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Sources of genetic variations
Ch. 21
Mutations1. mutations:
Is a change in the sequence of nucleotides in the DNAMay result in new alleles or new genesIs usually harmfulMust be in the gametes to be passed on to offspring
Mutations are more likely to be favorable when the environment is changing
Duplications Duplication mutations can lead
to an expanded genome with more loci that can take on new functions
Duplication mutations are thought to be the reason why certain mammals have more genes for detecting odors than others
Mutation rates Plants and animals
average about 1 mutation for every 100,000 genes per generation
Mutations happen more quickly in organisms that reproduce quickly, like bacteria
2. Sexual Recombination of DNA
During crossing over and fertilization
Produces variations that make adaptation possible
Sexual reproduction results in unique combinations of genes every generation
Quick Checks What is “sexual recombination”? What processes need to occur in cells
for “sexual recombination” to occur?
Shifts in allele frequency
Most evolutionary change the result of…1. Natural selection2. Genetic drift3. Gene flow
Natural Selection Results in the
differential success of organisms in being able to reproduce and pass on their genes to the next generation
Genetic Drift Fluctuations in allele frequencies in a small population from
one generation to the next The smaller the size of the population, the more likely there is to be a
major shift in allele frequencies
Genetic DriftEvolution by drift is aimless, not adaptive, because it is chance alone
Drift is common in two population events: Genetic bottlenecks and Founder events.
3 initial breeding pairs - shift in gene pool
The Bottleneck Effect when a population
undergoes a drastic reduction in size as a result of chance events (fire, flood, drought).
It is completely arbitrary and unrelated to phenotype.
The Bottleneck Effect
Lack of variation means less adaptability
Some alleles may be overrepresented in the survivors, some underrepresented, and some not represented at all.
Humans sometimes create bottlenecks in other species
The Founder Effect
a small group of individuals becomes separated from the larger population. They may establish a new population with a gene pool that is not representative of the larger population. Helps account for the high frequency of certain disorders in isolated populations
Gene Flow
Gene flow - movement of genes between populations. Gain or loss of alleles from a population due to migration of fertile individuals, or from the transfer of gametes.
Tends to reduce differences between populations
Gene flow increases the variability of the gene pool by adding new alleles.
Bioflix directions Natural selection Genetic Drift Gene Flow Title, explanation,
example
Natural selection
Explanation:
Example:
Genetic Drift
Explanation:
Example:
Gene Flow
Explanation:
Example:
Natural Selection in more detail
21
Variation in a population
Discrete characters - “either/or” traits, usually controlled by a single gene E.g. - widow’s peak or no widow’s
peak Phenotypic polymorphism - the
population has 2 or more distinct morphs for a trait that are fairly common in the population
Genetic polymorphisms - the presence of 2 or more distinct alleles in the gene pool of a population
May result in phenotypes that vary along a continuum, like height in humans
Geographic variation Most species
have geographic variations in their gene pools between different populations Due to
environmental differences
Survival of the fittest
Fitness - the contribution an individual makes to the gene pool of the next generation for advantageous adaptations
Relative fitness - the contribution of a genotype to the next generation. It is dependent upon the genetic and environmental context in which it was expressed (is it still an advantage to have that gene?)
Modes of Selection Directional
Most common during times of change or migration Favors one end of the phenotype range
Modes of Selection Disruptive
Occurs when conditions favor the two extremes in the phenotype range
Most likely to result in speciation
Modes of Selection Stabilizing -
Favors the average phenotype Reduces variation in a population
Why don’t “negative” phenotypes eventually disappear?
Diploids - variation “hides” in recessive alleles of heterozygotes
Balancing selection - natural selection that favors two or more phenotypes in a population, called balanced polymorphism
The Heterozygote Advantage
When individuals that are heterozygous for a trait have greater fitness than homozygotes
Example: sickle cell anemia AA - normal RBCs but prone to the
worst effects of malaria aa - have sickle cell anemia Aa - do not have sickle cell, are
protected against the worst effects of malaria
Frequency Dependent Selection The fitness of any one morph declines if
that morph becomes too common in the population Common in predator-prey relationships
Sexual Selection Natural selection for mating success Sexual dimorphism - the obvious
differences between males & females in a population
Intrasexual selection - members of the same sex compete with each other for mates Antlers, large size, etc
Intersexual selection - mate choice - individuals of one sex are choosy in selecting their mate Peacock tails, etc. Reflects upon the overall
fitness of the individual (only healthy males can grow that big tail)
Natural Selection cannot create perfect organisms
1. New phenotypes are the result of many small changes to previous phenotypes
2. As the environment changes, previous adaptations may not be favorable anymore
3. Selection can only act on variations that already exist in the gene pool