Evolution #101 Flashcards
(134 cards)
1
Q
What is Natural Selection?
A
environmental or selective pressures act on the phenotype of individuals in a population
lead to changes in allele frequency.
Certain alleles will be favoured and others are selected against.
2
Q
Variation -
A
- presence of variation in a population is what allows natural selection to occur.
naturally present in sexually reproducing organisms and then selection allows organisms with certain characteristics to survive and then pass on their genes to the next generation.
The features which are selected to ensure that the organism is well suited or adapted to its environment
3
Q
Selection: Biotic Factors
A
- Predators
- Disease-causing organisms
- Competition from members of the same or different species
- Symbiotic relationships with other organisms
4
Q
Selection: Abiotic Factors
A
- Rainfall
- Temperature
- Nutrient levels
- Light intensity
- Medical drugs like antibiotics, or chemicals like pesticides
5
Q
Mutations
A
permanent changes in the sequence of nucleotides of DNA
source of genetic variation in the species, new forms of a particular gene (alleles).
Mutations in germline cells can be passed on to the next generation.
6
Q
Variation in Offspring
A
asexually reproducing organisms = offspring genetically identical, unless mutation has occurred.
sexually reproducing organisms = offspring not genetically identical
7
Q
Four main factors which contribute to variation among offspring:
A
Mutations
Crossing over
Independent assortment
Random fertilisation
sexual reproduction allows for the ‘re-shuffling’ of genes from parents to give rise to new combinations in their offspring.
8
Q
Gene pool
A
The gene pool of a population is the sum of all of the alleles of all of the individuals in that population.
9
Q
Population
A
is a group of individuals of the one species that breed together in a particular habitat.
10
Q
The genotype of an organism
A
determines the appearance or phenotype of the individual.
11
Q
Allele frequency
A
the relative proportion of a particular allele in the population.
12
Q
Large/small gene pool
A
Populations with large gene pools = large diversity of alleles , more likely to survive in harsh conditions.
Populations with; small gene pool = are at greater risk of extinction. less genetic diversity
13
Q
Changes in the gene pool of a population can be brought about by several factors:
A
Gene Flow – Movement of alleles between different populations of the same species.
Increased gene flow between populations reduces differences between populations.
Genetic Drift – changes in the frequency of alleles in a population.
14
Q
Speciation
A
process that gives rise to the origin of new species.
result from an accumulation of genetic changes influenced by different selection pressures or genetic drift in geographically isolated populations
15
Q
Allopatric Speciation
A
involves members of the original population being separated by a geographical barrier.
16
Q
Sympatric speciation
A
is where one population gives rise to two or more species while still inhabiting the same region
- no geographical isolation.
If occurs = gene flow still needs to be prevented, leading to reproductive isolation.
17
Q
Sympatric speciation can occur through hybridisation and…
A
Asexual Reproduction
18
Q
Sympatric speciation can occur through Polyploidy
A
where organisms possess more than two sets of chromosomes.
result of a malfunction during meiosis where chromosomes fail to separate properly.
organism can’t interbreed with an organism with the normal diploid number
- gene flow is immediately stopped.
19
Q
Convergent Evolution
A
occurs when different species of organisms develop similar (analogous) features with similar functions.
can occur as different groups of organisms often live in similar habitats with similar selective pressures.
then evolve to have similar structures/ behaviours
Species evolved in particular environments in similar will evolve similar features, undergone convergent evolution.
20
Q
Analogous Features
A
have evolved separately in different lines of evolution.
serve the same function in different species but have evolved separately.
has occurred when such features evolve independently in unrelated groups or separate species.
Birds, bats and butterflies are unrelated have all evolved wings for flight independently of one another.
21
Q
Divergent Evolution
A
process by which interbreedingspeciesdiverged into two or more evolutionary groups.
these groups of species used to be similar and related. However, they became more and more dissimilar through time.
homologous structures = indicate a species is diverging from its ancestor.
need not have the same function as that of those of the species’ ancestors.
22
Q
Divergent Evolution: The forelimbs of humans and bats are
A
homologous structures.
used differently, the basic skeletal structure is the same and they are derived from the same embryonic origin.
indicate plausible evolution from a common ancestor.
