The Mechanisms of Evolution

Life’s History: Seen in Fossils & Relics

• Broad patterns– Change through time

• communities change• habitats change

– Each of Earth’s biotas replaced a similar, but distinct biota

Modern Life: Seen by Direct Observation

• Modern life exhibits characteristic patterns– species are variable– in the short term

• species are stable• environments are stable

Biological Evolution

• encompasses the changes in Earth’s biotas– detected in remnants of the changes found in

the modern biota

The Contribution of Charles Darwin• Darwin proposed a mechanism by which

evolution may have occurred– based on observations in South America

• SA flora & fauna differed from European• temperate SA forms resembled tropical SA forms more than temperate European forms

The Contribution of Charles Darwin• Darwin proposed a mechanism by which

evolution may have occurred– based on observations in South America,

especially the Galápagos Islands• ~600 miles west of Ecuador• each with dramatically different conditions and communities

Darwin’s map

Darwin’s Travels

Figure 23.1

The Contribution of Charles Darwin• Darwin observed stable, variable populations

– each possessed adaptations (n) to its environment

• Darwin imagined the different island populations came from a founding population– populations underwent adaptation (v) and

now thrive under different conditions

The Contribution of Charles Darwin• In 1859 Darwin proposed a mechanism by

which adaptation may have occurred– species change over time (are not

immutable)– some changes enable species to more

effectively inhabit their environments– adaptive changes occur by natural selection

The Concept of Natural Selection• artificial selection of domesticated species

mimics natural selection– artificial selection

• breeders retain desirable individuals and remove undesirable individuals

Artificial Selection - PracticalFigure 23.4

Artificial Selection - HobbyFigure 23.2

Artificial Selection - ExperimentalFigure 23.5

The Concept of Natural Selection• artificial selection of domesticated species

mimics natural selection– natural selection occurs

• when some individuals produce more offspring than other individuals

• because more individuals are produced than the environment can support–those best-suited to the conditions

survive & reproduce, others don’t

The Concept of Natural Selection

• natural selection is a conservative process– in a stable environment, “average”

individuals will survive and reproduce– in a changing environment changes, those

best-suited to new conditions will survive and reproduce

The Contribution of Charles Darwin• Darwin did not know the mechanisms of

heredity

Evolution: change in the genetic composition of a population over time

• evolution is population-based• phenotypic variation in a population is due to

genotypic differences in individuals• evolution results from differential success of

individuals with different heritable phenotypes

Evolution: change in the genetic composition of a population over time

• at one genetic locus– an individual has two alleles– a population may have many alleles– the sum of all alleles for all loci in a

population is its gene pool

a population’s gene pool

for the X locus:

Figure 23.3

a population’s gene pool

for the X locus:

Figure 23.3allele

frequenciesX1 = 0.2X2 = 0.5X3 = 0.3

a population’s gene pool for the X locus:Figure 23.3

genotypefrequenciesX1X1 = 0.1 X1X2 = 0.1 X1X3 = 0.1 X2X2 = 0.3 X2X3 = 0.3 X3X3 = 0.1

Evolution: change in the genetic composition of a population over time

• a population’s genetic structure– allele frequencies – genotype frequencies

Evolution: change in the genetic composition of a population over time

• a genotype’s or phenotype’s relative contribution to the next generation = fitness– depends on the survival and reproductive

success of individuals with it

the mathematics of population genetics• for a population with only two alleles, A & a,

at a locus– the frequency of allele A is p

and– the frequency of a is q = 1- p

• allele frequencies can be calculated from genotype frequenciesp = (2NAA + NAa)/2Nand q = (2Naa + NAa)/2N

the mathematics of population genetics• equal allele frequencies do not imply equal

genotype frequencie• Figure 23.6

the mathematics of population genetics• undisrupted, a population’s genetic structure

remains the same over time

undisrupted, a

population’s genetic structure

remains the same over time

Figure 23.7p = 0.55

q = 0.45 q = 0.45

p = 0.55

Hardy and Weinberg did the math

• a population in Hardy-Weinberg equilibrium– has allele frequencies p & q– has genotype frequencies p2, q2 and 2pqand – succeeding generations will have the same

genetic structure

Hardy and Weinberg did the math

• Hardy-Weinberg equilibrium requires– random mating– a large population size– no migration– negligible mutation– stabilizing natural selection

Hardy-Weinberg agents of evolution• changes in a population’s genetic structure

occur because of agents of evolution– mutation

• spontaneous, random changes • usually detrimental or neutral• may be pre-adaptive• natural rates are very low• rates of accumulation vary

Hardy-Weinberg agents of evolution• changes in a population’s genetic structure

occur because of agents of evolution– gene flow

• migration incorporates new alleles or changes allele frequencies

• migration is typical among populations of the same species

bottlenecks shrink populations abruptlyFigure 23.8

Prairie Chicken -millions to <hundred

Figure 23.9

• changes in a population’s genetic structure occur because of agents of evolution– random genetic drift:

• chance events that alter allele frequencies–most likely in small populations–bottlenecks shrink populations abruptly–the founder effect occurs when a small sub-population is displaced

Hardy-Weinberg agents of evolution

founder effect occurs when a

smallsub-population

isdisplaced

Figure 23.10

Hardy-Weinberg agents of evolution• changes in a population’s genetic structure

occur because of agents of evolution– random genetic drift:

• chance events that alter allele frequencies–bottlenecks & the founder effect

produce low allelic variation compared to the parent population

Hardy-Weinberg agents of evolution• changes in a population’s genetic structure

occur because of agents of evolution– assortative mating

• one genotype prefers another genotype• results in changed genotype frequencies

Assortative Mating in PrimulaFigure 23.11

Hardy and Weinberg did the math• a population in Hardy-Weinberg equilibrium

– has allele frequencies p & q– has genotype frequencies p2, q2 and 2pqand – succeeding generations with have the same

genetic structure

IF…

Hardy and Weinberg did the math• Hardy-Weinberg equilibrium requires

– random mating– a large population size– no migration– negligible mutation– stabilizing natural selection

Hardy-Weinberg agents of evolution• changes in a population’s genetic structure

occur because of agents of evolution– natural selection

• enhanced reproductive success by individuals with particular genotypes

• may lead to a change in allele frequency• leads to adaptation (v.)

Natural Selection May Have Different Effects Under Different Circumstances

Figure 23.12

Figure 23.13

See page 460

Figure 23.14

two food sources that differ significantly in

hardness

produce a bimodal distribution

in

beak sizesFigure 23.15