EVOLUTION

Chapter 13

Darwin’s Theory of Evolution

Evolution, or change over time, 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.

Voyage of the Beagle

Voyage of Beagle

Dates: February 12th, 1831 Ship: H.M.S. Beagle Destination: Voyage around the world. Findings: evidence to propose a

revolutionary hypothesis about how life changes over time as a result of an organisms environmental conditions.

Science Before Darwin’s Voyage Most people believed each species was a divine

creation. Scientists were trying to explain the origin of

fossils.Some tried to explain it by altering traditional

explanations.Others (including Darwin’s grandfather)

proposed mechanisms to explain how living things change over time.

1809: French scientist Jean Baptiste Lamarck proposed a hypothesis for how organisms change over generations.He believed that over the lifetime of an

individual, physical features increased in size b/c of use or reduce in size b/c of disuse.

He also believed these changes were then passed on to offspring.

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Science Before Darwin’s Voyage

This is now known to be incorrect, however he did correctly point out that the changes that occur in species are due to the physical conditions of life- this refers to an organisms environmental conditions.

Darwin’s Observations During Darwin’s voyage on the Beagle, he

found evidence that species were not unchanging.

Darwin was reading a book that discussed how changes in the Earth could occur over many years.

While he was out, he started to notice evidence of this. Fossils- he found fossils in South America

of extinct armadillos. These armadillos closely resembled, but were not identical to the armadillos still living in the area.

Fossils

The Galapagos Islands Located 620 miles off the coast of

Ecuador. He noticed that many plants/animals

resembled those of the nearby coast of South America.

Darwin hypothesized that the most simple reason for this was that these animals had migrated to the islands and then changed over time.

Eventually Darwin called this “descent with modification” – what we now refer to as evolution.

Animals found in the Galapagos

Land Tortoises

Darwin Finches

Blue-Footed Booby

Marine Iguanas

Animals

The Journey Home

When Darwin was 27, he returned home, but did not report his findings.

He continued to study his data, until his confidence grew that he was correct in his conclusions.

His hypothesis: Separate species may have arose from an original ancestor

Population Growth Thomas Malthus-19th century

English economist If population grew (more

babies born than die) Insufficient living space Food runs out (populations

are able to increase faster than food supply increases).

He proposed that human populations do not grow unchecked because of death caused by disease, war, and famine that slows population growth.

Darwin applied this theory to animals.

Growth of Populations Population: includes all of the individuals of a

species that live in a specific geographical area AND can interbreed (breed with one another).

Evolution by Natural Selection Darwin realized that Mathus’ conclusions about

human population growth applied to all species.Every organism has the potential to produce

many offspring during its life time.But, not all of them will survive.

Natural Selection After gathering all of his information, Darwin

made a conclusion: “individuals that have physical or behavioral traits that better suit their environment are more likely to survive and will reproduce more successfully than those that do not have such traits”. Darwin called this natural selection.Over time, the number of individuals that

carry favorable characteristics that are ALSO inherited will increase in a population. Therefore, the population will change. This changes = evolution.

Publication of Origin of Species 1844 Darwin wrote down

his ideas about evolution & natural selection, but didn’t publish his data due to the controversy that other scientist were facing concerning this topic- he wasn’t confident enough.

Finally, he wrote the book & it was published in Nov 1859.

Darwin’s theory of evolution by natural selection is supported by 4 major points.1) Inherited variation exist w/in the genes of every

population or species (the result of random mutation & translation errors).

2) In a particular environment, some individuals of a population or species are better suited to survive (as a result of variation) and have more offspring (natural selection).

3) Over time, the traits that make certain individuals of a population able to survive and reproduce tend to spread in that population.

4) There is overwhelming evidence from fossils & many other sources that living species evolved from organisms that are extinct.

Darwin’s Ideas Updated Change within populations:

Darwin’s main point was that in any population, the individuals that are best suited to survive & do well in their environment will produce the most offspring. So, the traits of those individuals will become more common in each new generation.

Now, we know that genes are responsible for inherited traits.

Natural selection causes the frequency of certain alleles in a population to increase or decrease over time.

Mutations & the recombination of alleles that occurs during sexual reproduction provide for endless sources of new variation.

Darwin’s Ideas Updated

Species Formation Because the environment differs from place

to place, populations of the same species living in different locations tend to evolve in different directions.

Reproductive Isolation is the condition in which two populations of the same species do not breed with one another b/c of geographic separation, a difference in mating periods, or other barriers to reproduction.

