Biology 1b Notes

CHAPTER 6

(Pg. 120-125)

REVISIONS OF DARWIN'S THEORY

NEO-DARWINISM

* Most serious weakness in Darwin's theory was his failure to identify correctly the

mechanism of inheritance.

- He saw heredity as a blending phenomenon

- Neo-Darwinism: Darwin's theory as revised by Weismann

~ Darwin's original theory(Lamarckian inheritance) was rejected by Weismann

who experimentally showed that modifications of an organism during its

lifetime do not change its heredity.

EMERGENCE OF MODERN DARWINISM: THE SYNTHETIC THEORY

* Population geneticists study evolution as a change in genetic composition of

populations.

* With the establishment of population genetics, evolutionary biology became divided

into two different subfields:

- Microevolution: pertains to evolutionary changes in frequencies of different allelic forms of genes

- Macroevolution: refers to evolution on a grand scale, encompassing the origins of new organismal structures and designs, evolutionary trends, adaptive radiation, phylogenetic relationships of species, and mass extinction.

Microevolution: Genetic Variation and Change Within Species

* Microevolution is the study of genetic change occurring within natural populations.

* Polymorphism: occurrence of different allelic forms of a gene in a population.

* Gene pool: collectively formed by all alleles of all genes possessed by members of a

population.

* Allelic frequency: the relative frequency of a particular allelic form of a gene in a

population.

GENETIC EQUILIBRIUM

* Hardy-Weinberg equilibrium: forms the foundation for population genetics.

According to this theory, the hereditary process alone does not produce evolutionary

change.

HOW GENETIC EQULIBRIUM IS UPSET

1. Random genetic drift

2. Nonrandom mating

3. Recurring mutation

4. Migration

5. Natural selection

+ interactions among these factors

1. Genetic drift: Chance fluctuation in allelic frequency from one generation to the next, including loss of alleles from the population.

2. Nonrandom Mating:

- Positive assortative mating: Individuals mate preferentially w/ others of the

same genotype, such as albinos mating w/ other albinos.

- Inbreeding: Preferential mating among close relatives (also increases

homozygosity).

3. Migration: prevents different populations of a species from diverging.

4. Natural Selection:

- Can change both allelic frequencies and genotypic frequencies in a population.

- Acts on the whole animal, not on isolated traits ‡ organism that possesses a

superior combination of traits will be favored.

- Relative fitness: Having an genotype at a gene that has an advantage in

survival and reproduction in the population.

- Sexual selection: Selection of traits that are advantageous for obtaining mates

but may be harmful for survival.

5. Interactions of Selection, Drift, and Migration:

- Interaction of selection,genetic drift, and migration in this example produces

evolutionary change qualitatively different from what would result if any of

these three factors acted alone.

MEASURING GENETIC VARIATION W/IN POPULATIONS

* Protein Polymorphism: Different allelic forms of genes encode proteins that may

differ slightly in their amino acid sequence.

QUANTITATIVE VARIATION

* Stabilizing selection: To favor average values of the trait and to disfavor extreme

ones.

* Directional selection: Favors an extreme value of the phenotype and causes the

population average to shift toward it over time.

* Disruptive selection: Two different extreme phenotypes are simultaneously favored,

but the average is disfavored.

- population then becomes bimodal (two very phenotypes

predominate)

(Pg. 102-106); (Pg. 118-120)

.

ORIGINS OF DARWINIAN EVOLUTIONARY THEORY

PRE-DARWINIAN EVOLUTIONARY IDEAS

* Greek philosophers (notably Xenophanes, Empedocles, and Aristotle) developed an

early idea of evolutionary change.