Historical and Current Theories to Explain Variation Why do organisms differ and why do they change over time?

Jean-Baptist Lamarck ( ) Believed that evolution did occur and proceeded with natural laws Believed there were two forces at work in evolution of a species 1.A force that drove organisms up a ladder of complexity 2.A force that adapted them to local environments through the “use and disuse” of certain characteristics of their bodies in order to make them different from other organisms. These characteristics were then inherited to the next generation (assuming that either both parents had the new trait or at least the producer of the offspring )

Use and Disuse of Characteristics… Lamarck wrote in approximately 1809 “In every animal which has not passed the limit of its development, a more frequent and continuous use of any organ gradually strengthens, develops and enlarges that organ, and gives it a power proportional to the length of time it has been used, while the permanent disuse of any organ imperceptibly weakens and deteriorates it, and progressively diminishes its functional capacity until it finally disappears.” Example: Lamarck may have believed that at one time it may have been possible that giraffes did not have long necks. Through the constant reaching high in the trees for leaves, the neck was able to actually grow in length over time. That new neck length could then be passed on to offspring. (termed “inheritance of acquired characteristics”)

Thomas Robert Malthus ( ) Malthus studied the growth of populations and noted that populations will most likely grow until they have reached their resource limit He studied only human populations. When resources increased, then human population size expanded. When resources were in demand then human population size decreased. He termed this “mans struggle for existence” and was dependant on cost/benefit decisions that humans made regarding sex, work and children. Darwin related Malthus’ theories, without the factor of human intelligence, to plants and animals and it helped Darwin recognize competition of resources between populations of the same species and between species

Sir Charles Lyell (1797 – 1875) Was the foremost geologist of his day and supported a theory called “uniformitarianism” –Assumption that natural processes operating in the past are the same as those that can be seen happening today and that preset day geological processes can be used to predict past processes Carried the belief that the earth’s features have been formed through minute changes over a long period of time Was a strong influence on Darwin’s theories as the two were good friends.

In volume three of his geology books Lyell completely rejected Lamarcks idea of “use and disuse” and instead proposed the idea that there may have been centers of creation all around the world to explain diversity and territory of species. Lyell became good friends with Darwin, and actually encouraged him to publish “On the Origin of Species”. In volume 10 of his series he actually started to support the idea of evolution because Darwin’s ideas of change over long periods of time were in line with his geological studies.

Charles Darwin (1809 – 1882) Darwin traveled around the world for 5 years on the HMS Beagle and where he noted a rich variety of geological features, fossils and living organisms and became very interested in finding a reason for such variety Darwin read the works of Malthus whilst on his travels and he had the idea that favorable traits would be preserved and unfavorable traits would be destroyed which would result in new species (1838) Darwin began to believe that all species of life must have evolved over time from common ancestors through a process later termed “natural selection” which proposed a logical explanation for the diversity of life In 1859 Darwin published “On the Origin of Species”

Natural Selection The process by which favorable heritable traits (according to environment) became more common in successive generations of a population of reproducing organisms and unfavorable heritable traits become less common More to come on the specifics of natural selection….