Transcript Topic D (Evolution)

Option D
D1 Origins of Life on Earth
Pre-biotic Earth
• The Solar System originated 4.57 BYA
• The Earth originated 4.5 BYA
– Formed by collisions of materials over 100 MY,
creating a planet with oceans of liquid magma and a
hot dense atmosphere
– Cooling of the Earth took over 50 MY and the loss
of the dense atmosphere
Pre-biotic Earth
• Pre-Biotic Earth 4.4-4.0 BYA
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–
–
–
–
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Continents of solid rock forming
Oceans of water forming
High temperatures
High UV light levels
Reducing atmosphere (no O2)
Frequent storms with lightning
• Life on Earth originated 3.5-4.0 BYA
– Earliest organisms were bacteria
– Stromatolites- banded domes of sediment strikingly similar
to the layered mats constructed by colonies of bacteria and
cyanobacteria
Spontaneous Origin of Life
Topic D.1.1 Describe four processes needed for the
spontaneous origin of life on Earth.
• Chemical reactions to produce simple organic
molecules, such as amino acids, from inorganic
molecules, such as water, CO2, and ammonia.
• Assembling of these simple organic molecules
into polymers, for example, polypeptides from
amino acids
Spontaneous Origin of Life
• Formation of polymers that can self replicatethis allows inheritance of characteristics
• Development of membranes, to form spherical
droplets, with an internal chemistry different
from the surroundings, including polymers that
held genetic information.
Spontaneous Origin of Life
• The product of these 4 processes would have
been cell-like structures that natural selection
could have operated on.
Miller and Urey
Topic D.1.2. Outline the experiments of Miller and Urey
into the origin of organic compounds.
– In the 1920’s, A.I. Oparin of Russia and J.B.S. Haldane of
Great Britain independently postulated that conditions on the
primitive Earth favored chemical reactions that synthesized
organic compounds from inorganic precursors present in the
early atmosphere and seas. Oparin-Haldane Hypothesis
– It cannot happen in the modern world, because the present
atmosphere is rich in oxygen, the oxidizing atmosphere of
today is not conducive to the spontaneous synthesis of
complex molecules because the oxygen attacks chemical
bonds, extracting electrons.
Miller and Urey
• In 1953, Stanley Miller and Harold Urey tested the
Oparin-Haldane Hypothesis using the conditions of prebiotic Earth.
A. A warm flask of water simulated the primeval sea
B. The atmosphere consisted of H2O, H2, CH4, and NH3.
C. Sparks were discharged to mimic lightning
D. A condenser cooled the atmosphere, raining water and any dissolved
compounds back to the miniature sea.
E. As material circulated through the apparatus the solution in the flask
changed from clear to murky brown
F. After 1 week the contents of the flask were examined and found a
variety of organic compounds including some amino acids that make
up proteins of organisms.
What are their conclusions?
• Organic compounds (amino acids) were formed
from inorganic compounds.
• Organic compounds could have existed on prebiotic Earth.
• Life might have arisen from non-living material.
Panspermia
Topic D.1.3. State that comets may have delivered organic
compounds to Earth.
• Panspermia- the theory concerned with the arrival of
material from outer space.
• Hundreds of meteorites and comets hitting the early
Earth brought with them organic molecules formed by
abiotic reactions in outer space.
• Extraterrestrial organic compounds, including amino
acids, have been found in modern meteorites, and it
seems likely that these bodies could have seeded the
early Earth with organic compounds.
Origin of Life
4. Discuss possible locations where conditions
would have allowed the synthesis of organic
compounds.
• Miller and Urey’s experiment suggest organic
compounds could have synthesized in Pre-Biotic
Earth.
– Lightning, high temps, oceans forming, Reducing
atmosphere
Origin of Life
• There are hydrothermal vents deep in the
oceans, with chemicals welling up from the rocks
below.
• Around these vents, there are unusual chemical
conditions, which might have allowed the
spontaneous synthesis of the first organic
compounds.
Origin of Life
• Panspermia- Tests have shown that meteorites
do contain organic compounds and proto-cells.
• Pre-biotic earth was bombarded with
meteorites, comets and interplanetary dust,
which might have brought organic compounds
that became organized into the first living
organisms.
RNA
Topic D.1.5. Outline two properties of RNA that
would have allowed it to play a role in the origin
of life.