Reproduction Isolation Two populations of the same species do not

breed with one another because of geographic separation, a difference in mating periods, or other barrier to reproduction

These isolated populations will become more different over time until they are no longer able to breed with one another

The Tempo of Evolution

Evolution is understood to be a gradual process that occurs continuously.

This is called gradualism. Some scientists believe that evolution of

some species may actually occur in short spurts, followed by long periods of no change. This change is called punctuated equilibrium.

The Tempo of Evolution

Natural Selection Over time, natural

selection results in changes in inherited characteristics of a population. These changes increase a species fitness in its environment

Natural Selection

Descent with modification = Natural Selection

Evidence of Evolution

The Fossil Record Geographic Distribution of Living Things Homologous Body Structures Vestigial Organs or Structures Similarities in Early Development Changes in DNA

Evidence of Evolution The Fossil Record Geographic

Distribution of Living Things-similar environments have similar types of organisms

Homologous Body Structures

Similarities in Early Development

Section 2: Evidence of Evolution Fossils: offer the most direct evidence that

evolution takes place.A preserved/mineralized remain or imprint of

an organism that lived long ago.Provide an actual record of Earth’s past

life-forms. Change over time can be seen in the fossil

record.• ie. Fossilized species found in older

rocks are different from those found in newer rocks.

After observing the differences in fossils, Darwin predicted that intermediate forms between groups of organisms would eventually be found.Some have been found.Some have not.

These intermediate forms are called transitional fossils.Any fossil which gives us information about a

transition from one species to another.

Examples of transitional fossils

Darwin’s theory is almost universally accepted by scientists as the best available explanation for the biological diversity on Earth:Based on supporting evidence, most

scientists agree on these 3 points:The Earth is about 4.5 million years old.Organisms have inhabited the Earth for

most of its history.All organisms living today share

common ancestry with earlier, simpler life forms.

Studying fossils

Paleontologist: scientists who study fossils.They analyze the sediments around the fossils.They use radiometric dating of certain rocks and

minerals to arrange fossils in order from oldest to youngest.

This creates a visual pattern of evolution.

Anatomy & Development

Studying the anatomy of different types of organisms often reveals basic similarities in body structures even through the structures function may differ between organisms.Vestigial structuresHomologous structuresEmbryonic development

Homologous Structures-structures that have different mature forms in different organisms, but develop from the same embryonic tissue.

Vestigial organs-organs that serve no useful function in an organism

i.e.appendix, miniature legs, arms

Similarities in Early Development

CHANGES IN DNA Natural Selection at Work

Mutation – a change in the nucleotide-base sequence of a gene or DNA molecule

If species have changed over time, then the genes that determine the species’ characteristics should also have changed by mutation

Section 3- Examples of Evolution Natural Selection at Work

In Darwin’s writings, he offered examples of how natural selection could have shaped life on Earth.

KEY POINT: The environment dictates the direction and amount of change. If the environment changes in the

future, the set of characteristics that most help an individual reproduce successfully may also change.

Factors in Natural Selection The process is driven by 4 important points

that are true for all real populations.All populations have genetic variationThe environment presents challenges to

successful reproduction. Individuals tend to produce more

offspring than the environment can support.

Individuals that are better able to cope with the challenges tend to leave more offspring than those who aren’t.

An example Evolution of antibiotic resistant diseases.

The bacteria needs to survive to reproduce.

Eventually, a mutation will occur in its genome that will allow even one bacteria cell to fight off the antibiotics. As it reproduces, more antibiotic resistant cells are made and the antibiotic currently used becomes ineffective.

Darwin’s Finches: different beaks utilized during food shortage.

An example

An example

Formation of a new species Species formation occurs in stages. Natural selection favors changes that

increase reproductive success. So, species are improved to “fit” into their

environment (ie. Fitness). Accumulation of differences between

groups- divergence. Divergence leads to the formation of new

species. Formation of new species- speciation.

Forming subspecies In environments differ enough, then

separate populations of the same species can become so different that they become a new species.

Populations of the same species that differ genetically b/c of adaptations to different living conditions become subspecies.

Once they are different enough, the subspecies may become a completely separate species.Due to reproductive barriers that form.

Summary of Darwin’s Theory

Individuals in nature differ from one another

Organisms in nature produce more offspring than can survive, and many of those who do not survive do not reproduce.

Summary of Darwin’s Theory

Because more organisms are produce than can survive, each species must struggle for resources

Each organism is unique, each has advantages and disadvantages in the struggle for existence

Summary (cont.)

Individuals best suited for the environment survive and reproduce most successful

Species change over time

Summary (cont.)

Species alive today descended with modification from species that lived in the past

All organisms on earth are united into a single family tree of life by common descent