Download UNIT 4: Evolution

UNIT 4: Evolution
Mr. Tamashiro
Biology I
Define evolution
• Evolution is the cumulative change in the heritable
characteristics of a population.
• If we accept not only that species can evolve, but also that
new species arise by evolution from preexisting ones, then
the whole of life can be seen as unified by its common
origins.
• Variation within our species is the result of different
selection pressures operating in different parts of the
world, yet this variation is not so vast to justify a construct
such as race having a biological or scientific basis.
State conditions for Hardy-Weinberg equilibrium in a population
• Derived in 1908,
• Hardy-Weinberg asserts that the genetic structure of a
non-evolving population remains constant over the
generations.
– If mating in a large population occurs randomly without
the influence of:
• natural selection,
• the migration of genes from neighboring populations,
• or the occurrence of mutations,
then, the frequency of alleles and of genotypes will
remain constant over time.
State conditions for Hardy-Weinberg equilibrium in a population
and why these conditions are not likely to appear in nature.
• Such conditions are so restrictive that they are not
likely to occur in nature precisely as predicted, but the
Hardy-Weinberg equilibrium equation often gives an
excellent approximation for a limited number of
generations in sizeable, randomly mating populations.
• Even though genetic recombination is taken into
account, mutations, gene flow between populations,
and environmental changes influencing pressures of
selection on a population do not cease to occur in the
natural world.
State that evolution is the result of genetic changes that occur in
constantly changing environments.
• Populations tend to produce more offspring than the
environment can support.
• If the mortality rate remains lower than the natality rate
then a population will keep growing.
• As more offspring are produced, there will be less
resources available to other members of the population.
• If there is an over production of offspring this will result in a
struggle for survival within the species as the resources
become scarce and individuals in the population will start
to compete for these.
• This results in an increase in mortality rate as the weaker
individuals in the population will lose out on these vital
resources that are essential for their survival.
OUTLINE THE EVIDENCE FOR EVOLUTION PROVIDED BY THE
FOSSIL RECORD, SELECTIVE BREEDING OF DOMESTICATED
ANIMALS AND HOMOLOGOUS STRUCTURES.
Outline the evidence for evolution
provided by the fossil record
• Fossils, selective breeding and homologous
structures have provided scientists with
evidence that support the theory of evolution.
• As they started to study fossils they realized
that these were not identical but had
similarities with existing organisms. This
suggested that organisms changed over time.
Fossil record:
• A fossil is the ancient preserved remains of an
organism.
• The fossil can be dated from the age of the rock
formation.
• Sequences of fossil can show the gradual change of an
organism over geological time.
• Continuous fossil records are rare with most containing
large time gaps until subsequent discoveries are made.
• Evolution and the fossil record This is an excellent site
with links to all aspects of this section of the syllabus
including the fossil record.
Outline the evidence for evolution provided by, selective
breeding of domesticated animals
• As the domestic breeds have
similar characteristics to the wild
ones and can still breed with
them.
• As selected wild individuals with
desirable characteristics were
bred, over time this resulted in a
more desirable species from a
human point of view.
• This suggests that not only have
these animals evolved but also
that they can evolve rapidly.
An example of this is the
taming of wild wolves and
their selective breeding in
order to produce the
domestic dogs we know
today.
Selective breeding:
•
man has selectively breed animals
and plants for thousands of years.
•
If an animal posses a characteristic
that is considered useful or valuable
then this animal is selected for
breading.
•
The hope then is that this
characteristic will be present in the
next generation and at a higher
frequency than before.
•
In subsequent generations it may
even then be possible to select from
an even more advantageous
characteristic.
Outline the evidence for evolution provided by homologous
structures.
• Finally scientists have found a number of
homologous structures within different
species.
• Many bones in the limbs are common to a
number of species and therefore suggests
that these have evolved from one common
ancestor.
Homologous structures:
• All of life is connected
through evolutionary
history and consequently
those organisms more
closely connected might
reasonably be expected to
share common structures or
homologous.
