Bio U3L1 Flashcards
Microevolution
Is a change in allele frequencies in a
population over generations
What is bottle neck
refers to a sharp reduction in the size of a population due to environmental events or other factors that significantly decrease genetic diversity.
Three mechanisms cause allele frequency change
Natural selection
Genetic drift
Gene flow
What causes adaptive evolution?
Natural selection
What is a prerequisite for evolution?
Variation in heritable traits
How is genetic variation among individuals caused?
differences in genes or other DNA segments
What is phenotype the product of?
Inherited genotype and environmental influences
How are phenotypic differences determined?
Either by a single gene - classified on an either or basis
Or by two or more genes - vary along a continuum within a population
How can genetic variation be measured?
Gene variability or nucleotide variability
Average heterozygosity
Measures the average percent of loci that are heterozygous in population
How is Nucleotide variability measured
By comparing DNA sequences of pairs of individuals (rarely results in phenotypic variation)
Example of phenotypic variation that does not result from genetic differences but from environmental influences
Caterpillars can have different appearances due to chemicals in diet and not their genotype
Only genetically determined variation can have
evolutionary consequences
population
localized group of individuals
capable of interbreeding and producing fertile offspring
gene pool
consists of all the alleles for all loci in a
population
A locus is fixed if
all individuals in a population are
homozygous for the same allele
How many alleles and what is used to represent their frequencies
2
p and q
Frequency of all alleles in a population will add up to
1
p + q = 1
Describe The Hardy-Weinberg Principle
If p and q represent relative frequencies of only two possible alleles in a population at a particular locus, then
– p^2 + 2pq + q^2 = 1
– where p^2 and q^2 represent the frequencies of the homozygous genotypes, and 2pq represents frequency of the heterozygous genotype
What does the hardy weinberg equilibrium describe
- Describes a hypothetical population that is not evolving
- In real populations, allele and genotype frequencies do change over time
- The 5 conditions are rarely met in nature
What are the 5 conditions required for hardy weinberg and what are the consequences if condition does not hold
- No mutations: Gene pool is modified if mutations occur or if entire genes are deleted or duplicated
- Random mating: If individuals mate within a subset of the population such as a neighbour or close relatives (inbreeding) random mixing of gametes does not occur and gene frequencies change
- No natural selection: Allele frequencies change when individuals with different genotypes show consistent differences in their survival or reproduction success
- Extremely large population size: allele frequencies fluctuate by chance over time (genetic drift)
- No gene flow: By moving alleles into or out of populations, gene flow can alter allele frequencies
Red short-horned cattle are homozygous for the red allele, white cattle are homozygous for the white allele, and roan cattle are heterozygotes. Population A consists
of 36% red, 16% white, and 48% roan cattle. What are the allele frequencies?
Allele counting method since we do not know if this is a weinberg question
Assume 100 individuals = 200 alleles
36% = 36/100 are red and 36x2 is 72 (diploid so multiply by 2)
48/100 are roan ( red + white)
72 + 48 = 120/200 = 0.6
Do the same thing with white
32 + 48 = 80/200 = 0.4
q^2 is for dominant or recessive genes?
recessive
In a population of 500 people, 6 have a
rare autosomal recessive disorder. How many people are carriers? Assume weinberg assumptions are met.
of carriers = 2pq x 500
q^2 = 6/500
q = √(6/500)
p + q = 1
p = 1- √(6/500)
# of carriers = 2(1-√(6/500)) (√(6/500)) x 500
# of carriers = 97.5
Three major factors alter allele frequencies and bring about
most evolutionary change:
Natural selection
Genetic drift
Gene flow
Differential success in reproduction results in
certain alleles being passed to the next generation in greater proportions
Natural selection can cause adaptive evolution, an improvement in the
Give an example
match between organisms and their
environment
an allele conferring resistance to DDT in fruit flies increased in frequency after DDT was used widely in agriculture
Genetic Drift
describes how allele frequencies
fluctuate unpredictably from one generation to the next. The smaller a sample, the greater the chance of
random deviation from a predicted result. Genetic drift tends to reduce genetic variation through
losses of alleles
Reduces variation
founder effect
occurs when a few individuals
become isolated from a larger population
Allele frequencies in the small founder population can be different from those in larger parent population
Type of genetic drift
bottleneck effect
Type of genetic drift
sudden reduction in population size due to a change in the environment. The resulting gene pool may no longer be reflective of the original population’s gene pool
* If the population remains small, it may be further affected by genetic drift
List the effects of genetic drift
- Genetic drift is significant in small populations
- Genetic drift can cause 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
consists of movement of alleles among
populations. Alleles can be transferred through movement of fertile individuals or gametes (for example, pollen)
* Gene flow tends to reduce variation among populations over time
can decrease or increase the fitness of a population
Phenylketonuria (PKU) is extremely
common in Ireland, affecting approximately 1 in 4500 live births. Assuming that the population is in Hardy-Weinberg equilibrium, what is the frequency of PKU carriers?
Freq of carriers = 2pq
q = √1/4500
p = 1 - √1/4500
Freq of carriers = 2 x (1-√1/4500)(√1/4500)
= 2.9%
Examples of gene flow increasing or decreasing fitness
Increase: Insecticides have been used to target mosquitoes carrying
West Nile virus and malaria. Alleles have evolved in some mosquito populations that confer resistance to these insecticides. The flow of insecticide resistance alleles into a population causes an increase in fitness
Decrease: Mating causes gene flow between the central and eastern populations. Immigration from mainland introduces alleles that
decrease fitness on the island
Evolution by natural selection involves
Chance: New genetic variations arise by chance
Sorting: Beneficial alleles are “sorted” and favoured by natural
selection
Relative fitness
Is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals
Three modes of selection
Directional selection: favours individuals at one end of the phenotypic range
Disruptive selection: favours individuals at both extremes of the phenotypic range
Stabilizing selection: favours intermediate variants and acts against extreme phenotypes
Why is adaptive evolution a continuous process?
Environment can change
Balancing selection
occurs when natural selection
maintains stable frequencies of two or more phenotypic forms in a population
Balancing selection includes
Heterozygote advantage
Frequency-dependent selection
frequency-dependent selection
the fitness of a phenotype declines if it becomes too common in the population
Heterozygote advantage
occurs when heterozygotes
have a higher fitness
can result from stabilizing or
directional selection
Why Natural Selection Cannot Fashion
Perfect Organisms
- Selection can act only on existing variations
- Evolution is limited by historical constraints
- Adaptations are often compromises
- Chance, natural selection, and the environment interact
