Download - Biological Science Chapter 5
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Chapter 5Evolution and Gene
Frequencies
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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
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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.
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• These sources of variation include:1. The independent assortment of chromosomes
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2. The crossing over
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3. the chance fertilization of an egg by a sperm cell
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4.Rearrangements in the number and structure of chromosomes.
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5. Mutations of existing alleles
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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.
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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.
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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.
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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
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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.
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Bottleneck effect- if population sizes are restored,
they will have only the remnant of the original gene pool.
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Gene flow-changes in relative allelic frequency from the
migration of individuals.
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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.
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mutation
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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.
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2.Disruptive selection- occurs when natural selection favors both extremes of continuous variation.
-an intermediate phenotype is selected against.
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3. Stabilizing selection- occurs when individuals at both phenotypic extremes are selected against.
-narrows the phenotypic range.
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Neutralist/ Selectionist Controversy
Both natural selection and neutral evolution occur, but they may not be equally important in all circumstances.
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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.
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Species and Speciation Species – a group of populations in which genes are actually, or
potentially, exchanged through interbreeding.
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Speciation – the formation of new species
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Reproductive isolation- gene flow among populations or subpopulations does not occur.
Occurs in different ways: Premating isolation – prevents mating from taking place.
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Postmating isolation – prevents successful fertilization and development, even though mating may have occured.
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Allopatric speciation-occurs when subpopulations become geographically
isolated from one another.
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Parapatric Speciation- occurs in small population called
demes
Sympatric Speciation- occurs within a single population
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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
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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