• Group of organisms closely
related share a common
form or derived trait which
has been inherited from the
common ancestor.
Homologous structures:
•
This classic example of homologous structures is
the pentadactyl limb of the vertebrate.
–
–
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a) Humerus
b) Radius
c) Ulna
•
In each example the bones are modified and
adapted to the locomotion of the animal.
•
In homologous structures it is normal to find that
parts of the structure will be modified, enlarged or
reduced (vestigial).
•
Divergence: The pentadactyl limb structure shows
adaptation and modification from a common limb
(ancestor) structure.
•
Convergence: Two organisms with different
ancestors have a limb structure that fulfills the
same function but has evolved form different
origins. Examples Wing of a bird and the wing of an
insect.
Explain the use of comparative embryology, DNA or protein sequence
comparisons, and other independent sources of data to create a
branching diagram (cladogram) that shows probable evolutionary
relationships.
cladogram
• is a diagram used in cladistics which shows
ancestral relations between organisms, to
represent the evolutionary tree of life.
• Although traditionally such cladograms were
generated largely on the basis of
morphological characters, DNA and RNA
sequencing data and computational
phylogenetics are now very commonly used in
the generation of cladograms.
Explain the use of comparative embryology, DNA or protein
sequence comparisons, and other independent sources of data
to create a branching diagram (cladogram)
•
Early embryos of animals show surprisingly similar
features, revealing a common ancestry.
•
Protein molecules essentially constitute the bottom
line in studying the phenotypes of organisms.
•
Similarities and differences in the amino acid
sequences of the same molecule (e.g. hemoglobin)
taken from different species produce a phylogeny.
The phylogeny revealed by studying protein
structure reflects the same phylogeny as
comparative anatomy and embryology, but with a
much finer resolution.
•
The ultimate is the comparison of the base
sequences of variable regions of DNA (in particular
mitochondrial DNA) taken from different organisms.
•
The analysis of DNA provides a molecular clock
against which the geological clock can be compared.
State how several independent molecular clocks, calibrated against each
other and combined with evidence from the fossil record, can help to
estimate how long ago various groups of organisms diverged evolutionarily
from one another.
•
The molecular clock (based on the molecular
clock hypothesis (MCH)) is a technique in
molecular evolution that uses fossil
constraints and rates of molecular change to
deduce the time in geologic history when
two species or other taxa diverged.
•
It is used to estimate the time of occurrence
of events called speciation or radiation.
•
The molecular data used for such
calculations is usually nucleotide sequences
for DNA or amino acid sequences for
proteins.
•
It is sometimes called a gene clock or
evolutionary clock.
Identify evidence of change in species using fossils, DNA sequences,
anatomical similarities, physiological similarities, and embryology.
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In embryology, the developing fetus is studied, and
similarities with other organisms are observed.
For example, annelids and mollusks are very
dissimilar as adults.
If, however, the embryo of a ragworm and a whelk
are studied, one sees that for much of their
development they are remarkably similar. Even the
larvae of these two species are very much alike.
This suggests that they both belong to a common
ancestor.
It is not, however, true that a developing organism
replays its evolutionary stages as an embryo.
There are some similarities with the more
conserved regions, but embryonic development is
subjected to evolutionary pressures as much as
other areas of the life cycle.
Read more: Evidence of Evolution - Species, Similar,
Evolutionary, Organisms, Molecular, and Common
http://science.jrank.org/pages/2610/EvolutionEvidence.html#ixzz15Sk01JKN
Identify evidence of change in species using fossils, DNA
sequences, anatomical similarities, physiological similarities, and
embryology.
Embryological Suggestions of Common
Ancestry
Evolution Genetics Embryo's and our Common Ancestor's
http://www.youtube.com/watch?v=h3Lkac890c0
State that populations tend to produce more offspring than the
environment can support.
• This increases the chance of survival of the
population as a whole
• a single death is less disastrous in a population
of 1,000 than it is in a population of 10.
Explain that the consequence of the potential overproduction of
offspring is a struggle for survival.
• Populations of living organisms tend to increase
exponentially.
