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Ch. 17
Part 4
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Darwin-Wallace Theory of Evolution
1856 Charles Darwin and Alfred Wallace
Natural Selection believed to be mechanism by which evolution occurred
No knowledge of genetics, alleles, or inheritance
Theory based on observations and deductions
Observation
Organisms produce more offspring than are needed to
replace parents
• Deduction
• There is competition for survival “struggle” for
existence
Natural Populations tend to remain stable in size over
long periods of time
There is variation among the individuals of a given
species
• Best adapted variants will be selected for by natural
conditions operating at the time (natural selection)
• Best variants have selective advantage
• “Survival of Fittest”
Natural Selection can act on variation within a population to bring about
changes in allele frequencies
Species and Speciation
• Species:
• A group of organisms with similar
morphological, physiological,
biochemical, and behavioral features
• Can interbreed to produce FERTILE
offspring
• Reproductively isolated from other
species
• Morphological features structural features
• Physiological features  features regarding
how the body works
• Biochemical features  include sequences of
bases in DNA molecules AND sequences of
amino acids
• Speciation is getting one
group of species to split into
new, different groups
• This large group MUST
become separated or
ISOLATED in some way so
they can NO longer
interbreed…
How to Determine if organisms are the same or different species….
• Species need to be tested to find out if they can interbreed successfully &
produce fertile offspring
• Problems:
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Organisms dead (fossils/museum animals)
Same sex
Little access to facilities to test
No time to test
Organisms will not breed in captivity
Reproduce asexually
Immature (not at reproductive age)
Key determination if two organisms belong to the same species or not:
inability to interbreed with each other successfully….in order to for one group
of organisms to produce another group of organisms that cannot interbreed
successfully, the two groups must under go REPRODUCTIVE ISOLATION….
How to MAINTAIN Reproductive Isolation
• Isolating Mechanisms
• Review: What is a ZYGOTE???
• Pre-zygotic isolating mechanisms (before a zygote is
even made)
• Ways to PREVENT fertilization
1.
2.
3.
4.
Behavioral isolation
Mechanical isolation
Gametic isolation
Incompatibility of pollen and stigma in plants
• Post-zygotic isolating mechanisms (after zygote is
made)
• Ways to prevent the formation of FERTILE offspring
5.
6.
7.
Hybrid inviability
Hybrid sterility
Hybrid breakdown
Pre-zygotic isolating mechanisms (before a
zygote is even made)
1.
Behavioral isolation
2.
Mechanical isolation
3.
Gametic isolation
4.
Incompatibility of pollen and stigma with plants
• Species A doesn’t recognize species B’s mating ritual
• Species and mates/flowers at different time or season than
species B (temporal isolation)
• Reproductive parts are incompatible
• Animals physically unable to mate
• Male gametes do not survive in the environment of the female
gamete
• Female gametes do not recognize male gametes
• Gametes cannot fuse with each other
5.
6.
7.
Post-zygotic isolating mechanisms
(after zygote is made)
Hybrid inviability
• Failure of cell division in zygote
• Zygote fails to develop properly
Non-viable offspring
• Offspring dies soon after birth
Hybrid sterility and breakdown
• Hybrid becomes functional adult but they are reproductively sterile
• Eggs or sperm are non-existent or dysfunctional
• Viable but sterile offspring
• Hybrids produced actually DO produce offspring but their offspring have reduced viability or fertility
• Species: group of individuals capable of
interbreeding and producing viable offspring
• Speciation: formation of a new species
2 MAJOR processes of speciation:
1.Allopatric Speciation
2.Sympatric Speciation
Allopatric speciation
• speciation occurred in different
regions
• Key: geographical separation
• Geographical isolation
Sympatric speciation
• one population of one species became two
species while in the same geographic
region with no physical separation
• (temporal isolation, reproductive
isolation, behavioral isolation)
1. Allopatric Speciation
• Geographical Isolation
• Geographic barrier prevents
interbreeding between members
of the population
• Barrier leads to reproductive
isolation
• Allele frequencies change (due to
natural selection, genetic drift, or
mutation
• If gene pool changes too much,
interbreeding between
populations will no longer be
possible= NEW SPECIES
Allopatric =
different places
• Selection pressures in one area very different from
original area
• Results in different alleles being selected for
• Over time morphological, physiological, behavioral
features in new area population become so different
from original population
• Two populations can no longer mate
•
new species has evolved
2. Sympatric Speciation
• New species formed WITHOUT geographic barrier
• Due to:
• Reproductive isolation
• Behavioral isolation
• Temporal isolation
• How Sympatric Speciation Can Occur:
• Polyploidy
• Polyploid organism: organism that contains MORE than 2 complete sets of
chromosomes in its cells
• Cause:
• Meiosis gone wrong
• Results:
• One or more gametes end up with 2 sets of chromosomes instead of 1
• If fertilization occurs between two of these gametes: 2N + 2N =
4 N  TETRAPLOID
• If fertilization occurs with another normal gamete, you get 1N +
2N = 3N  TRIPLOID
Some Common Polyploids….
