Chapter 17 Notes - Evolution of Populations
Darwin based his evolutionary theory on heritable variation, but he had no idea where the variation came from or how heredity worked
Mendel’s work was published during Darwin’s lifetime, however, no one realized how it related to Darwin’s work
Biologists connected the work of Mendel to Darwin in the 1930s
20th century geneticists discovered that heritable traits are controlled by genes and carried on chromosomes - changes in genes and chromosomes generate variation
individual organisms don’t evolve → populations evolve from generation to generation
population - a group of individuals of the same species that mate and produce offspring
plants and animals have two sets of genes in their nuclei, one from each parent
alleles — specific forms of genes — may vary from individual to individual
natural selection works directly on phenotype, not genotype
it acts on an organism’s characteristics, not directly on its alleles
it “selects” an entire organism, not a single gene or group of genes, to survive and reproduce or die without reproducing
some individuals have phenotypes that are better suited to their environment than phenotypes of other individuals
better suited individuals have higher fitness, produce more offspring, and pass more copies of their genes to the next generation that poorly suited individuals do
inherited variation can lead to natural selection because it can result in differential reproductive success for individuals with different phenotypes
members of a population interbreed with one another — they share a common group of genes called a gene pool
gene pool - a gene pool consists of all genes, including all alleles for each gene, that are present in a population
gene pools are studied by examining the number of different alleles they contain
allele frequency - the number of times an allele occurs in a gene pool
allele frequency has nothing to do with whether it is dominant or recessive, just whether it occurs more frequently
in genetic terms, evolution is any change in the frequency of alleles in a population over time
two main sources of genetic variation:
Mutation: any change in a sequence of DNA
caused by a mistake in replication or radiation and chemicals in the environment
some mutations are harmless, others affect an organism’s fitness
fitness - the ability to survive and reproduce
Gene shuffling from sexual reproduction:
MEIOSIS
Review
Segregation - separation of alleles during meiosis
Independent Assortment - genes for different traits assort independently
Crossing over - homologous chromosomes exchange segments, increases variation
sexual reproduction is a major source of genetic variation
The number of phenotypes produced for a given trait depends on how many genes control the trait
single-gene trait: any trait controlled by one gene that has two alleles
ex. widow’s peak
only 2 possible phenotypes
polygenic traits: traits controlled by two or more genes
ex. skin color, height
traits have more than 2 alleles
many possible phenotypes — not just 2
Natural selection never acts on a single gene — it acts on an entire organism because the whole organism either survives to reproduce or dies before reproducing — populations evolve over time, not single individuals
natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution
ex. brown lizards reproduce, but because of mutations, black and red babies are born
the black is more advantageous because it absorbs more sunlight — warm up faster, more energy, run faster
the red is very visible to predators and doesn’t survive to reproduce like the black lizard does
The effects of natural selection are more complex on polygenic traits because there are many possible phenotypes and genotypes
often display a range of phenotypes that form a curve — fitness of individuals may vary from one end of a vurve to the other
Natural selection can affect the distributions of phenotypes in any of 3 ways:
Directional Selection:
when individuals at one end of the curve have a higher fitness than individuals in the middle or at the other end
curve moves in one direction
Stabilizing Selection:
when individuals near the center of the curve have higher fitness than individuals at the two other ends
curve gets narrower
Disruptive Selection:
when individuals at the upper and lower end of the curve have a higher fitness
curve dips in the middle
Genetic Drift: In small populations, chance alone can cause a change in the allele frequency in a population
Ex.
founder effect - when allele frequencies change because of the migration of a small number of individuals in a population
Evolution vs Genetic Equilibrium:
Hardy-Weinberg Principle: allele frequencies in a population will remain constant unless one ore more factors cause those frequencies to change
genetic equilibrium - the situation in which allele frequencies remain constant
5 conditions disrupt genetic equilibrium & cause evolution to occur
Non-random mating - sexual selection - individuals select mates based on size, strength, or coloration
genes for traits selected for or against are not in equilibrium
Small population size - genetic drink affects small populations
Gene flow from immigration or emigration - changes in allele frequency can be produced by gene flow
this causes evolution to occur
Mutations - can introduce new alleles into the gene pool, changing allele frequencies and causing evolution to occur
Natural selection - if different genotypes have different fitness, natural selection will disrupt genetic equilibrium and evolution will occur
species - a population or a group of populations whose members interbreed and produce fertile offspring
interbreeding permits any genetic change that occurs to spread throughout populations of a species
if some members of population stop breeding with other members, the species gene pool can split
once members of two populations stop interbreeding, changes in one gene pool cannot spread to the other — reproductive isolation
speciation - when populations become reproductively isolated, they may evolve into two separate species
3 ways for reproductive isolation to occur:
Behavior Isolation: two populations have differences in courtship rituals or other reproductive strategies that involve behavior
ex. Eastern and Western Meadowlarks use different songs to attract males
Geographic Isolation: two populations are separated by geographic barriers such as rivers and mountains
Temporal Isolation: reproduction occurs at different times of the year
ex. 3 different orchids live in the rain forest. Each produces a flower that only blooms one day and must be pollinated on that day to produce seeds. Because the three species flowers bloom on different days they cannot pollinate each other
Testing Natural Selection in Nature:
Darwin’s Finches - natural selection shaped the bird beaks on each island depending on the food source
Speciation in Darwin’s Finches:
Speciation in the Galapagos finches occurred by founding of a new population, geographic isolation, changes in the new population’s gene pool, reproductive isolation, and ecological competition
Studying Evolution Since Darwin:
Scientific evidence since Darwin’s time supports the theory that living species descended with modification from common ancestors that lived in the ancient past
research is limited to what we can observe or find in fossils
