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LS1 - Chapter 21 - Evolution: How Genotypes and Phenotypes Change Over Time
Terms in this set (88)
What is phenotype?
an observable trait
What factors influence phenotype?
genotype and the environment
What is genotype?
the set of alleles that an individual possesses at relevant genetic loci
What is a gene pool?
all the alleles present in all individuals in a species
What is population genetics?
the study of genetic variation in natural populations
What is a population?
interbreeding groups of organisms of the same species living in the same geographical area at the same time
What are the two sources of genetic variation?
mutation and recombination
How does recombination increase genetic variation?
it shuffles mutations to create new combinations of mutations
Mutations can be categorized as occurring in what two types of cells?
somatic or germ-line
What does somatic mean?
occurring in the body's tissues
What does germ-line mean?
occurring in the reproductive cells
From an evolutionary viewpoint, what kind of mutation are we interested in? Why?
germ-line mutation because it is passed on to the next generation
From an evolutionary viewpoint, what kind of mutation are we NOT interested in?
somatic mutation because it only affects the cells descended from the one cell in which the mutation originally arose, and thus affects only that one individual
What is a mutation?
a random change occurring in the DNA sequence
What are deleterious mutations?
How are some mutations neutral?
they occur in non-coding regions of the genome or do not alter the protein formed
How do advantageous mutations affect an organism?
they improve their carriers' chances of survival or reproduction
How is allele frequency calculated?
number of allele a / total number of alleles in population
What does it mean for an allele to be fixed?
the allele has an allele frequency of 100%
What are three ways to measure genotype and allele frequencies?
observable traits, gel electrophoresis, and DNA sequencing
Why does using observable traits to determine genotype frequency rarely work?
1) Many traits are encoded by a large number of genes
2) The phenotype is a product of both genotype and the environment
Before gel electrophoresis and DNA sequencing, population geneticists could only limit themselves to working with what type of phenotypes?
those that are encoded by a single gene
How does gel electrophoresis help population geneticists determine genotype?
Proteins move at different speeds through the gel when the current is turned on because of different charge and sizes. Individuals with different genotypes will have different proteins encoded so gel electrophoresis provides a visual picture of genetic variation.
What are two limitations of using gel electrophoresis to determine genotype?
1) Limited to enzymes because we need to be able to stain specificaly for enzyme activity
2) Could only detect mutations that resulted in an amino acid substitution that caused the protein to move at different speeds in the gel
What technique provided population geneticists an unambiguous means of detecting all genetic variation in a stretch of DNA?
How does DNA sequencing detect genetic variation?
differences in the DNA sequence itself is genetic variation
What is evolution?
a change in allele or genotype frequency from one generation to the next
Evolution acts on populations or individuals?
What situation does the Hardy-Weinberg equilibrium describe?
a situation in which evolution is not occurring in a population for a particular gene
What are the conditions for the Hardy-Weinberg equilibrium?
1) No differences in the survival and reproductive success of individuals [no natural selection]
2) No migration [no gene flow]
3) No mutations
4) Sufficiently large (technically infinite) population to prevent sampling error [no genetic drift]
5) Random mating [no non-random mating/sexual selection]
What is genetic drift?
a change in the frequency of an allele due to random change
Is genetic drift more apparent in small or large populations?
What two relationships can predict allele frequencies from genotype frequencies and vice-versa if the Hardy-Weinberg equilibrium holds true?
p + q = 1
p² + 2pq + q² = 1
What is selection?
the differential success of alleles
What about natural selection did Darwin find inspiration from Thomas Malthus?
Natural populations can increase geometrically, but limited resources cause competition between individuals. Individuals better adapted to their environment win this competition and their advantageous traits are passed on.
What about natural selection did Darwin find inspiration from geology?
a small change (such as a change in allele frequency) over time can lead to large changes
What is fitness?
the measure of the extent to which an individual's genotype is represented in the next generation
Why did evolutionary biologists initially argue that Mendel's discoveries did not apply to most genetic variation?
The traits studied by Mendel were discrete, but most of the variation we see in natural populations is continuous.
How could Mendel's discrete traits account for the continuous variation seen in natural populations? Who realized it?
Ronald Fisher realized that instead of a single gene affecting a trait, many traits were affected by multiple genes.
What is the Modern Synthesis?
The synthesis between Darwin's theory of natural selection and Mendelian genetics.
Mendelian genetics provided the mechanism by which natural selection passed on advantageous traits.
What is positive selection?
natural selection that increases the frequency of a favorable allele
Advantageous mutations will experience what type of selection?
