Evolution Guided Reading

Charles Darwin

• Charles Darwin was a naturalist who traveled on the British ship HMS Beagle on a five year trip around the world

• Darwin observed plants and animals he had never seen before and wondered why they were so different from those in England.

• These observations led him to develop the theory of evolution by natural selection

Darwin’s Observations

• Included:– The diversity of living things– The remains of ancient organisms– Characteristics of organisms on the

Galapagos Islands

Diversity

• Darwin was amazed by the diversity– Insects that looked like flowers– Ants that marched like armies– Sloths that moved very slowly and hung from

trees

• Today scientists have identified more than 1.7 million species of organisms on Earth

• What is a species?

Species

• A group of similar organisms that can mate with each other and produce fertile offspring– Fertile: can have their own babies

Fossils

• Fossil: a preserved remains or traces of an organism that lived in the past

• The fossils were similar to animals he saw, but he wondered why they had differences. – What had happened to the creatures from the

past?

Galapagos Organisms

• The Galapagos Islands are a chain of islands off the coast of South America

• Here, Darwin observed that there were many similarities and differences between the organisms on the islands and those in South America

Important Differences of the Galapagos Organisms

• Iguanas had large claws that allowed them to grip slippery rocks where they fed on seaweed– Iguanas on the mainland had smaller claws to

climb trees and eat leaves

• Darwin hypothesized that a small number of different plant and animal species had come to the islands from the mainland

• What is this process called?

Dispersal

• The movement of organisms from one location to another– Some might have blown out to sea during a

storm– Some may have set adrift on a fallen log

• Once the plants and animals reached the islands, they reproduced.

• Eventually their offspring became different from the mainland species

Comparisons Among the Islands

• Darwin noticed many differences among organisms from one island to the next– Tortoises on one island had dome-shaped

shells– On another island they had saddle-shaped

shells

Adaptations

• Like the tortoises, Finches on the Galapagos were noticeably different from one island to the next

• The most obvious differences were in the sizes and shapes of the beaks.

• Why did birds on different islands have different shaped beaks?

Adaptations

• They had adapted to the different food sources on their specific island– adaptation: a trait that helps an organism

survive and reproduce

• Finches with narrow, needle-like beaks ate insects

• Finches with strong, wide beaks ate seeds

Adaptations

• What are some other adaptations we have talked about?

• Adaptations can be for – getting food: speed, sharp teeth – avoiding being eaten: poison, bad taste– aid in reproduction: bright colors of a flower

attract insects

Evolution

• Darwin reasoned that plants or animals that arrived on the Galapagos Islands faced conditions that were different from those on the mainland.

• He hypothesized that the species gradually changed over many generations and became better adapted to the new conditions.

• The gradual change in a species over time is called evolution.

Scientific Theory

• Darwin’s ideas are referred to as the theory of evolution

• A scientific theory is a well-tested concept that explains a wide range of observations

• After his many observations, Darwin concluded that the organisms had changed over time, but he did not know how the changes happened.

Selective Breeding

• Darwin studied other examples of changes in living things to help him understand evolution– He studied animals produced by selective

breeding such as horses

Natural Selection

• Darwin published the book The Origin of Species in which he proposed that evolution occurs by means of natural selection– Natural Selection: the process by which

individuals that are better adapted to their environment are more likely to survive and reproduce than other members of the same species

Factors that affect natural selection

• Overproduction– Species produce more offspring than can

possibly survive

• Variations– Members of a species differ from one another

in many of their traits, this is called variation

• Competition– Food and other resources are limited

Selection

• Some variations made individuals better adapted to their environment– These individuals are more likely to survive

and reproduce– The offspring will inherit these helpful

characteristics and will be more likely to survive and reproduce

• Over time helpful variations may gradually accumulate while unfavorable ones may disappear

Genes and Natural Selection

• Darwin could not explain what caused variations or how they were passed on

• Today we know that they can result from mutation and the shuffling of alleles during meiosis

• Genes are passed from parent to offspring on chromosomes, because of this, only traits that are controlled by genes can be acted upon by natural selection

Environmental change

• Change in environment can lead to natural selection– Peppered moths of Manchester England

• Moths had been adapted to living on the lichen covered trees

• Industrial revolution put soot into the air which turned the trees darker and killed the lichen

• Now the darker moths which once stood out and were eaten first, were able to blend in to the newly darkened trees

