Mechanisms/evidence for evolution Flashcards
What are 5 causes for variation?
- Random assortment
- Crossing over
- Non-dis-junction
- Random fertilization
- Mutations
What is random assortment?
- Chromosomes are sorted into daughter cells randomly during meiosis, so there are many possible combinations of chromosomes that can come from the mother and father
What is crossing over?
- Process where during meiosis, pieces of chromatids may be broken off and attached to a different chromatids
- This results in a changed sequence, or recombination of the alleles along the resulting chromosome
What is non-dis-junction?
- One or more members of a chromosome pair fail to separate during meiosis
- This results in gametes that have more or less than the correct number of chromosomes
- If such gametes are involved in fertilisation, the resulting embryo will have the incorrect number of chromosomes
What is random fertilization?
Chance alone is responsible for which sperm meets which egg
What are mutations?
- Sudden and permanent changes in the DNA of a chromosome and may result in totally new characteristics in an individual
What is a species?
Organisms belonging to the same species who are capable of producing fertile offspring under natural conditions
What is a population?
A group of organisms of the same species living together in a particular place at a particular time
What is a gene pool?
- The sum of all the alleles in a given population
- Can change over time
Allele frequencies
- Can increase or decrease
- Different populations have different allele frequencies
- EG. Scandinavians have a high allele frequency for blue eyes and blond hair
- EG. Chinese have a high allele frequency for straight dark hair
What is evolution?
A gradual change in phenotype thought to be caused by a change in allele frequency
What happens when there is a change in allele frequency?
Changes in allele frequency → phenotypic changes → the gene pool changes
Changes to allele frequency can be brought around by:
- Mutations
- Natural selection
- Random genetic drift
- Migration
- Barriers to gene flow
- Genetic diseases
What is a mutation?
- A sudden and permanent change brought about by a change in the sequence of bases in a strand of DNA
- Gene or chromosomal mutations
What are somatic mutations?
- Body cells experience a mutation
- Body cells arising from the mutant cell inherits the mutation
- Subsequent offspring do not inherit the mutation
What are germinal mutations?
- If a mutation occurs in a gamete, then any offspring resulting from this gamete will inherit the mutation
- This mutation can then be inherited by following generations
- This will change the allele frequency in the long term
- Mutations may or may not affect the survival chances of an offspring
- Mutations change allele frequencies
Natural selection
- There is competition between individuals
- Selection pressures make some genetic traits more favorable for survival
- Those with the traits survive and reproduce
- Favorable traits are passed onto offspring
- The allele frequency of favorable traits increase
Random genetic drift
- Only usually occurs in small populations
- By chance (not because it is advantageous) the allele frequency in a population changes
- Some random event (not associated with an increased chance of survival eg. An earthquake) change the allele frequency
An example of random genetic drift
EG. The Dunkers in Germany
- Small religious group who only intermarry within the population
- Their allele frequencies for blood groupings, mid-digital hair, ear lobes and handedness are markedly different from the general population
- These features have no survival advantage
Islander group polulations
- Have high IA allele frequency
- No IB alleles
- Mainlanders are the reverse
- Blood groupings do not provide a survival advantage
The founder effect
- A sub-group of random genetic drift
- A small group moves away from the original population to begin a new population
- The allele frequency of the emigrating group just happens to be different from the frequencies of the original population
Achromatopsia
- Inherited total colour blindness
- An example of random genetic drift
- After a typhoon, only 20 people survived on a Micronesian Island
- One of these was heterozygous for Achromatopsia
- The current population now has a high frequency for this allele
Migration
- A gene flow from one population to another
- As individuals join a population, they change the allele frequencies
- Large migrations have a considerable impact on allele frequencies
Barriers to gene flow
- Can stop the interbreeding between populations
- Isolated populations may be subject to different environments with different selection pressures
