Darwin’s Theory of Evolution  

  Life is extremely diverse, with an estimated 20 million or more species (mostly insects), divided into six different kingdoms. 

 

Darwin’s Epic Journey Geologists were just beginning to suggest that the earth was changing over time.  Biologists were suggesting that evolution would explain how modern organisms evolved over long periods of time through descent from a common ancestor.

Observations aboard the Beagle As Darwin travelled aboard the Beagle he observed three distinct patterns of biological diversity; species vary globally, species vary locally and species vary over time.

   

Species Vary Globally Darwin noticed that similar yet different animals inhabited similar habitats around the world. Example: flightless, ground dwelling birds (Rheas) lived in grasslands of South America. They looked and acted like ostriches but ostriches could only be found in Africa. Darwin also found Emus in Australia’s grassland and they,too were similar to ostriches.

  Emu                                  Ostrich                     Rheas

Species Vary Locally Darwin noticed different, yet related, animals lived in different habitats within a local area. Example: while travelling the Galapagos Islands Darwin noticed that island slightly differed in their ecological conditions. He found giant tortoises varied slightly in their shells from island to island

Species Vary Over Time         Darwin collected fossils and he noticed that some fossils of extinct animals were similar to living species. Example: A fossil of a giant armoured animal was very similar to that of a present day armadillo. 

   

16.2 An Ancient, Changing Earth Hutton recognized that certain geological processes were responsible for certain geological features. Example: Hutton proposed that forces pushed the earth surface upwards and this gave rise to mountains

  Dacite columns that formed tens of thousands of years ago when a lava flow cooled rapidly against a glacier. Photo by Jackie Beckett, © American Museum of Natural History.

Uniformitarianism Lyell’s principle of uniformitarianism stated that geological processes we see today must be the same forces that shaped earth millions of years ago. Example: ancient rivers carved channels and canyons just as our present rivers do today.

   

  Darwin realized that if earth could change over time living things could change over time.

Change over time  

Lamarck’s Evolutionary Hypothesis Lamarck suggested that all living things urge to become more perfect and complex.  As a result organisms change and acquire characteristics to make them more successful in their environment. This was known as “acquired characteristics”.

   

  Example: the black – necked stilt’s long legs were a result of the bird’s ability to stretch their legs. Over time, future generations would have their legs  get longer. Lamarck also suggested that these “acquired traits could be passed on to their offspring. This was known as “inheritance of acquired characteristics”.

    Black-necked Stilt

Population Growth Malthus suggested that human populations tend to grow faster than food supply. He reasoned that war, hunger and disease would keep population in check. Darwin reasoned that the same notion could be true for other living things.He wondered why others of a same species survived and others perished. 

   

Artificial Selection Darwin took up breeding plants and pigeons to test the idea of artificial selection. The animals and plants provided the variations and humans selected those variations they find useful. Darwin realized that variation in a population provided the raw material for evolution.

   

Evolution by Natural Selection: Competition Darwin used Malthus’s reading to come up with the notion of a struggle for existence. All members of a population must compete for necessities of life.

Variation and fitness Darwin thought that the variation in a population played a role in who survived.  Some variations are better suited for the environment than others and this increased the organism’s ability to live and reproduce.  These organisms had better adaptations.

   

  Those organisms with the best adaptations were more likely to reproduce, meaning they had better fitness. The difference in reproduction rates refers to the “survival of the fittest”.  

   

  Natural selection takes into account that there are variations in a population, those best suited will survive and produce more offspring. From generation to generation, populations continue to change as they become better adapted, or as the environment changes. 

Common Descent Darwin suggested that over many generations, adaptations could cause successful species to evolve into news species. This explains the diversity of life and that species change over time. Descent with modification proposes that all living species are descended from common ancestors.  A tree of life links all living things.

   

Complete Finch activity. Ref. p 472 in M&L  

Evidence of Evolution: Biogeography Biogeography is the study of where organisms live now and where they and their ancestors lived in the past. Darwin saw two patterns related to biogeography. One pattern had closely related species differing slightly when the climate changed. Populations on the island were slightly different from the mainland species. 

   

  The second pattern had very distantly related species develop similarly in similar environments.  Similar habitats around the world had similar plants and animals that were very distantly related.  Darwin suggested that environmental pressures had caused very distantly related species to develop similar adaptations.

   

Comparing Anatomy and Embryology Darwin suggested that animals with similar structures evolved from a common ancestor. The structures shared by a related species that have been inherited from a common ancestor are called homologous structures. Similarities and differences among homologous structures indicate how recently these species shared a common ancestor.

   

  Vestigial structures are inherited from ancestors but have lost much of their function if not all their function. Dolphins have hipbones that of are no use, yet they may have played a role in their locomotion while on land.

   

   

  Embryological development of vertebrates is very similar, showing evidence that organisms have descended from a common ancestor.

   

Genetics and Molecular Biology Our advancements in genetics indicate that all living things share the same DNA and RNA to carry information from one generation to the next. Genetics provides some of the strongest supporting evidence of evolution.

   

  Homologous molecules show structural and chemical similarities at the molecular level. An example is cytochrome c, a protein which functions in cellular respiration for all types of cells.