Before After

Evidence of Evolution

• Fossils

• Similarities in Early development

• Similar body structures– Similar body structures that related species

have inherited from a common ancestor are called homologous structures

Similarities in early development

• A opossum, chicken, fish and salamander all have a tail and tiny slits along their throats during the early stages of development

• http://www.pbs.org/wgbh/nova/beta/evolution/guess-embryo.html

Similarities in Body Structures

• Fish, amphibians, reptiles, birds, and mammals are classified in one group because they have similar body structure

• Scientists have recently discovered fossils of ancient whale-like creatures that have legs and walked on land

• http://www.livescience.com/animals/090203-pregnant-whale-fossil.html

Inferring Species Relationships

• Scientists have combined the evidence from DNA, protein structure, fossils, early development, and body structure to determine evolutionary relationships among species– The more closely related species are, the

more similar their DNA sequences• Elephant shrew is more closely related to

elephants than to rodents

Combining Evidence

• Dogs are more similar to wolves than they are to coyotes

• Giant pandas are more closely related to bears

• Lesser pandas are more closely related to raccoons

• Branching trees can be used to show how scientists think different groups of organisms are related

Branching Tree

Did birds evolve from Pterosaurs?

What is the common ancestor of Crocodilians and Modern Birds?

Are modern birds more closely related to Archaeopteryx or to the first reptiles?

How Do New Species Form

• A new species can form when a group of individuals remains isolated from the rest of its species long enough to evolve different traits

• Isolation can occur from a river, a volcano, or a mountain range

Populations of squirrel separated by Grand Canyon

Fossil Record

• Formation of fossils is rare– Most form when organisms that die become buried in

sediments• Sediments are particles of soil and rock

• Bone and shell easily become fossils• Petrified Fossils: remains of organisms that are

actually changed into rock• Molds are a hollow space in sediment left after

the hard parts of the organism has dissolved• Casts are a copy of the shape made from a mold

Determining a Fossil’s Age

• Scientists can reconstruct the history of life on earth

• Two methods– Relative dating– Radioactive dating

Relative Dating

• In layers of sedimentary rock, the oldest layer is usually at the bottom. Each higher layer is younger than the layers below it– Think of a pile of clothes in the corner of your

room; the oldest clothes are at the bottom of the pile, and the ones you wore yesterday are thrown on the top

• This method can only help scientists determine whether one fossil is older than another

Radioactive Dating

• Scientists use unstable elements that decay, called radioactive elements, to determine the actual age of a fossil

• The half-life of a radioactive element is the time it takes for half of the atoms in a sample to decay– For example Potassium-40 breaks down into Argon-

40 over time

• Scientists compare the amount of a radioactive element in a sample to what it breaks down into to find the age.

What is the half-life of strontium-90? How do you know?

If you started with 8 grams of strontium-90, how long would it take until there was only one gram left?

• Half life = 28 years– Every 28 years, half of what you have will break down

• 8/2 = 4 – How long did this take?

• 28 years

• 4/2 = 2– This took another 28 years, so how many years total?

• 56 years

• 2/2 = 1– This took another 28 years in addition to the 56. How

long did it take to get to 1 g?• 84 years

What Do Fossils Reveal

• The millions of fossils that scientists have collected are called the fossil record

• There are gaps in the fossil record because not all organisms can be or have been fossilized.

• Fossils provide a way for scientists to learn about extinct species

The Geologic Time Scale

• A calendar of Earth’s history• Largest span of time is the Precambrian Time

– Covers the first 4 billion years of Earth’s history– Scientists know very little of this time because there

are few fossils

• After Precambrian Time, the time scale is divided into three major blocks of time or Eras– Eras are divided into shorter periods– The T Rex lived in the Mesozoic Era during the

Cretaceous period

Three Eras

•Paleozoic

•Mesozoic

•Cenozoic

Unanswered Questions

• Causes of mass extinctions– When many types of organisms become

extinct at the same time– Several have taken place

• End of the Cretaceous Period about 65 million years ago

• Rate at which evolution occurs– Two theories

• Gradualism and Punctuated Equilibira

Gradualism

• Evolution occurs slowly but steadily

• Tiny changes gradually add up to major changes over very long periods of time– Should have intermediate forms between

fossil organism and its descendants– Often long periods of time in which fossils

show little or no change then suddenly fossils appear that are distinctly different

• Could be explained by gaps in the record

Punctuated Equilibria

• This theory accounts for the gaps in the fossil record

• Says species evolve quickly during relatively short periods– These periods are separated by long periods

of little or no change

• Today scientists believe evolution can occur gradually at some times and more rapidly at others