22b Darwin, Agentsevolution

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

– Varieties of life forms

Figure 1.4C-F


• All organisms have evolutionary adaptations

– Inherited characteristics that enhance their ability to survive and reproduce

• blue-footed booby

• Large, webbed feet help propel the bird throughwater at high speeds

Clown, Fool, or Well Adapted?


– A streamlined shape, large tail, and nostrils that close are useful for diving

– Specialized salt-secreting glands manage salt intake while at sea


• Charles Darwin synthesized the Theory of Evolution by natural selection

– Theory vs hypothesis

• Evolution is the core theme of biology

Evolution explains the unity and diversity of life

Figure 1.6A


• The voyage of the Beagle

Figure 13.1B

NorthAmerica

Great Britain Europe

Africa

Equator

Australia

Tasmania

NewZealand

Cape ofGood Hope

SouthAmerica

And

es

Cape HornTierra del Fuego

GalápagosIslands

PacificOcean

AtlanticOcean


• species are fixed

• Earth is about 6,ooo yrs old

Prevalent ideas at Darwin’s time


New ideas proposed

• Fossils indicated the earth was very old

• Lyell, a geologist, argued that land forms changed constantly.

• Lamarck proposed that organisms changed and these changes were passed to progeny.


• Darwin became convinced that the Earth was old and continually changing

– He concluded that living things also change, or evolve over generations

– He also stated that living species descended from earlier life-forms: descent with modification

• Mex. marine snail shells on high mtns


• Darwin observed that

– organisms produce more offspring than the environment can support

– organisms vary in many characteristics

– these variations can be inherited

Darwin proposed natural selection as the mechanism of evolution


• natural selection explains the mechanism of evolution

Figure 1.6B

(1) Population with varied inherited traits

(2) Elimination of individuals with certain traits

(3) Reproduction of survivors

Pesticide-resistant insects

Antibiotic-resistant bacteria


• Charles Darwin, 1874

Figure 13.1x2

• Alfred Wallace


• Darwin cartoon

Figure 13.1x3


• Evolution happens when populations of organisms with inherited variations are exposed to environmental factors that favor the reproductive success of some individuals over others–Natural selection is the editing mechanism

–Evolution is based on adaptations

Figure 1.6C


– Hominid skull

Fossils provide strong evidence for evolution

Figure 13.2A, B

– Petrified trees


– Ammonite casts

– Fossilized organic matter in a leaf

Figure 13.2C, D


– Scorpion in amber

– “Ice Man”

– acid bogs

Figure 13.2E, F


• Mammoth tusks

Figure 13.2x4


• fossils show that organisms have appeared in a historical sequence

• Many fossils link early extinct species with species living today

– hind leg bones of fossil whales

Figure 13.2G, H


– Biogeography

– Comparative anatomy

– Comparative embryology

Other evidence for evolution

Figure 13.3A

Human Cat Whale Bat


– Molecular biology - protein “clocks”

Figure 13.3B

Human Rhesus monkey Mouse Chicken Frog Lamprey

Last commonancestor lived26 million yearsago (MYA),based onfossil evidence

80 MYA

275 MYA

330 MYA

450 MYA


Populations are the units of evolution

Figure 13.6

1.What is evolving? gene pool, microevolution

2.Four agents of evolution

3. Types of natural selection


Populations are the units of evolution

• A population is a group of interbreeding individuals

• A species is a group of populations whose individuals can interbreed and produce fertile offspring

Figure 13.6


• gene pool = total collection of genes in a population at any one time

• Microevolution is a change in the relative frequencies of alleles in a gene pool

What is evolving?

Four agents of microevolution

1. Mutation changes alleles

2. Genetic drift = random changes in allele frequency Bottleneck

Founder effect

LARGE POPULATION = 10,000 SMALL POPULATION = 10

allele frequency =1,00010,000 = 10% allele frequency =

110 = 10%

50% of population survives,including 450 allele carriers

50% of population survives,with no allele carrier amongthem

allele frequency =450

5,000 = 9% allele frequency =05 = 0%

little change in allele frequency(no alleles lost)

dramatic change in allele frequency(potential to lose one allele)

Genetic drift - effects of population size:


Founder effect

Figure 13.11B, C

Bottleneck effect


3. Gene flow can change a gene pool due to the movement of genes into or out of a population

ex. Migration 4. Natural selection leads to

differential reproductive success

Nonrandom mating changes genotype frequencybut not allele frequency.


• Natural selection

- results in the accumulation of traits that adapt a population to its environment

- the only agent of evolution that results in adaptation.


• Darwinian fitness is an individual’s contribution to the gene pool of the next generation compared to other individuals; i.e., number of progeny

• Production of fertile offspring is the only score that counts in natural selection

What is an organism’s evolutionary fitness?