• RNA is thought to have served as the first genes,
not DNA.
• DNA  RNA Proteins: the mechanisms for
this is too complicated to have evolved all at
once.
• Genes cannot be replicated without enzymes, and
enzymes cannot be made without genes.
• The first genes were short strands of RNA that
began self-replicating in the prebiotic world.
RNA
• RNA also has been shown to act as an enzyme,
called ribozyme.
• RNA has catalytic properties
• RNA can catalyze the formation of more RNA
(rRNA, tRNA, and mRNA)
• RNA can bind amino acids and form peptide
linkages
• RNA can transcribe into DNA using reverse
transcriptase
Protobionts
Topic D.1.6 State that living cells may have been preceded
by protobionts, with an internal chemical environment
different from their surrounding.
• This is biochemical evolution.
• Coacervate droplets self-assembles when a solution of
polypeptides, nucleic acids, and polysaccharides is
shaken.
• Coacervates can contain polynucleotides (RNA)
• Assembly of chains of amino acids can form
• Formation of proteins
• Alignment of lipids and the formation of a membrane
• Synthesis of ATP and anaerobic respiration
• Asexual reproduction
Prokaryotes
Topic D.1.7 Outline the contribution of prokaryotes
to the creation of an oxygen rich atmosphere.
• The first organisms on earth were
photosynthetic prokaryotes.
• Oxygen is a waste product of photosynthesis.
• Oxygen concentrations build up over time
First Humans
Extinction of dinosaurs
Origin of reptiles
Plants colonize land
0
Cenozoic
Mesozoic
Paleozoic
500
Origin of multicellular organisms
(oldest animal fossils)
2500
3500
Oldest eukaryotic fossils
Precambrian
1500
Accumulation of atmospheric oxygen
from photosynthetic cyanobacteria
Oldest prokaryotic fossils
Origin of life?
Earth cool enough for crust to solidify
4500
Origin of Earth
Endosymbiotic Theory
Topic D.1.8 Discuss the endosymbiotic theory for the
origin of eukaryotes.
• Eukaryotic cells contain membrane bound organelles.
• According to the Endosymbiotic Theory proposed by
Lynn Margulis of the University of Massachusetts, both
the Mitochondria and Chloroplasts have evolved from
independent prokaryotic cells, which were taken into a
larger heterotrophic cell by endocytosis.
• Instead of being digested, the cells were kept alive and
continued to carry out aerobic respiration and
photosynthesis.
Endosymbiotic Theory
Endosymbiotic Theory
• The characteristics of mitochondria and chloroplasts that
support the Endosymbiotic Theory are:
• Similar in size to Bacteria
• They grow and divide like cells.
• They have a circular naked loop of DNA.
• They synthesize some of their own proteins using 70S
ribosomes.
• They have double membranes, as expected when cells are
taken into a vesicle by endocytosis.
• Reproduce by binary fission.
• Cristae are similar to mesosomes of prokaryotes.
• Thylakoids are similar to structures containing chlorophyll
in photosynthetic prokaryotes.
D2 Species and
Speciation
Allele Frequency and Gene Pool
Topic D.2.1 Define Allele Frequency and Gene
Pool.
• Allele Frequency- is the frequency of an allele,
as a proportion of all alleles of the gene in the
population.
• Allele frequency can range from 0.0 to 1.0
–
•
Usually expressed as a percentage or a proportion
Gene Pool- is all the genes in an interbreeding
population.
Evolution
Topic D.2.2 State that evolution involves a change
in allele frequency in a population’s gene pool
over a number of generations.
Species
Topic D.2.3 Discuss the definition of a species.
• What is a species?
• A species is a potentially interbreeding population
having a common gene pool.
• Typological species concept- species are static,
nonvariable assemblages of organisms that conform to
a common morphological plan
• Plato and Aristotle
–
•
Today we use it as the type specimen
Problems:
–
What are type characteristics?
Species
• Morphological species concept- Species are
distinguished from each other by their
morphological characteristics.
• Useful for fossils
• Problems:
– Sexual dimorphism
– Cryptic Species
– Geographic variation
Species
• Biological species concept- a group of actually or
potentially interbreeding populations, with a common
gene pool, which are reproductively, isolated from
other such groups. Ernst Mayr (1963)
• Problems:
– Sibling species- species that cannot interbreed, but show no
significant differences in appearance.