23
Q
Advantages of Divergent Evolution
A
allows species with common ancestral origin to be able to adapt to their own habitats.
promotes biodiversity.
- leads to speciation,
- and a wide range of different organisms thriving in diverse habitats.
24
Q
Adaptive Radiation
A
when a single or small group of ancestral species rapidly diversifies into a large number of descendant species.
25
Divergent vs Adaptive Radiation
adaptive radiation - small-scale evolution over a shorter span of time
divergent evolution - evolution of species diverging from its ancestors over a relatively long span of time.
adaptive radiation may lead to divergent evolution over time as the species become more and more distinct from its ancestors.
26
Convergent Comparison
Species evolved from a relatively recent common ancestor.
Gives rise to homologous structures; species diverge.
Species become more different over time.
Species are relatively closely related, as they share a recent common ancestor.
27
Divergent Comparison
Species have relatively different common ancestors.
Gives rise to analogous structures; species converge
Species appear to become more similar over time.
Species are less closely related, do not have a recent common ancestor.
28
Issue of reduced gene pool
reduced genetic biodiversity = decreased chance of a population to survive and evolve in the face of changing environmental selective pressures.
large gene pool - more likely that at least some members of the population will survive, pass on favoured genes to the next generation.
29
Succession
gradual process by which species composition in a community changes.
brought about by modification of the habitat caused by the organisms that live there.
30
Succession: organisms can change the environment
can provide food or shelter for other organisms. E.g. trees are a source of food and shelter.
change the soil structure by causing the breakdown of rock or increasing the amount of organic matter in the soil.
decompose dead material and make nutrients available for others.
31
Example of succession
For example, if a lava flow kills all life in a particular area, the order which species begin to grow in the environment can be tracked over time.
The first organisms which establish the area are usually able to survive in harsh conditions e.g. lichens.
These colonizing species can then change the environment to allow other organisms to survive.
32
There are two main types of succession:
Primary succession and secondary
33
Primary Succession
Primary succession is where organisms establish themselves in a brand new environment.
34
Example on Primary Succession
For example, in an area where glaciers are retreating and revealing land with no life present.
Species such as lichens would be able to survive on the harsh rock environment and would slowly change the environment, forming soils.
This soil would then allow species like mosses to survive and over time, with further breakdown and decomposition occurring, the conditions become ripe for grasslands and shrubs.
As these plants begin to survive in the environment, birds and insects can survive and change the environment further, which then allows for other plant species to be introduced.
Over the succession period one type of community is replaced by another, until the ecosystem matures.
At the climax stage of the community, it is dominated by generally larger, longer living and slower growing species.
35
Secondary Succession
Secondary succession is where changes occur in an environment which has been exposed to damage or a disaster e.g. fire, land clearance.
The same pattern in the emergence of species will occur in secondary succession as in primary succession.
Secondary succession is usually much quicker than primary succession as there is a greater mix of soils and usually a greater mix of species and plants available to recolonize the area.
36
Succession Characteristics
As the ecosystem matures there is more biomass, but productivity decreases.
There are usually more species in mature ecosystems.
As the ecosystem matures the number of heterotrophic species tends to increase more than the number of autotrophic species.
Mature ecosystems are more efficient at recycling nutrients.
Mature ecosystems have organisms that tend to be more specialized.
37
Succession, Adaptive Radiation and Divergent Evolution…
When new niches become available to a species, for example as a result of succession or following an environmental change, different selection pressures may lead to divergent evolution or adaptive radiation.
38
Evidence shows that life has existed on Earth for around...
3.5 billion years.
39
All living things share many features:
Cell membrane composed of lipids
RNA and DNA
Proteins composed of the same amino acids
Water composition by mass is 50%-80% for all cells and water has the same functions in all cells.
40
Prokaryotes date back too...
3.5 billion years
41
The oldest fossils are...
are the cyanobacteria from rocks in Western Australia dated about 3.5 billion years old.
These cyanobacteria formed stromatolites.
Cyanobacteria are photosynthetic, and therefore were important for evolution on Earth as they released oxygen into the atmosphere.
