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CH 23: The Evolution of Populations
Terms:
Population genetics: is the study of allele frequency distribution and change under the influence of the
four main evolutionary processes. (the processes are: natural selection, genetic drift, mutation and gene
flow.)
Modern synthesis: the ideas or biological reasons for the cause of evolution.
Population: it is all the organisms that belong to the same group or species and live in the same
geographical area.
Species: it is a classification of and organism. “A species is often defined as a group of organisms
capable of interbreeding and producing fertile offspring”
Gene pool: it is the complete set of unique alleles in a species or population.
Hardy Weinberg theorem: a gene pool of a population that is not evolving.
Hardy Weinberg equilibrium: this principle states that the frequencies of alleles and genotypes in
population will remain constant from generation to generation, provided that only Mendelian
segregation and recombination of alleles are at work.
Hardy Weinberg equation: it is the equation that is used to discover the probable genotype frequencies
in a population and to track their changes from one generation to another.
Microevolution: Are the changes in allele frequencies that occur over time within a population. (This is
due to the four processes: mutation, selection, gene flow and genetic drift.
Genetic drift: is the change in the frequency of gene caring allele in a population due to random
sampling.
Bottleneck effect: is an evolutionary event in which significant percentage of la population or spices is
killed or otherwise prevented from reproducing.
Founder effect: is the loss of genetic variation that occurs when a new population is established by a
very small number of small individuals from a larger population.
Natural selection: is the gradual, nonrandom process by which traits are more common in a population
as a function of differential ones. It is the key of evolution. The fittest survive.
Gene flow: is the transfer of alleles form one population to another.
Mutation: a change in DNA
Polymorphic: it is when two or more clearly different phenotypes exist in the same population of a
species
Gene diversity: refers to the total number of genetic characteristics in the genetic makeup of a species.
Nucleotide diversity: “is a concept in a molecular genetics which is used to measure the degree of
polymorphism within a population”
Geographic variation: the variation of a species which is dependent on climate or other geographical
conditions.
Cline: “describes an ecotone in which a series of bio communities display a continuous gradient.”
Balanced polymorphism: a system of genes in which two alleles are maintained in stable equilibrium.
Heterozygote advantage: “describes the case in which the heterozygote genotype has a higher relative
fitness than wither the homozygote dominant or homozygote recessive genotype.
Frequency-dependent selection: the fitness of a phenotype is dependent on its frequency relative to
other phenotypes in a given population.
Neutral variation: is one that has no adaptive value one way or the other.
Darwinian fitness: “a biological condition in which a competing varies is increasing in frequency relative
to other competing variants in a population.”
Relative fitness: survive and reproduce ability
Directional selection: it is a way of natural selection in which a single phenotype is favored, causing the
allele frequency to continuously shift in one direction.
Diversifying selection: descries changes in population genetics in which extreme values for a trait are
favored over other values.
Stabilizing selection: is a type of natural selecting in which genetic diversity decreases as the population
stabilizes on a particular trait value.
Sexual dimorphism: distinct difference in size or appearance between the sexes of an animal in addition
to differences between the sexual organs themselves.
Intrasexual selection: meaning selecting within the same sex, individuals of one sex compete directly for
mates of the opposite sex.
Intersexual selection: (mate choice) individuals of one sex (usually the females) are choosy in selecting
their mates from the other sex.
The Questions
1. Explain why is it incorrect to say that individual organisms evolve.:: Because the evolutionary
impact of natural selection is only apparent in the changes in a population of organisms over
time.
2. Explain what is meant by “the modern synthesis”.:: it is the synthesis of genetics and evolution.
Darwin provided abundant evidence that life on Earth has evolved over time, but he could not
explain how organisms pass heritable traits to their offspring then Mendel did. He proposed a
particulate model of inheritance, which stated that organisms transmit discreet heritable units
to the offspring. So these ideas combine are the ideas of evolution.
3. Define a population; define a species.:: Population: all the inhabitants of a particular town, area,
or country. :: Species: a group of living organisms consisting of similar individuals capable of
exchanging genes or interbreeding.