– More offspring are produced than the environment
can support. There is a struggle for important
resources such as food and space. Intraspecific
competition. Some individuals survive and others die.
– Characteristics in organisms differ from one another.
Some have characteristics which make them better
suited to survive in their environment. These are the
most likely to survive.
Identify and illustrate that long-term survival of species is dependent on a
resource base that may be limited.
• Refer back to populations:
• Carrying capacity
Analyze the effects of genetic drift on the diversity of organisms in a
population.
• Genetic Drift
• It is the change brought
about in the gene
frequency of a
population by various
factors.
• It may be involved in
the elimination of genes
pertaining to certain
characters.
Explain how reproductive or geographic isolation affects
speciation.
•
The environment may impose an external barrier to reproduction, such as a river or mountain
range, between two incipient species but that external barrier alone will not make them separate,
full-fledged species.
•
Allopatry may start the process off, but the evolution of internal (i.e., genetically-based) barriers to
gene flow is necessary for speciation to be complete.
•
If internal barriers to gene flow do not evolve, individuals from the two parts of the population will
freely interbreed if they come back into contact.
•
Whatever genetic differences may have evolved will disappear as their genes mix back together.
•
Speciation requires that the two incipient species be unable to produce viable offspring together or
that they avoid mating with members of the other group.
•
Here are some of the barriers to gene flow that may contribute to speciation. They result from
natural selection, sexual selection, or even genetic drift:
The evolution of different mating location, mating time, or
mating rituals:
• Genetically-based changes to
these aspects of mating could
complete the process of
reproductive isolation and
speciation.
• For example, bowerbirds (shown
to the right) construct elaborate
bowers and decorate them with
different colors in order to woo
females.
• If two incipient species evolved
differences in this mating ritual, it
might permanently isolate them
and complete the process of
speciation.
Different species of bowerbird construct elaborate
bowers and decorate them with different colors in
order to woo females. The Satin bowerbird (left)
builds a channel between upright sticks, and
decorates with bright blue objects, while the
MacGregor’s Bowerbird (right) builds a tall tower of
sticks and decorates with bits of charcoal.
Evolutionary changes in mating rituals, such as
bower construction, can contribute to speciation.
Lack of "fit" between sexual organs:
• Hard to imagine for us,
but a big issue for
insects with variablyshaped genitalia!
These damselfly penises illustrate just how
complex insect genitalia may be.
Offspring inviability or sterility:
• All that courting and mating is
wasted if the offspring of
matings between the two
groups do not survive or
cannot reproduce.
• In our fruit-flies-in-rottenbananas-in-a-hurricane
example, allopatry kicked off
the speciation process, but
different selection pressures
on the island caused the island
population to diverge
genetically from the mainland
population.
Geographic isolation can instigate a
speciation event — but genetic changes
are necessary to complete the process.
Offspring inviability or sterility:
•
What might have caused that to happen?
Perhaps, different fruits were abundant on
the island. The island population was
selected to specialize on a particular type of
fruit and evolved a different food preference
from the mainland flies.
•
Could this small difference be a barrier to
gene flow with the mainland flies? Yes, if the
flies find mates by hanging out on preferred
foods, then if they return to the mainland,
they will not end up mating with mainland
flies because of this different food
preference.
Gene flow would be greatly reduced; and
once gene flow between the two species is
stopped or reduced, larger genetic
differences between the species can
accumulate.
•
Differing selection pressures on the two islands
can complete the differentiation of the new
species.
Geographic patterns:
• If allopatric speciation happens,
we’d predict that populations of
the same species in different
geographic locations would be
genetically different.
• For example, many species
exhibit regional "varieties" that
are slightly different genetically
and in appearance, as in the case
of the Northern Spotted Owl and
the Mexican Spotted Owl.
• Also, ring species are convincing
examples of how genetic
differences may arise through
reduced gene flow and
geographic distance.
Spotted owl subspecies living in different
geographic locations show some genetic and
morphological differences. This observation is
consistent with the idea that new species form
through geographic isolation.
Explain how sexual reproduction
promotes variation in a species.