Are the majority of mutations to FUNCTIONAL genes deleterious, neutral, or advantageous?
What is negative selection?
natural selection that decreases the frequency of a harmful allele
Why does natural selection NOT ALWAYS immediately eliminate all deleterious mutations?
Sometimes they are recessive and not expressed as a heterozygote, and natural selection will not select against that allele unless it is homozygous recessive
What is balancing selection?
natural selection that acts to maintain two or more alleles in a population
Ex. sickle cell
What is heterozygote advantage?
a form of balancing selection in which the heterozygote's fitness is higher than that of either homozygotes
When viewing changes over time in a trait rather than individual alleles, natural selection can take what three forms?
stabilizing, directional, and disruptive
What is stabilizing selection?
a form of natural selection that selects against the extremes
What is directional selection?
a form of natural selection that selects for an extreme (leads to a change in a trait over time)
What is disruptive selection?
a form of natural selection that selects for both extremes
What type of selection is artificial selection? How is it analagous to natural selection
a form of directional selection in which the competitive element of natural selection is removed
What type of selection keeps a trait the same over time?
What type of selection changes a trait over time?
What is sexual selection?
selection of traits that increase an individual's access to reproductive opportunities (sometimes acts against natural selection, Ex. peacock feathers are metabolically expensive to produce, advertise to predators, etc. so natural selection acts against them but sexual selection acts for them)
What evolutionary mechanism(s) lead to adaptation?
What evolutionary mechanism(s) lead to changes in allele frequency?
natural selection, gene flow, mutation, genetic drift, and sexual selection
What evolutionary mechanism(s) are considered non-adaptive evolutionary mechanisms?
gene flow, mutation, and genetic drift
What is gene flow?
the movement of alleles from one population to another
What evolutionary mechanism reduces genetic variation between populations?
Migration (increases/decreases) genetic variation between populations.
What are two cases of genetic drift?
population bottleneck and founder effect
What is a population bottleneck?
when a population falls to just a few individuals and genetic drift acts upon that population
What is the founder effect?
when a new population is split off from a larger population and genetic drift acts upon the smaller population
Does natural selection act upon neutral mutations?
What acts upon neutral mutations?
The extent of genetic difference or genetic divergence between two species is a function of what?
the time they have been genetically isolated from each other;
the longer they have been isolated, the greater the genetic divergence
What is the molecular clock?
the correlation between the time two species have been evolutionarily separated and the amount of genetic divergence between them
Why do the rates of molecular clocks vary from gene to gene?
Because of differences in the intensity of negative selection among different genes
The genes with the slowest molecular clocks are the ones that what?
negative selection acts very strongly on to eliminate any mutation
The genes with the fastest molecular clocks are the ones that what?
negative selection does NOT act very strongly on to eliminate any mutation
What is a pseudogene?
a gene that is no longer functoinal
Do pseudogenes have fast or slow molecular clocks?
fast molecular clocks because, since it is no longer a functional gene, all mutations are neutral and not eliminated by natural selection
What are abiotic factors?
(non-living) physical factors of the environment
What are biotic factors?
living factors of the environment
What was Jean-Baptiste Lamarck's idea of evolution?
Use or disuse of traits could cause traits to be altered over an individual's lifetime and that altered trait could be inherited by offspring
What four observations led to Darwin's theory of natural selection?
1) All populations have the ability to grow exponentially.
2) A population cannot grow exponentially due to resource limitation. [so more individuals are born than survive which leads to competition]
3) Variation exists within a population.
4) This variation is inheritable.
What is the Norm of Reaction?
how environment affects phenotype across a range of environmental conditions
[Ex. hydrangea color determined by pH of the soil]
What does a steep/flat Norm of Reaction mean?
Steep Norm of Reaction means there is a wide range of phenotypic expression due to environment
What is Darwin's Theory of Natural Selection?
Variation leads to differential survival & differential reproductive success for individuals
Can fitness change if the environment changes?
What is the difference between adaptation and acclimation?
Adapt refers to a genetic change while acclimate refers to no genetic change
Do individuals/populations adapt/acclimate?
What is microevolution?
a change in the relative frequencies of alleles in a gene pool over time
What is assortative mating?
What are the mechanisms of evolution?
1) natural selection
2) gene flow
4) genetic drift
5) non-random mating
What is the law of independent assortment?
alleles are assorted randomly during gamete formation [i.e. alleles are passed on randomly from parent to offspring]
Charles Darwin was influenced by writings of what two people? How were their writings influential?
Thomas Malthus - populations grow exponentially, but are limited by linear increase in resources.
Charles Lyell (geology) - small changes lead to large changes over time
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