Selection and Evolution Flashcards
Phenotypic variation
difference in phenotypes between organisms of the same species
Phenotypic variation can be explained by
genetic or environmental factors
Example of phenotypic variation explained by genetic factors
the four different blood groups observed in human populations are due to different individuals within the population having two of three possible alleles for the single ABO gene
Example of phenotypic variation explained by environmental factors
clones of plants with exactly the same genetic information (DNA) will grow to different heights when grown in different environmental conditions
Example of phenotypic variation explained by a combination of genetic and environmental factors
the recessive allele that causes sickle cell anaemia has a high frequency in populations where malaria is prevalent due to heterozygous individuals being resistant to malaria
The complete phenotype of an organism is determined by
the expression of its genotype and the interaction of the environment on this
Phenotype = Genotype + Environment
genetic variation
-small differences in DNA base sequences between individual organisms within a species population
-is transferred from one generation to the next
-it generates phenotypic variation
Genetic variation is the result of
A new combination of alleles in a gamete or individual is caused by the following processes:
-Independent assortment of homologous chromosomes during metaphase I
-Crossing over of non-sister chromatids during prophase I
-Random fusion of gametes during fertilization
-Mutation
Mutation results
-In the generation of new alleles
-The new allele may be advantageous, disadvantageous or have no apparent effect on phenotype (due to the fact that the genetic code is degenerate)
-New alleles are not always seen in the individual that they first occur in
-They can remain hidden (not expressed) within a population for several generations before they contribute to phenotypic variation
The varying effects of genes on an organism’s phenotype
-The phenotype may be affected by a single gene or by several
-The effect that the gene has on the phenotype may be large, small and/or additive
Variation in phenotype caused solely by environmental pressures or factors cannot
be inherited by an organism’s offspring- only alterations to the genetic component of gametes will ever be inherited, environmental factors don’t impact the DNA
Discontinuous variation
-Qualitative (categoric) differences in the phenotypes of individuals within a population
-They are discrete and distinguishable categories, usually with no intermediates
Continuous variation
-Quantitative differences in the phenotypes of individuals within a population for particular characteristics
-Continuous variation have a range of values existing between two extremes within which the phenotype will fall
Genetic basis of discontinuous variation
-This type of variation occurs solely due to genetic factors, the environment has no direct effect
-Different genes have different effects on the phenotype causing variation
-Different alleles at a single gene locus have a large effect on the phenotype causing variation
Genetic basis of continuous variation
-This type of variation is caused by an interaction between genetics and the environment
-Different alleles at a single locus have a small effect on the phenotype
-Different genes can have the same effect on the phenotype and these add together to have an additive effect
polygenes
a large number of genes that have a combined effect on a phenotype
An example of the additive effect of genes
-The height of a plant is controlled by two unlinked genes H / h and T / t
-The two genes have an additive effect
-The recessive alleles h and t contribute x cm to the plants’ height
-The dominant alleles H and T contribute 2x cm to the plants’ height
-The following genotypes will have the following phenotypes:
*h h t t : x + x + x + x = 4x cm
*H H T T : 2x + 2x + 2x + 2x = 8x cm
*H h T t : 2x + x + 2x + x = 6x cm
*H H T t : 2x + 2x + 2x + x = 7x cm
*H h T T : 2x + x + 2x + 2x = 7x cm
*h h T t : x + x + 2x + x = 5x cm
*H h t t : 2x + x + x + x = 5x cm
t-test
A statistical test that can be used to compare the means of two sets of data and determine whether they are significantly different or not
Conditions that data sets need to meet in order to conduct a t-test
*follow a rough normal distribution
*be continuous
*standard deviations should be approximately equal
Before you can conduct a t-test you need
-The standard deviation (s) to be calculated beforehand for each data set
-A null hypothesis should be given (for example, there is no statistically significant difference between the means of sample 1 and sample 2)
If there is a statistically significant difference between the means of two sets of data then
the observation is not down to chance and the null hypothesis can be rejected
Formula for calculating standard deviation
S = √Σ(x-x̄)^2/n-1
where:
S = sample standard deviation
x = observation
x̄ = mean
n = sample size (number of observations)
Steps in a t-test
Step 1: Calculate the mean for each data set
Step 2: Calculate the standard deviation for each set of data, s1 = standard deviation of sample 1 and s2 = standard deviation of sample 2
Step 3: Square the standard deviation and divide by n (the number of observations) in each sample, for both samples
Step 4: Add the values from step 3 together and take the square root
Step 5: Divide the difference between the two means (see step 1) with the value calculated in step 4 to get the t value
Step 6: Calculate the degrees of freedom (v) for the whole data set
