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Charles Darwin and Evolution

Lecture notes for a high school CP biology course on Darwin and Evolution. Credits to Miller and Levine Biology Text
by

Kelly Quinlan

on 11 February 2014

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Transcript of Charles Darwin and Evolution

Charles Darwin and Evolution
Darwin's Epic Journey
As a boy, Darwin wasn’t a star student.

Yet Charles would one day come up with one
of the most important scientific theories of
all time; the
Theory of Evolution
.


At the time, geologists were suggesting that Earth was ancient and had changed over time, and biologists were suggesting that life had also changed.









Darwin developed the
theory of biological evolution
that explains how modern organisms evolved over time and descend from common ancestors.
Charles Darwin was born
in England in 1809. He grew up at a
time when the scientific view of the natural world was shifting dramatically.
What type of changes do you think were happening during this time? Think about technology advances and the politics of the time...
The process of changing over time is called

evolution.
Darwin’s Epic Journey
Darwin sailed on a ship called the
Beagle
for a 5-year voyage mapping the coastline of South America. Darwin planned to collect specimens of plants and animals on the voyage.

This would be one of the most
important scientific voyages in history.
Species Vary Globally
Darwin noticed that places around the world that had similar habitats often had similar animals.


Darwin also noticed that different, yet related, animal species often occupied different habitats within a local area.
Rhea, South America
Ostrich, Africa
Emu, Australia
According to Darwin's observations, how likely would it be that similar squirrel-like species would be found in other similar habitats to ours? (like out west or in Europe)
Species Vary Locally
Darwin noticed that different, yet related, animal species often occupied different habitats within a local area.

For example, Darwin found two species of rheas living in South America: one in Argentina’s grasslands and the other in the colder, harsher grass and scrubland to the south.
Species Vary Locally
Other examples of local variation came from the
Galápagos Islands
, Pacific coast of South America.

These islands are close to one another, yet they have different ecological conditions. Several islands were home to distinct varieties of giant land tortoises.

Darwin saw differences among the tortoises
that inhabit the islands, specifically, that the
tortoises’ shells varied in predictable
ways from one island to another.
Greater Rhea
Lesser Rhea
Species Vary Locally
Darwin also observed that different islands had different varieties of mockingbirds, all of which resembled mockingbirds Darwin had seen in South America.

In addition, Darwin noticed several types of small brown birds on the islands with beaks of different shapes. He didn’t consider these smaller birds to be unusual or important—at first.
Species Vary Over Time
Darwin also collected
fossils
, which are the preserved remains or traces of ancient organisms.

Darwin noticed that some fossils of extinct animals were similar to living species.
Darwin wondered if the armadillo might be related to the ancient glyptodont.

He wondered: Why had glyptodonts disappeared? And why did they resemble armadillos?
Putting the Pieces of the Puzzle Together
On the voyage home, Darwin thought about the patterns he’d seen...

The Galápagos mockingbirds turned out to belong to three separate species found nowhere else. And the little brown birds were actually all species of finches, also found nowhere else, though they resembled a South American finch species.

The same was true of Galápagos
tortoises, marine iguanas, and
many plants that Darwin had
collected on the islands.
Putting the Pieces of the Puzzle Together
Darwin began to wonder whether different Galápagos species might have evolved from South American ancestors.

He spent years actively researching and filling notebooks with ideas about species and evolution.

The evidence suggested that species are not fixed and that they could change by some natural process.
What differences do you see between these tortoise shells?
What ecological differences in the tortoises habitats may have selected for these differences?
Darwin Presents His Case
Darwin knew that his theory was radical, so he wanted to gather as much evidence as he could to support his ideas before he made them public.

In 1858, Darwin reviewed work containing similar ideas about evolution by
Alfred Wallace
, a naturalist working in Malaysia. Not wanting to get “scooped,” Darwin decided to move
forward with his own work.

Wallace’s essay was presented with some
of Darwin’s observations at a scientific
meeting in 1858. The next year,
Darwin published his first complete
work on evolution:
The Origin of Species
.
After his voyage, Darwin wrote
up a complete draft of his ideas about natural selection, but he put the work aside and didn’t publish it for another 20 years.
Why do you think he waited so long to publish such ground breaking findings??
"Descending from the apes! My dear, let us hope that it is not true, but if it is, let us pray that it will not become generally known."
Evolution by Natural Selection

occurs in any situation in which:
- More individuals are born than can survive
.

(
the struggle for existence
)
- There is natural heritable variation
.

(
variation and adaptation
)
- There is varied fitness among individuals
.


.
(
survival of the fittest
).
Under what conditions does natural selection occur?
Natural Selection
normal colored birch tree
Dark, soot covered birch tree from pollution
The Struggle for Existence
If more individuals are produced than can survive, members of a population must compete to obtain food, living space, and other limited necessities of life.
Variation and Adaptation
Individuals have natural variations among their heritable traits (phenotypes), and some of those variants make an organism better suited to life in their environment than others.

Any characteristic that increases an organism’s ability to survive and reproduce is called an
adaptation
.
Adaptations can involve
body parts
or structures, like a tiger’s claws;
colors
, like those that make camouflage or mimicry possible; or
physiological functions
, like the way a plant carries out photosynthesis.
Many adaptations also involve behaviors, such as the avoidance strategies prey species use.
Survival of the Fittest
Fitness
describes how well an organism can survive and reproduce in its environment.

Individuals with adaptations that help it survive and reproduce and are said to have
high fitness
.