42
What several processes were necessary for life to begin on Earth?
Synthesis of simple organic molecules e.g. amino acids and ultimately the synthesis of larger organic polymers e.g. proteins.
The origin of molecules that can self replicate. I.e. RNA and DNA.
Packaging of these molecules into membranes with an internal environment different in composition from their surroundings.
These structures are known to form spontaneously.
43
It is likely that Earth was subjected to a...
a lengthy barrage of meteor showers and emissions from volcanoes which meant the atmosphere contained the precursors needed to make organic molecules.
44
Evidence also suggests that lfie may have orinagted at the bottom of the sea. How is this possible?
Hydrothermal vents warm ocean water to temperatures suitable for life processes and release hydrogen-rich molecules and minerals that early organisms could have used to obtain energy
45
What came first DNA or RNA?
DNA is the main hereditary unit of all forms of life today and it could be hypothesized that it was necessary for DNA to be present in the first cells.
The problem with this is that protein enzymes are required for DNA to be copied and replicated. It is likely that RNA appeared before DNA and proteins in the evolution of life.
46
The key function of RNA and ribosomes in simple cells - RNA can self replicate
RNA can self-replicate
RNA is able to store information in a sequence composed of four recurring nucleotides (similar to DNA)
Short sequences of RNA are able to be accurately duplicated to form identical RNA molecules.
47
The key function of RNA and ribosomes in simple cells - RNA can act as a catalyst
Modern cells use RNA catalysts (called ribozymes) to remove introns from mRNA and help synthesise new RNA molecules
In ribosomes, rRNA functions as the catalytic component and plays a pivotal role in peptide bond formation
48
Prokaryotic vs. Eukaryotic Cells
Prokaryotes are the simplest cells which lack a nucleus and membrane bound organelles.
It is thought that prokaryotes evolved before eukaryotes.
One of the pieces of evidence which supports this is the fossil record as the oldest known microfossils are the cyanobacteria found as stromatolites.
49
Evidence supporting what cell came first.
Fossils – earliest eukaryote fossil found, a multicellular alga – 2.1 billion years old
Biomarkers – eukaryotic biomarkers have been found in sedimentary rocks in the Earth’s crust indicating 2.7 billion years ago. (Sterols – a molecule only found in eukaryotic membranes)
Comparison of cell structure:
DNA in eukaryotes has a higher level of organization
Eukaryotes have membrane-bound organelles
Eukaryotic cells are bigger
50
Mechanisms for Evolution of Eukaryotic Cells from Prokaryotic Cells
The oldest and most primitive eukaryotes are called Protists.
It is believed that eukaryotic cells probably originated as a collection of prokaryotes.
51
The evolution of prokaryotic cells would have occurred in two steps:
The first step involved folding of the membrane to form the endo-membrane system. This step is thought to have formed the endoplasmic reticulum and most other organelles.
The second step was the process of endosymbiosis. Chloroplasts and mitochondria seemed to have evolved from prokaryotes which became incorporated after being engulfed by another cell.
52
Evidence supporting the evolution of prokaryotes
this theory includes that mitochondria and chloroplasts have inner membranes similar to plasma membranes of prokaryotes, they replicate by splitting through binary fission, they have their own circular DNA and have their own protein manufacturing system including ribosomes.
53
Endosymbiosis
A symbiotic relationship where one organism lives inside the other is known as endosymbiosis. Primary endosymbiosis refers to the original internalization of prokaryotes by an ancestral eukaryotic cell, resulting in the formation of the mitochondria and chloroplasts.
54
Evolution
Evolution is defined as the change in heritable characteristics of biological populations over successive generations.
Evolution can be recognized and measured using changes in the structure or base sequence of DNA.
These also give rise to changes in the proteins coded for, and therefore the phenotype of the organism.
55
Amino acid sequences
DNA and protein sequences should usually show greater similarity between closely related groups of organisms compared to distantly related groups.
Scientists can compare the sequence of amino acids in proteins of the same type.
If different species produce proteins with very similar amino acid sequences, their DNA would also be very similar, and the species would therefore be derived from a recent common ancestor.