Step 7: Look at a table that relates t values to the probability that the differences between data sets is due to chance to find where the t value for the degrees of freedom (v) calculated lies
Step 8: The greater the t value calculated (for any degree of freedom), the lower the probability of chance causing any significant difference between the two sample means
*Identify where the t value calculated lies with respect to the confidence levels provided
If the t value is greater than the critical value (obtained from the table at the critical probability of 0.05) then any difference between the two data sets is less likely to be due to chance, so the null hypothesis can be rejected
*If the t value is less than the critical value given at a confidence of 5% / the probability that any difference is down to chance is above 0.05; then an assumption can be made that the differences between the means of the two sets of data are not significant and the null hypothesis is accepted
t=
|x̄1-x̄2|/√(S1^2/n1 + S2^2/n2)
provided in the exam paper
v=
(n1 - 1) + (n2 - 1)
exponential growth
the growth in a population due to the offspring of every individual surviving to adulthood and reproducing
when does exponential growth occur
when there are no environmental factors or population checks acting on the population (for example, when there are plentiful resources and no disease)
Environmental factors
limit population sizes by reducing the rate of population growth whenever a population reaches a certain size
Environmental factors can be
biotic or abiotic
Biotic factors involve
other living organisms; so things like predation, competition for resources and disease
Abiotic factors involve
the nonliving parts of an environment for example light availability, water supply and soil pH
Examples of selection pressures
-environmental factors
-phenotypic variation
How selection pressure affects phenotypic variation
Selection pressures increase the chance of individuals with a specific phenotype surviving and reproducing over others
higher fitness
organisms with the favoured phenotypes possess adaptations that make them better suited to their environment
fitness
an organism’s ability to survive and pass on its alleles to offspring
When selection pressures act over several generations of a species
they have an effect on the frequency of alleles in a population through natural selection
natural selection
the process by which individuals with a fitter phenotype can adapt better to their environment and are more likely to survive and pass on their alleles to their offspring so that the advantageous alleles increase in frequency over time and generations
Types of selection pressure
-Stabilising
-Disruptive
-Directional
Stabilising selection
natural selection that keeps allele frequencies relatively constant over generations - means that allele frequencies stay as they are unless there is a change in the environment
Directional selection
natural selection that produces a gradual change in allele frequencies over several generations
when does directional selection usually happen
when there is a change in the environment or a new selection pressure which leads to a specific allele becoming advantageous
Disruptive selection
natural selection that maintains high frequencies of two different sets of alleles; in other words, individuals with intermediate phenotypes or alleles are selected against
Disruptive selection maintains
polymorphism
polymorphism
the continued existence of two or more distinct phenotypes in species
events that cause a change in allele frequencies due to chance
-genetic drift
-founder’s effect
-bottleneck effect
How can natural selection cause changes to allele frequencies
-Directional selection produces a gradual change in allele frequencies over several generations
-This is because there’s always phenotypic variation within a population and selection pressure in the environment
-Some individuals in a population may have a phenotype that aids their survival in the presence of a selection pressure
-The phenotype is produced by particular alleles, so individuals with the favoured phenotype are fitter and so more likely to reproduce and pass on the advantageous alleles to their offspring
-Those who do not possess the advantageous allele or phenotype are less likely to survive and pass on their alleles to their offspring
-So over time and several generations, the frequency of the advantageous allele increases and the frequency of other alleles decreases
genetic drift
the gradual change in allele frequencies in a small population, where some alleles are lost or favoured just by chance and not by natural selection
How genetic drift causes changes to allele frequencies
-When a population is very small chance can affect which alleles get passed on to the next generation
-This is because meiosis results in haploid gametes, meaning that a fertilisation event only passes on half of the alleles of an individual; the half that gets passed on is the result of random fertilisation, and the other half of the alleles may be lost to the next generation
-Over time some alleles can be lost or passed on purely by chance
Why genetic drift is more likely to affect allele frequencies in a small population
the chances of a certain allele simply being lost by chance as a result of random fertilisation are much greater if only 10 pairs of organisms are breeding than if there were 100 pairs of organisms breeding
gene pool
the complete range of DNA base sequences in all the organisms in a species or population
founder effect
the reduction in a gene pool compared to the main populations of a species resulting from only two or three individuals (with only a selection of the alleles in the gene pool) starting of a new population
