Chapter 23 Flashcards
When is evolution apparent?
in changes in a population over time, not individual animals
What is microevolution? (2)
change in allele frequencies in a population over generations
Evolution in its smallest scale
What are the three main mechanisms of microevolution?
natural selection
genetic drift
gene flow
How does natural selection differ from genetic drift and gene flow in regards to microevolution?
The only mechanism that consistently improves the match between organisms and the environment
What is genetic drift?
chance events that alter allele frequencies
What is gene flow?
transfer of alleles between population
What is genetic variation?
Differences among individuals in the composition of their genes or other DNA sequences
How does genetic variation differ with the amount of genes?
Single gene- either-or effect is produced (purple or white flowers)
two or more genes- produces a difference in gradations along a continuum
How is gene variation quantified?
the average percentage of loci that are heterozygous
How can gene variation be measured in the molecular level?
nucleotide variation
Why doesn’t nucleotide variation always result in phenotypic variability?
differences occur within introns
What are five sources of genetic variation?
Mutation
Gene duplication
Production of new alleles and genes
Sexual reproduction
Produced rapidly by organisms with short generation times
How does new alleles arise?
mutation, or change in nucleotide sequence
Can mutations be predicted?
Cannot be predicted how and which DNA will be altered
Where does mutations occur, and how can it be passed down?
The majority of mutations occur in somatic cells in animals
Only mutations in gametes can be passed to offspring
What is point mutation, and an example?
change in one base
Ex- sickle cell disease
How beneficial or harmful are mutations?
Most mutations are slightly harmful
What is heterozygote protection?
when heterozygous individuals mask harmful recessive alleles
What is neutral variation?
point mutations in introns
How does gene duplication occur? (3)
Error in mitosis
Slippage during DNA replication
Activities of transposable elements
When are gene duplications harmful?
Large duplications can be harmful
How can small amounts of gene duplication affect genetic variation?
Smaller duplication may persist over generations and allow mutations
How is gene duplication correlated with evolution, and an example?
Played a major role in evolution
human ancestors had only one gene to smell, now we have 350
How can prokaryotes evolve quickly?
Prokaryotes have lower average mutations but have such more generations that variations arise quickly
Why is HIV so hard to cure?
HIV mutates even more quickly since it has short generation times and RNA genome, making Single-drug treatment is less effective
How does gene variation arise from sexual reproduction?
Gene variation results from combination of alleles from parents
Methods during sexual reproduction where gene variation arises (3)
Crossing over
Independent assortment of chromosomes
fertilization
What is a population, and how are they separated?
a group of individuals of the same species that live in the same area and interbreed to produce fertile offspring
isolated from others geographically
what is a gene pool?
all copies of every type of allele at every locus in all members of the population
What does a gene pool characterize?
Characterizes a population’s genetic makeup
What is frequency proportion?
number of specific allele/number of all allele x 100%
What is a fixed allele?
an allele existing for a particular locus in a population- all individuals are homozygous for this allele
What is the Hardy-Weinberg equilibrium? (2)
a population that is not evolving
Allele and genotype frequencies will remain constant from generation to generation- only if Mendelian segregation and recombination of alleles at work
What is the Hardy-Weinberg equation?
P2 (homozygous dominant) + 2pq (heterozygous) + q2 = 1
How does the Hardy-Weinberg equation signify equilibrium?
The actual population of homozygous dominant must equal p2 and so on for the population to be in equilibrium
What are the conditions for Hardy-Weinberg equilibrium? (5)
no mutation
random mating
no natural selection
extremely large population size
no gene flow
What happens if conditions of the Hardy-Weinberg equilibrium is not met?
evolution occurs
How does mutation allow evolution?
A gene pool is modified if mutations alter alleles or if entire genes are deleted or duplicated
How does non-random mating allow evolution?
If individuals mate within a subset population, such as inbreeding, random mixing of gametes does not occur, and genotype frequencies change
How does natural selection allow evolution?
Differences in survival and reproductive success of individuals carrying different genotypes can alter allele frequencies
How does a small population size allow evolution?
The smaller the population, the more likely allele frequencies will fluctuate by chance from one generation to the next- aka genetic drift
How does gene flow allow evolution?
Moving alleles into or out of populations, gene flow can alter allele frequencies
How is the Hardy-Weinberg Equation applied?
in medicine
ex- Estimating % of the population carrying alleles for inherited diseases like PKU
What does not alter allele frequency? (2)
New mutations
non-random mating
Why doesn’t new mutations alter allele frequencies?
Mutations are rare, so change is small
Why doesn’t non-random mating alter allele frequencies?
Non-random mating can alter frequencies of homo/heterozygous, but not allele frequencies in the gene pool
What is natural selection based on?
differential success in survival and reproduction
What is natural selection?
