Download Evolution & Populations

Survey
yes no Was this document useful for you?
   Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Document related concepts

Organisms at high altitude wikipedia, lookup

Evolutionary history of life wikipedia, lookup

Hybrid (biology) wikipedia, lookup

Genetic drift wikipedia, lookup

Introduction to evolution wikipedia, lookup

Theistic evolution wikipedia, lookup

Hologenome theory of evolution wikipedia, lookup

Natural selection wikipedia, lookup

Inclusive fitness wikipedia, lookup

The eclipse of Darwinism wikipedia, lookup

Speciation wikipedia, lookup

Evolution of sexual reproduction wikipedia, lookup

Evidence of common descent wikipedia, lookup

Transcript
What is evolution?
•Evolution is a process that results in heritable changes in
a population spread over many generations.
•“Evolution can be precisely defined as any change in the
frequency of alleles within a gene pool from one
generation to the next."
How does Evolution Occur?
• Jean Baptiste Lamarck 1744-1829 was one
of the first to propose that organisms
change through time1. The
Principle of Use and Disuse
– A change in the environment produced
a need to change in the animals
– If an animal uses one part of its body
frequently, that part will become
stronger
– If they don’t use a part, it will become
smaller and disappear.
animation
Lamarck cont’d
2. The Principle of
Inheritance of
Acquired Traits
He assumed these
acquired traits could
be passed on to
offspring.
Charles Darwin
• Influences:
– geologist Charles Lyell
• Wrote Principles of Geology; claimed
that geologic forces produced changes
on Earth in the past and those same
forces will continue to produce
changes in the future.
– Economist Thomas Malthus
• Wrote in Essay on the Principle of
Population that “population growth
would always overpower food supply
growth, creating perpetual states of
hunger, disease, and struggle”
If evolution was a car, the theory of natural
selection would be the engine
• The basic ideas of evolution were discussed
long before there was any scientific research
to support them.
• The evolutionary concept was never able to
gain any real steam because it lacked a
mechanism.
– Scientists wanted to believe that species
evolved from one form to another, but
had no plausible process to make it
happen.
– The theory of natural selection is just that!
animation
Darwin’s Theory of Natural Selection
1. Overpopulation-
All species have the potential to
overpopulate the earth, but
usually remain stable over time.
2. VariationDue to variation, different
individuals among populations
have different traits
3. Competitiona large # of the population must
die at an early age.
Darwin’s Idea Cont.
4. Survival of the Fittest-
members of a population whose
trait makes them better adapted
to their environment are more
likely to survive to reproductive
age and produce more offspring.
5. Reproductionmembers of a population whose
trait makes them better adapted
are more likely to reproduce.
6. Speciationgiven time, the process of evolution by natural
selection can account for the formation of new
species and thus for the diversity of life on earth.
So where does all this variation
come from?
-August Weismann
Kinds of variation:- environmental
versus genetic
Sources of variation:- mutation,
genetic recombination
Vocabulary to understand:
• Population
– a group of individuals living in the same geographical area
and sharing a common gene pool
• Gene Pool
– the sum of all genetic information carried by the members of
a population
• Mutation
– any heritable change in DNA (gene-level and the
chromosome-level)
• Gene Flow
– The transfer of alleles between populations through
interbreeding
• Natural Selection
– Differential survival and reproduction of organisms with a
result of increase in frequency of best adapted traits.
• Nonrandom Mating
This stud
bull is theselection
product of generations of selective breeding that has
– artificial
resulted in a double muscled freak that can hardly walk
Populations and Gene Pools
What’s the difference between
microevolution and macroevolution?
• Microevolution is change within species which
can occur over dozens or hundreds of
generations.
• Macroevolution usually involves much longer
periods of time and includes the origin of new
species.
What is a gene pool?
The collection of
alleles available
among
reproductive
members of a
population
Evolution occurs on the population level
• A population is a group of
individuals of the same species in a
given area whose members can
interbreed.
• Because the individuals of a
population can interbreed, they
share a common group of genes
known as the gene pool.
• Each gene pool contains all the
alleles for all the traits of all the
population.
