Chapter 5Evolution and Gene

Frequencies

Populations and Gene Pools

Population- are groups of individuals of the same

species that occupy a given area.- share unique set of genes

Alleles – varying expressions of genes at each locus

Gene pool - the sum of all the alleles

for all traits in a sexually reproducing population is pool of hereditary resources for the entire population.

• These sources of variation include:1. The independent assortment of chromosomes

2. The crossing over

3. the chance fertilization of an egg by a sperm cell

4.Rearrangements in the number and structure of chromosomes.

5. Mutations of existing alleles

MUST EVOLUTION HAPPEN?The rate of evolution is slow, and sometimes it is rapid.

Theories of population genetics- the study of the genetic events in gene

pools.

The Hardy- Weinberg Theorem

Godfrey H. HardyWilhelm Weinberg

- it states that the mixing of alleles at meiosis and their subsequent recombination do not alter the relative frequencies of the alleles in future generations, if ccertain assumption are met.

The asssumption of the Hardy- Weinberg theorem are as follows:1. The population size must be large.2. Sexual reproduction within the population

must be random.3. Individuals cannot migrate into, or out of,

the population.4. Mutations must not occur.

EVOLUTIONNARY MECHANISMPopulation size, Genetic drift, and Neutral evolutionPopulation size

- the smaller population, the more significant may be.Genetic drift

- chance events influencing the frequencies of genes in populationsNeutral evolution

- gene frequencies are changing independently of natural selection

Two special cases of genetic drift have influenced the genetic makeup of some populations.

Founder effect - the new colony that emerges from

the founding individuals is likely to have distinctive genetic makeup with far less variation than the larger population.

Bottleneck effect- if population sizes are restored,

they will have only the remnant of the original gene pool.

Gene flow-changes in relative allelic frequency from the

migration of individuals.

Mutation

-changes in the structure of genes and the chromosomes.-origin of all new alleles and a source of variation that may prove adaptive for an animal.

MUTATION PRESSURE-measure of the tendency for gene frequencies to change through mutation.

mutation

Natural selection reexamined:

MODES OF SELECTION

Natural selection may affect a range of phenotypes in tree ways:

1. Directional selection- occurs when individuals at one phenotypic extreme are at a disadvantage compared to all other individuals in the population.

2.Disruptive selection- occurs when natural selection favors both extremes of continuous variation.

-an intermediate phenotype is selected against.

3. Stabilizing selection- occurs when individuals at both phenotypic extremes are selected against.

-narrows the phenotypic range.

Neutralist/ Selectionist Controversy

Both natural selection and neutral evolution occur, but they may not be equally important in all circumstances.

Balanced Polymorphism and Heterozygote Superiority

Balanced polymorphism- occurs when different phenotypes are maintained at relatively stable frequencies in the population.

Heterozygote superiority- when the heterozygote is more fit than either homozygote.

Species and Speciation Species – a group of populations in which genes are actually, or

potentially, exchanged through interbreeding.

Speciation – the formation of new species

Reproductive isolation- gene flow among populations or subpopulations does not occur.

Occurs in different ways: Premating isolation – prevents mating from taking place.

Postmating isolation – prevents successful fertilization and development, even though mating may have occured.

Allopatric speciation-occurs when subpopulations become geographically

isolated from one another.

Parapatric Speciation- occurs in small population called

demes

Sympatric Speciation- occurs within a single population

Rates of Evolution- it is a measurement of the change in an

evolutionary lineage over time

Phyletic Gradualism- evolutionary change as occuring over

millions of years- changes are gradual over long periods

Punctuated Equilibrium- rapid periods of change interrupt long

periods of stasis

Molecular BiologyAll evolutionary change results from changes in the base sequences in DNA and amino acids in proteins

Gene Duplication- the accidental duplication of a gene on a

chromosome is one way that extra genetic material can arise

Mosaic Evolution- the occurence within a given population of

organisms of different rates of evolutionary change various body structures aand functions