EVOLUTION OF POPULATIONSChapter 23
CLARIFYING EVOLUTION Natural selection ACTS on individuals Evolutionary impact of affects populations over
time Grants Finches
Drought = large, deep beaks
Seeds = large, hard Average beak size
increase More large in population
Population evolved Beak not change in life
Mutations ultimatesource of new alleles
GENETIC VARIATION
Mutation Must be in gametes to be passed to offspring Point mutations
Phenotype & environment Wobble bases and introns
Altering gene number or sequence Nondisjunction Olfactory receptors
Sexual reproduction Allelic reshuffling
Crossing over, independent assortment, and fertilization
Makes evolution possible
POPULATIONS
Species interbreeding to produce offspring in an area
Genetic makeup is the gene pool Alleles for all loci in
all individuals Each allele has a
frequency (proportion)
HARDY-WEINBERG PRINCIPLE
Frequency alleles & genotypes remain constant Determines if evolution is occurring
Conditions necessary No mutation
Usually not a huge effect anyway Infinitely large, isolated population
No movement in or out of population to change allelic frequency
Mating is random Neither allele gives reproductive (or early
survival) advantage over the other Can apply to some, all or no genes
HARDY-WEINBERG EQUILIBRIUM
2 alleles p is more common, q is less common
Allelic frequency p + q = 1
Genotypic frequency p2 + 2pq + q2 = 1
Phenotypic frequency Same or different then genotypic
HARDY-WEINBERG PRACTICE
Work with examples in section, end of chapter, and study guide to become familiar with equation
Wildflowers with 2 alleles (Cr Cw) demonstrating incomplete dominance, what does this mean? 320 red, 160 pink, 20 white
500 individuals, 1000 copies of genes for flower color (2n) Frequencies of each allele, genotype, & phenotype?
Blood Type Genotype Number of Individuals M LMLM 700 MN LMLN 650 N LNLN 150
MICROEVOLUTION
Change in allelic frequency in a population over generations
3 mechanisms Natural selection
Improves match between individual and environment Genetic drift
Chance events that alter allele frequencies Gene flow
Transfer of alleles between populations
GENETIC DRIFT
Chance events cause allele frequencies to fluctuate unpredictably
Doesn’t work to produce adaptations Founder effect and bottleneck effect are examples
FOUNDER EFFECT
Isolated individuals form a new population Gene pool differs from source
Few members blown to a new island or an earthquake splits a population Chance where some individuals and
their alleles, but not others are separated
BOTTLENECK EFFECT Sudden event
drastically reduces population
Chance allowed certain alleles to survive
Recovery may show low variation levels
Humans can impose Cheetahs
Skin grafts and immunity
Low sperm count Variability too low to
flourish
GENE FLOW
Transfer of alleles into or out of a population Fertile individuals and their alleles move
Reduces genetic differences between populations Significant enough, 2 populations can = 1 Human populations
Introduces new alleles to population Natural selection can increase frequency
RELATIVE FITNESS
Contribution an individual makes to the gene pool of the next generation relative to the contributions of others Natural selection is not survival of the fittest Individuals with most viable, fittest offspring pass
on the most genes Survival doesn’t guarantee reproductive success
Selection favors individuals with phenotypic traits that provide higher reproductive success than others
MODES OF SELECTION
Dark rocksBeak size in finches
Birth weight
SEXUAL SELECTION
Certain inherited characteristics enhance finding mates
Creates sexual dimorphism Differences (2°) that don’t have direct effect on
fitness Include size, color, ornamentation, and behavior
Intrasexual selection (within same sex) Males defend status through force or psychologically
Intersexual selection (between sexes) Female choice depends on showiness of male
Not always beneficial, pose risks by making more visible = tradeoff
Females want mates with ‘good genes’
Midshipman Fish Male singers or
sneakers Singing induces egg
laying Male resumes
singing Attract more mates
Sneakers hangout and sneak in to fertilize eggs Resemble females
NATURAL SELECTION ISN’T PERFECT
Selection can only act on existing variations Evolution limited by ancestry
Doesn’t scrap existing structure, adapts to new ones
Often compromises Interaction of chance, natural selection, and
the environment Chance moves 1 organism to new
environmentColorado, but not necessarily to best fit environment
New species are ‘better than’