- Results in different gene pools
Types of barriers to gene flow
Geographical barriers → ocean, river, canyon
Socio-cultural barriers → government, religion, race, income level
Genetic diseases
- Can changes the allele frequencies in a population
- It is expected that the frequency of a fatal allele will decrease in a population overtime
- This is not always the case
Tay-sachs disease
- A recessive autosomal trait
- Homozygotes lack an enzyme that results in the accumulation of a fatty substance in the nervous system
- Most frequently occurs in individuals of Jewish decent form Eastern Europe (1 in every 2500 births)
- Frequency worldwide occurs 1 in every 500 000 births
- Death usually occurs by the age of four or five
Reasons for this change in allele frequency (Tay-sachs)
- Jewish populations have tended to be small and isolated, increasing the chances of genetic drift
- Those who are heterozygous for Tay-sachs, have increased resistance to tuberculosis (TB)
- If this is the case, heterozygotes have an advantage in situations where TB is prevalent
- Individuals with two normal alleles would be more susceptible to TB, and would possibly die, while individuals with two Tay-sachs alleles would die in early life
- Heterozygotes would have a survival advantage and would be more likely to reproduce and pass on their alleles to the next generation
Sickle-cell anaemia
- Causes the flattening (sickling) of erythrocytes preventing them from carrying oxygen
- Fatal in homozygotes
- The allele frequency is unexpectedly high in some African populations
- It was discovered that heterozygotes have a resistance to malaria
- Homozygotes for sickle-cell anaemia would die from the disease
- Homozygotes for healthy RBCs die from malaria
- Heterozygotes have the greatest survival
Thalassemia
- A recessive disease in which anaemia results from defects in the formation of haemoglobin
- A reduction in the amount and shape of red blood cells
- Homozygotes recessive have two defective genes, which can be fatal
- Patients require regular blood transfusions and special drugs to remove excess iron
- Homozygotes dominant do not have the disease
Heterozygote for thalassemia
- High allele frequency along the Mediterranean coast
- A carrier of the disease
- They are normal except during surgery or pregnancy, where they may suffer from low haemoglobin
- Have a slight change in the shape of their red blood cells and this allows them to be resistant to malaria
- This is known as thalassemia minor
Evolution
- A gradual change brought about by changes in gene frequencies
- Thought to be responsible for the development of species (speciation)
- Generally brought about by an increase in the frequency of advantageous alleles and a decrease in the frequency of disadvantageous alleles
3 observations made by Charles Darwin (in 1858)
- Variation exists and are passed from one generation to the next
- Birth rate exceeds resource availability
- Natures balance: although the birth rate of organisms was very high, each species number tended to remain at a relatively constant level
Conclusion from these observations (Charles Darwin)
Conclusion that because the excessive birth rate and limited resources, there must be a struggle for existence
Survival of the fittest
- More organisms with favorable characteristics survived, while many of those with unfavorable characteristics died before they had an opportunity to reproduce and pass on the unfavorable characteristics
- This is possible because variation exists within any species
The environment does not cause the mutation or change in the organism
- An individual already has the mutation, but in its original environment, the mutation does not offer any advantage
- The environment changes, and suddenly having the mutation is an advantage
4 steps to speciation
- Variation (exists)
- Isolation (occurs)
- Selection (applied/occurs)
- Speciation (resulting changes in gene frequencies make it impossible for the two groups to interbreed and produce fertile offspring)
Types of evidence for evolution
- Fossils
- Comparative studies
a. Comparative biochemistry
- DNA
- Mitochondrial DNA
- Protein sequences
- Genomics
b. Comparative anatomy
- Embryology
- Homologous structures
- Vestigial organs - Geographical distribution
Comparative DNA
- All living organisms use the same bases in DNA
- This supports the idea that organisms are related to each other
- This supports the idea that all organisms have a common ancestor
- When speciation occurs, the new species would have very similar DNA
- There should be a gradual difference in the DNA as organisms become more distinctly related
- Closely related organisms should have more similar DNA
What is a Genome?
