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STRUCTURE OF DNA
I. INTRODUCTION
A. Evolution explains the unity and diversity of life
1. Unity is explained as all living things share the same fundamental
characteristics because they are descended from a common ancestor
2. Diversity is explained as all populations adapt to their varying habitats
B. Terms
1. Population
a) All the members of a single species, occupying a particular area at
the same time
2. Gene pool
a) In a population, a gene pool represents all the various alleles of all
the gene loci in all the members
3. Species
a) Biological species
(1) A group of interbreeding populations that share a gene pool and are
reproductively isolated from other species
(a) Must produce viable, fertile offspring
(2) Subpopulations of the same species exchange genes, but different
species do not exchange genes
b) Morphospecies
(1) Used for asexually reproducing organisms and extinct organisms
found in the fossil record
4. Evolution
a) Changes in the genetic make up of a population from one generation
to the next
b) Changes in the allelic or genotypic frequencies of a population over
time
II. CONDITIONS MAINTAINING A HARDY-WEINBERG EQUILIBRIUM
A. Large population
B. No mutations
C. No gene flow
D. Random mating
E. No selective pressure
III. CONDITIONS LEADING TO MICROEVOLUTION
A. Small population
1. Results in what is termed genetic drift
a) Changes in genetic make up of a population do to sampling errors
associated with small numbers
b) Results in genetic diversity
(1) Many genes may become fixed in the population
2. Founder's effect
a) When a few individuals found a colony, only a fraction of the total
genetic diversity of the original gene pool is represented
3. Bottle neck effect
a) A population subjected to near extinction because of a indiscrimnat
killing
b) Prevents the majority of types of genotypes from participating in the
production of the next generation
B. Mutations
1. Source of all new alleles (variation)
2. May be advantageous, disadvantageous or neutral
C. Gene flow
1. Movement of alleles between populations
D. Non-random mating
1. Interbreeding / selfing
a) Breeding with closely related individuals or self-fertilization
b) Changes genotypic frequency, not allelic frequency
c) Increased number of homozygotes
(1) More likely to see recessive traits expressed in these populations
2. Assortive mating
a) Like organisms breeding only with similar organisms
b) Changes genotypic frequency, not allelic frequency
c) Increased number of homozygotes
(1) More likely to see recessive traits expressed in these populations
E. Selective pressure
1. Some alleles may be disadvantageous in an environment and selected
against (not passed on), while others may be advantageous and selected for
IV. NATURAL SELECTION
A. Contributors
1. Charles Lyell
a) Introduced the concept of gradualism
2. Jean Baptiste Lamark
a) Introduced the idea of evolution
b) Theory of transmission of acquired characteristics was shown to be
incorrect
3. Charles Darwin
a) Introduced the idea of natural selection as the driving force of
evolution
4. Gregory Mendel
a) Introduced the idea of genes as the mechanism of transmission of
traits
B. Observation and Inferences by Charles Darwin
1. Observation 1
a) All species produce more offspring then the environment can support
2. Inference 1
a) This leads to a struggle for resources, with only a fraction of offspring
surviving each generation
3. Observation 2
a) There is much variation within a population
4. Observation 3
a) Much of this variation is hereditable
5. Inference 2
a) Survival is not random, but rather on inherited characteristics
b) Those individuals whose inherited characteristics fit them best to their
environment are more likely to leave more offspring than less fit
individuals
6. Inference 3
a) This unequal ability of individuals to survive and reproduce can lead
to gradual changes in a population, with favorable characteristics
accumulating over time
V. SELECTION PATTERNS
A. Stabilizing selection
1. The intermediate phenotype is favored by natural selection over the
extremes
B. Disruptive selection
1. Two or more extreme phenotypes are favored over intermediate phenotype
C. Directional selection
1. Occurs when an extreme phenotype is favored, and the distribution curve
shifts in that direction
VI. BARRIERS TO GENE FLOW BETWEEN SPECIES
A. Prezygotic barriers
1. Spatial isolation
a) Located in different habitats
2. Temporal isolation
a) Breed at different times
3. Behavioral isolation
a) Species specific signals do not attrack other species
4. Mechanical isolation
a) Genitalia unsuitable to one another
5. Gamete isolation
a) Gametes won't fuse
B. Post zygotic barriers
1. Reduced hybrid viability
2. Reduced hybrid fertility
3. Hybrid breakdown
VII.
SPECIATION
A. Allopatric
1. A population becomes separated physically and the subpopulations evolve
separately into different species
B. Sympatric
1. A population evolves into separate species while in the same area
2. Polyploidy is one mechanism that can result in sympatric speciation
a) Autopolyploidy is when an extra set of chromosomes comes from the
same species
b) Allopolypoidy is when an extra set of chromosomes comes from a
different species