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Transcript of Evolution
over long periods, natural selection produces organisms that have different structures, and that establish different niches
(or occupy different habitats).
What is evolution?
has many common uses, but in biology,
variation means the difference in the physical traits among individuals in a group of organisms.
Inherited Variation and Artificial Selection
Descent with Modification
The individual who contributed more to our
understanding of evolution
more than anyone was
Evolution by Natural Selection
Darwin was convinced that a process like
artificial selection worked in nature.
He realized that
high birth rates and a shortage of life's basic needs
force organisms into a competition
Variation and Natural Selection in Nature
Some of the most important studies showing
natural selection in action
that Darwin observed in the
Humans share the Earth with
of other kinds of
of every imaginable shape, size and habitat. This
variety of living things is called biological diversity.
But how did all these different organisms arise? How are they related? And how can scientific explanation account for the diversity of life?
The answer is
a collection of scientific facts, observations, and hypotheses
evolutionary theory. 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.
Darwin set sail from England for a
voyage around the world
. During his travels,
Darwin made numerous observations and collected evidence that led him to propose a revolutionary hypothesis about the way life changes over time.
, now supported by a huge body of evidence, has become
the theory of evolution.
Whenever the ship anchored, Darwin went ashore to
collect plant and animal specimens
that he added to an ever
. It was his
curiosity and analytical nature
that were keys to his success as a scientist.
Patterns of Diversity
Darwin was intrigued by the fact that
so many plants and animals seemed remarkably well suited to whatever environment they inhabited.
Darwin was also puzzled by where
different species lived, and did not live
. He visited
Argentina and Australia
, for example, which had
similar grassland ecosystems.
grasslands were inhabited by very different animals.
neither Argentina nor Australia was home to the sorts of animals that lived in European grasslands.
For Darwin, these patterns posed challenging questions.
Why were there no rabbits in Australia, despite the presence of habitats that seemed perfect for them?
Living Organisms and Fossils
Darwin soon realized that
part of the puzzle
posed by the natural world. In many places during his voyage,
Darwin collected the preserved remains of ancient animals, called fossils.
Some of those
fossils resembled organisms that were still alive, while others looked completely unlike any creature he had ever seen.
This made Darwin wonder
why so many of those species had disappeared, and how they were related to living species.
The Galapagos Islands
Of all the places Darwin visited, the one that influenced Darwin the most was a
group of small islands
located west of South America called
the Galapagos Islands.
Darwin noted that
although they were close together, the islands had very different climates, and thus a very different assortment of plants and animals.
Darwin was fascinated in particular by the
tortoises and marine iguanas in the Galapagos
He learned that the
giant tortoises varied in predictable ways from one island to another, and that their shell type could be used to identify which island a particular tortoise inhabited .
For example, some
cows give more milk
than others. Since farmers want
milk production, they would
for cows that
produce the most milk
. Darwin termed this process
In artificial selection, nature provided the variation, and humans selected those variations that they found useful.
single ancestral plant,
breeders selecting for enlarged flower buds, leaf buds, leaves, or stems have produced all these plants.
struggle for existence
means that members of each species
compete regularly to obtain food, living space, and other necessities of life.
In this struggle,
the predators that are faster or have a particular way of ensnaring other organisms can catch more prey.
prey that are faster, better camouflaged, or better protected can avoid being caught.
struggle for existence was central to Darwin's theory
Survival of the Fittest
A key factor in the struggle for existence is
how well suited an organism is to its environment.
Darwin called the
ability of an individual to survive and reproduce in its specific environment fitness.
Darwin proposed that
fitness is the result of adaptations. An
adaptation is any inherited characteristic that increases an organism's chance of survival.
Successful adaptations enable organisms to become better suited to their environment and thus better able to survive and reproduce.
Adaptations can be anatomical (structural) characteristics, like a giraffe's long neck.
can also include an
organism's physiological processes (functions), such as the way in which a plant performs photosynthesis.
More complex features, such as
some animals live and hunt in groups, can also be adaptations.
The concept of
is central to the process of
evolution by natural selection. Generation after generation, individuals compete to survive and produce offspring.
Because each individual differs from other members of its species, each has unique advantages and disadvantages.
Individuals with characteristics that are
not well suited
to their environment
(low levels of fitness)
die or leave few offspring.
Individuals that are
to their environment
(adaptations that enable fitness), survive and reproduce most successfully.
Darwin called this process
survival of the fittest.
Because of its similarities to artificial selection, Darwin referred to the
survival of the fittest as natural selection.
In both artificial selection and natural selection, only certain individuals of a population produce new individuals.
takes place without human control or direction
natural selection results in changes in the inherited characteristics of a population.
changes increase a species' fitness
in its environment.
As a result, species today look different from their ancestors.
Each living species has descended, with changes, from other species over time.
is known as
descent with modification.
Descent with modification also implies that all living organisms are related to one another.
This is the
According to this principle,
all species (living and extinct) were derived from common ancestors.
a single "tree of life" links all living things.