56
Amino acid sequences: cytochromes
Cytochromes, such as cytochrome c, are proteins which are crucial to survival in most species as these are needed for aerobic respiration. The protein varies from one species to another, but the degree of similarity indicates the closeness of the evolutionary relationship.
57
DNA Hybridisation
Another method used to compare the DNA of different species is called DNA hybridisation.
DNA from two species is heated and this separates the two DNA strands
Upon cooling the single strands of DNA recombine to form the DNA double helix again
The degree of bonding between one species’ DNA and another gives a measure of how closely related the two species are
58
DNA Hybridisation; similarity
If the two species are closely related, the DNA sequence will be very similar and the two strands will be difficult to separate.
If the two strands are poorly matched, they will be easier to separate.
- This means the two species are distantly related because the DNA sequence is very dissimilar
59
DNA profiling
DNA profiling can be used to compare specific genes between species.
Profiles obtained through gel electrophoresis can be used to compare regions of the DNA.
Similarities and differences can be used to help establish patterns of evolutionary relatedness.
60
DNA Sequencing
DNA sequencing is a technique used to work out the actual sequence of bases in a specific gene.
Software can then be used to create a phylogenetic tree of related organisms using DNA sequences.
61
Phylogenetic Trees
shows the evolutionary relationships between a variety of organisms based on similarities and differences in both genetic and physical features.
The DNA sequences will vary from one species to another and the degree of closeness can be used to determine the closeness of the evolutionary relationship.
62
Benefits of Comparative Genomics
Major contributions to the understanding of organisms, including the human genome.
Identifying genes that are essential to life and contributing to the understanding of how gene signals control gene function across a range of species.
A powerful tool to study evolution and the evolutionary relatedness between organisms, and humans.
Wide ranging applications in agriculture and biotechnology.
63
Species
is a population where the members have similar characteristics and the capacity to interbreed and produce fertile offspring.
64
Problems in the Definition of a Species - Asexual Reproduction
Asexual reproduction
Organisms that produce offspring that don’t involve the production of sperm and eggs undergo asexual reproduction.
This means the species definition of interbreeding doesn’t apply.
65
Problems in the Definition of a Species - Overlap of Population and Hybrids
Sometimes it is difficult to apply the species concept because it is difficult to examine the extent of interbreeding. See page 388 of your workbook for an example.
66
Problems in the Definition of a Species - Fossils
Trying to classify fossils into particular species will not fit with the current definition of species.
No evidence is available regarding interbreeding.
Biologists use morphological features in this case.
67
Pre-Zygotic Reproductive Isolation
these mechanisms prevent reproduction before the zygote is formed.
68
Types of Pre-Zygotic -
Temporal (Time) Isolation
Breeding cycles or time of fertility differs from one population of species to the next.
Ecological Isolation
Ecological, or habitat, isolation occurs when two species that could interbreed do not because the species live in different areas.
Behavioural Isolation
This occurs in populations with highly specific calls or rituals that are directed and responded to only by other members of a particular population.
Mechanical (Morphological) Isolation
This is related to compatible sexual organs.
Gamete Isolation
Incompatibility of sperm and eggs from populations of different species.
69
Post-zygotic Reproductive Isolation
Post-zygotic mechanisms prevent a zygote from developing into a viable, fertile adult hybrid
70
Post-zygotic Reproductive Isolation Factors - Hybrid inviability
When hybrid zygotes are formed and begin to develop, there are many instances where these individuals are weak and compete poorly for resources.
This means they are unlikely to reach reproductive age and contribute to the gene pool.
71
Post-zygotic Reproductive Isolation Factors - Hybrid sterility
This is when the hybrid formed is sterile.
An example of this is when a female horse mates with a male donkey, producing a mule.
Horses have a diploid number of 64 and donkeys 62. The mule then has 63 chromosomes in each of its cells and it can’t undergo meiosis to form gametes.
72
Mutations
Mutations are permanent changes in the sequence of nucleotides of DNA and are the ultimate source of genetic variation in the species.
Mutations are the source of new forms of a particular gene (alleles).