– Hybridization between different species
– Species that only reproduce asexually
– Fossils
Species
• Phylogenetic species concept- monophyletic and
genomically coherent clusters of individual organisms
that are descended from a single ancestral taxon and
show a high degree of overall similarity in many
independent characteristics, diagnosable by a
discriminative phenotypic property. A species is a
monophyletic group.
• Problems:
– DNA, Amino acids, or morphology
– Different techniques for analyzing sequence data
– Gene tree vs. Species tree
Gene Pools
Topic D.2.4 Describe three examples of barriers between gene
pools.
Topic D.2.6 Compare Allopatric and Sympatric Speciation.
• The formation of a new species is called speciation.
• New species are formed when a pre-existing species splits.
• This usually involves the isolation of a population of the
remainder of its species and thus the isolation of its gene pool.
• The isolated population will gradually diverge from the rest of
the species if natural selection acts differently on it.
• Eventually the isolated population will not be able to
interbreed with the rest of the species—it has become a new
species.
Speciation
• Allopatric speciation- species are isolated
geographically.
• Geographic Isolation- When members of a species
migrate to a new area, forming a population that is
geographically isolated from the rest of the population.
• Migration- members of a species move to a new
location that is geographically isolated from the original
territory.
• The Galapagos Finches
• Lava lizards in the Galapagos
• Adaptive radiation- the evolution of many diversely
adapted species from a common ancestor.
Speciation
• Sympatric speciation- species are not isolated
geographically. A subpopulation becomes
reproductively isolated in the midst of its parent
population.
Speciation
• Behavioral Isolation
– Example: Apple Maggot Fly (Rhagoletis pomonella)
– It originally laid its eggs on Hawthorn fruits, but some
individuals started to infest non-native apple trees as well.
The fruits ripen at different times, thus the adults emerge and
mate at different times.
– Now you have two separate breeding populations of the apple
maggot fly. There are differences in allele frequencies, but
they have not been classified as different species as of yet.
Speciation
• Hybrid Infertility- barriers between gene
pools- often due to polyploidy.
• Polyploidy- extra sets of chromosomes
• Plants may evolve into new species in one
generation by a polyploid event.
• Autopolyploid- More than one set of
chromosomes evolves from a single species.
• Allopolyploid- More than one set of
chromosomes evolves from different species.
Speciation
Topic D.2.5 Explain how polyploidy can
contribute to speciation.
• Rumex- most species have 20
chromosomes.
• Rumex obtusifolius has 40
• Rumex crispus has 60
• Rumex hydrolapathum has 200
Speciation
Topic D.1.7 Outline the process of adaptive
radiation.
• Adaptive Radiation- process in which many
related species evolve from one ancestor
– Example: Darwin’s Finches
– One finch or mating pair makes it to an island
– They have no predetors and unlimited resources.
– They are able to reproduce as much as possible
Speciation
• Variation exists and they can spread out to other
niches.
• They adapt to fill all the niches.
Speciation
8. Compare convergent and divergent evolution.
• Convergent Evolution- similar structures or form
but not closely related
– Shark and porpoise
• Divergent Evolution- 2 or more related species
that look different because of habitat
– All mammals have the same ancestor but look very
different.
Evolution in Process
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•
Homologous- similar feature from the same
ancestor- limbs
Analogous- similar feature because of
function but different ancestor- wing of bird
and insect
Pace of Evolution
Topic D.2.9 Discuss ideas on the pace of evolution
including gradualism and punctuated equilibrium.
– Gradualism is the slow change from one form to
another. Punctuated equilibrium, however, implies
long periods with no change and short periods of
rapid evolution. Mention could be made of the
effects of volcanic eruptions and meteor impacts in
affecting evolution on Earth.
Pace of Evolution
• Over long periods of time, many advantageous
alleles will appear and spread through a species.
These micro-evolutionary steps together
constitute macroevolution.
• Eventually the amount of change becomes so
great that the species is no longer the same and
one species have evolved into another.
• Gradualism- slow change from one form to
another. Evolution proceeds slowly, but over long
periods of time larger changes can gradually take
place. This does not fit with the fossil record.