• Variation is essential for natural selection and therefore
for evolution.
• Although mutation is the original source of new genes
or alleles, sexual reproduction promotes variation by
allowing the formation of new combinations of alleles.
• Two stages in sexual reproduction promote variation.
– Meiosis allows a huge variety of genetically different
gametes to be produced by each individual
– Fertilization allows alleles from two different individuals to
be brought together in one new individual.
Heredity
• Every organism requires a set of coded
instructions for specifying its traits.
• For offspring to resemble their parents, there
must be a reliable way to transfer information
from one generation to the next.
• Heredity is the passage of these instructions
from one generation to another. The DNA
molecule provides the mechanism for
transferring these instructions.
Compare and contrast the characteristics of asexual
and sexual reproduction.
Asexual Heredity
• In asexually reproducing
organisms, all the genes
come from a single parent.
• As asexually produced
offspring are produced by
the cell division process of
mitosis, all offspring are
normally genetically
identical to the parent.
Sexual Heredity
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In sexually reproducing organisms, the
new individual receives half of the
genetic information from its mother
through the egg and half from its father
from his sperm.
Sexually produced offspring resemble,
but are not identical to, either of their
parents.
Some reasons for these variations
between sexually reproduced offspring
and their parents include crossing over
when gametes are formed in each
parent and genetic recombination,
which is the combining of the genetic
instructions of both parents into a new
combination in the offspring when
fertilization occurs.
Explain how natural selection leads to
evolution.
• Greater survival and reproductive success of individuals with favorable
heritable variations can lead to change in the characteristics of a
population.
• Natural selection is the process by which species adapt to their
environment.
• Natural selection leads to evolutionary change when individuals with
certain characteristics have a greater survival or reproductive rate than
other individuals in a population and pass on these inheritable genetic
characteristics to their offspring.
• The reason that natural selection is important is that it’s the central idea,
stemming from Charles Darwin and Alfred Russel Wallace, that explains
design in nature.
• It is the one process that is responsible for the evolution of adaptations of
organisms to their environment.
Explain how natural selection leads to
evolution.
• Simply put, natural
selection is a consistent
difference in survival
and reproduction
between different
genotypes, or even
different genes, in what
we could call
reproductive success.
Charles Darwin studied beak variation of
finches on the Galapagos Islands as evidence
of natural selection.
Illustration from BSCS, Biological Science:
Molecules to Man, 1963.
Explain why natural selection acts on the phenotype
rather than the genotype of an organism.
• Since Natural Selection acts only on the phenotype
variability is maintained in the population.
• When Natural Selection is operating in a population
it actually tends to reduce variation by removing
individuals (and their genes) which exhibit extreme
phenotypes
• For example: very tall, very small etc.
Explain how natural selection determines the differential survival of groups of
organisms.
• Natural Selection can cause evolution only when something upsets
the dynamic equilibrium in a population and genes which once
were not favored help individuals within a population adapt to the
new conditions.
• In order to understand now this can come about we need to define
3 terms: gene pool, allele frequency, and genetic equilibrium.
• • A gene pool is all the genes in all the individuals of a population.
• • Allele frequency is the proportion of each allele in the gene pool
• • Genetic equilibrium occurs when allele frequencies in a
population remain the same from one generation to the next.
Explain how natural selection determines the differential survival
of groups of organisms.
Changes in Genetic Equilibrium (Microevolution)
• What are the mechanisms that cause the genetic
equilibrium to change?
• The mechanisms include:
• Mutation -- provides the variations that can be acted upon
by natural selection. Even though mutations are:
– relatively rare
– rarely beneficial
– often recessive and hidden
Explain how natural selection determines the differential survival
of groups of organisms.
•
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Mutations are the only source of new genetic material added to a gene pool.
In a species all other mechanisms of evolution merely shuffle the genetic material
that is already present. These include:
– Genetic recombination during cross over events in meiosis and the shuffling of genes caused
by the random pairing of gametes during fertilization can bring about a change in gene
frequencies.