Mutations in germline cells can be passed on to the next generation.
73
Example of bacteria natural selection
Evolution of species is a slow process, but in a few cases evolution can be observed over a relatively short time period.
This is seen in the evolution of strains of bacteria which are resistant to antibiotics.
Individual variation exists between bacteria, with some being more resistant to certain antibiotics than others.
Antibiotics act as selective agents and bacteria that have the resistant gene are more likely to survive and over time the frequency of the gene for resistance is increased
74
Speciation
A species is a population or group of populations where the members have the capacity to interbreed and produce fertile offspring.
Speciation is the name of the process that gives rise to the origin of new species.
Speciation may result from an accumulation of genetic changes influenced by different selection pressures or genetic drift in geographically isolated populations
75
Allopatric Speciation
Allopatric speciation involves members of the original population being separated by a geographical barrier.
76
Sympatric speciation
is where one population gives rise to two or more species while still inhabiting the same region – no geographical isolation.
If sympatric speciation occurs, gene flow still needs to be prevented or at least reduced, leading to reproductive isolation. Rare with animals and more common in plants and bacteria
77
Sympatric speciation can occur through microhabitats
Groups occupying different niches or micro-habitats
| A microhabitat is small and differs from some surrounding, more exclusive habitat
78
Sympatric speciation can occur through hybridisation and...
Asexual Reproduction
79
Sympatric speciation can occur through Polyploidy
Polyploidy is a condition where organisms possess more than two sets of chromosomes. This is a common condition in many plants as well as certain groups of fish and amphibians.
Polyploidy is a result of a malfunction during meiosis where chromosomes fail to separate properly.
The polyploid organism can’t interbreed with an organism with the normal diploid number – gene flow is immediately stopped.
80
Convergent Evolution
Convergent evolution occurs when different species of organisms develop similar (analogous) features with similar functions.
This can occur as different groups of organisms often live in similar habitats with similar selective pressures. These organisms then evolve to have similar structures/ behaviours to be able to carry out life’s processes.
Species that have evolved in particular environments in similar ways will evolve similar features and are said to have undergone convergent evolution.
81
Analogous Features
Analogous features are those that have evolved separately in different lines of evolution.
These features serve the same function in different species but have evolved separately.
Convergent evolution has occurred when such features evolve independently in unrelated groups or separate species.
Examples:
Wings: Birds, bats and butterflies are unrelated have all evolved wings for flight independently of one another.
Eyes: Cephalopods including squid, cuttlefish and the octopus have evolved a ‘camera eye’ that has the same features as the vertebrate camera eye.
82
Divergent Evolution
Divergent evolution refers to the process by which interbreeding species diverged into two or more evolutionary groups.
It means that these groups of species used to be similar and related. However, they became more and more dissimilar through time.
Divergent evolution may also pertain to the process of tracing back two or more species to their common ancestor and knowing how these species have diversified or diverged.
In divergent evolution, homologous structures are structures indicating a species is diverging from its ancestor. These structures need not have the same function as that of those of the species’ ancestors.
83
Divergent Evolution: The forelimbs of humans and bats are
homologous structures. Although they are used differently, the basic skeletal structure is the same and they are derived from the same embryonic origin.
Their similarity in this regard could indicate plausible evolution from a common ancestor.
84
Advantages of Divergent Evolution
This shows that divergent evolution allows species with common ancestral origin to be able to adapt to their own habitats.
It is likely to ease competition among them since they evolve traits that make them explicitly suited to their habitat and ecological niche.
Divergent evolution also helps in promoting biodiversity. Since it leads to speciation, it could lead to a wide range of different organisms thriving in diverse habitats.
85
Adaptive Radiation
the process whereby seevral new species from a recent ancestral soruce diversify. Each of these species is adpated to utlizie or occupy a vacant adaptive zone.
This zone, thus, serves as ana eoclogical oppunrinty for certain gorups of organism to diversify into different forms, often in a rapid process. Simialr to divergent evolution, adaptive radiation lead to speciation.