Pace of Evolution
• The fossil record shows periods of periods of stability,
with fossils showing little change, followed by periods of
sudden major change.
• The periods of stability may be due to equilibrium where
living organisms become well adapted to their
environment so natural selection acts to maintain their
characteristics.
• The periods of sudden change that punctuated the
equilibrium may correspond with rapid environmental
change, caused for example by volcanic eruptions or
meteor impacts.
• New adaptations would be necessary to cope with new
environmental conditions, hence strong directional
selection and rapid evolution—Punctuated equilibrium.
Transient Polymorphism
Topic D.2.10 Describe one example of transient
polymorphism.
• Populations of ladybug that changed from
having red wings with black spots to black wings
are an example of transient polymorphism.
Adalia bipunctata
• Adalia bipunctata- Ladybeetle- 2 spotted small beetle,
which usually has 2 small black spots on its red wings.
• The red is a type of aposematic coloration. Melanic
forms also exist, with solid black wings.
• The melanic forms absorb heat more efficiently and
therefore have a selective advantage when sunlight
levels are low.
• The melanic form became more common in industrial
areas of Britain, but declined again after the 1960’s.
• If the air is dark with smoke the melanic form can warm
up faster, but if there is no advantage to the melanin,
then the Aposematic coloration is more of an advantage.
Balanced Polymorphism
Topic D.2.11 Describe Sickle Cell Anemia as an
example of Balanced Polymorphism.
• Sickle cell anemia is an example of balanced
polymorphism.
Hardy-Weinberg
• Heterozygotes (HbA HbS) do not develop sickle
cell anemia and are resistant to Malaria.
• The sickle cell allele has increased in frequency
to high levels in some areas. Parts of Africa, as
many as 40% of the population are carriers of
the sickle cell allele.
D3
Evidence for Evolution
Geographical Distribution
Topic D.3.1 Describe the evidence for evolution as
shown by the geographical distribution of living
organisms, including the distribution of
placental, marsupial, and monotreme mammals.
Wallace’s Line
There are huge
differences in
the types of
land animals
that are found
on either side of
Wallace’s Line.
Placentals vs. Marsupials vs. Monotremes
• Placental mammals are found on the Asian side
• Mainly marsupial (pouched mammals) and monotreme
(egg laying mammals- only three living monotreme
species - the platypus, the short-beaked echidna, and
the long-beaked echidna) mammals are found on the
Australasian side.
Placentals vs. Marsupials vs. Monotremes
• The land masses on the two sides of the
boundary separated about 100 MYA and came
together by continental drift about 15MYA. The
mammals on the separated landmasses followed
different evolutionary paths, so different types
evolved.
Australian vs. African Moles
• In similar habitats where natural selection acts in
the same way on different organisms, the results
are sometimes strikingly similar, for example the
marsupial mole of Australia and the golden mole of
Africa.
Armadillos
• Armadillos- only found in the Americas.
Contempory armadillos are modified descendants
of earlier species that occupied these continents
and the fossil record confirms this.
Fossils
Topic D.3.2 Outline how remains of past living organisms have
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been preserved. Include petrified remains, prints, and moulds
and preservation in amber, tar, peat and ice.
Sediments that turn to rock- accumulates both in the sea and
on land.
If hard parts of animals such as shells or bones form part of the
sediment, they will be preserved as a cast.
Minerals sometimes seep into the soft parts of an organism as
it decays and harden to form a petrified replica of the
organism.
The remains of past living organisms can be trapped and
preserved in various ways:
Fossils
• Resins-which turn to amber
Fossils
• Frozen in ice or snow- besides animals, plants can
be preserved this way---How?
Fossils
• Tar pits- form when crude oil seeps to the surface through
fissures in the Earth's crust; the light fraction of the oil
evaporates, leaving behind the heavy tar, or asphalt, in
sticky pools.
• Found wooly mammoths fossils, plants, mollusks, and
insects.
Fossils
• Petrified fossils- A cast is formed as minerals or
other sediments fill and harden within the
sedimentary cavity formed as the original organism
deteriorates.
Fossils
• Acid peat, which prevents decay- Peat- soil material
consisting of partially decomposed organic matter; found
in swamps and bogs in various parts of the temperate
zone.
• It is formed by the slow decay of successive layers of
aquatic and semi aquatic plants, e.g., sedges, reeds,
rushes, and mosses.
