Evolution of Populations Flashcards
Natural selection acts on
individuals
only ____ evolve by natural selection
populations
The smallest unit of evolutionary change
population
Microevolution
a change in allele frequencies in populations over generations
Alleles
different forms of a gene, corresponding to different DNA sequences in each form (one allele per chromosome)
Population
a localized group of individuals of a single species that interbreed (share alleles) and produce fertile offspring
Gene pool
all the alleles present in all individuals in the population
Gene variation is advantageous to a population because:
enables adaptation of the population to the environment via natural selection
Phenotype is a product of
genotype and environmental influences
New genes and alleles arise by
mutation and gene duplication
Mutations
changes in an individuals DNA sequence
Mutations can be caused by
i) small-scale (e.g. point mutation) or chromosomal (e.g. insertion/deletion) errors in DNA
replication
ii) structural damage to DNA (e.g. radiation)
Sources of genetic variation
mutations, sexual preproduction
Sexual reproduction
can shuffle existing alleles into new combinations
Adaptation
feature or trait (created through natural selection) that provides an advantage (higher relative fitness) to an individual possessing it.
relative fitness
the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals
Three modes of natural selection
directional selection, disruptive selection, stabilizing selection
Directional selection
favors individuals at one end of the phenotypic range and occurs in response to consistent selective pressure
Disruptive selection
favors individuals at both extremes of the phenotypic range (results in 2 or more divergent phenotypes)
Stabilizing Selection
favors intermediate or common variants (mean of the population stays the same and variation of the population decreases)
Factor that alter allele frequencies in populations
natural selection, genetic drift, gene flow
Genetic drift
random changes in allele frequency in a population and is more likely in small populations
Genetic drift does not create
evolutionary adaptations
Examples of genetic drift
bottleneck effect, founder effect
Rare alleles are more likely lost due to
genetic drift
Bottleneck effect
a sudden reduction in population size due to a change in the environment and allele frequency in the next generation is different than the previous generation
Founder effect
occurs when a few individuals become isolated from a larger population which has a small fraction of the total gene pool present in the original population
Key points of genetic drift
- Genetic drift has the greatest impact in small populations.
- Genetic drift causes allele frequencies to change at random.
- Genetic drift can lead to a loss of genetic variation within populations.
- Genetic drift can cause harmful alleles to become fixed.
Gene flow
gene flow is the movement of alleles between populations of the same species (by fertile individuals or games like pollen)
Gene flow reduces ___ between populations over time
reduces variation
Gene flow can ____ the fitness of a population
increase/decrease
Neutral variation
genetic variation that does not confer a selective advantage or disadvantage
Genetic variation in populations is maintained by
Mutation.
− Recombination (crossing-over of chromosomes during meiosis).
− Independent assortment (of alleles) during meiosis.
− Random mating between organisms (sexual reproduction).
− Random fertilization (sexual reproduction).
− Recessive alleles are hidden from selection in heterozygote individuals.
− Disruptive selection (natural selection).
− Gene flow (between populations).
− Balancing selection.
Balancing selection
occurs when natural selection maintains stable frequencies of two or more alleles in a population
Mechanisms of balancing selection
− Heterozygote advantage
− Frequency-dependent selection
Heterozygote advantage
occurs when heterozygotes have higher fitness than do bot homozygotes
example of Heterozygote advantage
sickle cell disease
Frequency dependent selection
the fitness of a phenotype declines if it becomes too abundant in the population where the rare genotype has a selective advantage
