeds
    • long beak to catch insects
    • one using a cactus needle to catch insects
  • Each new species had a common ancestor that responded to new island conditions producing new species.

Other evidence for common ancestry

Prominent scientists note difficulty distinguishing between embryo’s of a mammal, bird or reptile. Biologists today still recognize many similar embryonic features gill slits, prototail

Other Evidence of Common Ancestry

Similar features found in different species that have different function. These features are said to be homologous

Bats have wing bones very similar to those bone of animals with walking feet or whale fins

Vestigial organs no longer function in one species are still used in others

Muscles that move ears in cats no longer function in many humans.

Selective Breeding and Natural Selection

Darwin observed that human beings were able to produce new breeds of dog by selective breeding. Mate only those dog that possess a desired trait.

Horticulturalists develop new species of flowers by cross breeding

So why can’t nature impose environmental constraints on organism

Natural Selection as Darwin described was a process whereby species that withstood environmental pressure would live to reproduce and pass on those successful traits.

Mechanism for Darwin’s Evolution: Natural Selection

He wrote in an abstract to the Linnean Society in 1859,

" As many more individuals of each species are born than can possibly survive; and as consequently, there is a frequently reoccurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of live, will have a better chance of surviving, and thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to propagate it’s new and modified form"

Important Discoveries after Darwin

In Darwin's time there was not explanation for how inheritance worked. Gregor Mendel’s Particulate inheritance

Heredity factors are passed in pairs – one from each parent while maintaining their individual characteristics. See Pea Plant Flower experiment

These factors are now know as genes segments of DNA stored as chromosomes in the nucleus of each cell.

Evolutionary Trends: Radiation

Darwin believed a organic evolution occurred gradually. New species are derived by slowly by selective pressures like geographic isolation.

  • Darwin’s Finches
  • Tortoises
  • Mesozoic Ammonites gradual change from straight shell to complexly sutured & curled shell.

Evidence for this gradualistic evolution is lacking in the fossil record. Intermediate species do not always appear.

Rapid Radiation: Punctuated Equilibrium

Some groups may change little for most of their history but, then a rapid change and production of new species can occur.


Extinction is disappearance of species as a result of extreme limiting factors:  Fossil record not longer contains evidence of their existence.

  • Predation
  • Disease
  • Competition
  • Environmental restrictions like drastic climate change, meteorite impact or geographic isolation – mountain building or faulting due to plate tectonic activity 

Pseudoextinction is a form of extinction by evolving into a different species.

During mass extinction, many species are lost over a very short period of time

Rates of Extinction vary greatly


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