• One of the principal types of peat is moss peat is an
acidifying agent.
Half-Life
Topic D.3.1 4 Define Half-life
• The number of years it takes for half of an
original sample of radioactive material to decay
or undergo radioactive transformation.
• It is unaffected by temperature, pressure, and
other environmental variables.
Radioisotopes
3. Outline the method for dating rocks and fossils using
radioisotope, with reference to 14C and 40K.
(Knowledge of the degree of accuracy and the choice
of isotope to use is expected. Details of the apparatus
used are not required.)
• Radiometric dating involves the use of isotope series,
such as rubidium/strontium, thorium/lead,
potassium/argon, argon/argon, or uranium/lead, all of
which have very long half-lives, ranging from 0.7 to
48.6 billion years.
• Subtle differences in the relative proportions of the
two isotopes can give good dates for rocks of any age.
• Radiometric dating has an error factor of less than
10%.
14C and Uranium 238
• 14C has a half-life of 5730 years; it is best used
for young fossils (< 20,000 years old)
• Uranium-238 has a half-life of 4.5 BY and can be
used to date rocks hundreds or thousands of
millions years old.
Amino Acids
• Amino acids exist in two isomers with either lefthanded (L) or right-handed (D) symmetry.
• After an organism dies its population of L amino acids
is slowly converted into proteins, resulting in a mixture
of L and D amino acids.
• Knowing the rate at which this chemical conversion
occurs (Racemization) then allows us to calculate the
ratio of L:D and determine how long the fossil has been
dead.
• However, racemization is temperature sensitive.
40K
• 40K has a half-life of 1.25 BY and it decays to:
• 40Ca by emitting a beta particle with no attendant
gamma radiation (89% of the time)
• The gas 40Ar by electron capture with emission of
energetic gamma ray (11% of the time)
• Potassium-Argon dating is the only viable dating
technique for dating very old archaeological material
(550 million years to 3.8 American billion years)
Decay Curve
Topic D.3.5 Deduce the approximate age of
materials based on a simple decay curve for a
radioisotope.
Evolution
Topic D.3.6 Outline the palaeontological evidence for
evolution using one example.
• Fossil of Acanthostega- 365 MYA fossil. It has
similarities to other vertebrates (backbone and 4
limbs), but it has 8 fingers and 7 toes. It is not identical
to any existing organism.
• This suggests that vertebrates and other organisms
change over time. Acanthostega is an example of a
“missing link”.
• Although it has 4 legs like most amphibians, reptiles,
and mammals, it also has a fish-like tail and gills and
lived in water.
• This shows that land vertebrates could have evolved
from aquatic animals.
Acanthostega
DNA
Topic D.3.7 Explain the biochemical evidence provided by
the universality of DNA and protein structures for the
common ancestry of living organisms.
• There are remarkable similarities between living
organisms in their biochemistry.
• All use DNA (or RNA) as their genetic material
• All use the same universal genetic code, with only a few
insignificant variations
• All use the same 20 amino acids in their proteins
• All use left, and not right-handed amino acids.
• These similarities suggest that all organisms have
evolved from a common ancestor that had these
characteristics.
Phylogeny
Topic D.3.8 Explain how variations in specific
molecules can indicate phylogeny.
• Phylogeny- the evolutionary history of a group of
organisms.
• The phylogeny of many groups of organisms has
been studied by comparing the structure of a
protein or other biochemical that they contain,
usually DNA.
Phylogeny
• Ex. Amino acid sequence of the polypeptide of
hemoglobin has been compared in many
vertebrates.
• Differences accumulate gradually over long
periods of time and this is a roughly constant
rate: evolutionary (molecular) clock.
Phylogeny
Evolutionary Clock
Topic D.3.9 Discuss how biochemical variations can
be used as an evolutionary clock.
• Evolutionary (Molecular) clock- the rate at which
mutations accumulate in a given gene are at a
constant rate and therefore you can date how
long two organisms have been diverged.
Homologous structures
Topic D.3.10 Explain the evidence for evolution provided
by homologous anatomical structures, including
vertebrate embryos and the pentadactyl limb.
• Homologous anatomical structures are structures
derived from the same part of a common ancestor.
• There are also remarkable similarities between some
groups of organisms in their structure.
