81 terms

Chapter 23

Campbell Biology, Biology
Natural selection changes allele frequencies because some _______ survive and reproduce more successfully than others.
There are 40 individuals in population 1, all with genotype A1A1, and there are 25 individuals in population 2, all with genotype A2A2. Assume that these populations are located far from each other and that their environmental conditions are very similar. Based on the information given here, the observed genetic variation is most likely an example of
genetic drift.
A fruit fly population has a gene with two alleles, A1 and A2. Tests show that 70% of the gametes produced in the population contain the A1 allele. If the population is in Hardy-Weinberg equilibrium, what proportion of the flies carry both A1 and A2?
In a large population of randomly breeding organisms, the frequency of a recessive allele is initially 0.3. There is no migration and no selection. Humans enter this ecosystem and selectively hunt individuals showing the dominant trait. When the gene frequency is reexamined at the end of the year, _____.
-the frequency of the homozygous dominants will go down, the frequency of the heterozygous genotype will go down, and the frequency of the homozygous recessives will go up
-the frequency of the homozygous dominants will go up, the frequency of the heterozygous genotype will go down, and the frequency of the homozygous recessives will go down
-the frequency of the homozygous dominants will go down, the frequency of the heterozygous genotype will go up, and the frequency of the homozygous recessives will remain the same
-the frequency of the homozygous dominants will go down, the frequency of the heterozygous genotype will remain the same, and the frequency of the homozygous recessives will go up
-the frequency of the homozygous dominants will go up, the frequency of the heterozygous genotype will go down, and the frequency of the homozygous recessives will go up
the frequency of the homozygous dominants will go down, the frequency of the heterozygous genotype will go down, and the frequency of the homozygous recessives will go up
In the Hardy-Weinberg equation, p2 represents _____.
-the frequency of dominant alleles in the population
-the frequency of homozygous recessive individuals in the population
-the expected frequency of homozygous dominant individuals in the population
-the frequency of heterozygous dominant individuals in the population
-the total alleles in the gene pool
-the expected frequency of homozygous dominant individuals in the population
In a large population of bonobos, the frequency of the recessive allele is initially 0.1. There is no migration and no selection. What is the frequency of the dominant allele? Assume that there are two alleles of this gene.
p (the frequency of the dominant allele) = 1 − q (the frequency of the recessive allele), or 0.9.
In a population in Hardy-Weinberg equilibrium, 1% of the individuals in a population show the recessive trait of a certain characteristic. In this situation, what is the value of p?
In the equation for Hardy-Weinberg equilibrium, 1 represents _____.
the sum of the frequencies of the genotypes for a particular gene locus
What does natural selection act on? What actually evolves?
Natural selection acts on individuals, but only populations evolve.
individuals either survive or don't survive.
What is microevolution?
change in allele frequencies in a population over generations (population is then better able to survive)
What are the three mechanisms that can cause allele frequency change?
-Natural selection
-Genetic drift - chance events that alter allele frequencies
-Gene flow - transfer of alleles b/w populations
Does genetic drift and genetic flow cause cause animals to evolve?
-Genetic Drift and Gene flow are chance events that are not adaptive changes and does not help organisms better able to survive.
- Only natural selection causes adoptive evolution.
-it is the only thing that consistently improves relationship b/w population and environment, "adaptation"
-Genetic drift and gene flow do not consistently lead to adaptive evolution as they can increase or decrease the match between an organism and its environment
Genetic variation makes evolution possible
Variation in heritable traits (i.e., caused by genetic variation) is a prerequisite for evolution (although Darwin didn't necc know how organisms pass heritable traits down to offspring. Also obv this is true because you need differences to select).
What causes genetic variation?
-differences in genes or other DNA segments
What is phenotype?
product of inherited genotype and environmental influences. Environmentally influenced phentoypes, i.e. a bodybuilder's buffness, is not heritable. Nothing acquired over a lifetime can be passed.
Two types of variation within a population : discrete and quantitative
-discrete characters can be classed in an either or basis (Many discrete characteristics are determined by a single locus w diff alleles that produce distinct phenotypes)
-quantitative characters vary along a continuum within a population
Two ways to measure genetic variability IN a population: gene variability and neucleotide variability
-gene variability - % of different alleles in population, also by average heterozygosity - average percent of loci that are heterozygous in population
-nucleotide variability - measured by comparing DNA suquence

-gene variability much larger than nucleotide variability because each gene is made up of many nucleotides
What is a locus?
specific location of a gene or DNA sequence on a chromosome
What is an allele?