How the founder effect can cause changes to allele frequencies
-The founder effect occurs when a small number of individuals from a large parent population start a new population
-It can come about as the result of chance such as a storm separating a small group of individuals from the main population
-As the new population is made up of only a few individuals from the original population only some of the total alleles from the parent population will be present; not all of the gene pool is present in the smaller population
-Because the population that results from the founder effect is very small it is more susceptible to the effects of genetic drift
evolutionary bottleneck
a period when the number of species falls to a very low level, resulting in the loss of a large number of alleles and therefore a reduction in the gene pool of the species
How the bottleneck effect can cause changes to allele frequencies
-A major environmental event can greatly reduce the number of individuals in a population which in turn reduces the genetic diversity in the population as alleles are lost
-The surviving individuals end up breeding and reproducing with close relatives which makes it more susceptible to the effects of genetic drift
Antibiotics
chemical substances that inhibit or kill bacterial cells with little or no harm to human tissue
bactericidal antibiotics
antibiotic that kills bacterica
bacteriostatic antibiotics
antibiotic that inhibits the growth processes of bacteria
how are the alleles that confer resistance to the effects of the antibiotic generated
through random mutation
What happens when a new allele arises in bacteria
It is immediately displayed in its phenotype because each bacteria contains only one loop of DNA that has only one copy of each gene
When an antibiotic is present in bacterial culture
-Individuals with the allele for antibiotic resistance have a massive selective advantage so they are more likely to survive, reproduce and pass their genome (including resistance alleles) on
-Those without alleles are less likely to survive and reproduce
-Over several generations, the entire population of bacteria may be antibiotic-resistant
What happens when we overuse antibiotics
selective pressure is exerted on the bacteria, which supports the evolution of antibiotic resistance
How do bacteria pass on alleles for antibiotic resistance
1)through reproduction (vertical gene transfer)
2)through the transfer of plasmids (a small circular piece of DNA that is not the main chromosome that often contains alleles for antibiotic resistance) so antibiotic resistance can be passed from one species of bacteria to another species (horizontal gene transfer)
Hardy-Weinberg principle
states that if certain conditions are met then the allele frequencies of a gene within a population will not change from one generation to the next
The Hardy-Weinberg equation allows for
-the calculation of allele and genotype frequencies within populations
-predictions to be made about how these frequencies will change in future generations
7 conditions or assumptions for the Hardy-Weinberg principle
1)Organisms are diploid
2)Organisms reproduce by sexual reproduction only
3)There is no overlap between generations, i.e. parents do not mate with offspring
4)Mating is random
5)The population is large
6)There is no migration (no individuals entering the population (immigration) or leaving (emigration)), mutation, or selection (natural and artificial selection)
7)Allele frequencies are equal in both sexes
Why is the Hardy-Weinberg principle more useful when building models and making predictions
because the assumptions listed are very rarely, if ever, all present in nature
When using the Hardy-Weinberg equation frequencies are represented as
proportions of the population; a proportion is a number out of 1
The frequency of alleles can be represented;
this is the proportion of all of the alleles in a population that are of a particular form
The letter p represents the frequency of
the dominant allele
The letter q represents the frequency of
the recessive allele
p + q =
1
The frequency of genotypes can be represented;
this is the proportion of all of the individuals with a particular genotype
The chance of an individual being homozygous dominant is
p^2 i.e., the offspring would inherit dominant alleles from both parents so p x p = p^2
The chance of an individual being heterozygous is
2pq i.e., offspring could inherit a dominant allele from the father and a recessive allele from the mother (p x q) or offspring could inherit a dominant allele from the mother and a recessive allele from the father (p x q) so 2pq
The chance of an individual being homozygous recessive is
q^2 i.e., the offspring would inherit recessive alleles from both parents so q x q = q2
p2 + q2 + 2pq =
1
artificial selection/selective breeding
the process by which humans choose organisms with desirable traits and selectively breed them together to enhance the expression of these desirable traits over time and many generations
why is knowledge of the alleles that contribute to the expression of the desired traits not required for selective breeding
individuals are selected to breed by their phenotypes, and not their genotypes
How do breeders accidentally enhance other traits (that might negatively impact the health of the organism) that are genetically linked to the desirable trait
They don’t fully understand the genetics
Examples of artificial selection include:
-Increased milk yield from cattle
-Faster racehorses
-Disease-resistant crops
Principles of selective breeding
- The population shows phenotypic variation - there are individuals with different phenotypes/traits
- Breeder selects an individual with the desired phenotype