– Genetic drift results in changes in allele frequencies because of random (chance) fluctuations
possible in small populations. Flip a coin 4 times and it's possible that it could come up heads
every time. It is much less possible (actually impossible) that a coin tossed 1000 times would
always come up heads. The larger the sample size the more likely the expected ratio (500
heads: 500 tails)
– Gene flow occurs when individuals from different populations of the same species migrate
from one group to another. Since genes are carried along with the organisms the gene pool
must increase or decrease in size.
– Gene flow between different species although once thought to be impossible, then rare, is
now acknowledged as a major source of change in gene frequency, especially in prokaryotes
Explain how natural selection determines the differential survival
of groups of organisms.
Variation in Natural Selection
•
When the frequencies of genes change by whatever
mechanism Natural Selection can operate on the
individuals to establish a new genetic equilibrium in
one of the following ways:
–
Stabilizing Selection
–
Directional Selection
–
Disruptive Selection
Stabilizing Selection
•
favors the average individual in a population.
•
Variation at the extremes is selected against by
opposing biotic or abiotic factors in the
environment.
•
An example might be if an organism is too small to
compete for a mate or too large to hide from a
predator. Only those organisms of average size are
least vulnerable.
Directional selection.
•
If one of the opposing forces in the environment
dominates selection occurs favoring genes which
help the population overcome the new pressure
until genetic equilibrium is reestablished.
•
An example is a pesticide (such as DDT) which kills
99% of a mosquito population. The population of
the one percent that survives quickly increases
along with the gene or genes which helped them
overcome the toxic effects of the pesticide.
Disruptive selection.
•
In some cases the average condition becomes the
target of environmental pressure.
•
Individuals at either extreme are better able to
cope with the change and their genes increase in
frequency.
Explain two examples of evolution in response to environmental change;
one must be antibiotic resistance in bacteria. (Other examples could include:
the changes in size and shape of the beaks of Galapagos finches; pesticide
resistance, industrial melanism or heavymetal tolerance in plants.)
• Before Penicillin was invented,
bacteria was the leading cause of
death.
• However, once it began to be
used, since it's an antibiotic,
some individuals of bacteria may
carry the gene Penicillinase,
which codes for an enzyme that
deactivates Penicillin, making
them resistant to an antibiotic
such as Penicillin.
• Thus, when it is indeed used, they
will be the only ones left to
reproduce and new bacteria will
also be resistant to the antibiotic.
• The Peppered Moth is another
example of evolution in response
to environmental change.
• When Britain began
industrialising, soot would come
from factories and land on trees.
• A species of peppered moth with
a lighter color vanished and those
with a darker color flourished
because they could hide
themselves easily.
Distinguish between the accommodation of an individual organism to its
environment and the gradual adaptation of a lineage of organisms through
genetic change.
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Living organisms may adapt to changing
environments through nongenetic changes in
their structure, metabolism, or behavior or
through natural selection of favorable
combinations of alleles governing any or all
of these processes.
Genetic and behavioral adaptations are
sometimes difficult to identify or to
distinguish without studying the organism
over a long time.
Physical changes are slow to develop in most
organisms, requiring careful measurements
over many years.
Examining fossil ancestors of an organism
may help provide clues for detecting
adaptation through genetic change.
Genetic change can institute behavioral
changes, making it all the more complicated
to determine whether a change is solely a
behavioral accommodation to environmental
change.
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Through the use of print and online
resources in library-media centers, students
can research the effects of encroaching
urbanization on undeveloped land and
consider the effects on specific species, such
as the coyote (not endangered) and the
California condor (endangered).
Such examples can illustrate how some
organisms adapt to their environments
through learned changes in behavior, and
others are unsuccessful in learning survival
skills.
Over a long time, organisms can also adapt
to changing environments through genetic
changes, some of which may include
genetically determined changes in behavior.
Such changes may be difficult to recognize
because a long time must elapse before the
changes become evident.
Studies of the origins of desert pup fish or
blind cave fish may help students understand
how gradual genetic changes in an organism
lead to adaptations to changes in its habitat.