86
Divergent vs Adaptive Radiation
The two concepts differ in a way that adaptive radiation deals more with small-scale evolution over a shorter span of time whereas divergent evolution looks through the evolution of species diverging from its ancestors over a relatively long span of time.
Nevertheless, adaptive radiation may lead to divergent evolution over time as the species become more and more distinct from its ancestors.
87
Convergent Comparison
Species evolved from a relatively recent common ancestor.
Gives rise to homologous structures; species diverge.
Species become more different over time.
Species are relatively closely related, as they share a recent common ancestor.
88
Divergent Comparison
Species have relatively different common ancestors.
Gives rise to analogous structures; species converge
Species appear to become more similar over time.
Species are less closely related, do not have a recent common ancestor.
89
Issue of reduced gene pool
The issue of reduced genetic biodiversity relates to the ability of the population to survive and evolve in the face of changing environmental selective pressures.
With a large gene pool it is more likely that at least some members of the population will survive and pass on favoured genes to the next generation.
90
Succession
Succession is the gradual process by which species composition in a community changes.
Succession is brought about by modification of the habitat caused by the organisms that live there.
91
Succession: organisms can change the environment
can provide food or shelter for other organisms. E.g. trees are a source of food and shelter.
They can change the soil structure by causing the breakdown of rock or increasing the amount of organic matter in the soil.
They can decompose dead material and make nutrients available for others.
92
Example of succession
For example, if a lava flow kills all life in a particular area, the order which species begin to grow in the environment can be tracked over time.
The first organisms which establish the area are usually able to survive in harsh conditions e.g. lichens.
These colonizing species can then change the environment to allow other organisms to survive.
93
There are two main types of succession:
Primary succession and secondary
94
Primary Succession
Primary succession is where organisms establish themselves in a brand new environment.
95
Example on Primary Succession
For example, in an area where glaciers are retreating and revealing land with no life present.
Species such as lichens would be able to survive on the harsh rock environment and would slowly change the environment, forming soils.
This soil would then allow species like mosses to survive and over time, with further breakdown and decomposition occurring, the conditions become ripe for grasslands and shrubs.
As these plants begin to survive in the environment, birds and insects can survive and change the environment further, which then allows for other plant species to be introduced.
Over the succession period one type of community is replaced by another, until the ecosystem matures.
At the climax stage of the community, it is dominated by generally larger, longer living and slower growing species.
96
Secondary Succession
Secondary succession is where changes occur in an environment which has been exposed to damage or a disaster e.g. fire, land clearance.
The same pattern in the emergence of species will occur in secondary succession as in primary succession.
Secondary succession is usually much quicker than primary succession as there is a greater mix of soils and usually a greater mix of species and plants available to recolonize the area.
97
Succession Characteristics
As the ecosystem matures there is more biomass, but productivity decreases.
There are usually more species in mature ecosystems.
As the ecosystem matures the number of heterotrophic species tends to increase more than the number of autotrophic species.
Mature ecosystems are more efficient at recycling nutrients.
Mature ecosystems have organisms that tend to be more specialized.
98
Succession, Adaptive Radiation and Divergent Evolution…
When new niches become available to a species, for example as a result of succession or following an environmental change, different selection pressures may lead to divergent evolution or adaptive radiation.
99
Evidence shows that life has existed on Earth for around...
3.5 billion years.
100
All living things share many features:
Cell membrane composed of lipids
RNA and DNA
Proteins composed of the same amino acids
Water composition by mass is 50%-80% for all cells and water has the same functions in all cells.
101
Prokaryotes date back too...
3.5 billion years
102
The oldest fossils are...
are the cyanobacteria from rocks in Western Australia dated about 3.5 billion years old.
These cyanobacteria formed stromatolites.
Cyanobacteria are photosynthetic, and therefore were important for evolution on Earth as they released oxygen into the atmosphere.
103
What several processes were necessary for life to begin on Earth?
Synthesis of simple organic molecules e.g. amino acids and ultimately the synthesis of larger organic polymers e.g. proteins.
The origin of molecules that can self replicate. I.e. RNA and DNA.
Packaging of these molecules into membranes with an internal environment different in composition from their surroundings.
These structures are known to form spontaneously.
