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Historical Context Organisms are unchanging

Aristotle & Old Testament

Georges Cuvier (1812) extinctions happen. The older the life form, the more different to current version.

Hutton and Lyell: geological features are produced gradually over vast time scales

Lamark proposes organisms change through use and disuse and inheretance of acquired characteristics

Wallace proposes a “natural selection” mechanism

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Figure 22.2

1812Cuvier publishes hisextensive studies ofvertebrate fossils.

1809Charles Darwin

is born.

1809Lamarck publishes his

hypothesis of evolution.

1798Malthus publishes

“Essay on the Principleof Population.”

1795Hutton proposes

his principle ofgradualism.

1790 18701831–1836

Darwin travelsaround the world

on HMS Beagle. 1844Darwin writes hisessay on descentwith modification.

1859On the Origin ofSpecies is published.

1830Lyell publishesPrinciples of Geology.

Sketch of a flyingfrog by Wallace

1858While studying species in theMalay Archipelago, Wallace(shown above in 1848) sendsDarwin his hypothesis ofnatural selection.

Marine iguanain the

GalápagosIslands

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Artificial Selection, Natural Selection, and Adaptation

Darwin noted that humans have modified other species by selecting and breeding individuals with desired traits, a process called artificial selection

Darwin drew two inferences from two observations

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Figure 22.UN03

Observations

Individuals in a populationvary in their heritable

characteristics.

Organisms produce moreoffspring than the

environment can support.

Inferences

Individuals that are well suitedto their environment tend to leave more

offspring than other individuals.

Over time, favorable traitsaccumulate in the population.

and

Selection & differential

reproduction

Adaptive evolution

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Evolution explains:

The unity of life

Why there is so much underlying similarities in apparently different organisms

The diversity of life

Why there are so many different types of living thing

The match between organisms and their environment (adaptations)

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Figure 22.13

Soapberry bug with beakinserted in balloon vinefruit

Field Study

Results

BeakOn native species,balloon vine(southern Florida)

Average for museum specimens

On introducedspecies,goldenrain tree(central Florida)

Nu

mb

er o

f in

div

idu

als

0

2

4

6

8

10

0

2

4

6

8

10

6 7 8 9 10 11Beak length (mm)

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Figure 22.10

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Natural selection does not create new traits, but edits or selects for traits already present in the population

The current, local environment determines which traits will be selected for or selected against in any specific population

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Modern Synthesisaka neo-darwinism

Modern Synthesis = darwin’s natural selection + modern genetics

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Microevolution is a change in allele frequencies in a population over generations

Three mechanisms cause allele frequency change

• Natural selection (including sexual selection)

• Produces consistent adaptive evolution

• Genetic drift

• Gene flow

• Artificial selection

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DefinitionsA population is a localized group of individuals capable of interbreeding and producing fertile offspring

A gene pool consists of all the alleles for all loci in a population

Gene a segment of DNA coding for an RNA/protein

Allele different versions of a gene

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Effects of Genetic Drift: A Summary

1. Genetic drift is significant in small populations

2. Genetic drift can cause allele frequencies to change at random

3. Genetic drift can lead to a loss of genetic variation within populations

4. Genetic drift can cause harmful alleles to become fixed

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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 variation among populations over time

can be adaptive or not

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Figure 23.12

Parus major

Centralpopulation

NORTH SEA EasternpopulationVlieland,

the NetherlandsN

2 km

Population in whichthe surviving femaleseventually bred

CentralEastern

Su

rviv

al r

ate

(%)

Females born incentral population

50

40

30

20

10

0Females born in

eastern population

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Figure 23.13

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

Originalpopulation

Originalpopulation

Evolvedpopulation

Phenotypes (fur color)F

req

uen

cy o

fin

div

idu

als

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Sexual Selection

Sexual selection is natural selection for mating success

It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics

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Why Natural Selection Cannot Fashion Perfect Organisms

1. Selection can act only on existing variations

2. Evolution is limited by historical constraints

3. Adaptations are often compromises

4. Chance, natural selection, and the environment interact