Darwin’s Theory of Evolution

• Evolution, is change over time, OR is the process by which modern organisms have descended from ancient organisms.

• A scientific theory is a well-supported testable explanation of phenomena that have occurred in the natural world.

Evolution is a Theory – Just like Gravity!

• Evolution is a well supported explanation of phenomena that have occurred in the natural world

• A theory in science is a well tested hypothesis, not just a guess

Evolution is when organisms change over time. So, modern organisms descended from ancient

ones

Charles Darwin: born in England in 1809

Sailed around the world 1831-1836

Voyage of the Beagle

How do you think Darwin came up with his theory?

Darwin’s Journey

Based on the excerpts you read from Darwin’s journal, The Voyage of the Beagle, what factors may have contributed to Darwin’s theory of evolution?

What did Darwin’s Travels reveal

• The diversity of living species was far greater than anyone had previously known!!– Diversity = Many different

kinds of species

• These observations led him to develop the theory of evolution!!

1. Patterns of Diversity

• Darwin visited Argentina and Australia which had similar grassland ecosystems.

– those grasslands were inhabited by very different animals.

– neither Argentina nor Australia was home to the sorts of animals that lived in European grasslands.

Examples of Diversity

• He saw rabbits in England but not in Australia even though the environment was similar

2. Living Organisms and Fossils

• Darwin collected the preserved remains of ancient organisms, called fossils.

• Some of those fossils resembled organisms that were still alive today.

Living Organisms and Fossils

 • Others looked completely unlike any creature he

had ever seen.

• As Darwin studied fossils, new questions arose. – Why had so many of these species disappeared?

– How were they related to living species?

Types of fossils Cast moldTrace fossils ImprintsFrozen or amber Petrified fossils

Fossils

Imprint

The Galapagos Island• A group of islands of the northwest coast of

South America

• The islands are close together but have very different climates

• Darwin observed that the characteristics of many plants and animals varied among the different islands.

• Examples from the Islands:

Galapagos finchesLand tortoises

Blue footed booby

More examples on the Galapagos Islands

Marine Iguanas

4. The Journey Home

• Darwin Observed that – Many islands close together had

different climates.

– Characteristics of many plants and animals varied greatly among the islands

Darwin finally published his ideas in 1859 in his book “The Origin of Species” that summarized all his

findings from his trip around the world.

LamarckTheory of acquired characteristics

• Lamarck said organisms acquired traits by using their bodies in new ways

• These new characteristics were passed to offspring

• Lamarck was totally wrong!

Evidence of Evolution

1. Fossil Record

2. Geographic Distribution of Living Species

3. Homologous Body structures

4. Similarities in Embryology

1. The Fossil Record

Fossil Record provides evidence that living things have evolved

Fossils show the history of life on earth and how different groups of organisms have changed over time

Relative vs. Absolute Dating

Relative Dating

Can determine a fossil’s relative age

Performed by estimating fossil age compared with that of other fossils

Drawbacks – provides no information about age in years

2. 2. Geographic Distribution of Living Species

Similar animals in different locations were the product of different lines of descent

Structures that have different mature forms but develop from the same embryonic tissues.

Wings and legs all descended from fish fins.

e.g. Wing of bat, human arm, leg of turtle

Turtle

Alligator

Bird

Homologous body structures

Homologous Body Structures

Vestigial Organs

– traces of homologous organs in other species

– Organ that serves no useful function

e.g. Appendix in man

In their early stages of development, many animals look similar, providing evidence that they shared a common ancestry.

Similarities in Embryology

Embryological development

Artificial Selection Nature provides variation, humans select

variations that are useful. Example - a farmer breeds only his best livestock.

1. Evolution by Natural Selection

What do you think natural selection mean? (“Nature chooses”, chooses what? Best adaptations for survival)

The Struggle for Existence-members of each species have to compete for food, shelter, other life necessities in order to survive.

Survival of the Fittest-Some individuals are better suited for the environment. Organisms with most favorable adaptation will survive.

Natural selection is the process by which individual organisms

with favorable traits are more likely to survive and reproduce.

Fitness is the ability of an individual to survive and reproduce in its specific environment.

Adaptation: an inherited characteristic that increases an organism’s chance of survival.