Lamarck and the Long Necks
Darwin was not the first
to propose a theory explaining the variety of life on earth. One of the
most widely accepted hypotheses of evolution
in Darwin's day was proposed by
Jean-Baptiste de Lamarck.
In the 18th century,
Lamarck had proposed that acquired traits were inherited and passed on to their offspring.
For example, in the case of
Lamarck's theory said that giraffes
had long necks
because they were
constantly reaching for higher leaves while feeding.
This idea is referred to as the
"law of use and disuse"
"use it or lose it."
Lamarck, giraffes have long necks because they constantly use them.
He thought that
by selective use or disuse of organs, organisms acquired certain traits during their lifetime.
These traits could then be
passed on to their offspring, and over time this process led to change in a species.
Tendency Toward Perfection
Lamarck proposed that all organisms have
an innate tendency toward complexity and perfection.
As a result, they are
continually changing and acquiring features
that help them
live more successfully
in their environment.
ancestors of birds,
in Lamarck's opinion,
acquired an urge to fly.
After many generations,
birds kept trying to fly, and because of this desire their wings increased in size and became more suited to flying.
Use and Disuse
Because of this
tendency towards perfection,
Lamarck proposed that
organisms could alter the size or shape of particular organs by using their bodies in new ways.
Inheritance of Acquired Traits
Like many biologists of his time,
Lamarck thought that acquired characteristics could be inherited.
For example, if during its lifetime an animal
somehow altered a body structure
(like the crabs getting a larger claw or the giraffes getting a longer neck),
it would pass on that change to its offspring.
Using this reasoning, if you spent much of your life lifting weights to build muscles, your children would inherit big muscles, too.
Evaluating Lamarck's Hypothesis
We know now that
Lamarck's hypothesis was wrong.
(that is, changes at a "macro" level in body cells),
cannot be passed on to haploid cells.
For example, if you were to lose one of your fingers, your offspring would not inherit this trait.
However, Lamarck was
one of the first to develop a scientific hypothesis of evolution,
and to realize that
organisms were adapted to their environments
. He paved the way for the work of later biologists.
so different from one another
first saw them, he
did not realize they were all a type of finch.
greatly in the
sizes and shapes of their beaks
and in their
Some species fed on small seeds
, while others ate
with thick shells.
One species used
from dead wood.
Once Darwin discovered that these birds were
, he hypothesized that they had
descended from a common ancestor.
He proposed that
over time natural selection shaped the beaks of different bird populations as they adapted to eat different foods.
However, in order for
beak size and shape to evolve,
in beak size and shape
must produce differences in fitness
that cause natural selection.
Also, when a population is exposed to
environmental change, or "stress,"
those who are
better equipped to compete
more likely to survive
and pass on their
This type of
was especially evident in the
since they had such
different climate conditions.
different food and weather types allowed for the obvious changes in finch beaks.
led to speciation (the formation of new species).
group of organisms that breed
with one another and
produce fertile offspring.
enough changes take place
it is possible for the organisms to
no longer be able to successfully mate
with each other; thus
resulting in new species.
Camouflage and The Peppered Moths
In England during the 1850s, there was
a large population of peppered moths
. In most areas, exactly
half of them were dark
, or carried
, while the
other half carried "light" alleles
All was fine in these cities until
, due primarily to the
burning of coal, changed the environment.
light tree bark
black with soot
, which made the
light colored moths
against the trees.
This meant that the
light colored moths
impossible for a predator to miss
! As a result, the
predators gobbled up light colored moths
as fast as they could reproduce,
often before they reached the age where they could reproduce
. All of the
difficult for the predators to see them
, which meant that the
dark colored moths survived to reproduce.
This meant that
when the dark moths reproduced, they had more and more offspring carrying the dark allele.
The important thing to remember is that the
had to do with
. One day, a moth was
born with dark coloration
, and since the mutation
did not kill
the organism, it was kept.
Over time, this
one moth had offspring
, and these were
also dark colored
. Up until this point,
the dark and light colored moths lived happily side-by-side until something from the outside (the environment) changed.
came about by
dark moths an edge
in the face of the Industrial Revolution when there was
intensive pollution due to the burning of coal.
abundance of soot
easier for predators to spot the light-colored moths against the darkened trees,
Eventually, over time,
these two different populations might change so much
that they could
no longer reproduce together. This would result in two different species.
Moths aren't the only ones who hide by blending in with their surroundings (the adaptation of camouflage).
try to look like another animal
. For example,
non-poisonous snakes will rattle their tail and flatten their head to look poisonous to a predator.
The monarch butterfly (poisonous), and the viceroy butterfly (non-poisonous)
Chemical warfare (think skunks and stink bugs) is another method in which animals avoid being eaten by predators.
The Process of Speciation
change the relative frequencies of alleles in a population.
But how does this relate to speciation?
of individuals has a
shared gene pool
that occurs in
spread through the population
genetic change increases fitness
will eventually be
many individuals of that population.
Given the definition of a
, what must happen for a species to
evolve into two new species
for them to
become new species.
new species evolve, populations become reproductively isolated from each other.
members of two populations can no longer interbreed
reproductive isolation has occurred.
