1

Evolution By Natural Selection

2

Isn’t Evolution Just A Theory?Video clip: learning and teaching evolution NP-VT 576.8 min 0-7

• How does the scientific meaning of a term like theory differ from the way it is used in everyday life?

• Can the “facts” of science change over time? If so, how?

3

Evolution

• Scientific theories– Concepts based on the conclusions of

observations and experiments.– A well tested explanation that unifies a broad

range of observations.– Supported by a large number of observations

and not yet found lacking.

4

Evolution

Cell All organisms are composed of cells.

Biogenesis Life comes only from life.

Gene Organisms contain coded information that dictates their form, function, and behavior.

Evolution All living things have a common ancestor. Genetic changes in a population or species over generations have led to the diversity of life on Earth.

5

Introduction• Evolution: change over time by which modern organisms

have descended from ancient organisms.

6

How Does Evolution Really Work?Video clip: learning and teaching evolution NP-VT 576.8 min 20-27

• What are the 4 components of natural selection?

• What determines an individual hummingbird’s beak length?

• What factors in the environment might select for beak length and shape within the hummingbird population?

7

Introduction• Natural selection:

– Individuals that are better suited to their environment are more likely to survive and reproduce than those that do not have such traits.

– testable scientific explanation of the process by which life on earth has changed over time.

8

Modern understanding of Natural selection

• In nature populations produce more offspring than environment can support

struggle for existence

survival of the “fittest”

9

Modern understanding of Natural selection

1) Populations of a species have genetic variation

-random mutation creates new alleles for the gene pool

-Sexual reproduction shuffles genetics to create unique individuals

10

Evolution by Natural selection overview

2) Certain phenotypes are better suited to an environment

11

Modern understanding of Natural selection

3) Over time, the genetic traits that make certain individuals better at surviving & reproducing have more offspring

12

Modern understanding of Natural selection

4) Natural selection selects for phenotypes (physical characteristics) - ex. Lizard color coded for by genes.

• Alters the gene frequency (microevolution)

Predict what it will the gene frequency be at Gen 40

13

Natural selection and Heterozygous individuals

• Lethal recessive alleles – how can they hide in a gene pool?– Dominant allele: functioning chloride ion channel healthy lungs

– Recessive cystic fibrosis allele: nonfunctioning ion channel airways clogged with mucus and early death

• Sickle cell Heterozygous advantage– Natural protection against malaria parasite

– No complications of being homozygous

For sickle cell

X

14

How Do We Know Evolution HappensVideo clip: learning and teaching evolution NP-VT 576.8 min 14-20

• How do fossils give us a picture of change over time?

• What distinguishing feature of the fossil Pakicetus skull identified it as related to a whale? Why was this surprising?

• Why do scientists seek fossils that are intermediate in form and time between modern forms and their probable earliest ancestors?

15

Review of Current Evidence of Evolution

*

Has amnion (can have offspring away from water)

Has paired appendages & spinal coard

Has a paired legs

Has a Dorsal nerve cord

warm blooded

Has mammary glands

Has opposable thumb

Has foramenMagnum forward*

*the opening between which the brain and the spinal cord make connections

frog

monkey

rabbitchicken

turtle

human

tuna

Lamprey

Vestigial thigh bones

Vestigial Tail bone

16

Review of Current Evidence of Evolution

*

Fossils from50 mya

Modern

Baleen whale

Change over m

any gen

erations

Fossils from50 mya

Modern

Baleen whale

Change over m

any gen

erations

17

Genetic Drift

• Genetic drift may occur when a small group of individuals colonizes a new habitat.

• Individuals may carry alleles in different relative frequencies than did the larger population from which they came.

• The new population will be genetically different from the parent population.

18

Species and Speciation

Species: a group of organisms that breed with one another and produce fertile offspring

Speciation: the formation of new species

Isolating Mechanisms: As a new species evolves, populations become isolated from one another

19

Isolation

• Temporal Isolationprezygotic

– can live in same place but breeds at different times

– Frog example

20

Isolation

Behavioral isolation: courtship ritualsprezygotic

21

Isolation

22

Isolation

• Geographic isolation:

– population split by barrier (ex: river, mountain…)

Evidence Common ancestor Separated 10,000 years ago

different sides of the Grand Canyon

23

Macroevolution: back to the big picture

• Macroevolution refers to large-scale evolutionary patterns and processes that occur over long periods of time.

24

Macroevolution: back to the big pictureAuxiliary video 4 Billion Year war – 1hr movie: McKay

Extinction

• More than 99% of all species that have ever lived are now extinct.

• In the past, most researchers looked for a single, major cause for each mass extinction.

• Many paleontologists now think that mass extinctions were caused by several factors.

25

Macroevolution: back to the big picture

• What effects have mass extinctions had on the history of life? Mass extinctions have:– provided ecological opportunities for

organisms that survived

– resulted in bursts of evolution that produced many new species

• Punctuated equilibrium is a pattern of evolution in which long stable periods are interrupted by brief periods of more rapid change.