Individuals with characteristics that are
NOT well-suited to their environment
either die without reproducing or leave
few offspring and are said to have
low fitness
.
Adaptations
For example, a crane will display defensive behavior to scare off a predator.
Look how big and scary I am!!
ex. retractable claws
This difference in rates of survival and reproduction is called
survival of the fittest
. In evolutionary terms, survival means reproducing and passing adaptations on to the next generation.
Camouflage
Mimicry
Natural Selection
The process by which organisms with variations most suited to their local environment survive and leave more offspring.
From generation to generation, populations continue to change as they become better adapted, or as their environment changes.
Natural Selection
This hypothetical population of grasshoppers changes over time as a result of natural selection. Explain what is happening in each picture in relation to the caption below.
Common Descent
According to the principle of common descent, all species—living and extinct—are descended from ancient common ancestors.
Just as well-adapted individuals survive and reproduce, well-adapted species also survive over time. Over many generations, adaptation could cause successful species to evolve into new species.
Common Descent
Darwin based his explanation for the diversity of life on the idea that species change over time.

This page from one of Darwin’s notebooks shows the first evolutionary tree ever drawn. This sketch shows Darwin’s explanation for how descent with modification could produce the diversity of life.

A single “tree of life” links all living things
.
Warm up
According to Darwin's findings, what are the 3 ways in which species vary?
- the wife of Bishop of Worcester after hearing about The Origin of Species
During Darwin's time, scientists
did not have the technology or understanding to test Darwin’s theories. Fields like genetics and molecular biology didn’t exist yet!



In the 150 years since Darwin published The Origin of Species, discoveries in all these fields have supported Darwin’s ideas about evolution.
Evidence of Evolution
Biogeography
Patterns in the distribution of living and fossil species tell us how modern organisms evolved from their ancestors.
Two biogeographical patterns are significant to Darwin’s theory:
Closely Related but Different
Distantly Related but Similar
On the other hand, similar habitats around the world are often home to animals and plants that are only
distantly related
.

Darwin noted that similar large, flightless birds inhabit similar grasslands in South America, Australia, and Africa.

Differences in body structures among those animals show that they evolved from different ancestors. Yet, similarities among those animals provide evidence that similar environmental pressures had caused distantly-related species to develop similar adaptations.
For example, natural selection produced variation in shell shape among the giant land tortoises that inhabit the islands.
The Age of Earth
Evolution takes a VERY long time. Current radioactive dating of rocks and fossils indicates that Earth is about
4.5 billion years old
—plenty of time for evolution by natural selection to take place.
Recent Fossil Finds
Since Darwin's time, paleontologists have discovered hundreds of fossils that document intermediate stages in the evolution of many different groups of modern species.
Comparing Anatomy and Embryology
Evolution explains the existence of homologous structures adapted for different purposes as the result of descent with modification from a common ancestor.
For example, the front limbs of amphibians, reptiles, birds, and mammals contain the same basic bones.
Homologous Structures
Darwin proposed that animals with similar structures evolved from a common ancestor with a basic version of that structure.

Structures that are shared by related species and that have been inherited from a common ancestor are called
homologous structures
.
Homologous bones, as shown by color-coding, support the differently-shaped front limbs of modern vertebrates.
Analogous Structures


Body parts that share a common function, but not structure, are called
analogous structures
. The wing of a bee and the wing of a bird are analogous structures.
Vestigial Structures


Over time, natural selection in the Galapagos islands produced variations among populations that resulted in
different, but closely related
, island species.
Not all homologous structures have important functions.

Vestigial structures
are inherited from ancestors, but have lost much or all of their original function due to different selection pressures acting on the descendant.
In whale ancestors, the pelvis played a role in walking. However, as the whale's ancestors adapted to life at sea, this function was lost.
Vestigial Structures
The appendix in humans or the legs of a three-toed skink are examples of vestigial structures.
Why would an organism possess structures with little or no function?
One possibility is that the presence of a vestigial structure does not affect an organism’s fitness. In that case, natural selection would not eliminate it.
Embryology
The same groups of embryonic cells develop in the same order to produce many homologous tissues and organs in vertebrates (animals with back bones).

Similar patterns of
embryological development
provide further evidence that
organisms have descended
from a common ancestor.
Genetics and Molecular Biology
For example:
Life’s Common Genetic Code
All living cells use information coded in DNA and RNA to carry information from one generation to the next and to direct protein synthesis.
Similar genetic code is indisputable evidence that all living organisms descended from common ancient ancestors.
Homologous Molecules
Homologous proteins share extensive structural and chemical similarities.

One homologous protein is cytochrome c, which functions in cellular respiration. Remarkably similar versions of cytochrome c are found in almost all living cells: from yeast and bacteria to plants, animals and humans.

This evidence shows that all
organisms share an ancient
common ancestor.


Such profound biochemical similarities are best explained by Darwin’s conclusion:
Living organisms evolved through descent with modification from a common ancestor.
For example, bacteria that live in a hot spring are very different from animals, yet many of their genes, and therefore the proteins coded by those genes, are similar to those of animals.
Examples
These snails are all the same species of Cuban Tree Snails
What characteristics to all these snails have in common?
What variations do you see?
What controls an organism's traits?
While on the Beagle, Darwin Observed 3 ways that species vary...
The strongest evidence supporting evolutionary theory comes from genetics.

We now understand how genetic mutations occur during sexual reproduction to produce the heritable variation on which natural selection operates.
Homologous skeletons in primates

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