The complete set of DNA in an organism
Junk DNA
- Have no apparent function and appear to serve no purpose
- Contain non-coding sequences of bases in the DNA
- The more closely related the organisms, the more ‘Junk DNA’ they have in common
- Supports the idea of a common ancestor
Endogenous retroviruses (ERV)
- A viral sequence that becomes part of an organism’s genome
- Store their genetic information as RNA, not DNA
upon entering a cell, a retrovirus copies it’s RNA genome into DNA (reverse transcription) - The DNA becomes inserted into one of the host cell’s chromosomes, which is then passed onto future generations (in a gamete)
- ERV’s make up 8% of the human genome
- More closely related organisms have more ERVs in common
- Distantly related organisms have fewer ERVs in common
- This supports the idea of a common ancestor
Mitochondria
- Organelles in the cell where the aerobic phase of respiration occurs to release energy for use by the cell
- Contains small amounts of DNA called mitochondrial DNA
Mitochondrial DNA
- Does not form threads, instead forms circular molecules
- 5-10 molecules in each mitochondrion
- Inherited from the maternal parent (sperm mitochondria is destroyed in fertilisation)
- Mutates at a faster rate than nuclear DNA and has gradually evolved
- Can be used to determine the closeness of the relationship between organisms
- Supports the idea of a common ancestor
Number of genes in mitochondrial DNA
- Has 37 genes, essential for the mitochondrion to function normally
- 24 of the genes contain the code for making transfer RNA molecules involved in protein synthesis
- 13 genes that code for enzymes necessary for reactions in cellular respiration
Protein sequences
- By comparing the type and sequence of amino acids in similar proteins form different species, the degree of similarity can be determined
- Animals from the same species have identical amino acid sequences in their proteins
Ubiquitous proteins
- Found everywhere
- Carry out the same function in all organisms (from bacteria to humans
Cytochrome C
- An example of a ubiquitous protein
- Performs an essential step in the production of cellular energy
- Contains 104 amino acids
- 37 of these amino acids have been found at the same position in every cytochrome C molecule that has ever been tested
- This supports the idea of a common ancestor
Annotation
The process by which genes and DNA sequences of a species are identified
Comparative genomics
- Differences and similarities between species are identified
- A high level of similarity or a smaller amount of time since the two species diverged from a common ancestor shows a close relationship between organisms
- Can be depicted by the construction of a phylogenetic tree or a cladogram
What do poly-genetic trees represent?
The evolutionary relationship among groups of organisms that are believed to have a common ancestor
What do the tips represent?
A species
What do the nodes represent?
A common ancestor or when an organism has undergone speciation
What do two descendants that split from the same node represent?
- Each other’s closest relatives
- Have a lot of evolutionary history
- Have a common ancestor that is unique to them
What is the single branch point from which all branches originate?
- Called the root of the tree
- The nodes closest to the root of the tree represent a common ancestor for all organisms in the tree
The longer the line….
The more time that has gone by
Explain how closely related organisms are using DNA sequences
- The less differences there are in DNA sequences, the closer the organisms are on the polygenetic tree
What is embryology?
- Comparing the very early stages of the development of organisms
- Although it is easy to distinguish between adults of different species, it is difficult to tell the difference between the embryos of different species
- All vertebrate embryos have gill arches and sacs, regardless of the type of respiratory organs found in the adult
- These embryos also lack appendages and have substantial tails
- This supports the idea of a common ancestor
What are homologous structures?
Organs that are similar in structure but may be used for a different function
Front limb
The high degree of similarity between these structures supports the idea of a common ancestor
Vestigial organs
- Structures that are reduced in size and now appear to have no function
- The presence of organs not required (non-functioning) supports the idea of a common ancestor
Examples of vestigial organs
- Nictating membrane
- Muscles to move ears
- Pointed canines
- Nipples on males
- Wisdom teeth
- Appendix
- Coccyx
- Erector pili muscles
- Wings in emus
- Pelvis and femurs in whales
Geographical distribution
- Isolated regions often evolve their own distinctive plant and animal populations e.g. Marsupials in Australia or different finches on different islands of the Galapagos
- Over many generations, natural selection would have favoured those characteristics that aided in survival in a particular set of conditions
- This also supports the theory of evolution
- Over years (and generations) random traits that provide an advantage in that particular area (environment) become common due to natural selection
- The allele frequency for particular traits become more common in particular areas
What is a fossil?