At that point, the
populations have separate gene pools
respond to natural selection as separate units.
can develop in a
including behavioral isolation, geographic isolation, and temporal isolation.
One type of
isolating mechanism, behavioral isolation
, occurs when
two populations are capable of interbreeding, but have different behaviors or courtship rituals.
For example, the
eastern and western meadowlark
similar birds with overlapping habitats.
However, they will
not mate with each other
because they use
different songs to attract mates.
geographic isolation two populations are separated by geographic barriers such as rivers, mountains, or bodies of water.
For example, about
10,000 years ago
Colorado River split the Abert squirrel population into two
Two separate gene pools were formed, and genetic changes that appeared in one group were not passed on to the other.
Natural selection worked separately on each group,
and led to the
, the Kaibab squirrel. They have very
similar anatomical and physiological characteristics
, indicating that they are
but they also have
, in which
two or more species reproduce at different times.
three similar species of orchid
all live in the same rain forest, and
each species releases pollen only on a single day
. Because the three species
release pollen on different days,
cannot mate with each other.
Body Structures, Vestigial Organs, and Embryology
evidence of evolution
can be found in
. By Darwin's time, researchers had noticed striking
body parts of animals with backbones.
For example, although the
limbs of reptiles, birds, and mammals (with their arms, wings, legs, and flippers) vary greatly in form and function, they are all constructed from the same basic bones.
Each of these limbs has
adapted in ways that enable organisms to survive in different environments
. However, despite these
limb bones all develop from the same clumps of cells in embryos.
different mature forms but develop from the same embryonic tissues
This means that
similar structures serve different functions.
a human's arm, a cat's leg, a whale's fin, and a bird's wing are all the same appendages, even though they have evolved to serve different purposes.
Homologous structures point towards a common ancestor.
sometimes animals have
features with the same function but that are structurally different.
a bat's wing and an insect's wing are both used to fly.
They therefore have the
same function, but have evolved totally independent of one another.
These are called
The early stages, or
many animals with backbones are very similar.
human embryo is ever identical to a fish or a bird embryo
many embryos look especially similar during early stages of development.
The reason for this is that
the same groups of embryonic cells develop in the same order and in similar patterns to produce the tissues and organs of all vertebrates.
common cells and tissues
, growing in similar ways,
produce homologous structures.
All vertebrates, including fish, amphibians, birds, and humans, show fish-like features called gill slits in the early stages.
not all homologous structures serve important functions.
of many animals are
so reduced in size
that they are just
vestiges, or traces, of homologous organs in other species.
vestigial organs may resemble miniature legs, tails, or other structures.
modern day whales
bones left over
from the time
when they had legs.
would an organism
possess organs with little or no function?
One possibility is that the
presence of a vestigial organ may not affect an organism's ability to survive and reproduce (human appendixes
, for example).
In that case,
natural selection would not cause the elimination of that organ.
Evolution as Genetic Change
Each time an organism
passes copies of its genes to its offspring
. We can therefore view
organism's success in passing genes
to the next generation.
natural selection never acts directly on genes
because it is
an entire organism, not a single gene,
survives and reproduces or dies without reproducing.
individuals survive and reproduce,
and which ones do
If an individual
dies without reproducing it does not contribute its genes
to the population's
. If an individual
produces many offspring, its alleles stay
gene pool and may increase in frequency.
This is why it is
populations, not individual organisms,
evolve over time
; and why
natural selection acts on phenotypes, not genotypes.
Natural selection can affect the
distributions of phenotypes
in three ways:
directional selection, stabilizing selection, or disruptive selection.
individuals at one end of the curve have higher fitness than individuals in the middle or at the other end.
range of phenotypes shifts
as some individuals
fail to survive and reproduce, while others succeed.
Sometimes this can be caused by
, like food or water, and how
an organism is at getting them.
For example, Darwin's finches with the
bigger, thicker beaks
were able to feed
larger, harder, thicker-shelled seeds.
causes all other types of seeds to run low, leaving
only the larger seeds,
the birds with the big-beak adaptation would be
more fit than the small-beaked birds
, and the
average beak size
of the population would probably
individuals near the center of the curve have higher fitness than individuals at either end of the curve.
This situation keeps the
center of the curve at its current position, but it narrows the overall graph.
For example, the
birth weight of human babies
is under the influence of
. This is because human babies born much smaller than average or much larger than average are likely to be born less healthy or have difficulty being born. Therefore,
the fitness of these extreme individuals are lower than that of more average sized individuals.
Disruptive selection is when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle.
This is when
acts more strongly against individuals of an
. If the
pressure of natural selection is strong enough and lasts long enough
, it can cause a
split in the curve,
which results in
two distinct phenotypes.
For example, suppose a population of birds lives in an area where
medium sized seeds become less common,
and large and small seeds become more common. In this situation,
birds with small or large beaks would have higher fitness over the medium beaks
; which might cause the populations to
split into two separate subgroups.