A variant of the DNA sequence at a given locus is called an allele.
Variation between different populations
-Most species exhibit geographic variation, differences between gene pools of separate populations
-They still produce viable, fertile offspring if allowed to mate in a laboratory
-Geographic population have different karyotypes of genes in populations that differ in location
-large examples are clines
What is a gene pool?
all genes and all alleles in a population
What is a cline?
-a gradual shift in gene frequency (a trait) as geography shifts across a range over the different populations.
-For example, as fish populations are in colder areas, allele frequence for temperature resistance increases
What are the two sources of genetic variation?
-gene duplication
What is a mutation?
A change in the nucleotide sequence of DNa. Mutations in gametes can be passed down to offspring. if in somatic cells, mutation does not add to allele frequency.
What is a point mutation?
Specific type of mutation, change in one gene, but can have significant effects to the phenotype.
-if on non coding regions of DNA, harmless mutation
-if on redundant genetic code or in code where molecules are not important, neutral
-mutationg that change protein production are often harmful but sometimes can be beneficial
Mutations in plants and animals
-In plants and fungi, many different cells produce gametes so mutations happen. In animals, only a few cells can produce gametes, hence mutations are limited. In the majority of animals, the mutations occur in somatic cells, so those changes are lost.
-Most mutations in animals are slightly harmful. Animals have had many years to adapt to their environment, so they are well suited to their environment already. Slight mutations will not likely improve phenotype. BUT since point mutations often happen in noncoding genes, these regions of mutations are harmless.
What is a gene number or position mutation? (Chromosomal mutations)
-where pieces of entire chromosomes are deleted, disrupted, or loci are rearranged
-these are typically harmful
-Duplication of small pieces of DNA increases genome size and is usually less harmful As long as you have 1 functional copy of the chromosome it's okay)
-Duplicated genes can take on new functions by further mutation
Mutation rates
-Mutation rates are low in animals and plants
-The average is about one mutation in every 100,000 genes per generation
-Mutations rates are often lower in prokaryotes and higher in viruses
What creates the largest combinations of alleles?
-Sexual reproduction
-can shuffle existing alleles into new combinations
-In organisms that reproduce sexually, recombination of alleles is more important than mutation in producing the genetic differences that make adaptation possible
-Sexual reproduction produces new genotypes that may make organism better adapt. (i.e. getting the best traits out of both parents).
Hardy Weinberg
--Hardy weinberg equation tests for whether natural selection factors are testing a population.
- It does this by finding what the genetic makeup of the population would be if it were NOT EVOLVING, and then comparing to population data.
-Frequencies of alleles remain constant from gen to gen
-Total number of copies for each heterozygous allele:
allele 1's occurence as a homogozygous trait in population x 2 + 1/2 of heterozygous population
-total number of copies for each homozygous allele: homozygous trait in population x 2
-frequence for each allele: the ratio of occurence in all the alleles available. i.e. total for allele 1 from above / (total for allele 1 plus allele 2 from above)
Sum of alelic frequences is always
1 (if you have covered all possible alleles for that trait).
-p^2 + 2pq + q^2 = 1
If allele frequencies and genotypes in populations are are not constant from generation to generation
It is said that the population is evolving
Hardy-Weinberg Equilibrium
-In a given population where gametes contribute to the next generation randomly, allele frequencies will not change (so no mutations)
-Mendelian inheritance preserves genetic variation in a populatIOn
-p and q are the constant frequencies in a gene pool
How do you calculate the frequency of a homozygous genotype?
p^2 or q^2
How do you calculate the frequency of a heterozygous genotype?
What are the five conditions of the Hardy Weinberg equation?
-In real populations, allele and genotype frequencies do change over time
-The five conditions for non-evolving populations are rarely met in nature:
1) No mutations -increases allele frequency
2) Random mating
3) No natural selection - rarely happens bc environment always changing
4) Extremely large population size
5) No gene flow - depends on population movement geographically and populations interacting no moveming in OR out means no gene flow
How can organisms be in HW equilibrium?
-Natural populations can evolve at some loci, while being in Hardy-Weinberg equilibrium at other loci - an important idea so it is not hard to find a population in H-W equilibrium ..but for a particular loci.
-Also some populations evolve so slowly that changes in alleles and genotype frequencies are hard to distinguish.
How does natural selection change allele frequency?
-Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions
-Selection results in alleles being passed to the next generation in different proportions than the present generation
-By consistently favoring some alleles over others, natural selection can cause adaptive evolution
How does genetic drift change allele frequency?
