Evolution Of Populations
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The Evolution of Populations Natural selection acts on individuals, but populations evolve
• Anagenesis (microevolution) – Is change in the genetic makeup of a population from generation to generation
20.1
Populations MAP AREA
CANADA
ALASKA
• are localized group of individuals that are capable of interbreeding and producing fertile offspring Beaufort Sea Porcupine herd range
N TE OR RR TH IT W E O S RI T ES
Fortymile herd range
20.2
ALASKA YUKON
• Fairbanks
• Whitehorse
The Gene Pool • Is the total group of genes in a population at any one time • Consists of all genes in all individuals of the population
20.2
The Hardy-Weinberg Theorem – Describes a population that is not evolving – States that 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
20.3
The Hardy-Weinberg Theorem • The five conditions for non-evolving populations are rarely met in nature. • They are: – – – – –
20.3
Extremely large population size No gene flow (breeding between populations) No mutations Random mating No natural selection
The Hardy-Weinberg Theorem p = dominant allele q = recessive allele
PARENTS
p + q = 1 (100%) OFFSPRING
p2 + 2pq + q2 = 1 (100%) Homozygous dominant
Heterozygous
Homozygous recessive
20.3
Mutation and Sexual Recombination (crossing over in meiosis) • produce the variation in gene pools that contributes to differences among individuals
20.5
Mutation • Mutations – Are changes in the nucleotide sequence of DNA
– Cause new genes and alleles to arise
• Mutation rates – Average about one mutation in every 100,000 genes per generation 20.5
• If humans have 25,000 genes, then one mutation would occur in every _______ individuals per generation.
Sexual Recombination – Is far more important than mutation in producing the genetic differences that make adaptation possible
20.5
Genetic drift – Describes how allele frequencies can fluctuate unpredictably from one generation to the next – Tends to reduce genetic variation CW CW
CRCR
CRCR
Only 5 of 10 plants leave offspring
CRCW
CWCW
CRCR
CW CW
CRCR CRCW
CRCR
CW CW
CRCW CRCW
Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3
20.6
CRCR
CRCW
CRCW
CRCR
CRCR Only 2 of 10 plants leave offspring
CRCR
CRCR CRCR
CRCR
CRCR
CRCR CRCR
CRCR CRCW
CRCW Generation 2 p = 0.5 q = 0.5
Figure 23.7
CRCR
CRCR
Generation 3 p = 1.0 q = 0.0
Genetic Drift in a small population
20.6
The Bottleneck Effect – A sudden change in the environment may drastically reduce the size of a population – The gene pool may no longer be reflective of the original population’s gene pool
20.7
Original population
Bottlenecking event
Surviving population
Understanding the bottleneck effect – Can increase understanding of how human activity affects other species A small population of approximately thirty grizzly bears lives isolated in the Selkirk Mountains of northern Idaho. Reduced in number by human activity, this population is rarely able to interbreed with surrounding larger populations resulting in a genetic bottleneck.
20.7
Grizzly Bear Distribution
20.7
The Founder Effect • The founder effect – Occurs when a few individuals become isolated from a larger population – Can affect allele frequencies in a population
Wolves often disperse far from their pack and start founder populations
20.7
Gene Flow – Results from the breeding of individuals from separate populations – Tends to reduce differences between populations over time
20.7
Relative Fitness – Is the contribution of a genotype to the next generation as compared to the contributions of alternative genotypes for the same locus – 0 for genotypes that are not passed to offspring – 1 for genotypes that are always passed to offspring 20.8
Why Natural Selection Cannot Fashion Perfect Organisms • Evolution is limited by historical constraints • Adaptations are often compromises • Chance and natural selection interact • Selection can only edit existing variations 20.9
• Anagenesis (microevolution) – Is change in the genetic makeup of a population from generation to generation
20.1
Populations MAP AREA
CANADA
ALASKA
• are localized group of individuals that are capable of interbreeding and producing fertile offspring Beaufort Sea Porcupine herd range
N TE OR RR TH IT W E O S RI T ES
Fortymile herd range
20.2
ALASKA YUKON
• Fairbanks
• Whitehorse
The Gene Pool • Is the total group of genes in a population at any one time • Consists of all genes in all individuals of the population
20.2
The Hardy-Weinberg Theorem – Describes a population that is not evolving – States that 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
20.3
The Hardy-Weinberg Theorem • The five conditions for non-evolving populations are rarely met in nature. • They are: – – – – –
20.3
Extremely large population size No gene flow (breeding between populations) No mutations Random mating No natural selection
The Hardy-Weinberg Theorem p = dominant allele q = recessive allele
PARENTS
p + q = 1 (100%) OFFSPRING
p2 + 2pq + q2 = 1 (100%) Homozygous dominant
Heterozygous
Homozygous recessive
20.3
Mutation and Sexual Recombination (crossing over in meiosis) • produce the variation in gene pools that contributes to differences among individuals
20.5
Mutation • Mutations – Are changes in the nucleotide sequence of DNA
– Cause new genes and alleles to arise
• Mutation rates – Average about one mutation in every 100,000 genes per generation 20.5
• If humans have 25,000 genes, then one mutation would occur in every _______ individuals per generation.
Sexual Recombination – Is far more important than mutation in producing the genetic differences that make adaptation possible
20.5
Genetic drift – Describes how allele frequencies can fluctuate unpredictably from one generation to the next – Tends to reduce genetic variation CW CW
CRCR
CRCR
Only 5 of 10 plants leave offspring
CRCW
CWCW
CRCR
CW CW
CRCR CRCW
CRCR
CW CW
CRCW CRCW
Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3
20.6
CRCR
CRCW
CRCW
CRCR
CRCR Only 2 of 10 plants leave offspring
CRCR
CRCR CRCR
CRCR
CRCR
CRCR CRCR
CRCR CRCW
CRCW Generation 2 p = 0.5 q = 0.5
Figure 23.7
CRCR
CRCR
Generation 3 p = 1.0 q = 0.0
Genetic Drift in a small population
20.6
The Bottleneck Effect – A sudden change in the environment may drastically reduce the size of a population – The gene pool may no longer be reflective of the original population’s gene pool
20.7
Original population
Bottlenecking event
Surviving population
Understanding the bottleneck effect – Can increase understanding of how human activity affects other species A small population of approximately thirty grizzly bears lives isolated in the Selkirk Mountains of northern Idaho. Reduced in number by human activity, this population is rarely able to interbreed with surrounding larger populations resulting in a genetic bottleneck.
20.7
Grizzly Bear Distribution
20.7
The Founder Effect • The founder effect – Occurs when a few individuals become isolated from a larger population – Can affect allele frequencies in a population
Wolves often disperse far from their pack and start founder populations
20.7
Gene Flow – Results from the breeding of individuals from separate populations – Tends to reduce differences between populations over time
20.7
Relative Fitness – Is the contribution of a genotype to the next generation as compared to the contributions of alternative genotypes for the same locus – 0 for genotypes that are not passed to offspring – 1 for genotypes that are always passed to offspring 20.8
Why Natural Selection Cannot Fashion Perfect Organisms • Evolution is limited by historical constraints • Adaptations are often compromises • Chance and natural selection interact • Selection can only edit existing variations 20.9
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