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’