• For evolution to occur in real
populations, some of the gene
frequencies must change with time.
Gene Frequencies and H-W
• The gene frequency of an allele is the number
of times an allele for a particular trait occurs
compared to the total number of alleles for
that trait.
Gene frequency = the number of a specific type
of allele / the total number of alleles in the
gene pool
• An important way of discovering why real
populations change with time is to construct a
model of a population that does not change.
– This is just what Hardy and Weinberg did.
– Their principle describes a hypothetical
situation in which there is no change in the
gene pool (frequencies of alleles), hence
no evolution.
The Hardy-Weinberg Theory
This law states an equilibrium of allele frequencies in a gene pool (using a
formula p2 + 2pq + q2) remains in effect in each succeeding generation of a
sexually reproducing population if five conditions are met:
1. Large populationThe population must be large to minimize
random sampling errors.
2. Random matingThere is no mating preference. For example
an AA male does not prefer an aa female.
3. No mutation
The alleles must not change.
4. No migration –
Exchange of genes between the population
and another population must not occur.
5. No natural selection- Natural selection
must not favor any particular individual.
• These conditions of the Hardy-Weinberg law are rarely met
• Allele frequencies in the gene pool of a population do
change from one generation to the next
– thus resulting in evolution.
• The H-W equation provides a baseline by which to judge
whether evolution has occurred.
The effect of natural selection on gene frequencies can be
quantified.
If a change in allele frequencies occurs over time, you can
assume evolution is happening.
A hypothetical “gene pool”
longthe
as the
conditions of
are met, the
• As
Find
Frequencies
ofHardy-Weinberg
A and a and the
population
can frequencies
increase in sizeofand
theAa
gene
frequencies
of A
genotypic
AA,
and
aa.
Solution:
and a will remain the same.
f(A)gene
= 12/30
0.4 = 40%
Thus, the
pool= does
not change.
f(a) = 18/30 = 0.6 = 60%
Then, p + q = 0.4 + 0.6 = 1
and p2 + 2pq + q2 = AA + Aa + aa
= .16 + .48 + .36 = 1
• Now, suppose more 'swimmers' dive in as shown in
What will the gene and genotypic frequencies be?
Solution:
f(A) = 12/34 = .35 = 35 %
f(a) = 22/34 = .65 = 65%
f(AA) = .12, f(Aa) = .23 and f (aa) = .42
• The results show that H-W Equilibrium was not maintained.
– The migration of swimmers (genes) into the pool
(population) resulted in a change in the population's gene
frequencies.
– If the migration were to stop and the other agents of
evolution (i.e., mutation, natural selection and nonrandom mating) did not occur, then the population would
maintain the new gene frequencies generation after
generation.
– It is important to note that a fifth factor affecting gene
frequencies is population size.
• The larger a population is, the number of changes that occur by
chance alone becomes insignificant.
• In the previous analogy, a small population was used to simplify
the explanation.
How much variation does a large
population of sexually reproducing
organisms have in their gene pool?
•Typically about 0.5% of the
DNA bases are variable
• In fruit flies there are 165
million base pairs so ~1
million nucleotides sites differ.
•In humans with 3 billion base
pairs there are about 15
million different/variable
nucleotides).
Mutation Rate
• Gene mutations result in new alleles, and are
the source of variation within populations.
• Due to DNA replication and DNA repair
mechanisms, mutation rates of individual
genes are low, but many genes + many
individuals= new mutations all the time.
Mutations and Peppered Moths
In 1850, f(L)= .95 and f(l) = .05
(light colored moths on light
colored trunks were
camouflaged)
In 1900, f(L) = .05 and f(l) = .95
50 generations later (light lichen
was killed by pollution, now the
dark tree trunk was exposed so
light colored moths became easy
prey for birds).
Positive, Negative, or Neutral:
Impacts of mutations
• Mutations are random and unpredictable
• Some are lethal and kill individuals before
they are born
• Some are harmful but masked by a
dominant allele (each of us carry 7-8 lethal
recessive genes)
• Some are neutral and have little to no affect
on an organisms long term survival.
• Some are beneficial and help the organism
survive to reproductive age and reproduce
Gene Flow
• the exchange of alleles
– Alleles move through populations by interbreeding, as
well as by migration of breeding individuals.
– Gene flow can increase variation within a population by
introducing new alleles produced in another population
Genetic Drift
• random changes in the gene pool
– Genetic drift causes gene pools of two isolated
