The Theory of Evolution by Natural Selection (1809-1882) Background Information In the 1800’s, most people held the view that each species was a divine creation that existed, unchanging, as it was originally created. Scientists, however, had begun to seek explanations for the origins of fossils.

Jean Lamarck (1744-1829) He believed that over the lifetime of an individual, physical features increase in size because of use or decrease in size because of disuse, furthermore, these changes are then passed on to the offspring; Lamarck also pointed out that change in species is linked to the “physical conditions of life”, referring to an organism’s environmental conditions.

Charles Darwin (1859) He was an English naturalist who traveled to the Galapagos Islands to study plants and animals. Darwin’s observations at Galapagos Island: extinct armadillo fossils resembled armadillos living in the area many plants & animals of the Galapagos Islands resembled those of the nearby coast of S. America He concluded the differences in appearances were due to evolution.

evolution – a change that occurs over a long period of time natural selection – the process by which individuals that have favorable variations and are better adapted to their environment survive and reproduce more successfully than less well adapted individuals do; in time the number of individuals that carry these favorable characteristics increases in a population and thus the nature of the population will change

Darwin’s Theory of Natural Selection is based on Four Key Principles: Overproduction: A population produces more offspring than can possibly survive. Inherited variations: Each individual in a population has it own set of traits. Inherited variations are differences in the traits of individuals of the same species. Darwin believed such variations were crucial for the survival of a species. Competition: There is competition among the individuals within a population. They must struggle for food, water, shelter, etc. Selection: Those individuals that win the struggle for survival will be able to reproduce. Darwin reasoned that those individuals who win the struggle have the traits (or adaptations) that are needed to survive.

Darwin’s Theory of Evolution basically states: inherited variations or differences exist within the genes of a population or species in a particular environment, some individuals of a population or species are better suited (or adapted) to survive and have more offspring over time, the traits that make certain individuals of a population able to survive and reproduce tend to spread in that population;

Species Formation species – a group of organisms that are closely related and can naturally mate to produce fertile offspring adaptations – a change that improves a population’s ability to survive; can be an anatomical, physiological, or behavioral change speciation – the formation of new species as a result of evolution by natural selection; occurs when a species becomes so different they can no longer interbreed and are eventually considered to be different species

Reproductive Isolation leads to Speciation. reproductive isolation – is the condition in which two populations of the same species do not breed with one another Ex: as two isolated populations of the same species become more different over time, they may eventually become unable to breed with one another; generally, when the individuals of two related populations can no longer breed with one another, the two populations are considered to be different species.

Factors that lead to Reproductive Isolation: Geographical isolation – physical barriers such as rivers or mountains separate a population Temporal Isolation – timing prevents reproduction between a population Behavioral Isolation – differences on courtship or mating behaviors

Models of Evolution: gradualism – a model of evolution in which there is a gradual change over a long period of time that leads to biological diversity punctuated equilibrium – a model of evolution in which short periods of drastic change occur in a species, includes mass extinctions and rapid speciation; this model is separated by long periods of little or no change

Gradualism vs. Punctuated Equilibrium