-The smaller a sample, the greater the chance of deviation from a predicted result
-Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next, especially in small populations
-Genetic drift tends to reduce genetic variation through losses of alleles
-Two types: Founder and bottleneck effects
What is Gene Drift 1: Founder Effect?
-a few individuals become isolated from larger population.
-one person that is isolated has a very rare allele
-a new population is formed, but in this population allele frequency for that rare allele is higher than larger population
-Accounts for high frequency of genetic disorders in isolated populations
What is Gene Drift 2: Bottleneck Effect?
-sudden reduction in population size due to the environment but
-people are left are a small subset, so rare alleles are lost
-the population is not a large, random sample that would have had the rare alleles at least a few times
-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
-Even if size is recovered, there may not be as much variability as the original population
Effects of all genetic drift
-Genetic drift is significant in small populations- chance events can cause alleles to be disproportionately over or underrepresented in the next generation. Especially in small populations, this is pronounced.
-Genetic drift causes allele frequencies to change at random - unlike natural selection, which favors alleles year over year, allele frequencies change year over year
-Genetic drift can lead to a loss of genetic variation within populations - because genetic drift causes allele frequencies to fluctuate. Can fluctuate to 0! This may effect how population can adapt to changing environment.
-Genetic drift can cause harmful alleles to become fixed - cause population survival to be threatened.
How does gene flow affect allele frequency
-Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen) from one population to another
-Gene flow tends to reduce variation among populations over time
-In fact, if it is extensive enough, the two populations could share one common gene pool (they'll all eventually have both genes)
-Gene flow can decrease the fitness of a population
-Gene flow can increase the fitness of a population (pass on resistance to new population for i.e.)
Why is gene flow important in human evolution?
Gene flow is important in human evolution. It has created human populations that have few genetic differences. No matter where on earth we live we are all one species.
Natural selection is the only mechanism that causes adaptive evolution
-Evolution by natural selection involves both change and "sorting"
-New genetic variations arise by chance
-Beneficial alleles are "sorted" and favored by natural selection
-Only natural selection consistently results in adaptive evolution
-Outcome of natural selection is NOT random! - favors alleles that provide advantage
How does natural selection bring about adaptive evolution?
-by acting on an organisms phenotype
-Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals (one individual may have a gene that makes them better suited than neighbor
-Selection favors certain genotypes by acting on the phenotypes of certain organisms
-eventually, Natural selection increases the frequencies of alleles that enhance survival and reproduction
-Adaptive evolution occurs as the match between an organism and its environment increases
-Because the environment can change, adaptive evolution is a continuous process
Why are "struggle for existence" and "survival of the fittest" misleading terms?
-imply direct competition among individuals
-"Fitness" depends on environment and the individual. Competition is rarely between individual and individual.
-Reproductive success is generally more subtle and depends on many factors
What are the three modes of natural selection when environments change?
What is directional selection?
Directional selection favors individuals at one end of the phenotypic range (thereby shifting populations curve to that range over time, i.e. if very tall is good, future population is tall)
What is disruptive selection?
Disruptive selection favors individuals at both extremes of the phenotypic range
-For example, salmon - the big guys and the little guys get to mate and produce offspring. Big guys fight their way through it, and little guys are sneaky.
What is stabilizing selection?
Stabilizing selection favors intermediate variants and acts against extreme phenotypes
-In an environment that doesn't change often or changes are minor, the extremes are lost.
Sexual selection
-Sexual selection is natural selection for mating success
-It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics
What is intersexual selection?
Intersexual selection, often called mate choice, occurs when individuals of one sex (usually females) are choosy in selecting their mates
What is intrasexual selection?
Intrasexual selection is competition among individuals of one sex (often males) for mates of the opposite sex
How do female preferences evolve?
The good genes hypothesis suggests that if a trait is related to male health, both the male trait and female preference for that trait should increase in frequency (for example blue feet in male blue-footed boobies)
What is neutral variation?
genetic variation that does not confer a selective advantage or disadvantage
What various mechanisms help to preserve genetic variation in a popualtion, despite natural selection?
-Why doesn't natural selection just keep ONLY the best traits and kick out all the bad ones?
-B/c of diploidy and Balancing selection and heterozygote advantage
-Diploidy maintains genetic variation in the form of hidden recessive alleles because heterozygotes can carry recessive alleles that are hidden from the effects of selection, that is not affecting the phenotype
-If phenotype is not affected, no natural selection can "pick" gene for those recessive genes.