Any preserved trace or evidence left by a previously living organism
e.g. bones, teeth, footprints, faeces, burrows or egg shells
What is an artefact?
- Any object made by a human
- Can also be a fossil
- E.g. cave paintings, ancient pottery or arrow heads and weapons
What do fossils show?
- The sequence of development in plant and animal species
- The sequence of different life forms as they appeared on earth
- The sequence of changes in speciation
Fossil formation
- Very few organisms form fossils
- Most dead organisms undergo bacterial decay
- Predators and scavengers also prevent fossil formation
Things that enhance fossil formation
- Drifting sand, mud and volcanic ash
- Rapid burial
- Alkaline soils produce the best fossils as the minerals in the bones are not dissolved
What happens during fossilisation?
- New minerals (from the soil) are deposited in the pres of the bone
- The bone becomes petrified, but details of the structure are preserved
What happens in wet, acidic soils?
No fossilisaiton occurs
What happens in wet, acidic soils, with no oxygen (peat)
This may result in complete preservation of both the bones and the soft tissues
What is dating fossils?
Determining the age of the fossils
What is absolute dating?
- Stating the actual age of the fossils
- E.g. He is 20 years’ old
What is relative dating?
- Giving comparisons between the age of fossils
- E.g. He is older than her
Methods of absolute dating
- Potassium – argon dating
- Carbon 14 dating
- Tree ring dating
Methods of relative dating
- Stratigraphy
2. Fluorine dating
Carbon-14 dating
- Based on the decay of the radioactive isotope, carbon 14
- Produced in the upper atmosphere by the action of cosmic radiation on nitrogen at about the same rate at which it decays
- When green plants undergo photosynthesis, carbon-14 is absorbed
- Animals then eat the plants and the carbon-14 becomes a part of the animal’s tissue
- At death, the amount of C14 begins to decline at a fixed, exponential rate
- Therefore, the age of the sample can be determined by the amount of C14 present
What does the rate at which carbon 14 forms =?
The rate at which it decays
The amount of C14 in the atmosphere =
Constant
The amount of C14 in living organisms =
Constant
The half-life of C14 = 5700 years
- In 5700 years, the amount of C14 is reduced by half (1/2)
- In another 5700 years, the amount of C14 is reduced by half again (1/4)
- In another 5700 years, the amount of C14 left is reduced by half again (1/8)
Positives of carbon 14 dating
- It is a form of absolute dating, and provides an exact age of the organism
Negatives of carbon 14 dating
- Normal radio carbon dating requires at least 3 grams of organic material
- Can only be used on organic materials
- Can only be tested on materials less than 70000 years’ old, otherwise there is not enough carbon 14 present test
Accelerator mass spectrometry (AMS) radiocarbon dating
Requires only 100 micrograms of organic matter
Potassium – argon dating
- Based on the decay of radioactive potassium to form calcium and argon
- Decay takes place at a slow but constant rate
- Found in magma and lava, and decays as soon as it hits the atmosphere
- Measures the amount of calcium and argon compared to potassium
- Young (new) volcanic rock has lots of K and little Ca/Ar
- Old rock has lots of Ca/Ar and little K
Positives of potassium – argon dating
- Absolute dating (exact age)
- Has a slow half life
Negatives of potassium – argon dating
- Can only be used on volcanic rocks
- Only useful for old rocks (> 200,000 years old)
- It assumes that the fossil inside is the same age as the rock
Tree ring dating
- Trees grow one concentric ring in the trunk per year
- Good year → fatter ring
- Bad year → thinner ring
- Count rings → know the tree’s age
- Use of live and dead bristle cone pines (USA) have provided marker rings up to about 8600 years’ old
- Only useful if the tree is
Stratigraphy
- Rule of superposition says deeper layers are older
- Must take into account folding of layers, faulting and erosion
Index fossils
- Widely distributed for a short period of time
- Can compare the age of fossils to known index fossils
Fossil formation requires (limitations):
- A quick burial of remains
- The presence of hard body parts
- An absence of decaying organisms
- A long period of stability