Individuals exhibit variations in charitable traits, and traits more suited to the environment often produce more offspring
What does natural selection result in?
alleles being passed in proportions that differ
What is adaptive evolution?
an evolution that results in a better match between organisms and their environment
What is genetic drift?
chance events causing allele frequencies to fluctuate unpredictably from one generation to the next
What is genetic drift common in?
small populations
What is an example of genetic drift?
white flowers being accidentally killed, leading to less white-flowered offspring
What increases chance of deviation from predicted results?
The smaller the number of events that occur
What is does small population lead to?
over/underrepresentation of an allele in the next generation
What two things can genetic drift cause?
allele frequencies to change at random
harmful alleles to become fixed
What can genetic drift lead to?
loss of genetic variation within populations
What is the founder effect?
when few individuals become isolated from a larger population, a smaller group can establish a new population with a different gene pool
What does the founder effect account for?
a high frequency of certain inherited disorders in isolated human populations
What is the bottleneck effect?
a sudden change in environment, like a flood, drastically reducing the size of the population
What is gene flow?
transfer of alleles into or out of a population due to movement of fertile individuals or their gametes
What can gene flow result in?
two populations combining into a single population with a common gene pool
What can natural selection increase?
frequencies of alleles that provide reproductive advantage
What is relative fitness?
contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals
What does relative fitness act on?
more directly on the phenotype than on the genotype
What does selection favor?
heritable phenotypic traits that provide higher reproductive success
What is directional selection?
when conditions favor individuals exhibiting one extreme of a phenotypic range
What is a directional selection frequency curve?
Shifts the population’s frequency curve of phenotypic character in one direction or the other
When is directional selection common, and an example?
Common when the environment changes or when members migrate to a new habitat
Ex- Galapagos finches
What is disruptive selection?
when conditions favor individuals at both extremes of a phenotypic range over individuals with intermediate phenotypes
What is an example of disruptive selection?
two beak sizes in one species
One for soft seeds, the other for hard seeds
Medium-sized beaks are inefficient in feeding on both
What is stabilizing selection?
acts against both extreme phenotypes and favors intermediate variants
What does stabilizing selection reduce?
Reduces variation and tends to maintain the status quo of a phenotype
What is an example of stabilizing selection?
baby weight, with smaller and larger weights having higher mortality
How does adaptations arise?
gradually as natural selection increases frequencies of alleles that enhance survival and reproduction
What kind of process is adaptive evolution, and why?
Since the environment changes over time, adaptive evolution is a continuous dynamic process
How does genetic drift affect adaptive evolution?
can both increase and decrease beneficial allele frequency
How does genetic flow affect adaptive evolution?
introduce both advantageous and disadvantageous alleles
What is sexual selection?
natural selection which individuals with certain inherited characteristics are more likely than other individuals to obtain mates
What is sexual dimorphism, and examples (2)?
difference in secondary sexual characteristics between males and females of the same species
Ex- size, color, ornamentation, behavior
What is intrasexual selection, and what is it common in?
selection within the same sex, competing directly for mates of the opposite sex
Common in males
What is intersexual selection, and what is it common in?
individuals of one sex being choosy in selecting their mates from the opposite sex
Often in female
Why did female preferences evolve? (2)
Female preferences for certain male characteristics may have evolved because the traits are correlated with good genes
“Attractive” male traits may be related to overall health as well
Why can’t natural selection cut all unfavorable alleles?
because recessive alleles are hidden in heterozygotes
What is balancing selection?
selection preserving variation at some loci, maintaining two or more forms in a population
What does balancing selection include?
Includes heterozygote advantage and frequency-dependent selection
What is heterozygote advantage?
when heterozygotes have greater fitness than either kind of homozygotes
How does natural selection correlate with heterozygote advantage?
Natural selection tends to maintain two or more alleles at certain loci
What is heterozygote advantage defined in?
terms of genotype
What kind of selection is heterozygote advantage?
Either a stabilizing or direction selection, depending on phenotype advantage
What is an example of heterozygote advantage?
sickle cell
Heterozygotes are protected from the severe effects of malaria
What is frequency-dependent selection?
fitness of a phenotype depends on how common it is in the population
What is an example of frequency-dependent selection?
scale eating fish oscillate between right and left mouths in response to prey guarding against the specific side
How does natural selection operate?
on a “better than” basis
Four reasons why natural selection cannot fashion perfect organisms
Selection can act only on existing variation
Evolution is limited by historical constraints
Adaptations are often compromises
Chance, natural selection, and the environment interact
How does natural selection act on existing variation?
Favors only the fittest phenotype currently in the population, which may not be the ideal trait
How is evolution limited by historical constraints?
Evolution only operates on traits an organism only has, working on existing structure
What is an example of adaptation compromise?
flexible limbs allow versatility, but make us prone to sprains, torn ligaments, and dislocation
How does chance and natural selection interact, and an example?
Chance events affect the evolution of the population
Ex- storm can transport insects to an unfavorable environment
How does the environment and natural selection interact?
The environment at a location can change unpredictably from year to year