-allows population to keep rare recessive alleles
balancing selection
-Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population
-Balancing selection includes:
1) Heterozygote advantage
2) Frequency-dependent selection
Heterozygote advantage
-Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes
-Natural selection will tend to maintain two or more alleles at that locus (thus the recessive and the dominant) - so it is good to have BOTH genes and both give good traits
-The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance
-Heterozygote advantage results in more genetic variation in the population.
In the end, why can't natural selection create the perfect organism?
1) Selection can act only on existing variations
2) Evolution is limited by historical constraints - what kind of selection has been there already? Again, you are limited by what the past has weeded out.
3) Adaptations are often compromises - so you get clumsy looking animals
4) Chance, natural selection, and the environment interact - environments always change! so you get a perfect organism for now....
-is the number / total population that has the recessive allele
is the frequency of the abnormal recessive allele
-the frequency of the normal dominant homozygous allele
the expected frequency of homozygous dominant individuals in the population
frenquency of population carrying q
Which type of mutation plays the most important role in increasing the number of genes in the gene pool?
Because the genetic code is redundant, even point mutations in genes that code for proteins may have little effect because they do not alter the amino acid composition. The chances are slight that such a mutation would have a profound effect on an organism.
In a certain group of African people, 4% are born with sickle-cell disease (homozygous recessive). If this group is in Hardy-Weinberg equilibrium, what percentage of the group has the selective advantage of being more resistant to malaria (heterozygous) than those individuals who are homozygous for normal hemoglobin or for sickle-cell disease?
An earthquake hits a small island. All but a small group of closely related lizards are eliminated, and the survivors spread out over the island. This is an instance of _____.
bottleneck effect . large catastrophes are bottleneck. isolated migrations are founder.
Which of the following scenarios would most likely result in the microevolution of a population of humans?
-All of the listed responses are correct.
-A colony of humans on the moon is isolated from Earth.
-The incidence of skin cancer in adults over age 40 rises significantly.
-Hundreds of thousands of people are killed in a natural disaster.
-Only random mating takes place among all the people that reproduce in North America.
A colony of humans on the moon is isolated from Earth. This is the only change that would be significant enough to cause microevolution.
Which of the following would seem to be an example of neutral variation?
-human fingerprints
-homozygosity of the cheetah population
-moth coloration
-polymorphism of the Gálapagos finches
-founder effect
human fingerprints. Everything is adaptive rather than neutral.
How does natural selection fashion organisms?
-Chance and the environment interact with natural selection, so that the best available traits are selected for.
-The best traits for the current environment arise in the population and are selected for.
-Sexual selection acts to make organisms better adapted to their environments.
-Brand new body plans arise often, giving natural selection many new possibilities to work with.
-Neutral traits are often selected for because they will eventually produce alleles that confer an advantage.
-Chance and the environment interact with natural selection, so that the best available traits are selected for.
-Natural selection can only act on variations that exist or arise in a population by chance. There is no guarantee that any of the existing alleles provide the best traits.
In sexually reproducing organisms, three processes lead to most genetic variation:
1)independent orientation of chromosomes in meiosis
2)crossing over of chromosomes in meiosis
3)random fertilization
Modern travel along with migration reduces the probability of _____ having an effect on the evolution of humans.
-genetic drift
-genetic drift includes isolated populations in the founder effect and the bottle neck effect
-Modern travel and migration decreases the chances of there being a small, isolated population.
Homologous pairs of chromosomes are lined up independently of other such pairs during _____.
Metaphase 1
Crossing over, resulting in an increase in genetic variation, occurs between _____.
-sister chromatids of homologous chromosomes
-nonsister chromatids of nonhomologous chromosomes
-sex cells and somatic cells
-sister chromatids of nonhomologous chromosomes
-nonsister chromatids of homologous chromosomes
During prophase I of meiosis, homologous chromosomes pair up very closely, and corresponding parts of two nonsister chromatids may trade places. This process of crossing over creates variation by producing chromosomes that combine the genes inherited from two parents.
In human gamete production there is an average of _____ crossover events per chromosome pair.
The original source of all genetic variation is _____.
Which type of selection tends to increase genetic variation?
disruptive selection
In evolutionary terms, an organism's fitness is measured by its _____.
-genetic variability
-mutation rate
-contribution to the gene pool of the next generation
-stability in the face of environmental change
contribution to the gene pool of the next generation
what is frequency dependent selection
Often the seleciton for a particular phetoype that is least common (the recessive phenotype now lives)