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Chapter 23
The Evoution of Populations
40
Biology
09/16/2008

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Term
microevolution
Definition
change in the genetic makeup of a population from generation to generation

example: drug resistance bacteria (penicillin), strands of HIV resistive to drugs

evolutionary change on its smallest scale
Term
population genetics
Definition
the study of how populations change genetically over time
Term
modern synthesis
Definition
a comprehensive theory of evolution that integrates ideas from many other fields
Term
population
Definition
a localized group of individuals that are capable of interbreeding and producing fertile offspring
Term
gene pool
Definition
the aggregate of genes in a population at any one time

- consists of all alleles at all gene loci in all individuals in a population
Term
Hardy-Weinberg Theorem
Definition
- the frequencies of alleles and genotypes in a population's gene pool remain constant from generation to generation, provided that only Mendelian segregation and recombination of alleles are at work
Term
Hardy-Weinberg Equilibrium
Definition
- the population will have the same allele frequencies from one generation to the next AND its genotype frequencies can be predicted from the allele frequencies

**assumes that individuals in a population donate gametes to the next generation at random AND mate at random
Term
Calculating genotype frequencies in the next generation according to HWP
Definition
( p + q ) X ( p + q ) = p^2 + 2pq + q^2

where p is the frequency of allele #1 and q is the frequency of allele #2
Term
Conditions for Hardy-Weinberg Equilibrium
Definition
1. Extremely Large Population Size

2. No Gene Flow

3. No mutations

4. Random Mating

5. No Natural Selection

** these conditions are rarely met
** departure from these conditions results in evolution
Term
mutations
Definition
- changes in the nucleotide sequence of DNA

- weak evolutionary force
Term
point mutation
Definition
- a change in one base in a gene

- mostly harmless

- example of significant impact: sickle-cell disease
Term
Insertion/Deletion mutation
Definition
"Indels" can cause frameshift mutations or add/subtract amino acids

- almost certain to be harmful
Term
Gene Duplication
Definition
- duplication of a short stretch of DNA

- caused by unequal crossovers during meiosis

- may maintain function, gain a new function, or lose function

*nearly always harmful
Term
Chromosome Inversions
Definition
Part of a chromosome breaks in two places, flips, and the reanneals

- ABCDEF -> ABEDCF
Term
Polyploidization
Definition
The condition whereby an organism has 2 or more sets of chromosomes, usually resulting in a failure of reduction division

- may cause instant speciation
Term
Sexual Recombination
Definition
- reshuffling of the existing alleles in a gene pool

- caused by random assortment of chromosomes and crossing over within chromosomes
Term
Three Major Factors That Cause Most Evolutionary Change:
Definition
1. Natural Selection
2. Genetic Drift
3. Gene Flow
Term
Genetic Drift
Definition
Unpredictable fluctuations in allele frequencies from one generation to the next because of a population's finite size

Over time drift tends to reduce genetic variation through losses of alleles from the gene pool
Term
2 Major Examples of Genetic Drift
Definition
1. bottleneck effect
2. founder effect
Term
The Bottleneck Effect
Definition
A sudden catastrophe drastically reducesthe size of the population and the surviving gene pool is no longer reflective of the original population's gene pool

- by chance, some alleles are over represented, some are under represented, and some are eliminated

example: 1890 hunting of northern elephant seals in CA reduced the population to 20 individuals ... although pop. size is now ~ 30,000 all have the same genes
Term
Founder Effect
Definition
- When a few individuals become isolated from a larger population, this smaller group may establish a new population whose gene pool is not reflective of the source population

*think: white guys on a boat at Plymouth rock produce a bunch of white guys
Term
gene flow
Definition
- genetic additions to and/or subtractions from a population resulting from the movement of fertile individuals or gametes

- causes a population to gain or lose alleles

- tends to reduce differences between populations over time

** think about humans moving much more freely about the world that in the past **
Term
phenotypic polymorphism
Definition
if two or more distinct "morphs" are each represented in high enough frequencies to be readily noticeable
Term
genetic polymorphism
Definition
- not distinct and separate morphs, but a continuum

example: heights of humans varies along a continuum
Term
geographic variation
Definition
differences between the gene pools of separate populations

example: house mice separated by mountains on an island will evolve in isolation from one another
Term
fitness
Definition
the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals
Term
relative fitness
Definition
the contribution of a genotype to the next generation compared to the contributions of alternative genotypes for the same locus
Term
Modes of Selection
Definition
1. Directional Selection
2. Disruptive Selection
3. Stabilizing Selection
Term
Directional Selection
Definition
- shifts the frequency curve for some phenotypic character in one direction or the other by favoring individuals that deviate from the average

- most common when a population's environment changes or when members of a population migrate to a new habitat with different environmental conditions than the former one

example: moth colors and the industrial revolution
Term
Disruptive Selection
Definition
- occurs when conditions favor individuals on BOTH extremes of a phenotypic range over individuals with intermediate phenotypes

- birds who live on soft seeds and hard seeds need either a small bill or a large-bill but a medium bill is useless
Term
Stabilizing Selection
Definition
- acts against extreme phenotypes and favors intermediate variants

- reduces variation and maintains the status quo for a particular phenotypic character

example: human babies lie in the range of 3-4 kg; babies who are much smaller or larger suffer higher rates of mortality
Term
Reasons for the preservation of genetic variation (3)
Definition
1. Diploidy
2. Balancing Selection
3. Heterozygote Advantage
Term
Diploidy
Definition
- genetic variation is hidden from selection in the form of recessive alleles

- recessive alleles survive in heterozygous carriers

example: cystic fibrosis is maintained in the population even though it sucks
Term
balancing selection
Definition
- occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population

- leads to balanced polymorphism

- includes heterozygote advantage and frequency-dependent selection
Term
heterozygote advantage
Definition
if individuals who are heterozygous at a particular gene locus have greater fitness than the homozygotes, natural selection will tend to maintain two more alleles at that locus

example: in Africa, the heterozygotes are resistant to malaria even though dominant individuals are much more susceptible to the disease while recessive individuals develop sickle-cell anemia
Term
Frequency-Dependent Selection
Definition
- the fitness of any one morph declines if it becomes too common in the population
Term
Sexual Selection
Definition
- natural selection for mating success

examples: select for size, color, and ornamentation
Term
Sexual Dimorphism
Definition
- marked differences between the sexes in secondary sexual characteristics which are not directly associated with reproductions
Term
Why Natural Selection Cannot Fashion Perfect Organisms: (4)
Definition
1. Evolution is limited by historical constraints

2. Adaptations are often compromises

3. Chance and natural selection interact

4. Selection can edit only existing variations
Term
Random Fixation
Definition
- a certain genotype gets passed on

- when the allele frequency hits either 1 or 0