populations to become dissimilar as some alleles are
lost and other are fixed
– This causes a reduction in the frequency of the
heterozygotes over time.
– The smaller the population size the faster the effect is
seen
Why does it affect small
populations more?
• In a small population, allele
frequencies are likely to be atypical
just by chance.
• If you were to toss a coin 1000 times
and get heads 750 times you would
be very surprised.
• If you tossed a coin 4 times and got 3
heads, you would not be so surprised!
• Chance has a greater influence upon
gene frequencies in a small
population
When is genetic drift likely to occur in
nature?
• Most populations are large enough that
genetic drift does not occur
• Some populations crash in numbers due to
natural disasters or over-harvesting by
humans(“bottleneck effect”- examples will
be provided later)
• Another example is when a new habitat (e.g.
an island) is colonized by just a few
dispersing individuals (“founder effect”)
• The suddenly small population may, purely by
chance, contain a different frequency of
different genotypes that the original large
population.
More on the Founder Effect
• Genetic drift can occur when a small number
of individuals from a large population
emigrate to a new area.
•The small number
of emigrants are
likely to have an
genetic structure
that differs, purely
by chance, from the
main population
An Example of Founder’s Effect:
•In the 1700’s 200 German Amish immigrated to
Pennsylvania to start a community.
•These people carried a high concentration of a mutation
which causes a form of polydactylism.
•Individuals in this group tended to marry within so
there's a greater likelihood that the recessive genes of the
founders will come together in the cells that produce
offspring.
Inbreeding
Breeding between close relatives.
This causes the gradual increase in
homozygosity.
Example:
In most species, related individuals share about 80% of
the same genes.
With cheetahs, this figure rises to approximately 99%.
The genetic inbreeding in cheetahs has led to low
survivorship (a large number of animals dying), poor
sperm quality, and greater susceptibility to disease.
More on the Bottleneck Effect
• From an original large population, only few survive
to repopulate the habitat.
• The few survivors are likely to have a genetic
structure (a set of genotype frequencies) that is
unrepresentative of the ancestral large population
Example of a genetic
bottleneck
Northern elephant seals have reduced genetic
variation probably because of a population bottleneck
humans inflicted on them in the 1890s.
Hunting reduced their population size to as few as 20
individuals at the end of the 19th century.
Their population has since rebounded to over
30,000—but their genes still carry the marks of this
bottleneck: they have much less genetic variation
than a population of southern elephant seals that
was not so intensely hunted
Comparing the 2
Effects:
Bottleneck Effect is
sampling error as a result
of only a few individuals
surviving a population
crash.
Founder Effect is
sampling error as a result
of movement of a few
individuals away from the
main population
What’s inbreeding
depression?
The fertility and survival are
reduced compared with
populations that are not inbred.
Caused by an increase in
homogeneity.
Outbreeding
enhancement/hybrid
vigor is when they manage a
species and bring back
heterogeneity.
In the adder case, introducing
snakes from other populations
Ex.
a population of 40 adders
experienced inbreeding
depression when farming
activities in Sweden isolated them
from other adder populations.
Higher proportions of stillborn and
deformed offspring were born in
the isolated population than in the
larger populations
Artificial Selection
• Much of what Darwin
learned about Natural
Selection, came from
his observations about
selectively breeding
crops and domestic
animals.
• This showed that
continued selection
was powerful enough
to bring about largescale changes within a
species.
A change in plant or
animal population by
selective breeding
Fitness
• The suitability of an organism
to a given environment
• This is often measured by the
number of offspring that an
individual has
• However, the offspring must
survive to contribute to the
following generation
•Consider two zebras:
• Both live in the savannah and must escape predation
by lions while also finding food & water. As babies,
they were protected by the herd
•Zebra 1- has a deformed rear leg
•Zebra 2- is fast and strong
• Soon after they both became
juveniles, a lion pride
attacked the heard.
– Zebra one, not being able to
run fast, was caught by the
lion pride.
– Zebra 2 escaped this, and
many more attacks. Lived to
have many offspring who
were also fast and strong
