The Environment and Change Over Time

Chapter 6

6-1

Fossil Evidence

of Evolution

The Fossil Record

● The fossil record is made up of all the fossils ever discovered on Earth.

● The fossil record provides evidence that species have changed over time.

● Based on fossil evidence, scientists can recreate the physical appearance of species that are no longer alive on Earth.

How do fossils form?

● Usually only the hard parts of an animal remain such as bones, teeth or shells

● In plants, leaves, stems, roots, or seeds can leave behind fossils.

● Fossils rarely occur, and most form when an organism dies and becomes buried in sediment.

Fossil Formation cont’

● The impression (imprint) of an organism found in a rock is called a mold when it hardens.

● A cast is a fossil copy of an organism in a rock

Trace Fossils- the preserved evidence of the activity of an organism.

Determining a Fossils age● Instead of dating fossils

directly, scientists date the rocks the fossils are embedded inside.

● If the age of the igneous layers is known, it is possible to estimate the age of the sedimentary layers—and the fossils they contain—between them.

● Deeper fossils look less like present day organisms

Fossils Over Time● The geologic time scale is a chart that

divides Earth’s history into different time units.

Extinctions

● Extinctions occur when the last individual organism of a species dies.

● A mass extinction occurs when many species become extinct within a few million years or less.

● Extinctions can occur when environments change and organisms can’t adapt quickly enough

Extinctions cont’

● The fossil record contains evidence that five mass extinction events have occurred during the Phanerozoic eon.

Extinctions cont’

● The fossil record contains evidence of the appearance of many new species over time.

● Biological evolution is the change over time in populations of related organisms

Biological evolution

● The fossil record shows evidence that horses descended from organisms for which only fossils exist today.

6-2

Theory of Evolution by

Natural Selection

In December 1831, the British naval ship HMS Beagle set sail from England for a 5 year long trip around the world.

Charles Darwin•Charles Darwin, a British Naturalist was on board.

•A naturalist is a person who studies plants and animals by observing them.

•His job was to learn as much as he could about the living things he saw on the voyage.

Darwin’s Observations

● During the voyage, he observed many plants and animals that he had never seen before.

● He wondered why there were so different than the plants and animals in England.

In 1835, the Beagle reached the

Galapagos Islands- a group of small

islands in the Pacific Ocean off the west

coast of South America

Were there differences between the Islands?

●Darwin studied many animals on the islands & noticed differences & similarities between them

Giant Galapagos Tortoises

For example, the tortoises on one island had dome shaped shells and on another island had saddle-shaped shells.

Why?

Darwin found that each island in the Galápagos had a different environment, and tortoises looked different depending on which island environment they inhabited.

Blue-footed booby

Red-footed booby

Land and Marine Iguanas

The Iguanas

● The Iguanas on the Islands had large claws and the Iguanas on the mainland had smaller claws.

● Why?● Large claws allowed the

Iguanas to hold on to the slippery rocks to feed on seaweed.

● Small claws allowed the Iguanas to climb trees where they ate leaves.

Cormorants (large sea birds) were able to fly on the mainland but could not fly on the Islands.

Mainland Cormorant

Flightless Cormorant

Similarities and Differences

● Darwin observed that many of the species on the Galapagos Islands were similar to those on the mainland.

● Why?

● Where did they come from?

How did they get there?

● Darwin inferred that some of the animals from mainland Ecuador came over to the Islands

● They might have been blown out to sea during a storm or drifted on a log millions of years ago

Darwin’s Finches● Finches on the Galapagos Islands

differed slightly from the finches in Ecuador too

● The finches on each of the Galapagos Islands were also different from each other

● Mainly the shape of their beaks and the food they ate were different depending what was available on their island

● The finches had to adapt to life on their island or they would die

Darwin’s Finches of the Galapagos

Darwin’s Theory● Darwin realized the Galapagos animals came

from common ancestors, but had changed over time & now had slight differences or variations.– A variation is a slight difference in an inherited trait

of individual members of a species.

● Variations arise naturally in populations, occurring in offspring as a result of sexual reproduction.– The #1 source of variations are genetic mutations,

which are changes in a gene

● Peppered Moths● Before 1850, black

peppered moths were rare and white were more common.

● Several species of birds eat the peppered moths.

● White moths blended into their surroundings and black moths were eaten more frequently.

Natural Selection in action

After the 1850’s…

● Soot and smoke from industrial areas blackened nearby trees

● Black moths became less visible and white moths stood out and became easy prey

● More black moths survived and produced more black moth offspring

● Population changed from mostly light-colored moths to mostly dark-colored moths

Darwin’s Theory● Darwin believed these variations occurred

through the process of Natural Selection● Natural Selection is the process by which

organisms with favorable traits that help them survive in their environments live longer, compete better, and reproduce more than those that do not have the better trait.

● Natural selection explains how populations change as their environments change.

