Unit 6 – Evolution I

Biology Lecture Notes

 

Section I - Evolution

 

I. Development of Modern Evolutionary Theory

 

            a. Modern evolutionary theory has its roots in the principals of natural selection formulated by

                        Charles Darwin and Alfred Wallace, culminating in Darwin’s On the Origins of Species

                         by Means of Natural Selection (1859)

 

                        Picture of Darwin

                        http://www.firstscience.com/site/articles/darwin_hl.asp

                        http://www.windows.ucar.edu/tour/link=/people/enlightenment/darwin.html

 

            b. The key elements of Darwin’s theories include:

                        1) Reproductive rates will generally outpace the rate of increase of food supplies

                        2) The presence of biological variation within all species

                        3) Constant competition among individuals for survival

                        4) Individuals with favorable traits are more likely to survive and reproduce

                        5) The environment determines which traits are favorable

                        6) Favorable traits are passed on to offspring at a higher rate than non-favorable traits,

                                    thus increasing in frequency through time and eventually producing new species

                        7) Geographical isolation may also lead to the formation of new species

 

            c. Examples of natural selection observed in modern times include:

                        1) Industrial melanism includes changes in frequency of pigmentation patterns of the

                                    peppered moth near Manchester, England

 

                        Industrial Melanism

                        http://smccd.net/accounts/bucher/melanism.htm

                        http://www.ucl.ac.uk/~ucbhdjm/courses/b242/OneGene/peppered.html

 

                        2) Darwin’s finches on the Galapagos Islands, thicker beaked individuals have better

                                    reproductive success during times of drought

 

                        Darwin’s Finches

                        http://www.rit.edu/~rhrsbi/GalapagosPages/DarwinFinch.html

                        http://www.biology-online.org/2/11_natural_selection.htm

                        http://www.darwinfoundation.org/articles/calif22069901.html

 

                        3) Antibiotic resistant strains of bacteria and other microorganisms

 

            d. These and other examples of evolution show certain common principals:

                        1) A trait must be inherited to have importance in natural selection

                        2) Natural selection cannot occur without variation in inherited characteristics

                        3) Fitness is a relative measure that changes with changing environmental conditions

 

II. Definition of Evolution

 

            a. Evolution is defined as a change in allele frequency from one generation to the next.

            b. Evolution only applies to populations, as individuals cannot change alleles

 

III. Factors that Produce and Redistribute Variation

 

            a. Mutations are alterations in genetic material

                        1) Mutations must occur in gametes in order to have evolutionary significance

                        2) Mutations are the principle source of new variation

                        3) Evolutionary changes can occur rapidly if mutations are coupled with natural selection

            b. Gene Flow is the exchange of genes between populations

                        1) High population mobility increases gene flow, and may have acted to spread genetic

                                    variation and reduce speciation, especially in humans

            c. Genetic Drift is caused by the random alteration of allele frequencies in a population, and is

                   tied to population size

                        1) Founder Effect – when a small founding population colonizes a new area and carries

                                    with it unique genetic characteristics that are amplified because of the small

                                    population size

                        2) Bottleneck Effect – when a large population is rapidly reduced to a smaller one due to

                                    war, famine, disease or other factors, and only a small portion of genetic variation

                                    survives the severe population reduction

            d. Recombination is the reshuffling of genes in the offspring of every generation as a result of

                   sexual reproduction

                        1) Does not cause evolution, but provides a source of variation

 

IV. Natural Selection Acts on Variation

 

            a. Natural selection acts on variation produced by mutation, gene flow, genetic drift and

                        recombination

            b. For variations to be selected for, the right environmental conditions must exist for those

                        variable characters to succeed

            c. A good example of evolution by natural selection in humans is the gene for sickle-cell anemia,

                   the HbS gene.

                        1) The HbS gene is very rare in most human populations, as it usually results in a fatal

                                    condition.

                        2) However, there is a geographical correlation between malarial areas and the frequency

                                    of HbS, with the frequencies approaching 40% in western and central Africa.

                        3) Heterozygotes for the HbS allele provide a natural resistance to malaria and confer

                                    greater reproductive success to those with the gene than to those without

 

            Sickle-cell Anemia

            http://medlib.med.utah.edu/WebPath/HEMEHTML/HEME015.html

            http://www.ncbi.nlm.nih.gov/books/bv.fcgi?call=bv.View..ShowSection&rid=gnd.section.98

 

V. Review of Genetic Factors and Evolutionary Factors

 

            a) The evolutionary process occurs at five different levels:

                        1) Mutations in the DNA are transferred to the gametes (the molecular level)

                        2) These mutations are carried in the chromosomes (the chromosomal level)

                        3) The chromosomes pass on the mutations to offspring (the cellular level)

                        4) The individual is subject to natural selection (the level of the organism)

                        5) The population undergoes evolution when allele frequencies change (the level of the

                                    population)