104
It is likely that Earth was subjected to a...
a lengthy barrage of meteor showers and emissions from volcanoes which meant the atmosphere contained the precursors needed to make organic molecules.
105
Evidence also suggests that lfie may have orinagted at the bottom of the sea. How is this possible?
Hydrothermal vents warm ocean water to temperatures suitable for life processes and release hydrogen-rich molecules and minerals that early organisms could have used to obtain energy
106
What came first DNA or RNA?
DNA is the main hereditary unit of all forms of life today and it could be hypothesized that it was necessary for DNA to be present in the first cells.
The problem with this is that protein enzymes are required for DNA to be copied and replicated. It is likely that RNA appeared before DNA and proteins in the evolution of life.
107
The key function of RNA and ribosomes in simple cells - RNA can self replicate
RNA can self-replicate
RNA is able to store information in a sequence composed of four recurring nucleotides (similar to DNA)
Short sequences of RNA are able to be accurately duplicated to form identical RNA molecules.
108
The key function of RNA and ribosomes in simple cells - RNA can act as a catalyst
Modern cells use RNA catalysts (called ribozymes) to remove introns from mRNA and help synthesise new RNA molecules
In ribosomes, rRNA functions as the catalytic component and plays a pivotal role in peptide bond formation
109
Prokaryotic vs. Eukaryotic Cells
Prokaryotes are the simplest cells which lack a nucleus and membrane bound organelles.
It is thought that prokaryotes evolved before eukaryotes.
One of the pieces of evidence which supports this is the fossil record as the oldest known microfossils are the cyanobacteria found as stromatolites.
110
Evidence supporting what cell came first.
Fossils – earliest eukaryote fossil found, a multicellular alga – 2.1 billion years old
Biomarkers – eukaryotic biomarkers have been found in sedimentary rocks in the Earth’s crust indicating 2.7 billion years ago. (Sterols – a molecule only found in eukaryotic membranes)
Comparison of cell structure:
DNA in eukaryotes has a higher level of organization
Eukaryotes have membrane-bound organelles
Eukaryotic cells are bigger
111
Mechanisms for Evolution of Eukaryotic Cells from Prokaryotic Cells
The oldest and most primitive eukaryotes are called Protists.
It is believed that eukaryotic cells probably originated as a collection of prokaryotes.
112
The evolution of prokaryotic cells would have occurred in two steps:
The first step involved folding of the membrane to form the endo-membrane system. This step is thought to have formed the endoplasmic reticulum and most other organelles.
The second step was the process of endosymbiosis. Chloroplasts and mitochondria seemed to have evolved from prokaryotes which became incorporated after being engulfed by another cell.
113
Evidence supporting the evolution of prokaryotes
this theory includes that mitochondria and chloroplasts have inner membranes similar to plasma membranes of prokaryotes, they replicate by splitting through binary fission, they have their own circular DNA and have their own protein manufacturing system including ribosomes.
114
Endosymbiosis
A symbiotic relationship where one organism lives inside the other is known as endosymbiosis. Primary endosymbiosis refers to the original internalization of prokaryotes by an ancestral eukaryotic cell, resulting in the formation of the mitochondria and chloroplasts.
115
Evolution
Evolution is defined as the change in heritable characteristics of biological populations over successive generations.
Evolution can be recognized and measured using changes in the structure or base sequence of DNA.
These also give rise to changes in the proteins coded for, and therefore the phenotype of the organism.
116
Amino acid sequences
DNA and protein sequences should usually show greater similarity between closely related groups of organisms compared to distantly related groups.
Scientists can compare the sequence of amino acids in proteins of the same type.
If different species produce proteins with very similar amino acid sequences, their DNA would also be very similar, and the species would therefore be derived from a recent common ancestor.
117
Amino acid sequences: cytochromes
Cytochromes, such as cytochrome c, are proteins which are crucial to survival in most species as these are needed for aerobic respiration. The protein varies from one species to another, but the degree of similarity indicates the closeness of the evolutionary relationship.
118
DNA Hybridisation
Another method used to compare the DNA of different species is called DNA hybridisation.
