Mechanisms of Evolutionx

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Transcript Mechanisms of Evolutionx

Mechanisms (Features) of Evolution
What is needed for Natural Selection to
occur?
Evolution of Populations
 Population: A particular species in one area
 By focusing on evolution of populations, we define evolution on the
smallest scale (microevolution)
 Because:
 Individuals do not evolve
 Populations are isolated from others of their own species; keeps
gene pool from being manipulated by outside traits
 Allele frequencies can be monitored over generations
o Allele: traits passed on from parents (You have two alleles for
each trait, one from each parent)
 Changing allele frequencies point to an evolving population
Genetic Variation
 The most important factor needed for NS to occur
 Random Mutations in DNA
 Mutation cannot be silent or non-beneficial to reproductive
success of individual
 Mutation must occur in cell line that produces gametes
 Less limiting in plants and fungi as many cell lines produce
gametes
 Mutations produced in somatic cells die with individual
Genetic Variation
 The most important factor needed for NS to occur
 Random Mutations in DNA
 This is likely how vertebrates evolved
 Invertebrates have a gene called the Hox complex that direct the
major development of body parts during fetal (embryological)
development
 520 million years ago a duplication of the Hox complex may have
provided genetic material associated with the first vertebrates
 A second duplication (leading to 4 Hox complexes in current
vertebrates) points to a linear evolution of these genes
 Hox Gene Evolution
Genetic Variation
 The most important factor needed for NS to occur
 Sexual Reproduction (only in organisms that reproduce in this
manner)
 Shuffles genetic material in individuals to contribute variability to population
 Crossing over during Meiosis – happens during synapsis in which
homologous chromosomes touch, exchanging chromatids and ensures that
specific variation of traits do not have to be inherited together
 Independent Assortment – homologous chromosomes from each parent do
not have to all line up on one side of metaphase plate; ensures that parental
DNA is shuffled in offspring creating genetic variability
 Random Fertilization – each gamete produced has a unique combination of
parental DNA and fertilized at random; ensures variability of offspring with
each combination of gametes
Genetic Variation
 The most important factor needed for NS to occur
 Asexual Reproduction – creates offspring that are genetically
identical to parents
 Advantageous only when environment is stable
Genetic Variation
 How can asexually reproducing organisms be
genetically variable?
 Random Mutations in DNA
 Mutation cannot be silent or non-beneficial to reproductive
success of individual
 Mutation must occur in cell line that produces gametes
 Horizontal Gene Transfer: transfer of genes between
organisms in a manner other than traditional reproduction
 Transformation
 Transduction
 Conjugation
Genetic Variation
 How can asexually reproducing organisms be genetically
variable?
 Bacterial Transformation: Direct uptake of DNA from environment by
bacteria through cell wall
 Can occur naturally in some bacteria but has to be induced in others

E.coli Lab
Genetic Variation
 How can asexually reproducing organisms be genetically
variable?
 Bacterial Transduction: Uptake of DNA through viruses known as
bacteriophages
 Bacteriophage: virus that infects bacteria


Happens naturally; once virus takes over bacterial cell, new viruses have
bacterial DNA incorporated into them; those viruses then go onto infect other
bacteria and are more readily accepted by the bacteria because of the
recombinant DNA
Now commonly used in gene therapy to insert therapeutic DNA or siRNA into
cells
Bacterial Transduction
Genetic Variation
 How can asexually reproducing organisms be genetically
variable?
 Bacterial Conjugation: transfer of genetic material (DNA or RNA)
between bacterial cells by direct cell-to-cell contact or by a bridge-like
connection between two cells
Bacterial Conjugation
Genetic Variation
 Genetic Drift: Chance events
that alter the allele frequencies
of a population with no regard
to how trait is related survival
 Not considered NS
 Example: You stepping on a
large population of green bugs;
trait has nothing to do with
survival, just an oops
 Extreme drift can lead to:
 Founders Effect
 Bottleneck Effect
Genetic Variation
 Founders Effect: A few individuals become isolated from original
population; allele frequency may differ from original population
 Not necessarily a negative effect unless frequency reflects less
variation
Genetic Variation
 Bottleneck Effect: A dramatic reduction in
population size that may not be
representative of allele frequencies original
population
 Generally reduces genetic variation
 Can lead to endangerment of species
 Example: Cheetahs
 Two Bottlenecking Events: Ice age (100,000 years
ago) and extreme hunting and poaching (100 years
ago)
 So genetically similar that tissue grafts are never
rejected between unrelated cats (less than 1%
genetic variation)
Genetic Variation
 Gene Flow: Transfer of alleles into
or out of a population due to
movement of fertile individuals or
gametes
 Not considered NS
 Introduced alleles would change
frequencies of next generation
 Transferred alleles can be both
beneficial and damaging to the
survival of population
 Reduces variation between
populations of different
geographic locations
 Example: HUMANS! Much more
mobile than previous generations;
much less variation based on
geographic location
Relative Fitness - Competition
 Variation doesn’t matter unless
organism that is passing on traits is in
some way improving its chances of
reproductive success
 Competition for Limited Resources
 Food
 Space
 Mates
 Survive long enough to have
offspring
 Interactions with environment such as
coloration or symbiotic relationships
with other organisms
Relative Fitness - Competition
 Sexual Selection: Form of
natural selection in which certain
individuals are more likely to
obtain mates
 Intrasexual selection: individuals
compete with member of the
same sex for mate
 More often occurs in males, but can
occur in females
 Intersexual selection: mate
choice; individuals are choosy in
selecting mates
 Often it is females choosing males
based on behavior or appearance
 Results in sexual dimorphism
Overall Requirements for NS to Occur
Reproduction
2. Heredity (ability to pass
on heritable traits)
3. Variation of traits in the
population
4. Relative Fitness of
members of the
population
1.