Homologous structures
• At an early stage, vertebrate embryos are very similar,
despite huge differences in the structure of the adults.
Homologous structures
• The limbs of vertebrates show striking similarities in
their bones, despite being used in many different ways.
The structure is called the pentadactyl limb.
• The most likely explanation for these structural
similarities is that the organisms have evolved from a
common ancestor.
Galapagos Finches
Topic D.3.11 Outline two modern examples of
observed evolution. One example must be the
changes to the size and shape of the beaks of
Galapagos finches.
Galapagos Finches
• Galapagos Finches- Daphne Major (one island inhabited by 2 species
of finches).
• Geospiza fortis has a short, wide beak and feeds on a variety of
seeds, including large hard ones.
• During 1982-1983, there was a severe El Niño, The available food
population increased and the population of G. fortis also increased,
reaching a peak in 1983.
• It dropped back in the drier years following and in 1987, was only
37% of its peak population from 1983. The period of heavy rain
changed the vegetation and until 1991 there were fewer plants
producing large, hard seed and more producing small soft ones.
• The diet of G. fortis changed. The population had longer, narrower
beaks than the average in 1983.
• The conclusion that this change in diet caused a change in beak
shape is supported by evidence from the other finch on the island.
G. scandens’ population rose and fell the same as G. fortis but
neither its diet nor the size of its beak changed.
D4-D6
D4
Human Evolution
Classification
1. State the full classification of human beings from
kingdom to sub-species.
• Domain
Eukarya
• Kingdom
Animalia
• Phylum
Chordata
• Class
Mammalia
• Order
Primate
• Family
Hominidae
• Genus
Homo
• Species
Homo sapiens
• Subspecies
Homo sapiens ssp. sapiens
Primates
2. Describe the major physical features, such as the
adaptations for tree life, that define humans as
primates.
• Grasping limbs, with long fingers and a separate
opposable thumb
• Mobile arms, with shoulder joints allowing
movement in three planes and the bones of the
shoulder girdle allowing weight to be transferred via
the arms
• Stereoscopic vision, with forward facing eyes on a
flattened face, giving overlapping fields of view
• Skull modified for upright posture
Anatomical and Biochemical Evidence
3. Discuss the anatomical and biochemical
evidence, which suggests that humans are a
bipedal and neotenous species of African ape
that spread to colonize new areas.
– Attention should be drawn to the main features
only. Neoteny in this case is in relation to the
delayed onset of puberty leading to the increased
period of parental care.
Bipedalism
• Bipedalism- walking on 2 legs
• Australopithecus afarensis shows partial
bipedalism
• Therefore this is an early development in human
evolution.
Adaptations for Bipedalism
• The foramen magnum, a hole in the skull
through which the spinal cord and brain
connect, moved forwards. This allows the head
to balance on the backbone.
• The arms became shorter and less powerful
• The legs became longer and stronger
• The knee changed to allow the leg to straighten
fully
• The foot became more rigid, with longer heel,
shorter toes and a non-opposable big toe.
Consequences of bipedalism
• Collecting food from bushes is easier
• Walking long distances while carrying food,
water, infants, tools, or weapons is easier.
• Tree climbing is more difficult
Neoteny
• Neoteny- keeping juvenile characteristics as an adult.
• Adult humans show similarities in appearance to baby
apes, with flat faces, large brain to body size ratio,
upright heads, and little body hair.
• This suggests that human evolution from an ape
ancestor might have involved a slowing down of
development, with a long childhood, delayed puberty
and retention of juvenile characteristics in adulthood.
Fossil trends
4. Outline the trends illustrated by the fossils of
Australopithecus including A. afarensis, A.
africanus, and A. robustus, and Homo including
H. habilis, H. erectus, H. neanderthalensis, and
H. sapiens.
• Many hominid fossils have been found, dated
and assigned to a species.
• These fossils show evolutionary trends,
including increasing adaptations to bipedalism
and increasing brain size.
Australopithecus afarensis
Australopithecus africanus
Australopithecus robustus
Homo habilis
Homo erectus
Homo neanderthalensis
Homo sapiens
Ecological Changes
5. Discuss the possible ecology of these species and
the ecological changes that may have
prompted their origin.
• 5 MYA Africa became drier and dense forests
were replaced by thinner woodland with
clearings.
