I.  History of the Theory of Natural Selection

We need to compare and contrast two different aspects of evolutionary theory:

First, the undeniable fact (proven by the fossil record and our own observations) that organisms DO change their appearence over time

Second, the theories which have been proposed to explain why these changes occur.

While we can disagree about the mechanisms underlying it, there is no question that evolution occurs.

The first testable ideas about the mechanismsm underlying the long-term change in organism appearance and function was advanced by Charles Darwin and Alfred Russel Wallace in the mid-1800's.

Timeline:

      1798:  Thomas Malthus presents idea that populations can grow more rapidly than food supply

      1831:  Darwin takes job as naturalist for the exploration ship Beagle

      1837:  Darwin develops hypothesis to explain mechanisms underlying evolutionary change

      1844:  Darwin first writes down his ideas regarding natural selection

      1856:  First draft of Origin of Species written, but not submitted for publication

      1858:  Wallace writes essay describing his theory of natural selection (almost identical to Darwin's),
                    based upon his field experiences in southeast Asia

      1859:  Origin of Species is published

 

(1)  Natural variation occurs within populations (differences exist between individuals)

(2)  Even though more may be born, only one offspring per parent will survive to adulthood
 
(3)  The most fit of the offpsring will be the ones that survive
 
(4)  If heritable, this increased fitness will be passed on to the next generation
 

Darwin and Wallace only differed in what they thought would select for the most fit offspring.

Darwin saw this happening as offspring struggled for health
 
Wallace saw this happening as offspring struggled for food

In the end, Wallace seems to be more correct
 

Darwin and Wallace felt that these changes would happen gradually as each succeeding generation became slightly more fit than the one preceeding them.  This is termed Gradualism.

However, they also did not realize how dynamic the Earth's environment is, as they assumed that only slow environmental changes would also occur.

By the early 1970's enough data had been gathered from the fossil record to suggest that Darwin and Wallace may not have been right.  Rather, it appears that many species do not change over long time periods, only to rapidly change at other times.

Stephen Jay Gould and Niles Eldredge presented these findings, and suggested that evolutionary changes happens by what they term Punctuated Equilibrium, which simply states that evolution will not always happen at the same rate.