Natural Selection

2. Components of Natural Selection

• Not all individuals will be able to reproduce.

• Due to environmental issues, illness, etc…

• DIFFERENTIAL REPRODUCTION

Birds eat green beetles, not brown ones.

What’s Left?

What’s the end result?

The brown beetles that are left will mate and have brown offspring.

The brown trait has a genetic basis.

This is called HEREDITY.

Components of Natural Selection

• Finally, the brown trait (which is more advantageous) allows the beetle to survive in order to reproduce.

• Eventually, all beetles in this population will be brown.

• This PHENOTYPE has been SELECTED over the green phenotype.

Image courtesy of http://evolution.berkeley.edu/evolibrary/article/_0_0

3. Descent with Modification• Descent with Modification- Natural selection

produces organisms that have different structures, establish different niches or occupy different habitats. – This causes today’s species to look different from their ancestors.

Common Descent- were derived from common ancestors

Summary of Darwin’s Theory

1. Organisms differ; variation is inherited.

2. Organisms produce more offspring than survive.

3. Organisms compete for resources.

4. Organisms with advantages survive to pass those advantages to their children.

5. Species alive today are descended with modifications from common ancestors.

Evolution of Populations

Occurs when there is a change in relative frequency of alleles

VOCABULARY REVIEW

• Evolution –

change over time

• NATURAL SELECTION –

Individuals better adapted to the environment are able to survive and reproductive.– “SURVIVAL OF THE FITTEST”

NEW VOCABULARY

• POPULATION –

Group of individuals of same species that interbreed.

• GENE POOL –

Common group of all genes present in a population.

Variation and Gene Pool

Combined genetic information of all members.

Allele frequency is number of times alleles occur in the population.

Evolution: Any change in the relative frequency of alleles in a population

Single-Gene vs. Polygenic TraitsSingle-Gene trait: any trait controlled by one gene

(example: tongue rolling)Natural selection on single-gene traits can lead to change in

allele frequencies and thus evolution. Distinct phenotype

Polygenic traits: any trait controlled by two or more genes (example: height in human) Many Phenotypes

Natural selection can affect the distribution of phenotypes in any of three ways:

• Directional selection- Individuals at one end of the curve have higher fitness than those in the middle. (Example: seed size and birds beak size)

• Stabilizing selection-Individuals near the center of the curve have higher fitness than those at either ends of the curve. (Example: weight of human infants at birth).

• Disruptive selection-Individuals at the upper and lower ends of the curve have higher fitness than those near the middle. (Eample: seed size and bird beak size).

Natural Selection on Polygenic Traits

• Shifts to

middle range

• Shifts to

2 extremes

• Shifts to

1 extreme

Genetic Variation

Gene FlowMovement of genes

from one population to another.

SexSexual reproduction

causes new combinations of genes.

MutationChanges in DNA

Sources of genetic Variation in Populations

processes can

lead to this:

Mutations -

change in DNA

sequence

Gene Shuffling –

from sexual

reproduction

Genetic Drift

• Suppose that some organism left behind a few more offspring than other organisms.

• The ones that are left are the “lucky” ones. But their genes may be no more advantageous than anyone else’s.

• Entirely random.• Doesn’t produce adaptations, only a

mixing of the gene pool.

Genetic Drift changes populations…….

• Random change in allele frequency causes an allele to become common

Founder Effect: genetic drift due to the migration of a small subgroup of a population. (example: fruit flies migrating from mainland to different Hawaiian islands.)

Hawaiian Honeycreepers

FOUNDER SPECIES

An example of adaptive radiation – these species all diverged from a common ancestor (founder species)

Conditions needed for Genetic Equilibrium

1

2

3

4

5

5

SPECIATION

Speciation is the formation of new species

• As new species evolve, population become reproductively isolated.

1. Reproductive Isolation – Members of two population cannot interbreed & produce fertile offspring.

ISOLATING MECHANISMS……..2. Behavioral Isolation - capable of

breeding but have differences in courtship rituals (EX. Meadowlarks)

3. Geographical Isolation – Separated by geographic barrier like rivers ,mountains, or bodies of water (ex: squirrel)

4. Temporal Isolation – Two or more species reproduce at different times.

Table 23.1a