• This could have prompted the evolution of
bipedalism, although early hominids probably
still lived partly in trees.
• Australopithecus had powerful jaws and teeth
indicating a mainly vegetarian diet.
Ecological Changes
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•
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2.5 MYA Africa became much cooler and drier.
Savannah grassland replaced forest.
This change of habitat may have prompted the
evolution of the first species of Homo, with the
development of increasingly sophisticated tools
and a change in diet that included meat
obtained by hunting and killing large animals.
Homo erectus and later species developed the
use of fire and were able to colonize colder
areas and survive during ice ages.
Fossil Record
6. Discuss the incompleteness of the fossil record and the
resulting uncertainties with respect to human
evolution.
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•
•
Knowledge of approximate dates and distribution for the
named species is expected. Details of sub-species or
particular groups (Cro-Magnon, Peking, etc) are not required.
Reasons for the incompleteness of the fossil record should be
included.
The question of where the earliest hominid ancestors
lived has still not been answered with certainty.
The closest existing relatives of humans are
chimpanzees and gorillas from Africa and orangutans
from Southeast Asia.
D5
NeoDarwinism
Mutations
1. State that mutations are changes to genes or
chromosomes due to chance, but with predicted
frequencies.
Gene and Chromosome Mutations
2. Outline phenylketonuria (PKU) and cystic fibrosis as examples of
gene mutation, and Klinefelter’s syndrome as an example of
chromosome mutation.
• Examples of gene mutations: PKU, Cystic fibrosis, and Sickle cell
anemia.
• Phenylketonuria (PKU) - caused by mutations of an autosomal
gene that code for phenylalanine hydroxylase.
• This enzyme converts the amino acid phenylalanine into tyrosine.
Without it, phenylalanine accumulates in the blood to a harmful
level that causes mental retardation and death in young children.
• Over 30 different alleles cause PKU.
• Natural selection has kept them at low frequencies in the human
population because, until screening and treatment (recently
became possible), children homozygous for PKU alleles died at an
early age.
Gene and Chromosome Mutations
• Cystic fibrosis- is the commonest genetic disease in
Europe. It is caused by mutations of a gene coding for a
chloride channel.
• This protein transports chloride ions across membranes
in epithelium cells. Without chloride channels in the
plasma membrane, mucus secreted by epithelia
becomes thick and sticky and tends to block airways of
the breathing system, causing respiratory infections.
• Although mutations of the chloride channel gene can
cause cystic fibrosis, 70% of cases are due to one
mutant allele, in which 3 bases coding for phenylalanine
have been deleted.
• The frequency of cystic fibrosis in Europe is 1 in 2500.
Gene and Chromosome Mutations
• Examples of Chromosome mutations: Klinefelter’s syndrome,
Down’s syndrome, and Turner’s syndrome.
• Chromosome mutations often cause infertility and so the
variation that they cause is not inherited. They are therefore not
usually significant in evolution.
• Klinefelter’s syndrome- Males having one or more extra X
chromosomes (XXY).
• Although recognizably male, those with the syndrome have low
testosterone levels and so are infertile and do not fully develop
the male secondary sexual characteristics. Slight mental
retardation and the development of female characteristics may
occur.
• Mental retardation usually only occurs in XXXY, XXYY, and XXXXY
forms.
• Is among the most common chromosomal mutations occurring in
1 in 500 to 1000 male births, these numbers maybe misleading
because not all men exhibit symptoms.
Gene and Chromosome Mutations
• Turner’s syndrome- Females having only one X
chromosome (XO). Recognizable female, short
stature and lack of ovarian development.
• Other characteristics are a webbed neck, arms
that turn out slightly at the elbow, and low
hairline in the back of the head.
• No mental retardation and with medical help
can carry a fetus to term.
• Is among the most common chromosomal
mutations occurring in 1 in 2500 female births.
Variation
3. Explain that variation in a population results from the
recombination of alleles during meiosis and fertilization.
• A new individual, produced by sexual reproduction
inherits genes from its two parents.
• If there is random mating, any two individuals, in an
interbreeding population could be the two parents, so
the individual could inherit any of the genes in the
interbreeding population.
• These genes are called the gene pool. A gene pool is all
the genes in an interbreeding population.
