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Evolution

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Miss Schwinge

on 5 February 2015

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Transcript of Evolution

Darwin proposed that
over long periods, natural selection produces organisms that have different structures, and that establish different niches
(or occupy different habitats).
What is evolution?
The word
"variation"
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
Evolution
Charles Darwin
The individual who contributed more to our
understanding of evolution
more than anyone was
Charles Darwin
.
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
would eventually
force organisms into a competition
for resources.
Variation and Natural Selection in Nature
Some of the most important studies showing
natural selection in action
involve
descendants
of the
finches
that Darwin observed in the
Galapagos Islands.
Humans share the Earth with
millions
of other kinds of
organisms
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
known as
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.
In
1831
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.
That
hypothesis
, 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
growing collection
. 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.
Yet those
grasslands were inhabited by very different animals.
Also,
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
living animals
represented just
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
land

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
more
milk production, they would
selectively mate
for cows that
produce the most milk
. Darwin termed this process
artificial selection.
In artificial selection, nature provided the variation, and humans selected those variations that they found useful.
From a
single ancestral plant,

breeders selecting for enlarged flower buds, leaf buds, leaves, or stems have produced all these plants.
The
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.
Those
prey that are faster, better camouflaged, or better protected can avoid being caught.
This
struggle for existence was central to Darwin's theory
of evolution.
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.
Adaptations
can also include an
organism's physiological processes (functions), such as the way in which a plant performs photosynthesis.
More complex features, such as
behavior
in which
some animals live and hunt in groups, can also be adaptations.
Natural Selection
The concept of
fitness
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)
, either
die or leave few offspring.
Individuals that are
better suited
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.
Natural selection

takes place without human control or direction
Over time,
natural selection results in changes in the inherited characteristics of a population.
These
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.
This
principle
is known as
descent with modification.
Descent with modification also implies that all living organisms are related to one another.
This is the
principle
known as
common descent.
According to this principle,
all species (living and extinct) were derived from common ancestors.
Therefore,
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
giraffes,
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"
or,
"use it or lose it."
According to
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.
The
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.

Acquired change
(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.
Those
finch species
looked
so different from one another
that when
Darwin
first saw them, he
did not realize they were all a type of finch.
The
species
he examined
differed
greatly in the
sizes and shapes of their beaks
and in their
feeding habits.
Some species fed on small seeds
, while others ate
large seeds

with thick shells.
One species used
cactus spines
to pry
insects
from dead wood.
Once Darwin discovered that these birds were
all finches
, 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,
the
differences
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
are
more likely to survive
and pass on their
genes.
This type of
natural selection
was especially evident in the
Galapagos Islands
since they had such
different climate conditions.
The
different food and weather types allowed for the obvious changes in finch beaks.
Eventually these
changes

led to speciation (the formation of new species).
A
"species"
is a
group of organisms that breed
with one another and
produce fertile offspring.
However, if
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
"dark" alleles
, while the
other half carried "light" alleles
.
All was fine in these cities until
air pollution
, due primarily to the
burning of coal, changed the environment.
The
light tree bark
became
black with soot
, which made the
light colored moths
very
obvious
against the trees.
This meant that the
light colored moths
were
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
.
However, the
dark moths
were just
fine
. All of the
soot
made it
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
moth coloration
had to do with
random mutation
. 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.
The
initial variation
came about by
chance,
and this
particular variation
gave the
dark moths an edge
in the face of the Industrial Revolution when there was
intensive pollution due to the burning of coal.
The
abundance of soot
made it
easier for predators to spot the light-colored moths against the darkened trees,
which mostly
removed them
from the
population.
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).
Animals
may also
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
Factors
such as
natural selection
and
chance events
can
change the relative frequencies of alleles in a population.
But how does this relate to speciation?
Because a
population
of individuals has a
shared gene pool
, a
genetic change
that occurs in
one individual
can
spread through the population
as that
individual
and its
offspring reproduce.
If a
genetic change increases fitness
, that
allele
will eventually be
found
in
many individuals of that population.
Given the definition of a
species
, what must happen for a species to
evolve into two new species
? The
gene pools
of
two populations
must
become separated
for them to
become new species.
As
new species evolve, populations become reproductively isolated from each other.
When the
members of two populations can no longer interbreed
and produce
fertile offspring,

reproductive isolation has occurred.
At that point, the
populations have separate gene pools
and they
respond to natural selection as separate units.
Reproductive isolation
can develop in a
variety
of ways,
including behavioral isolation, geographic isolation, and temporal isolation.
Behavioral 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
are very
similar birds with overlapping habitats.
However, they will
not mate with each other
because they use
different songs to attract mates.
Geographic Isolation
With
geographic isolation two populations are separated by geographic barriers such as rivers, mountains, or bodies of water.
For example, about
10,000 years ago
the
Colorado River split the Abert squirrel population into two
separate populations.
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
formation
of a
distinct subspecies
, the Kaibab squirrel. They have very
similar anatomical and physiological characteristics
, indicating that they are
closely related;
but they also have
significant differences.
Temporal Isolation
A third
isolating mechanism
is
temporal isolation
, in which
two or more species reproduce at different times.
For example,
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,
they
cannot mate with each other.
Body Structures, Vestigial Organs, and Embryology
Aside from
fossils
, further
evidence of evolution
can be found in
living animals
. By Darwin's time, researchers had noticed striking
anatomical similarities
among the
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
different functions
, these
limb bones all develop from the same clumps of cells in embryos.
Structures
that have
different mature forms but develop from the same embryonic tissues
are called
homologous structures.
This means that
similar structures serve different functions.
For example,
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.
In
contrast
to
homologous structures,
sometimes animals have
features with the same function but that are structurally different.
For example,
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
analogous structures.
The early stages, or
embryos
, of
many animals with backbones are very similar.
This does
not mean
that a
human embryo is ever identical to a fish or a bird embryo
, however,
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.
These
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.
But
not all homologous structures serve important functions.
The
organs
of many animals are
so reduced in size
that they are just
vestiges, or traces, of homologous organs in other species.
These
vestigial organs may resemble miniature legs, tails, or other structures.
For example,
modern day whales
still have
miniature pelvic

bones left over
from the time
when they had legs.
But
why
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
reproduces,
it
passes copies of its genes to its offspring
. We can therefore view
evolutionary fitness
as an
organism's success in passing genes
to the next generation.
However,
natural selection never acts directly on genes
because it is
an entire organism, not a single gene,
that either
survives and reproduces or dies without reproducing.
Therefore,
natural selection
can only
affect
which
individuals survive and reproduce,
and which ones do
not.
If an individual
dies without reproducing it does not contribute its genes
to the population's
gene pool
. If an individual
produces many offspring, its alleles stay
in the
gene pool and may increase in frequency.
This is why it is
populations, not individual organisms,
that can
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.
Directional selection

is when

individuals at one end of the curve have higher fitness than individuals in the middle or at the other end.
The
range of phenotypes shifts
as some individuals
fail to survive and reproduce, while others succeed.
Sometimes this can be caused by
limited resources
, like food or water, and how
well equipped
an organism is at getting them.
For example, Darwin's finches with the
bigger, thicker beaks
were able to feed
more easily
on the
larger, harder, thicker-shelled seeds.
If a
food shortage
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
increase.
Directional Selection
Stabilizing Selection
Stabilizing selection

is when

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
stabilizing selection
. 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
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
selection
acts more strongly against individuals of an
intermediate type
. 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.
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