Natural Selection: A Summary

• Individuals with certain heritable characteristics survive and reproduce at a higher rate than other individuals

• Natural selection increases the adaptation of organisms to their environment over time

• If an environment changes over time, natural selection may result in adaptation to these new conditions and may give rise to new species

Video: Seahorse CamouflageVideo: Seahorse Camouflage

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Fig. 22-12

(b) A stick mantid in Africa

(a) A flower mantid in Malaysia

Anatomical and Molecular Homologies

• Homologous structures are anatomical resemblances that represent variations on a structural theme present in a common ancestor

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 22-17

Humerus

Radius

Ulna

Carpals

Metacarpals

Phalanges

Human WhaleCat Bat

Convergent Evolution

• Convergent evolution is the evolution of similar, or analogous, features in distantly related groups

• Analogous traits arise when groups independently adapt to similar environments in similar ways

• Convergent evolution does not provide information about ancestry

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 22-20

Sugarglider

Flyingsquirrel

AUSTRALIA

NORTHAMERICA

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)

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

• 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

Genetic Drift

• The smaller a sample, the greater the chance of deviation from a predicted result

• Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next

• Genetic drift tends to reduce genetic variation through losses of alleles

Animation: Causes of Evolutionary ChangeAnimation: Causes of Evolutionary Change

Fig. 23-8-2

Generation 1p (frequency of CR) = 0.7q (frequency of CW

) = 0.3

Generation 2p = 0.5q = 0.5

CW CW

CR CR

CR CW

CR CR

CR CR

CR CR

CR CR

CR CW

CR CW

CR CW

CR CWCR CW

CR CW

CR CW

CW CW

CW CW

CW CW

CR CR

CR CR

CR CR

Fig. 23-9

Originalpopulation

Bottleneckingevent

Survivingpopulation

Case Study: Impact of Genetic Drift on the Greater Prairie Chicken

• Loss of prairie habitat caused a severe reduction in the population of greater prairie chickens in Illinois

• The surviving birds had low levels of genetic variation, and only 50% of their eggs hatched

Gene Flow

• Gene flow consists of the movement of alleles among populations

• Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen)

• Gene flow tends to reduce differences between populations over time

• Gene flow is more likely than mutation to alter allele frequencies directly

• Gene flow can decrease the fitness of a population• In bent grass, alleles for copper tolerance are

beneficial in populations near copper mines, but harmful to populations in other soils

• Windblown pollen moves these alleles between populations

• The movement of unfavorable alleles into a population results in a decrease in fit between organism and environment

Directional, Disruptive, and Stabilizing Selection

• Three modes of selection:– Directional selection favors individuals at one end

of the phenotypic range– Disruptive selection favors individuals at both

extremes of the phenotypic range– Stabilizing selection favors intermediate variants

and acts against extreme phenotypes

Fig. 23-13

Original population

(c) Stabilizing selection(b) Disruptive selection(a) Directional selection

Phenotypes (fur color)F

req

uen

cy o

f in

div

idu

als

Originalpopulation

Evolvedpopulation

• Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes

• Natural selection will tend to maintain two or more alleles at that locus

• The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance

Heterozygote Advantage

Fig. 23-17

0–2.5%

Distribution ofmalaria caused byPlasmodium falciparum(a parasitic unicellular eukaryote)

Frequencies of thesickle-cell allele

2.5–5.0%

7.5–10.0%

5.0–7.5%

>12.5%

10.0–12.5%

Conditions on early Earth made the origin of life possible

• Chemical and physical processes on early Earth may have produced very simple cells through a sequence of stages:1. Abiotic synthesis of small organic molecules2. Joining of these small molecules into

macromolecules3. Packaging of molecules into “protobionts”4. Origin of self-replicating molecules

Protobionts

• Replication and metabolism are key properties of life

• Protobionts are aggregates of abiotically produced molecules surrounded by a membrane or membrane-like structure

• Protobionts exhibit simple reproduction and metabolism and maintain an internal chemical environment

The First Eukaryotes

• The oldest fossils of eukaryotic cells date back 2.1 billion years

• The hypothesis of endosymbiosis proposes that mitochondria and plastids (chloroplasts and related organelles) were formerly small prokaryotes living within larger host cells

• An endosymbiont is a cell that lives within a host cell

Fig. 25-9-4

Ancestral photosyntheticeukaryote

Photosyntheticprokaryote

Mitochondrion

Plastid

Nucleus

Cytoplasm

DNAPlasma membrane

Endoplasmic reticulum

Nuclear envelope

Ancestralprokaryote

Aerobicheterotrophicprokaryote

Mitochondrion

Ancestralheterotrophiceukaryote

• Unit 4• C22 45&46, 70&71, 79&80C23 5&6, 11&12,

31&32, 44&46, 50&52, 58&60, 69&70, 89&90C24 14&18C25 5&12, 54&60, 93&screen shot