runners.
– So Zebra 2 was more fit than
Zebra 1, by surviving.
Adaptations that affect Fitness
• Camouflage– color is not necessarily relevant
– Pattern and shades matter
– Pattern breaks up an organism’s
outline
• Protective Coloration– Color of animal blends in with
environment
– However, this can Allows the
organism’s shadow to reveal
the outline
Camouflage and Protective Coloration: A
Model of Natural Selection
• Natural selection operates on the principle of “survival
of the fittest”.
• Fitness can be defined as the suitability of an organism
to a given environment.
• One might ask if one set of features favorable in one
environment might prove unfavorable in another
environment.
• In this lab you will test the following hypothesis:
– If survival is related to specific characteristics in a given
environment, then altering the environment will decrease the
survival rate.
• Once you have formed a conclusion to this hypothesis
and how it relates to the adaptations of camouflage and
protective coloration, you will apply this information to
the peppered moths of Manchester, England. You will
predict the direction of an adaptive shift and the
resulting gene frequencies.
Chapter 17
Origins of Life
Origins of Life
• Astrobiology is study of the
origin, evolution, distribution, &
destiny of life in the universe.
• It attempts to answer 3
fundamental questions:
– How did (does) life begin and
develop?
– Does life exist elsewhere in
the universe?
– What is life's future on Earth
and beyond?
Advances in the biological sciences, space exploration, and astronomy
make it possible for us to realistically attempt to answer them.
• We know that free oxygen probably was not present
in the early atmosphere.
– Compounds in early rocks do not contain oxygen……
significant amounts of oxygen would not have formed until
the process of photosynthesis was under way.
• Some of the ideas about the origin of life cannot be
stated in the form of hypotheses or tests;
– therefore, science is limited in it’s ability to investigate such
ideas.
• RNA is a more likely candidate for life formation that
DNA because DNA is more complex than RNA and
requires proteins for its replication.
– RNA can cataylze its own splicing.
• The conditions on early Earth were probably similar to
those on Venus and Mars,
– that is, containing high levels of CO2, water vapor, and N2 and small
amounts of H2.
• The formation of organic compounds necessary for life is
better understood than the formation of the first cells.
• Chemical evolution provides a mechanism by which life
could have originated from the development of increasingly
more complex chemicals in the primitive seas.
– Darwinian evolution provides a means by which organisms can
change over time and could not start until chemical evolution
resulted in the first life.
• We know that one characteristic of living things is their
need for energy.
– Early energy sources included lightning, UV radiation, and heat from
the radioactive decay within earth.
– These energy sources drove the chemical reactions that produced
the first life.
– Those organisms most efficient at using the energy had an
advantage over less efficient organisms.
Heterotroph Hypothesis
• In the 1920’s, Oparin and Haldane
– hypothesized that the early atmosphere had
methane, ammonia, hydrogen, and water
vapor in it.
– An energy source (radioactivity, lightening,
cosmic radiation, and/or heat from
volcanoes) reacted with those gases and
formed organic compounds.
• This hypothesis requires 3 major steps:
– Nonbiolobical processes had to supply some
organic molecules.
– Some process had to trigger the small
molecules to form polymers like nucleic
acids and proteins
– Some other processes had to organize the
polymers that could replicate itself.
Urey & Miller’s Experiment- 1952
• Can organic compounds be generated under conditions
similar to those that existed on primeval earth?
Conclusions:
The organic building
blocks of life (amino
acids and all the
building blocks for
nucleic acids)
formed in the liquid.
Early Organisms
• Because the early atmosphere lacked oxygen gasthe first organisms must have been anaerobic
(probably methanogens)
• There is some evidence of photoautotrophs dating
back to 4 billion years ago.
• The level of oxygen gas was probably insignificant
until about 2 billion years ago.
• Aerobic respiration may have evolved as a way to
produce large amounts of ATP from food sources.
Endosymbiotic Hypothesis
• The first microfossils of
eukaryotes are about 1.4
billion years old.
• This hypothesis proposes
that eukaryotes originated
from a symbiotic
relationship between large
anaerobic prokaryotes and
smaller aerobic or
photosynthetic prokaryotes
• The question of whether life was
created by a supernatural being cannot
