ap biology big idea #1 – part a – part 2
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AP BIOLOGY Big Idea #1 – Part A – Part 2. Natural Selection Acts on Phenotypes. Concept 23.2: The Hardy-Weinberg equation can be used to test whether a population is evolving. The first step in testing whether evolution is occurring in a population is to clarify what we mean by a population. - PowerPoint PPT PresentationTRANSCRIPT
AP BIOLOGYBig Idea #1 – Part A – Part 2
Natural Selection Acts on Phenotypes
Concept 23.2: The Hardy-Weinberg equation can be used to test whether a population is evolving
• The first step in testing whether evolution is occurring in a population is to clarify what we mean by a population
Gene Pools and Allele Frequencies
• A population is a localized group of individuals capable of interbreeding and producing fertile offspring
• A gene pool consists of all the alleles for all loci in a population
• A locus is fixed if all individuals in a population are homozygous for the same allele
Fig. 23-5Porcupine herd
Porcupineherd range
Beaufort Sea NO
RTH
WEST
TERR
ITOR
IES
MAPAREA
AL
AS
KA
CA
NA
DA
Fortymileherd range
Fortymile herd
AL
AS
KA
YU
KO
N
Fig. 23-5a
Porcupineherd range
Beaufort Sea NO
RTH
WEST
TERR
ITOR
IES
MAPAREA
AL
AS
KA
CA
NA
DA
Fortymileherd range
AL
AS
KA
YU
KO
N
The frequency of an allele in a population can be calculated:
– For diploid organisms, the total number of alleles at a locus is the total number of individuals x 2
– The total number of dominant alleles at a locus is 2 alleles for each homozygous dominant individual plus 1 allele for each heterozygous individual; the same logic applies for recessive alleles
• By convention, if there are 2 alleles at a locus, p and q are used to represent their frequencies
• The frequency of all alleles in a population will add up to 1
– For example, p + q = 1
The Hardy-Weinberg Principle
• The Hardy-Weinberg principle describes a population that is not evolving
• If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving
Hardy-Weinberg Equilibrium• The Hardy-Weinberg principle states
that frequencies of alleles and genotypes in a population remain constant from generation to generation
• In a given population where gametes contribute to the next generation randomly, allele frequencies will not change
• Mendelian inheritance preserves genetic variation in a population
Fig. 23-6
Frequencies of alleles
Alleles in the population
Gametes produced
Each egg: Each sperm:
80%chance
80%chance
20%chance
20%chance
q = frequency of
p = frequency of
CR allele = 0.8
CW allele = 0.2
• Hardy-Weinberg equilibrium describes the constant frequency of alleles in such a gene pool
• If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then
– p2 + 2pq + q2 = 1
– where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype
Fig. 23-7-1
SpermCR
(80%)
CW
(20 %
)
80% CR ( p = 0.8)
CW (20%)
20% CW (q = 0.2)
16% ( pq) CRCW
4% (q2) CW CW
CR
(80%
)
64% ( p2) CRCR
16% (qp) CRCW
Eg
gs
Fig. 23-7-2
Gametes of this generation:
64% CRCR, 32% CRCW, and 4% CWCW
64% CR + 16% CR = 80% CR = 0.8 = p
4% CW + 16% CW = 20% CW = 0.2 = q
Fig. 23-7-3
Gametes of this generation:
64% CRCR, 32% CRCW, and 4% CWCW
64% CR + 16% CR = 80% CR = 0.8 = p
4% CW + 16% CW = 20% CW = 0.2 = q
64% CRCR, 32% CRCW, and 4% CWCW plants
Genotypes in the next generation:
Fig. 23-7-4
Gametes of this generation:
64% CR CR, 32% CR CW, and 4% CW CW
64% CR + 16% CR = 80% CR = 0.8 = p
4% CW + 16% CW = 20% CW = 0.2 = q
64% CR CR, 32% CR CW, and 4% CW CW plants
Genotypes in the next generation:
SpermCR
(80%)
CW
( 20%
)
80% CR ( p = 0.8)
CW (20%)
20% CW (q = 0.2)
16% ( pq) CR CW
4% (q2) CW CW
CR
(80%
)
64% ( p2) CR CR
16% (qp) CR CW
Eg
gs
Conditions for Hardy-Weinberg Equilibrium
• The Hardy-Weinberg theorem describes a hypothetical population
• In real populations, allele and genotype frequencies do change over time
The five conditions for non-evolving populations are rarely met in nature:
– No mutations
– Random mating
– No natural selection
– Extremely large population size
– No gene flow* Natural populations can evolve at some loci, while being in Hardy-Weinberg equilibrium at other loci
Applying the Hardy-Weinberg Principle:
• We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium given that:
– The PKU gene mutation rate is low
– Mate selection is random with respect to whether or not an individual is a carrier for the PKU allele
– Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions
– The population is large
– Migration has no effect as many other populations have similar allele frequencies
• The occurrence of PKU is 1 per 10,000 births
– q2 = 0.0001
– q = 0.01
• The frequency of normal alleles is
– p = 1 – q = 1 – 0.01 = 0.99
• The frequency of carriers is
– 2pq = 2 x 0.99 x 0.01 = 0.0198
– or approximately 2% of the U.S. population
• Two processes, mutation and sexual reproduction, produce the variation in gene pools that contributes to differences among individuals
Concept 23.1: Mutation and sexual reproduction produce the genetic variation that makes evolution possible
Genetic Variation
• Variation in individual genotype leads to variation in individual phenotype
• Not all phenotypic variation is heritable
• Natural selection can only act on variation with a genetic component
Fig. 23-2
(a) (b)
Fig. 23-2a
(a)
Fig. 23-2b
(b)
Variation Within a Population
• Both discrete and quantitative characters contribute to variation within a population
• Discrete characters can be classified on an either-or basis
• Quantitative characters vary along a continuum within a population
• Population geneticists measure polymorphisms in a population by determining the amount of heterozygosity at the gene and molecular levels
• Average heterozygosity measures the average percent of loci that are heterozygous in a population
• Nucleotide variability is measured by comparing the DNA sequences of pairs of individuals
Variation Between Populations
• Most species exhibit geographic variation, differences between gene pools of separate populations or population subgroups
Fig. 23-3
13.17 19 XX10.169.128.11
1 2.4 3.14 5.18 6 7.15
9.10
1 2.19
11.12 13.17 15.18
3.8 4.16 5.14 6.7
XX
• Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis
Fig. 23-4
1.0
0.8
0.6
0.4
0.2
046 44 42 40 38 36 34 32 30
GeorgiaWarm (21°C)
Latitude (°N)
MaineCold (6°C)
Ldh
-B b
alle
le f
req
uen
cy
Mutation
• Mutations are changes in the nucleotide sequence of DNA
• Mutations cause new genes and alleles to arise
• Only mutations in cells that produce gametes can be passed to offspring
Animation: Genetic Variation from Sexual Recombination
Point Mutations
• A point mutation is a change in one base in a gene
The effects of point mutations can vary:• Mutations in noncoding regions of
DNA are often harmless
• Mutations in a gene might not affect protein production because of redundancy in the genetic code
Mutations That Alter Gene Number or Sequence
• Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful
• Duplication of large chromosome segments is usually harmful
• Duplication of small pieces of DNA is sometimes less harmful and increases the genome size
• 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
Sexual Reproduction• 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