many questions are still unanswered
Darwin based his evolutionary theory on heritable variation, but he had no idea where the variation came from or how heredity worked
Mendel’s work was published during Darwin’s lifetime, however, no one realized how it related to Darwin’s work
Biologists connected the work of Mendel to Darwin in the 1930s
20th century geneticists discovered that heritable traits are controlled by genes and carried on chromosomes - changes in genes and chromosomes generate variation
individual organisms don’t evolve → populations evolve from generation to generation
population - a group of individuals of the same species that mate and produce offspring
plants and animals have two sets of genes in their nuclei, one from each parent
alleles — specific forms of genes — may vary from individual to individual
natural selection works directly on phenotype, not genotype
it acts on an organism’s characteristics, not directly on its alleles
it “selects” an entire organism, not a single gene or group of genes, to survive and reproduce or die without reproducing
some individuals have phenotypes that are better suited to their environment than phenotypes of other individuals
better suited individuals have higher fitness, produce more offspring, and pass more copies of their genes to the next generation that poorly suited individuals do
inherited variation can lead to natural selection because it can result in differential reproductive success for individuals with different phenotypes
members of a population interbreed with one another — they share a common group of genes called a gene pool
gene pool - a gene pool consists of all genes, including all alleles for each gene, that are present in a population
gene pools are studied by examining the number of different alleles they contain
allele frequency - the number of times an allele occurs in a gene pool
allele frequency has nothing to do with whether it is dominant or recessive, just whether it occurs more frequently
in genetic terms, evolution is any change in the frequency of alleles in a population over time
two main sources of genetic variation:
Mutation: any change in a sequence of DNA
caused by a mistake in replication or radiation and chemicals in the environment
some mutations are harmless, others affect an organism’s fitness
fitness - the ability to survive and reproduce
Gene shuffling from sexual reproduction:
MEIOSIS
Review
Segregation - separation of alleles during meiosis
Independent Assortment - genes for different traits assort independently
Crossing over - homologous chromosomes exchange segments, increases variation
sexual reproduction is a major source of genetic variation
The number of phenotypes produced for a given trait depends on how many genes control the trait
single-gene trait: any trait controlled by one gene that has two alleles
ex. widow’s peak
only 2 possible phenotypes
polygenic traits: traits controlled by two or more genes
ex. skin color, height
traits have more than 2 alleles
many possible phenotypes — not just 2
Natural selection never acts on a single gene — it acts on an entire organism because the whole organism either survives to reproduce or dies before reproducing — populations evolve over time, not single individuals
natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution
ex. brown lizards reproduce, but because of mutations, black and red babies are born
the black is more advantageous because it absorbs more sunlight — warm up faster, more energy, run faster
the red is very visible to predators and doesn’t survive to reproduce like the black lizard does
The effects of natural selection are more complex on polygenic traits because there are many possible phenotypes and genotypes
often display a range of phenotypes that form a curve — fitness of individuals may vary from one end of a vurve to the other
Natural selection can affect the distributions of phenotypes in any of 3 ways:
Directional Selection:
when individuals at one end of the curve have a higher fitness than individuals in the middle or at the other end
curve moves in one direction
Stabilizing Selection:
when individuals near the center of the curve have higher fitness than individuals at the two other ends
curve gets narrower
Disruptive Selection:
when individuals at the upper and lower end of the curve have a higher fitness
curve dips in the middle
Genetic Drift: In small populations, chance alone can cause a change in the allele frequency in a population
Ex.
founder effect - when allele frequencies change because of the migration of a small number of individuals in a population
Evolution vs Genetic Equilibrium:
Hardy-Weinberg Principle: allele frequencies in a population will remain constant unless one ore more factors cause those frequencies to change
genetic equilibrium - the situation in which allele frequencies remain constant
5 conditions disrupt genetic equilibrium & cause evolution to occur
Non-random mating - sexual selection - individuals select mates based on size, strength, or coloration
genes for traits selected for or against are not in equilibrium
Small population size - genetic drink affects small populations
Gene flow from immigration or emigration - changes in allele frequency can be produced by gene flow
this causes evolution to occur
Mutations - can introduce new alleles into the gene pool, changing allele frequencies and causing evolution to occur
Natural selection - if different genotypes have different fitness, natural selection will disrupt genetic equilibrium and evolution will occur
species - a population or a group of populations whose members interbreed and produce fertile offspring
interbreeding permits any genetic change that occurs to spread throughout populations of a species
if some members of population stop breeding with other members, the species gene pool can split
once members of two populations stop interbreeding, changes in one gene pool cannot spread to the other — reproductive isolation
speciation - when populations become reproductively isolated, they may evolve into two separate species
3 ways for reproductive isolation to occur:
Behavior Isolation: two populations have differences in courtship rituals or other reproductive strategies that involve behavior
ex. Eastern and Western Meadowlarks use different songs to attract males
Geographic Isolation: two populations are separated by geographic barriers such as rivers and mountains
Temporal Isolation: reproduction occurs at different times of the year
ex. 3 different orchids live in the rain forest. Each produces a flower that only blooms one day and must be pollinated on that day to produce seeds. Because the three species flowers bloom on different days they cannot pollinate each other
Testing Natural Selection in Nature:
Darwin’s Finches - natural selection shaped the bird beaks on each island depending on the food source
Speciation in Darwin’s Finches:
Speciation in the Galapagos finches occurred by founding of a new population, geographic isolation, changes in the new population’s gene pool, reproductive isolation, and ecological competition
Studying Evolution Since Darwin:
Scientific evidence since Darwin’s time supports the theory that living species descended with modification from common ancestors that lived in the ancient past
research is limited to what we can observe or find in fossils
many questions are still unanswered