be investigated by the methods of
science because no controlled
experiments can be set up and run.
Chapter 18
Diversity and Variation
Natural Selection:
• Not all members of a population
necessarily have an equal chance of
surviving and reproducing.
• Some individuals are better adapted
to their environment than are
others.
• The better adapted individuals are
more "fit" and tend to survive and
reproduce, passing on their
adaptations to the next generation in
greater frequency than those
adaptations of the less "fit" members
of the population
How does natural selection shape a population?
• First, let’s make sure we understand variation
and bell shaped curves:
– In a bell curve, what does the y-axis
represent?
• The number of individuals
– In a bell curve, what does the x-axis
represent?
• the value of a particular characteristic–
could be size, color, etc.
– What does it mean when a variable exhibits
continuous variation?
• It has a wide range over which it varies,
like human height
Directional Selection
• tends to favor
phenotypes at
one extreme of
the range of
variation.
The graph
shows the
decrease in size
of pink salmon
caught in two
rivers in British
Columbia since
1950, driven by
selective fishing
for the large
individuals.
Stabilizing Selection:
• Organisms with extreme characteristics die or fail to
reproduce, resulting in populations of individuals that
possess intermediate characteristics.
•Infants weighing significantly less or more than 7.5
pounds have higher rates of infant mortality. Selection
works against both extremes.
Disruptive Selection
• favors individuals at
both extremes of
variation: selection is
against the middle of
the curve
What’s taxonomy?
The theories and techniques of describing, grouping
and naming organisms
•Biologists do not think of species simply as a long alphabetical list.
•Since Linnaeus, the father of modern taxonomy, species have been
arranged in a taxonomic hierarchy:
•Species are grouped in genera.
•The gray wolf species Canis lupus and the golden
jackal Canis aureus , for example, are grouped in the
genus Canis .
•Genera are grouped into families;
•the genus containing dogs and wolves combines
with several other genera, such as the fox genus
Vulpes , to make up the family Canidae.
•Several families combine to make up an Order
• Carnivora, in this example.
•Orders make a Class (Mammalia).
• Classes make a Phylum (Chordata).
• Phyla make up one of the five Kingdoms (Animalia).
Why do we classify organisms?
•In order to make sense of the diversity of
organisms, similar organisms together and
it is necessary to group organize these
groups in a non-overlapping hierarchical
arrangement.
•Morphological, physiological, metabolic,
ecological, genetic, and molecular
characteristics are all useful in taxonomy
because they reflect the organization and
activity of the genome.
•Nucleic acid structure is probably the best
indicator of relatedness because nucleic
acids are either the genetic material itself
or the products of gene transcription.
Why else is classification
important?
Practical and scientific reason—
•identifying wild relatives of crops–
that provide breeders with useful
alleles
•Benefits to public health--disease
control
•Identifying unknown species that
might help understand ecosystems
•Understanding the “unity of life”
"Of all the categories in this classification system, only the lowest level,
the species, is objective; that is, biologists can test whether two different
populations of individuals represent two different species by testing
whether they can interbreed successfully in nature. (p. 434, Postlethwait &
Hopson, 1995)
What’s a Species?
The basic taxonomic group is the species
All individuals and populations of a particular
type of organism that can interbreed with one
another.
Species may look different, but as long as they
can produce FERTILE offspring they are still
members of the same species.
The Species Concept
Species are interbreeding natural populations which
are reproductively isolated from other such groups.
Species remain separate from one
another in three basic ways:
• potential mates do not meet;
•potential mates meet but do not breed
•potential mates meet and breed but do
not produce fertile or viable offspring.
Appearance Isn’t Everything
•Organisms may appear to be alike and be different species.
•For example, Western meadowlarks (Sturnella neglecta) and
Eastern meadowlarks (Sturnella magna) look almost identical
to one another, yet do not interbreed with each other
•Thus, they are separate species according to this definition.
Evidence Used to Classify
Organisms
•
•
•
•
morphology
anatomy /development
the fossil record
molecular data
Placing Organisms in Groups
Ex. All animals with hair
who give birth to live young