Natural Selection (4 steps)

● 1859 Darwin wrote – The Origin of the Species– In it, he says that evolution occurs by natural

selection– Individuals that are better adapted to their

environment are more likely to survive and reproduce

– “survival of the fittest”

1. Reproduction

● Most species naturally produces more offspring than will survive to maturity

● Starvation, disease, predators affect the size of their population

● Limited number survive to reproduction age● Offspring of the strongest organisms inherit

traits that help them survive in their environment

2. Genetic Variation

● Offspring have slightly different traits from one another

● Some traits increase the chances of survival and reproduction, others decrease the chances of survival

● These variations are inherited

3. Competition

● Natural environments do not have enough food, water, and other resources

● Offspring fight for food, water, and other resources

● Some become prey for predators● Only the fittest (strongest) survive to

adulthood

4. Selection

● Offspring that are better adapted to the environment will be naturally “selected” to survive & reproduce to keep the species going

Natural Selection in 4 steps

● 1. Reproduction● Species produces more offspring than will survive● 2. Genetic Variation● Offspring have different traits● Some traits increase chances of survival● 3. Competition● Offspring fight for food, water, and other resources● Some become prey● The fittest survive to adulthood● 4. Selection● Offspring better adapted to environment are more

likely to reproduce

Adaptations

● Through natural selection, a helpful variation in one individual can spread to all members of a population.

● An adaptation is an inherited trait that increases an organism’s chance of surviving and reproducing in its environment.

● Adaptations can be structural or behavioral– Example: structures and behaviors for finding

food, for protection, and for moving from place to place

Adaptations to Habitat

● Arctic hare- – white fur in winter

provides camouflage from predators

• (structural adaptation)

Adaptations to Habitat

● Monkey-– Grasping tail acts as

an extra hand to aid movement through trees

• (structural adaptation)

Adaptations to Habitat

● Cactus– Waxy skin reduces water

loss from evaporation • (structural adaptation)

Adaptations to Habitat

● Black Bear– Hibernates to adapt to

seasonal changes • (behavioral adaptation)

Artificial Selection● The breeding of organisms for desired

characteristics is called selective breeding

● Darwin realized that changes caused by selective breeding were much like changes caused by natural selection, but done by humans.

● Humans can choose to breed individuals with desired traits– Farmers can choose to breed only corn

with plump corn kernels– Dog breeders choose which dogs to

breed by selecting traits

What is a Species?

● A species is a group of similar organisms that can mate with each other and produce fertile offspring.

● Species evolve so they can survive in their changing environment.

● Not all members of a species are strong enough to evolve and survive.

How do new species form?

● Isolation or complete separation occurs● Over time new traits are developed

● Formation of a new species can happen in 3 steps:

1. Separation

● A portion of a population becomes isolated from the rest of the population by:– newly formed canyon,

mountain range, lake

2. Adaptation

● If the population is divided, the environment may also change and so may the population that lives there

● The separated groups may adapt to better fit their environments

3. Division

● 2 groups may become so different that they can no longer interbreed

● They are no longer the same species

Darwin’s Finches may have evolved this way:1. Some finches left the mainland and reached one of

the islands (separation)2. The finches reproduced and adapted to the

environment (adaptation)3. Some finches flew to a second island (separation)4. The finches reproduced and adapted to a different

environment (adaptation)5. Some finches flew back to the first island but could

no longer interbreed with those finches (division)6. This process may have occurred over and over

again as the finches flew to the other islands

● 1.Separation– Portion of a population becomes isolated from the rest of the

population by:• Newly formed canyon• Mountain range• Lake

● 2.Adaptation– As environment changes, separated groups adapt to their

new environment● 3.Division

– Groups of a population become so different they can no longer interbreed

• Different species form

6-3

Biological Evidence of Evolution

Evidence for Evolution● Evolution is the process by

which populations accumulate inherited changes over time

– Because of evolution, scientists think that all living things share a common ancestor

Evidence for common ancestry● Homologous

structures are body parts of organisms that are similar in structure and position but different in function.

● Suggests a common ancestor

Homologous structures

Evidence for common ancestry

● Body parts that perform a similar function but differ in structure are analogous structures

Evidence for common ancestry

● Vestigial Structures are body parts that have lost their original function through evolution.

● Examples: – Human Tail Bone– Appendix– Tonsils– Wisdom teeth

Remnants of hind-limb bones are embedded inside

the whale’s body

●“Leftovers”

Comparing DNA

● Looking at similarities in DNA

● Human and chimpanzee DNA 99% identical

● Suggests common ancestor

How are these species alike?

● Turtle● Chicken● Rat

Early development

Turtle Chicken Rat

Early development

● All have a tail ● All have gill slits● They share a common ancestor

Comparing Embryos

● Similar appearance early in embryonic development

● The science of the development of embryos from fertilization to birth is called embryology

● Suggest common ancestor