DNA from two species is heated and this separates the two DNA strands
Upon cooling the single strands of DNA recombine to form the DNA double helix again
The degree of bonding between one species’ DNA and another gives a measure of how closely related the two species are
119
DNA Hybridisation; similarity
If the two species are closely related, the DNA sequence will be very similar and the two strands will be difficult to separate.
If the two strands are poorly matched, they will be easier to separate.
- This means the two species are distantly related because the DNA sequence is very dissimilar
120
DNA profiling
DNA profiling can be used to compare specific genes between species.
Profiles obtained through gel electrophoresis can be used to compare regions of the DNA.
Similarities and differences can be used to help establish patterns of evolutionary relatedness.
121
DNA Sequencing
DNA sequencing is a technique used to work out the actual sequence of bases in a specific gene.
Software can then be used to create a phylogenetic tree of related organisms using DNA sequences.
122
Phylogenetic Trees
shows the evolutionary relationships between a variety of organisms based on similarities and differences in both genetic and physical features.
The DNA sequences will vary from one species to another and the degree of closeness can be used to determine the closeness of the evolutionary relationship.
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Benefits of Comparative Genomics
Major contributions to the understanding of organisms, including the human genome.
Identifying genes that are essential to life and contributing to the understanding of how gene signals control gene function across a range of species.
A powerful tool to study evolution and the evolutionary relatedness between organisms, and humans.
Wide ranging applications in agriculture and biotechnology.
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Species
is a population where the members have similar characteristics and the capacity to interbreed and produce fertile offspring.
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Problems in the Definition of a Species - Asexual Reproduction
Asexual reproduction
Organisms that produce offspring that don’t involve the production of sperm and eggs undergo asexual reproduction.
This means the species definition of interbreeding doesn’t apply.
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Problems in the Definition of a Species - Overlap of Population and Hybrids
Sometimes it is difficult to apply the species concept because it is difficult to examine the extent of interbreeding. See page 388 of your workbook for an example.
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Problems in the Definition of a Species - Fossils
Trying to classify fossils into particular species will not fit with the current definition of species.
No evidence is available regarding interbreeding.
Biologists use morphological features in this case.
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Pre-Zygotic Reproductive Isolation
these mechanisms prevent reproduction before the zygote is formed.
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Types of Pre-Zygotic -
Temporal (Time) Isolation
Breeding cycles or time of fertility differs from one population of species to the next.
Ecological Isolation
Ecological, or habitat, isolation occurs when two species that could interbreed do not because the species live in different areas.
Behavioural Isolation
This occurs in populations with highly specific calls or rituals that are directed and responded to only by other members of a particular population.
Mechanical (Morphological) Isolation
This is related to compatible sexual organs.
Gamete Isolation
Incompatibility of sperm and eggs from populations of different species.
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Post-zygotic Reproductive Isolation
Post-zygotic mechanisms prevent a zygote from developing into a viable, fertile adult hybrid
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Post-zygotic Reproductive Isolation Factors - Hybrid inviability
When hybrid zygotes are formed and begin to develop, there are many instances where these individuals are weak and compete poorly for resources.
This means they are unlikely to reach reproductive age and contribute to the gene pool.
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Post-zygotic Reproductive Isolation Factors - Hybrid sterility
This is when the hybrid formed is sterile.
An example of this is when a female horse mates with a male donkey, producing a mule.
Horses have a diploid number of 64 and donkeys 62. The mule then has 63 chromosomes in each of its cells and it can’t undergo meiosis to form gametes.
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Mutations
Mutations are permanent changes in the sequence of nucleotides of DNA and are the ultimate source of genetic variation in the species.
Mutations are the source of new forms of a particular gene (alleles).
Mutations in germline cells can be passed on to the next generation.
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Example of bacteria natural selection
Evolution of species is a slow process, but in a few cases evolution can be observed over a relatively short time period.
This is seen in the evolution of strains of bacteria which are resistant to antibiotics.
Individual variation exists between bacteria, with some being more resistant to certain antibiotics than others.
Antibiotics act as selective agents and bacteria that have the resistant gene are more likely to survive and over time the frequency of the gene for resistance is increased