are classified as mammals
Using Homologous Structures
Similarities of biological structures that results
from evolution form a common ancestor.
Forearm of frogs, lizards, birds, humans, cats, etc.
Homologous structures are are also those that develop
from similar embryological origins
Adult fish, salamanders,
turtles, chickens, rabbits, and
humans bear virtually no
resemblance to one another.
Yet these animals are virtually
indistinguishable as embryos.
Can you pick out which one is
the human?
How about now?
Which one is the human
embryo?
Why should animals that have markedly different adult
forms and function develop from such similar embryos?
Far back in vertebrate
history, they all had a
common ancestor,
probably some type
of primitive fish, that
developed from a
similar type of
embryo.
As the various types of vertebrates evolved, they each retained
this basic vertebrate embryo as part of their life cycle– even
though its parts gave rise to different adult organs.
Analogous Structures
Similarities in form or function that are not a result of
evolution from a common ancestor, but is evidence of
convergent evolution (similar environmental pressures).
Ex. wings of
insect, bird, bat
and pterosaur
Analogous vs.
Homologous Structures
Chemical homologies
•Since characteristics of an organism are determined by its genetic
content, changes in organism over the course of evolution are reflected
in changes in the nucleotide sequence.
•The longer the period since two species have diverged from a common
ancestor, the greater the number of substitutions that are found in
corresponding genes and proteins between the species.
Ex. The sequence of amino acids in a protein molecule or of nucleotides
in DNA.
The Linnean classification system
The system uses
homologies to
group species into
larger and more
general categories.
Kingdom, phylum,
class, order,
family, genus,
species
Binomial nomenclature?
The scientific naming of
species whereby each
species receives a Latin or
Latinized name of two parts
• the first indicating the
genus and the second
being the specific epithet.
•For example, Juglans regia
is the English walnut;
Juglans nigra, the black
walnut.
How do you write a scientific name?
•Capitalize genus
•underline/italicize both names
•lower case species name
Ex.
American Black Bear- Ursus americanus
Brown Bear- Ursus arctos
•Subspecies Syrian (Brown) Bear (Ursus arctos syriacus)
•Subspecies Grizzly Bear, (Ursus arctos horribilis)
•Subspecies Kodiak Bear, (Ursus arctos middendorffi)
Ways to Classify Organisms
Current classification
systems attempt to reflect
the evolutionary
relationships of organisms
Orthodox• Originally, organisms classified using the Linnaean
system were grouped together based simply on physical
similarities
•Ex. Dividing birds into orders based on their
beaks and claws
• This method can be subjective
Phenetic principle of classification
•groups species according to their observable phenetic
attributes
• if two species look more like each other than either does
to any other species, they will be grouped together.
•Ex. a wolf and a dog (same genus) look phenetically more
alike than do a wolf and a dolphin which are in the same
Class (mammalia).
•NOTE: Nothing needs to be known about evolution in order for species to be
classified phenetically.
Phylogenetic principle of classification
•Only entities that have evolutionary
relations can be classified in this way
• It classifies species according to how
recently they share a common ancestor.
•Two species that share a more
recent common ancestor will be put
in a group at a lower level than two
species sharing a more distant
common ancestor
For instance the wolf and the dog opposite share a more recent common
ancestor than do a wolf and a dolphin: they are therefore both classified in
the genus Canis.
The phylogenetic principle is the basis behind the taxonomic school called
cladistics.
Cladistics
•Assumes each
group has an
ancestor that other
species do not
share
•Group species
according to
ancestry and
homologous
characteristics.
Ex. All mammal species have mammary glands, but no other organism do.
•Therefore all mammals must be descended from a species that has no other
living descendants.
•Mammals form a clade, or branch on this diagram of the history of the
animals.
•One way to discover how groups of
organisms are related to each other
(phylogeny) is to compare the
anatomical structures of many
different organisms.
•Recall, corresponding organs and
other body parts that are alike in
basic structure and origin are said to
be homologous structures
•for example, the front legs of a
horse, wings of a bird, flippers of
a whale, and the arms of a
person are all homologous to
each other.
•When different organisms share a