4. Explain how micro evolutionary change can affect a gene pool.:: the change in frequencies in a
population over generations can change a gene pool over time because the change in alleles will
then determine which are best and there for change to what is the fittest.
5. State the Hardy-Weinberg theorem.:: it is the principle that frequencies of alleles and
genotypes in a population remain constant from generation to generation.
6. Write the general Hardy Weinberg equation and use it to calculate allele and genotype
frequencies.:: p(to the power of 2) + 2pq +q( to the power of 2)=1 .
7. Explain why the Hardy Weinberg theorem is important conceptually and historically.:: Because
this principle states that the frequencies of alleles and genotypes in a population will remain
constant from generations to generations, provided that only Mendelian segregation and
recombination of alleles are at work. We can take a similar approach here, but instead of
considering the possible allele combinations from one genetic cross, our focus now is on the
combination of alleles in all of the genetic crosses in a population.
8. List the conditions a population must meet to maintain Hardy Weinberg equilibrium.:: No
mutations. Random mating. No natural selection. Extremely large population size. No gene flow.
9. Define microevolution.:: it is the change in allele frequencies in a populations.
10. Define evolution at the population level.:: population evolves not the individual.
11. Explain how genetic drift, gene flow, mutation, nonrandom mating an natural selection can
cause micro evolution.:: (1) by altering alleles or deleting or duplicatin entire genes, mutations
modify the gene pool. (2) If individuals mare preferentially with in a subset of the population,
such as their close relatives, random mixing of gametes does not occure, and the genotype
frequencies change. (3) Difference in the survival and reproductive success of individuals
carrying different genotypes can alter allele frequencies.(4) the smaller the population, the
more likely it is that allele frequencies will fluctuate by chance from one generation to the next
(genetic drift). (5) by moving alleles into or out of populations, gene flow can alter allele
frequencies.
12. Explain the role of population size in genetic drift.:: if a population is small and a allele change is
done then it would be dramatic due to the small size. If the population is big and there is a n
allele change then the genetic drift wont be as dramatic but wold take time to take affect.
13. Distinguish between the bottleneck effect and the founder effect.:: FOUNDER EFFEFT: When a
few individuals become isolated from a larger population whose gene pool differs from the
source population. BOTTLENECK EFFECT: a sudden change in the environment, such as a fire or
flood may drastically reduce the size of population. A severe drop in population size can cause
the bottle neck effect, so named because the population has passed through a restrictive “bottle
neck” in size. By change alone, certain alleles may be overrepresented among the survivors,
others may be underrepresented , and some may be absent altogether.
14. Explain why mutation has little quantitative effect on a large population.:: a mutation is a
change in an organisums DNA. If a mutation occurs in a large population it will most likely not be
one that survives the more common traits of those organisms in the population. That’s why its
not much of an effect because mutations are less likely to survive and be passed on.
15. Explain how quantitative and discrete characters contribute to variation within a population.::
well if there is a large number of a different species in a population then they are likely to
reproduce of springs of varieties. ( Example: a large amount of German Sheperds, Dolmations
and Winers. If they all mix then they would produce different species and that adds on to the
variation of dogs in that population)
16. Define polymorphism and morphs. Describe an example of polymorphism within the human
population.::POLYMORPHISM: when two or more clearly different phenotypes exist in the same
population. MORPHS: “the occurrence of more than one form or morph.” For example in
humans we have blacks, whites, Mexicans and Asians.
17. Distinguish between gene diversity and nucleotide diversity.:: GENE DIVERSITY: refers to the
total number of genetic characteristics in the genetic makeup of a species. NUCLEOTIDE
DIVERSITY: is a concept in molecular genetics which is used to measure the degree of
polymorphism within a population.
18. List some factors that can produce geographic variation among closely related populations.
Define cline.:: a fire, a mountain in between them, a lake.. a cline is “a series of bio communities
display a continuous gradient”
19. Explain why even though mutation can be a source of genetic variability, it contributes a
negligible amount to genetic variation in a population.:: for example if there is this population
and there are five different of species of a bird and a mutation occurs in one of them and kills off
all of those of the same species then in the population in general there is not that much of a
variety because now there are four species of birds left.
