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unit 7 chp 21 Cheat Sheet by

unit 7 chapter 21 notes

NATURAL SELECTION REMINDER

- indivi­duals DO NOT evolve
-it acts on indivi­duals but the evolut­ionary impact of evolution is only seen in a population of organisms over time!

GENETIC VARIATION

-indiv­idual variation occurs in al species and often reflects genetic variation, differ­ences among indivi­duals in the compos­ition of their genes or other DNA segments
- not all phenotypic variation is heritable. Phenotype is the prodcut of an inherited genotype and enviro­nmental influe­nces. Only the genetic component of variation has evolut­ionary conseq­uences
- genetic variation provides the raw material for evolut­ionary change, without genetic variation, evolution cannot occur

HETERO­ZYGOTE ADVANTAGE

occurs when the hetero­zygote genotype has a higher relative fitness than either the homozygous dominant or recessive genotype
EXAMPLE: Malaria and Sickle Cell
Homozygous dominant (normal)- die of malaria (HbHb)
Homozygous recessive- die of sickle cell anemia (HsHs)
Hetero­zygote Carriers (HbHs)- relatively free of malaria sick cell anemia. They survive more, therefore are more common in the population

MODES OF NATURAL SELECTION

direct­ional selection
favors one extreme
disruptive selection
favors the 2 extremes and not the middle
stabil­izing selection
favors the middle

GENE FLOW

the transfer of alleles into and out of a population due to the movement of fertile indivi­duals or their gametes
- tends to reduce differ­ences between popula­tions

HOW DO WE MEASURE CHANGE?

population
-localized group of indivi­duals that belong to the same species
species
a group of popula­tions who have the ability to interbreed and produce fertile offspring in nature
gene pool
a collection of alleles within the population
allele frequency
how common is that allele with the population
think: A vs a

GENETIC DRIFT

- chance events that cause allele freque­ncies to fluctuate unpred­ictably from one generation to the next especially in small popula­tions
- 2 ways genetic drift can occur in small popula­tions:
1. Founder Effect
2. Bottleneck Effect

CONDITIONS FOR H-W EQUILI­BRIUM

1. No mutations
2. Random mating
3. No natural selection
4. Extremely large population size
5. No gene flow
 

MICROE­VOL­UTION

the smallest scale we can define evolution occurs as changes in allele freque­ncies in a population over time
- allele: the different versions of the SAME GENE

CREATING A PERFECT ORGANISM

Natural Selection cannot create a perfect organisms because...
1. selection can act only on existing variation
2. evolution is limited by historical constr­aints
3. adapta­tions are often compro­mises
4. chance, natural selection, and the enviro­nment interact

ADAPTIVE EVOLUTION

natural selection consis­tently increases the freque­ncies of alleles that provide reprod­uctive advantage and thus leads to adaptive evolut­ion!!
-although we may refer to the relative fitness of a genotype, the entity that is subjected to natural selection is the whole organism, not the underlying genotype
-natural selection acts on the genotype indire­ctly, via how the genotype affects the phenotype

GENETIC DRIFT - 4 KEY POINTS

1. signif­icant in small popula­tions
2. causes allele freque­ncies to change at random
3. leads to a loss of genetic variation within popula­tions
4. can cause harmful alleles to become fixed

MUTATIONS, AND MATING

- although new mutations can modify allele freque­ncies, because mutations are rare, the change from generation to generation is very small
- mutation can ultimately have a large effect on allele freque­ncies when it produces new alleles that strongly influence fitness in a positive or negative way
- nonrandom mating can affect the freque­ncies of homozygous and hetero­zygous genotypes, but it usually has no effect on allele freque­ncies in the gene pool

HARDY-­WEI­NBERG PRINCIPLE

- describes the gene pool of a population that is not evolving
- states that the freque­ncies of alleles and genotypes in a population will remain constant from generation to generation as long as ONLY Mendelian segreg­ation and recomb­ination of alleles are at work

VARIATION WITHIN A POPULATION

population geneti­cists measure genetic variation in a population by determ­ining the amount of hetero­zyg­osity at the gene level and the molecular level of DNA (nucle­otide variab­ility)

VARIATION WITHIN A POPULATION

quanti­tative characters
most traits vary along a continuum within a population
usually result from the influence of 2+ genes on a single trait

VARIATION WITHIN A POPULATION

discrete characters
classified on an either-or basis
usually determined by a single gene locus
 

H-W EQUILI­BRIUM EQUATIONS

- useful in determ­ining how fast a population is changing
-based on a simple Punnett square where p is the frequency of the dominant allele and q is the frequency of the recessive allele
-frequency of dominant and recessive alleles MUST equal 1 (100%)
p + q = 1
frequency of the genotypes must equal 1 (100%)
p2 + 2pq + q2 = 1

SEXUAL SELECTION

Charles Darwin was the first scientist to invest­igate sexual select­ions, which is selection for mating success
-sexual dimorphism
differ­ences in secondary sex charac­ter­istics
-intras­exual selection
compet­ition among indivi­duals of one sex for mates of the opposite sex
-inters­exual selection
mate choice- one sex is choosy in selecting mates from the other sex

SOURCES OF GENETIC VARIATION

sexual reprod­uction
- through the process of sexual reprod­uction we are mixing up the genes (DNA) of 2 parents to create 1 offspring with a mixture of the parental traits

SOURCES OF GENETIC VARIATION

- mutations
- a change in the nucleotide sequence of an organisms DNA
* some mutations can alter gene number or position
 
= deletions
 
= duplic­ations
 
=trans­loc­ations
 
=inver­sions

NATURAL SELECTION

-based on differ­ential survival and reprod­uctive success
-indiv­iduals in a population vary in their heritable traits
-indiv­iduals with variations better suited to the enviro­nment tend to produce more offspring than those with variations that are less well suited
-as a result of selection, alleles are passed on to the next generation in freque­ncies different from their relative freque­ncies in the present population
-by consis­tently favoring some alleles over others, natural selection can cause adaptive evolution (evolution that results in a better match between organisms and their enviro­nment)

3 MAIN CAUSES OF ALLELE FREQUENCY CHANGES

1. Natural Selection
2. Genetic Drift
chance events that alter allele freque­ncies
3. Gene Flow
the transfer of alleles between popula­tions
 

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  • Published: 15th March, 2021

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