There are three general outcomes of natural selection

Figure 13.19

Freq

uenc

y of

indi

vidu

als

Originalpopulation

Phenotypes (fur color)

Originalpopulation

Evolvedpopulation

Stabilizing selection Directional selection Diversifying selection

beak depth

1976

1978Averagebeak depth,1978

Averagebeak depth,

1976

Beak depth (mm)

Shift of average beakdepth during drought

5 6 7 8 9 10 11 12 13 140

20

40

60

80Number of individuals

Infantdeaths

Infantbirths

Percentof infantdeaths

Percent ofbirths inpopulation

Birth weight in pounds

0

10

20

30

40

50

60

70

2 3 4 5 6 7 8 9 10 11880

5

10

15

20

Natural selection tends to reduce variability in populations.


1. The diploid condition preserves variation by “hiding” recessive alleles (Bb)

2. Balanced polymorphism (2+ phenotypes stable in population) may result from:

a. heterozygote advantage Aa > aa and AA

b. frequency-dependent selection

c. variation of environment for a population

Why doesn’t natural selection eliminate all genetic variation in populations?


• Many populations exhibit polymorphism and geographic variation

Figure 13.13


3. Some variations may be neutral, providing no apparent advantage or disadvantage

– Example: human fingerprint patterns

Not all genetic variation may be subject to natural selection

Figure 13.16


• Low genetic variability may reduce their capacity to survive as humans continue to alter the environment

– cheetah populations have extreme genetic uniformity

Endangered species often have reduced variation

Figure 13.17


• Sexual selection leads to the evolution of secondary sexual characteristics

• Sexual selection may produce sexual dimorphism

Why do male and female animals differ in appearance?

Figure 13.20A, B


• This is due to:

– historical constraints

– adaptive compromises

– chance events

– availability of variations

Natural selection cannot fashion perfect organisms


• appearance alone does not always define a species

Figure 14.1A

– Example: eastern and western meadowlarks

What is a species?


What is a species?

• Naturally interbreeding populations

- potentially interbreeding

- reproductively isolated from other species

What about asexually reproducing organisms?

Extinct species?

Shy species?


• When geographically isolated, species evolution may occur– gene pool then changes to cause reproductive isolation

= allopatric speciation

When does speciation occur?MECHANISMS OF SPECIATION

Figure 14.3


• A ring species may illustrate the process of speciation

Figure 14.1C

OREGONPOPULATION

1

2

COASTALPOPULATIONS

Yellow-eyed

Monterey 3

SierraNevada

Yellow-blotched

Gap in ring Large-

blotched

INLANDPOPULATIONS


Reproductive barriers between species

• Habitat - different locations

• Timing - mating, flowering

• Behavioral - mating rituals, no attraction

• Mechanical - structural differences

• Gametic - fail to unite

• Hybrid weak or infertile


• Hybrid sterility is one type of postzygotic barrier

– A horse and a donkey may produce a hybrid offspring, a mule

– Mules are sterile

Figure 14.2C


Sympatric speciation

• No geographical isolation

• Mutation creates reproductive isolation

• Polyploidization

• Hybridization


• Specialists - Galapagos finches

• Generalists - horseshoe crabs, cockroaches

• New environments

- ecological niche

When does speciation occur?


• Adaptive radiation on an island chain

- specialization for different niches

Figure 14.4B

Species Afrom mainland

1

A

2B

B

3BC 4

C

C5

BC

D

C D

Figure 15.9

Mediumground finch

Cactusground finch

Smalltree finch

Largeground finch

Smallground finch

Large cactusground finch

Sharp-beakedground finch

Vegetarianfinch

Seedeaters

Ground finches

Cactus flowereaters

Budeaters

Tree finches

Insecteaters

Mediumtree finch

Largetree finch

Mangrovefinch

Woodpeckerfinch

Greenwarbler finch

Graywarbler finch

Warbler finches

Common ancestor fromSouth America mainland

Figure 15.8

No predestined goal of evolution


• Continental drift is the slow, steady movement of Earth’s crustal plates on the hot mantle

Continental drift has played a major role in macroevolution

Figure 15.3A

PacificPlate

NorthAmerican

Plate

NazcaPlate

SouthAmerican

Plate

AfricanPlate

EurasianPlate

Splitdeveloping

Indo-AustralianPlate

Edge of one plate being pushed over edge of neighboring plate (zones of violent geologic events)

Antarctic Plate


• influenced the distribution of organisms

– Continental mergers triggered extinctions

– Separation of continents caused the isolation and diversification of organisms

Figure 15.3B

Mill

ions

of y

ears

ago

EurasiaCEN

OZO

ICM

ESO

ZOIC

PALE

OZO

IC

North America

AfricaIndiaSouth

America

AntarcticaAustralia

Laurasia

Gondwana

Pangaea


Speciation - how much change is needed?

• Gradual vs. jerky

• Evidence:

– Fossil record

– Genetic differences between species

– Homeotic genes


• homeotic genes control body development

• Single mutation can result in major differences in body structure

Figure 11.14

Mouse chromosomes

Mouse embryo (12 days)

Adult mouse

Fly chromosomes

Fruit fly embryo (10 hours)

Adult fruit fly

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22b Darwin, Agentsevolution

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