Tetraploids
• When two messed up gametes fuse
• instead of 1N + 1N = 2N…you get 2N + 2N = 4N
• STERILE (most of the time)
• Cannot produce gametes
• 4 of each chromosome
• All 4 try and pair up during meiosis 1 = big mess
• Difficult for cell to divide and produce gametes with equal number of
chromosomes
• Tetraploid can grow and develop into multicellular organism
that reproduces ASEXUALLY
• Uses mitosis to grow and develop and reproduce
• No pairing up of chromosomes required in mitosis (no tetrads)
• Occurs often in plants (NOT animals b/c animals cannot reproduce asexually)
Tetraploid Plants
• Some manage to produce gametes
• Diploid gametes
• (NOT haploid 4N  2N)
• If diploid gamete 2N fuses with a
normal plant gamete 1N you will get
an organism that is 3N (TRIPLOID)
• Grows and develops normally through
mitosis
• Sterile cannot go through mitosis to
split chromosomes evenly between
daughter cells
• Original tetraploid plant and diploid
plant that produced this TRIPLOID
offspring cannot successfully
interbreed
• New triploid offspring is considered a
NEW SPEICIES in just one generation
Types of Tetraploids: Autopolyploid vs. Allopolyploid
Types of Tetraploids: Autopolyploid vs. Allopolyploid
• Autopolyploid
• Polyploid that
contains four sets of
chromosomes all
from the same
species
• Meiosis not likely to
occur b/c all 4
chromosomes try to
pair up with each
other
• If self-fertilization
occurs tetraploid
Allopolyploidy
• Polyploid that contains 2 sets of
chromosomes from one species
and 2 sets of chromosomes from
another species
• Meiosis can occur more easily in
these plants b/c chromosomes
are not identical
• 2 chromosomes from one species
pair up with 2 chromosomes from
the other species
• Meiosis can reach a successful
conclusion (producing gametes
that are 2N…still not N but they
can lead to new triploid species)
• Considered Fertile
• Cannot interbreed with
individuals with parent species (it
is a new species)
• Ex. Cord grass Spartina anglica
Speciation through Allopolyploidy: Spartina anglica
• Spartina anglica
• Vigorous grass in salt marshes
• Before 1830
• Spartina maritima
• 1829  S. alterniflora imported from America
• S. maritimia and S. alterniflora interbred  hybrid created S. townsendii
• Diploid (2N)
• One set of chromosomes from S. maritimia
• One set of chromsomes from S. alterniflora
• B/c of different sets, they CANNOT pair up during meiosis = unsuccessful meiosis
• S. townsendii CANNOT interbreed with its parent species = NEW SPECIES
• Sterile BUT can reproduce ASEXUALLY
• Produce rhizomes (long, underground stems that propagate new plants)
1892 MAJOR Event
• S. townsendii produced cells with DOUBLE the number of
chromosomes (due to faulty cell division)
• Fusion of 2 abnormal gametes of S. townsendii produce TETRAPLOID plant
that is an ALLOTETRAPLOID
• 2 sets of chromosomes from S. maritima
• 2 sets of chromosomes from S. alterniflora
• Chromosomes CAN pair up during meiosis = FERTILE plant
• S. anglica
• More vigorous than other 3 species
Comparing Amino Acids Between Species
• Reveals similarities between species
• Number of differences between the
amino acid sequence of a particular
protein between 2 different species gives
a measure of how closely related the
species are
• Cytochrome C protein in ETC
• Humans, rats, and mice
• All three molecules of cytochrome c had 104
aa
• Sequences of mouse and rat cytochrome c
proteins were identical
• 9 aa in human cytochrome c are different
from mice or rat sequence
• Most of substitutions in human cytochrome
c are of amino acids with the same R group
• substitutions change codon but still produce