20. Describe the cause of nearly all genetic variation in a population.:: genetic drift when new
species are introduced into a population and genetic mutation is another one. Founder effect
and bottle neck effect are others. Gene flow the introduction of new alleles in a population are
factors of genetic variation.
21. Explain how genetic variation may preserved in a natural population.:: “The majority of
mutations are neutral with respect to the environment. This means that they are not subject to
selection pressure as long as the environment stays the same. Thus neutral mutations do not
tend to become either fixed in the gene pool, nor flushed from it. “
22. Briefly describe the neutral theory of molecular evolution and explain how changes in gene
frequency may be nonadaptive.:: “ most mutations in genetic code are neither harmful nor
beneficial to the organism.” ( I don’t understand this one much)
23. Distinguish between Darwinian fitness and relative fitness.:: Darwinian is the fittest survives the
best. Relative fitness is “ quantified as the average number of surviving progeny of a particular
genotype compared with average number of survingin progeny of competing genotypes after a
single generation.”
24. Describe what selection acts on and what factors contribute to the overall fitness of a
genotype.:: Natural selection- it is the gradual, nonrandom process by which biological traits
become either more or less common in a population as a function of differential reproduction of
their beares.
25. Describe examples of how an organism’s phenotype may be influenced by the environment.:: if
the environment provides food that only ( for example) a long beaked bird can eat then those
reproduce because they will survive unlike the short beak bird that can get the worms due to
their beaks. After time the long bird beaks will be the ones reproducing more
26. Distinguish among stabilizing selection, directional selecting, and diversifying selection.::
“Directional Selection occurs when selection favors one extreme trait value over the other
extreme. This typically results in a change in the mean value of the trait under selection.
Disruptive Selection occurs when selection favors the extreme trait values over the intermediate
trait values. In this case the variance increases as the population is divided into two distinct
groups. Disruptive selection plays an important role in speciation.
Stabilizing Selection occurs when selection favors the intermediate trait value over the extreme
values. Populations under this type of selection typically experience a decrease in the amount of
additive genetic variation for the trait under selection”
27. Describe the advantages and disadvantages of sexual reproduction.:: “Advantages of sexual
reproduction is that it results in genetic diversity amongst allorganisms of the species. As a
result, this allows for evolution by way of natural selection to occur for the reason that the
adaptations that allow a specific organism to become better adapted to its environment will
survive, thus this beneficial mutation will be passed to its offspring, and those lacking the
adaptation are less likely to survive. This in turn can be a disadvantage for asexual reproducers,
because negative mutations will be passed through generations and cannot leave the lineage.
This is attributed to the reason that offspring as a result of asexual reproduction contain the
identical genetic information from their parent as they are essentially a clone. Contrarily, this
can also be an advantage for organisms that reproduce asexually. Beneficial mutations, or
adaptations, that occur in a parent will be passed onto their offspring, the cause of bacterial
resistance.
A negative to sexual reproduction is that it requires two organisms, whereas asexual
reproduction does not. Due to this, asexual reproducers conserve energy for it is not necessary
for them to seek a mate in order to reproduce. Their rapid reproduction is attributed to this
factor.”
28. Define sexual dimorphism and explain how it can influence evolutionary change.:: Dimorphism is
the phenotypic difference between males and females of the same species. Because of the
phenotype in the same species now there are options of what traits will survive.
29. Distinguish between intrasexual selection and intersexual selection.:: “For intersexual
selection to work, one sex must evolve a feature alluring to the opposite sex, sometimes
resulting a "fashion fad" of intense selection in an arbitrary direction. Or, in the second case,
while natural selection can help animals develop ways of killing or escaping from
other species, intrasexual selection drives the selection of attributes that allow alpha males to
dominate their own breeding partners and rivals.”
30. Describe at least four reasons why natural seletion cannot breed perfect organisms.::
Mutation, gene flow, changing environment, competition,