large number of homologous
structures, it is considered strong
evidence that they are related to
each other.
Making a Cladogram
Step 1: determine which of the
characteristics each animal has.
Step 2: Draw a cladogram to
illustrate the ancestry of these
animals. The diagram should
reflect shared characteristics as
time proceeds.
SETS
#1
Backbone
TRAITS
Tuna
x
#2
Placenta
#3
Foramen magnum fwd. (large
opening at base of skull)
TOTALS of X’s
Horse
Human
x
x
x
x
x
1
2
3
Notice how the
different animals
are all at the
same time level
(across the top)
since they all live
today.
Practice making a cladogram
Vestigial Structures
•Why do whales have underdeveloped pelvis and leg bones?
•What’s up with the diminished toe bones in horses?
•What about our appendix? Wisdom teeth? Tail in embryos?
These are remains of ancestral structures or organs
which were, at one time, useful.
Chapter 19
Changes in Species
Speciation
Q: When are two populations new species?
A: When populations no longer interbreed they
are thought to be separate species.
•As natural selection proceeds, populations
occupying different environments will diverge
into races, subspecies, and finally separate
species.
•Barriers to gene flow between populations
isolate those populations, ultimately leading
to the formation of new and separate species
Reproductive Isolating Mechanisms
•Pre-zygotic Isolating Mechanisms-
these act before fertilization can happen
-Ecological:
-Different habitats
»Example Lions and Tigers
–Geographical:
–Mountains, islands, or rivers separate
»Chipmunks in the Grand Canyon
–Seasonal:
–Different breeding season
»Blue whales in northern and southern hemisphere
–Mechanical:
–Parts don’t fit
»bushbabies
–Behavioral:
–Different mating behavior
»crickets
Post-zygotic Isolating Mechanisms•these happen, even if fertilization happens
Gametic incompatibility
Sperm transfer takes place,
but egg is not fertilized.
Zygotic mortality
Egg is fertilized, but zygote does not develop.
Hybrid inviability
Hybrid embryo forms, but of reduced viability.
Hybrid sterility
Hybrid is viable, but resulting adult is sterile.
Hybrid breakdown
First generation (F1) hybrids are viable and fertile, but
further hybrid generations (F2 and backcrosses) may be
inviable or sterile.
Paths of Speciation:
• Allopatric Speciation
– Populations begin to
diverge when gene flow
between them is
restricted.
– Geographic isolation
is often the first step in
allopatric speciation.
Parapatric Speciation• The splitting of a population into 2 species under
conditions where members of each population reside
in adjacent areas
Sympatric Speciation
animation
• No Geographic isolation
• Refers to the formation of two or more descendant
species from a single ancestral species all occupying the
same geographic location.
• Most common in Plants- failure to reduce chromosome
number results in polyploid plants that reproduce
successfully only with other polyploids.
• Some scientists don't believe that it ever occurs-- they feel
that interbreeding would soon eliminate any genetic
differences that might appear.
Hybridization
• Two species mate and make a hybrid
that cannot mate with either parent.
Patterns of Evolution:
• Divergent Evolution- two species evolve from a
common ancestor and become different over time.
• Convergent Evolution- species with different
ancestors colonize similar habitats with a
superficial resemblance
Comparing Patterns of
Parallel
Evolution:
evolution occurs
when two species
evolve
independently of
each other, but
maintain the
same level of
similarity. Parallel
evolution usually
occurs between
unrelated species
that do not
occupy the same
or similar niches
in a given habitat.
Convergent evolution takes place when species of different
ancestry begin to share analogous traits because of a shared
environment or other selection pressure
When people
hear the word
"evolution," they
usually think of
divergent
evolution, the
pattern where
two species
gradually
become
increasingly
different.
• Adaptive Radiation- many species evolve from a
single ancestral line- this is an example of a divergent
evolutionary pattern
Extinction
Extinction occurs when
the population cannot
adapt to changing
environmental
conditions.
The golden toad of Costa
Rica’s Monteverde cloud
forest became extinct
because of climate
change.
video
Pace of Evolution
• Gradualism– Very gradually, over a long time, a population changes.
– Over a short period of time it is hard to notice.
– Change is slow, constant, and consistent.
– The steady uninterrupted process has small adaptive steps
– Darwin’s view
• Punctuated Equilibrium• change comes in spurts. There
is a period of very little change,
and then one or a few huge
changes occur
• Often due to major changes in
the environment.