Adaptations
4. State that adaptations (or micro-evolutionary
steps) may occur as the result of allele
frequency increasing in a population’s gene
pool over a number of generations.
Evolution
5. Describe how the evolution of one species into
another species involves the accumulation of many
advantages alleles in the gene pool of a population
over a period of time.
• If an allele increases the chances of survival and
reproduction of individuals that posses it, the
frequency of the allele in the gene pool will tend to
increase.
• Conversely, if the allele reduces the chance of survival
and reproduction, it will decrease in frequency.
• These changes are due to natural selection.
D6.
The Hardy-Weinberg
Principle
Hardy-Weinberg
5. State the Hardy-Weinberg principle and the
conditions which it applies.
• Large population (No Genetic Drift)
• Random Mating
• No mutation
• No Gene flow (No Immigration or Emigration)
• All phenotypes have equal fitness (No Selection)
Hardy-Weinberg
1. Describe an adaptation in terms of change in frequency of a
gene’s alleles.
• If an allele increases the chance of survival and reproduction it
should increase in frequency and vice versa.
• The Hardy-Weinberg equation can be used to test for natural
selection. If allele and genotype frequencies in a population
show that the Hardy-Weinberg principle is being followed for a
particular gene, this indicates no natural selection.
• Members of the population all have an equal chance of survival
whatever alleles of the gene they posses.
• The allele frequency will not change between one generation
and the next.
• If allele and genotype frequencies do not follow the HardyWeinberg principle, a possible reason is that natural favors one
allele over another.
Hardy-Weinberg
• Adaptations develop in populations as a result of
changes in allele frequencies in the gene pool.
(Microevolution)
• A population in which there are 2 alleles of a
gene in the gene pool is polymorphic. If one
allele is gradually replacing the other, the
population shows transient polymorphism.
• Populations of ladybug that changed from
having red wings with black spots to black wings
are an example of transient polymorphism.
Hardy-Weinberg
2. Explain how the Hardy-Weinberg equation
2pq +q2 = 1) is derived.
(p2 +
• If there are 2 alleles of a gene in a population, there are
3 possible genotypes:
• aa
homozygous recessive
• AA homozygous dominant
• Aa
heterozygous
• The frequency of the 2 alleles in the population is
usually represented by p and q.
• The total frequency of the alleles is 1. p + q = 1
Hardy-Weinberg
• If there is random mating in a population, the
chance of inheriting 2 copies of the first of the
two alleles is (p x p).
• The chance of inheriting 2 copies of the second
of the two alleles is (q x q).
• The expected frequency of the 2 homozygous
genotypes is p2 and q2.
• The expected frequency of the heterozygous
genotype is 2pq.
• The sum of all these frequencies is 1.
•
p2 + 2pq +q2 = 1
Hardy-Weinberg
• Example: MN blood group gene in a town in Japan.
The two alleles are codominant.
• Allele frequencies of the P1
• M= p= 0.525
N= q= 0.475
•
F1 Predicted F1 Actual
• MM p2= 0.276
0.274
• MN 2pq= 0.499
0.502
• NN
q2= 0.225
0.224
• The results show that the actual genotypes fit those
predicted by the Hardy-Weinberg equation very closely.
• They therefore follow the Hardy-Weinberg Principle.
Hardy-Weinberg
3. Calculate allele, genotype and phenotype
frequencies for two alleles of a gene, using the
Hardy-Weinberg equation.
• The ability to taste phenylthiocarbamide (PTC) is
due to a dominant allele (T) and non-tasting is
due to the recessive allele (t).
• 1600 people were tested in a survey.
• 461 were non-tasters- a frequency of 0.288.
• Their genotype was homozygous recessive (t t).
Hardy-Weinberg
• If q= frequency of t allele, q2 = 0.288 so q= 0.537
• If p= frequency of T allele, p= (1-q) = 0.463
• The frequency of homozygous dominants (T T)
and heterozygotes (T t) can be calculated.
• p2 = frequency of homozygous dominant
• p2 = 0.463 x 0.463 = 0.214
• 2pq = frequency of heterozygotes
• 2pq = 2 (0.463 x 0.537) = 0.497
Hardy-Weinberg
4. State that the Hardy-Weinberg principle can
also be used to calculate allele, genotype and
phenotype frequencies for genes with more
than two alleles.