Science 20 Unit D: Living Systems Developing a theory of Evolution

Scientific Theory Scientific theories develop over time They are formed from many observations, experiments, and models Theories may change over time as new information is gathered

“Theory of Evolution by Natural Selection” A well-supported and widely accepted explanation of the diversity and change that has occurred and been observed throughout Earth’s history

Developing the Theory Plato and Aristotle (~ BC) were two of the most influential Greek philosophers “All life existed in a perfected and unchanging form” This view of life dominated for over 2000 years Some Greek philosophers believed that life evolved gradually, but they did not have the influence of Plato or Aristotle.

Georges-Louis Leclerc, Comte de Buffon ( ) Naturalist published Historie Naturelle (the first public challenge of the Greek school of thought) Speculated that humans and apes could have a common ancestor Earth is much older than 6000 years

Georges Cuvier ( ) Paleontologist His observations: –older rock strata show show more dissimilar species from modern life –species have become extinct over time Cuvier proposed that the Earth had experienced several catastrophic events (floods, volcanoes) which resulted in the observed extinctions

Charles Lyell ( ) Geologist Lyell proposed that the Earth has been shaped by slow and continuous processes, rather than sudden catastrophic events The processes that shaped history are still occurring today Charles Darwin: could slow, subtle changes also occur in populations?

Buffon Lyell Cuvier These individuals contributed ideas and observations to the theory of evolution, but they did not propose a mechanism or explanation for what they saw

Jean-Baptiste Lamarck ( ) Compared current species with fossil forms Observed a “line of descent” - fossils became more complex over time The first scientist to propose a theory of evolution with a mechanism

Lamarck’s Theories (1)New, simple species are continually created by spontaneous generation (arising from non-living material) and gradually become more complex Organisms have a “force” or “desire” within that leads them to become better adapted to their environment

(2) The term “Lamarckism” refers to the concept of inheritance of acquired characteristics The use and disuse of certain structures could be passed on to offspring (eg) A giraffe must continually stretch its neck to reach its food, and so develops an elongated neck. This long neck is inherited by its offspring. (eg) Athletes will give birth to athletic children

Evaluating Lamarck’s Theories “pros” - he recognized the role of environment in driving evolutionary change “cons” - we know that acquired traits cannot be inherited. His theory did not consider populations or variation - change could be accomplished by the individual. By the end of the 1800’s, as biologists learned about cells, genes, and heredity, Lamarck’s theories were rejected

Darwin’s Journey In 1831, 22 year-old Charles Darwin ( ) left England on the HMS Beagle, a British survey ship. Darwin made many observations over the 5 year journey - he did not initially understand their significance, but they would later become very important to his theory of evolution by natural selection.

(1) The flora and fauna of South America were very distinct from those Darwin studied in Europe and England Why were there distinctive clusters of similar organisms in different parts of the world? Why weren’t they randomly distributed?

(2) Darwin observed fossils of extinct animals that looked very similar to living animals Why would living and fossilized organisms that looked similar be found within the same region? Why would one survive and not the other?

(3) The finches that Darwin observed on the Galapagos Islands closely resembled birds from the South American mainland. Why was there such a close resemblance? Could these Galapagos species have originated from the mainland species?

(4) Galapagos species looked identical at first, but actually varied between islands. For example, each finch was adapted to eating a different type of food based on beak size and shape. Why was there such a diversity of species in such a small area? Could there be a common ancestral form?

(5) Darwin observed fossil deposits of coral at elevations of 3000 m. A severe earthquake lifted the coastline by 2m. What role did geological forces play in the changing environment?

When Darwin returned to England, he began to organize these observations He also questioned experts from other fields of study He became very interested in artificial selection and wondered if the environment could have the same affect He gained more confidence in his theory, but was aware of how controversial its presentation would be

Theory of Evolution by Natural Selection Alfred Russel Wallace ( ) reached similar conclusions to Darwin through his work in Malaysia Both papers were made public at an event organized by Charles Lyell in 1858 Darwin published On the Origin of Species in 1859, complete with a mechanism to explain how evolution occurred (a process he called natural selection)

Darwin’s theory was based on 3 key observations and inferences (1) There is a great deal of variation within a species –Not all members of a species are created equal

(2) Every generation produces more offspring than can survive to reproduce –Individuals are in a constant struggle for survival. Resources are not infinite. –Individuals with favourable variations are more likely to survive and pass these on. Survival is not random. This is natural selection

(3) Populations tend to remain stable in size Because individuals with more favourable variations contribute the highest proportion of offspring to succeeding generations, their traits will become more common. This is evolution.

Example Imagine an ancient population of giraffes, each with different neck lengths Giraffes with long necks have a more consistent food supply and stay healthier These giraffes lead longer reproductive lives and are able to better care for and protect their young Over time, more and more giraffes have long necks

On the Origin of Species In his book, Darwin assembled a group of facts and observation that had previously seemed unrelated He proposed that –present forms of life have arisen by descent and modification from an ancestral species –The mechanism for modification is natural selection working for long periods of time

In Darwin’s words… As many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring 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 life, 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 its new and modified form