• Gaps in the fossil record
Coevolution
• a change in the genetic composition of one species (or
group) in response to a genetic change in another.
– a.k.a. “reciprocal evolutionary change” in
interacting species
Ex.
Central American Acacia species have hollow
thorns and pores at the bases of their leaves
that secrete nectar.
These hollow thorns are the exclusive nest-site
of some species of ant that drink the nectar.
But the ants are not just taking advantage of
the plant—they also defend their acacia plant
against herbivores.
Pollination
• For pollination to be effective, a relationship must be established
between the pollinator and the blossom to be pollinated, involving:
– The pollinator should visit this particular blossom regularly and get a
reward such as nectar for pollinating the plant.
– The plant must supply:
• Some kind of reward (food?) for the pollinator (nectar, pollen)
• Some kind of attractant to advertise the presence of the reward
This could be a direct attractant such as odor, color, shape, or texture
• A means of putting pollen onto the pollinator such that it is effectively
transferred to the next flower visited.
Examples
•The common snapdragons that many people plant
in their gardens are designed for a bumblebee of just
the right weight to trip the opening mechanism.
•Moth-pollinated plants often have spurs or tubes
the exact length of a certain moth’s “tongue.”
The Arms Race
Predator/prey coevolution can lead to an evolutionary
arms race.
• Murex snails, have evolved thick shells and spines to
avoid being eaten by animals such as crabs and fish.
• These predators have, in turn, evolved powerful
claws and jaws that compensate for the snails’ thick
shells and spines.
Evolution of Antibiotic Resistance:
• Forty or fifty years ago, thanks to antibiotics, scientists
thought medicine had all but eradicated infectious
agents as a major health threat.
• More recently, an upsurge of infectious disease is a
problem we have unwittingly created for ourselves b/c:
– rapid, frequent, and relatively cheap international travel allows
diseases to leap from continent to continent
– Many people have inadequate sanitation and lack of clean
drinking water
– We have overused the "miracle drugs“ to treat such diseases to
the point that they lose their potency
• Whenever antibiotics wage war on microorganisms, a few of
the enemy are able to survive the drug.
• Because microbes are always mutating, some random
mutation eventually will protect against the drug.
• Antibiotics used only when needed and as directed usually
overwhelm the bugs.
• Too much antibiotic use selects for more resistant
mutants.
• When patients cut short the full course of drugs, the resistant
strains have a chance to multiply and spread.
Animals and Antibiotic Resistance
• As much as 70% of the
antibiotics produced in the
United States today are for
use in food animals
• In addition to treating
disease, a lot of these drugs
are given to healthy animals
to prevent illness and to
promote growth.
– Small farms are giving way to
huge consolidated feedlots that
facilitate the rapid spread of
bacteria and disease. Feedlots
housing up to 100,000 cows*
•The economic equation. Livestock breeders say that reducing the
use of antibiotics in animals will significantly impact the cost of meat,
though environmentalists and others say the cost of antibiotic resistance
would be far greater.
* In 1945, the typical henhouse contained 500 birds. Today, the average ranges from 80,000 to 175,000. In 1980, there were
650,000 hog farms in the United States. In 2001, there were 81,000, including Circle Four Farms in Milford, Utah, a mega-facility
housing 500,000 hogs in a 35-square-mile area, according to the U.S. Department of Agriculture
The End
Richard Owen
• Born in England
• Trained as a surgeon
• Catalogued 13,000 human and animal anatomical
specimens
• Became a lecturer in comparative anatomy
• Worked with Darwin to catalogue the fossils he returned
with from the S.S. Beagle
– However, he became outspokenly oppositional towards Darwin’s
theory of Natural Selection
• He believed there must exist a common structural plan for
all vertebrates – but this was the idea of a Divide mind, not
an archetype
Thomas Henry Huxley
•
•
•
•
Born in England
Very little formal schooling
Navy officer turned surgical apprentice
Darwin’s Bulldog
– A passionate defender of Darwin’s theory of Natural Selection
– During the debate with Archbishop Wilberforce who ridiculed
evolution he was asked whether he was descended from an ape on
his grandmother's side or his grandfather's
– He responded by saying, "I would rather be the offspring of two
apes than be a man and afraid to face the truth."
• He also wrote essays on theology and philosophy from an
"agnostic" viewpoint