same amino acid
• Mice and rats closely related…common
ancestor
• Humans distantly related, common ancestor
with mice and rats is LESS recent
Comparing Nucleotide Sequences of mitochondrial DNA
• Human mitochondrial DNA inherited through the
female line
• Zygote contains mitochondrial DNA from the OVUM
• Mitochondrial DNA (mtDNA)
• Circular DNA
• Does not undergo crossing over
• Changes to sequence of nucleotides in mtDNA can
only occur because of mutation
• Mutates faster than nuclear DNA
• mtDNA does not contain histones (which protect
nuclear DNA)
• Oxidative phosphorylation in mitochondria produce
forms of free radicals (oxygen) that act as mutagens to
mtDNA
Homo Sapiens and mtDNA
• Different human populations =
differences in mtDNA sequences
• Evidence for origin of H. sapiens in Africa
and later species
• All modern humans are descendants of
“Mitochondrial Eve”
• Woman who lived in Africa between 150 000 and
200 000 years ago
• Derived from molecular clock hypothesis:
• assumes constant rate of mutation over time
• Assumes the greater the number of
differences in the sequence of nucleotides,
the longer ago the individuals shared a
common ancestor
• Clock is calibrated by comparing nucleotide
sequences of species whose date of
speciation can be estimated from fossil
evidence
Anole lizards and mtDNA
• mtDNA analysis of anole lizards in
Caribbean and adjacent mainland
show ALLOPATRIC SPECIATION
occurred
• Each island species of lizard is found on
one island or a small group of islands
• Three species of Anole lizards are more
closely related to A. porcatus than to
each other
• A. brunneus
• A. smaragdinus
• A. carolinensis
• Suggests that species have EACH
originated from separate events in
which a few individuals of A. porcatus
spread from Cuba to three different
places
Extinctions
• When a species disappears from Earth forever
• International Union for Conservation of Nature
(IUCN)
• Annual Red List threatened species world wide
• 2013 21,286 species are threatened
• 2015 more than 22,000 species threatened CLICK
HERE
• Mass Extinctions
• When a huge number of species becomes extinct at
one time
• Many times natural  asteroid colliding with Earth
Human Impact on Extinction
• On the bring of another mass extinction
• Due to habitat destruction
• Species are adapted to specific habitats
with a range of environmental conditions
• Human activity destroying habitats:
• Climate change
• Competition
• Habitat Loss due to:
• Draining wetlands
• Deforestation (rainforests)
• Pollution (air, soil, water)
• Hunting (sport and/or food)
High vs. Low Profile Endangered Species
• High Profile Endanger Species
• Mammalian species
• Panda, rhinos, tigers
• Tigers in China
• Global population: 5000
• Human population in tiger habitats (China and India) add pressure to remaining population
• Tiger products sought after by humans hunting/poaching
• African Southern White Rhino
• Global population: 100
• Green plants
• Low Profile Endanger Species
• Invertebrates
• Kerry slug
• Unknown threatened species
• Protoctists
• Prokaryotes
Extinctions
• Africa
• Western Black Rhinos
• Northern White Rhino
• Extinctions due to:
• Lack of political support for conservation
• Increasing demand for rhino horn
• Internationally organized criminal groups
targeting rhinos
How to Address Extinctions
• Stop concentration on high-profile
species
• Too difficult or too costly to preserve
• Focus conservation efforts on
other species that will yield a
greater degree of success
• Plants and animals that CAN be
saved
• Focus on conservation of ENTIRE
ecosystems rather than single
species