- Another individual with the desired phenotype is selected. The two chosen individuals should not be closely related to each other
- The two selected individuals are bred together
- The offspring produced reach maturity and are then tested for the desirable trait. Those that display the desired phenotype to the most significant degree are selected for further breeding
- The process continues for many generations: the best individuals from the offspring are chosen for breeding until all offspring display the desirable trait
the introduction of disease resistance to varieties of wheat
-Wheat crops can be badly affected by fungal diseases, for example, Fusarium is a fungus that causes “head blight” in wheat plants
-Fungal diseases are highly problematic for farmers as they destroy the wheat plant and reduce crop yield
-By using selective breeding to introduce a fungus-resistant allele from another species of wheat, the hybrid wheat plants are not susceptible to infection, and so yield increases
-Introducing the allele into the crop population can take many generations and collaboration with researchers and plant breeders
the introduction of disease resistance to varieties of rice
-Rice plants are prone to different bacterial and fungal diseases, examples include “bacterial blight” and “rice blast” caused by the Magnaporthe fungus
-These diseases all reduce the yield of the crop as they damage infected plants
-Scientists are currently working hard to create varieties of rice plants that are resistant to several bacterial and fungal diseases using selective breeding
Inbreeding in maize
-Maize plants have been heavily inbred (bred with plants with similar genotypes to their own) resulting in small and weaker maize plants that have less vigour
-Inbreeding depression causes:
*increase in the chance of harmful recessive alleles combining in an individual and being expressed in the phenotype
*increase in homozygosity in individuals (paired alleles at loci are identical)
*decreased growth and survivability
Outbreeding in maize
A farmer can prevent inbreeding depression by outbreeding
*This involves breeding individuals that are not closely related which produces taller and healthier maize plants
*decreases the chance of harmful recessive alleles combining in an individual and being expressed in the phenotype
*Increases heterozygosity (paired alleles at loci are different)
*leads to increased growth and survivability (known as hybrid vigour)
*crops of these plants have a greater yield
Why is uniformity important when growing a crop
-If outbreeding is carried out completely randomly, it can produce too much variation between plants within one field
-A farmer needs the plants to ripen at the same time and be of a similar height; the more variation there is, the less likely this is
What do farmers do in order to achieve heterozygosity and uniformity in their crops?
-Farmers buy sets of homozygous seeds from specialised companies and cross them to produce an F1 generation
-Different hybrids of maize are constantly being created and tested for desirables traits such as resistance to pests/disease, higher yields and good growth in poor conditions
Improving milk yield in cattle
-Over many years and generations farmers have selected female cows that have the highest milk yield and crossed them with male bulls related to high-yield females
-Over time this selective breeding has resulted in cows with greater milk yields, which has been of great economic benefit to farmers
How selective breeding of cows for increased milk yield doesn’t take the organisms’ survival into account
-Selective breeding usually focuses on only one, or a handful of, characteristics, often to the extreme. Little thought is given to other traits essential to an organism’s health
-In cows, it has been observed that selectively bred individuals are much more prone to ailments such as mastitis (inflammation of the udder), milk fever and lameness compared to those that were allowed to mate at random
Evolution
the formation of new species from pre-existing species over time, as a result of changes to gene pools from generation to generation
For a population to have evolved into a separate species it must
be genetically and reproductively isolated from the pre-existing species population
reasons for reproductive isolation occurring
due to mutations that lead to the incompatibility of gametes or sex organs, or differences in breeding behaviour
genetically isolated
when two populations can’t exchange exchange genes with each other in the production of offspring
isolated populations and allele frequencies
-Changes in the allele frequencies of isolated populations are not shared so they evolve independently of each other
-This can lead to the formation of two groups that are no longer successfully able to interbreed and that are said to be separate species
-The formation of new species in this way is known as speciation
How evolutionary relationships between species can be shown
By analysing and comparing the sequence of the DNA found in the nucleus, mitochondria and chloroplasts of cells to creating family trees
Information that can provided by differences between the nucleotide sequences (DNA) of different species
-The more similar the sequence the more closely related the species are
-Two groups of organisms with very similar DNA will have separated into separate species more recently than two groups with less similarity in their DNA sequences
When analysing DNA from the mitochondria is is important to remember that
-A zygote only contains the mitochondria of the egg and none from the sperm so only maternal mitochondrial DNA is present in a zygote
-There is no crossing over that occurs in mitochondrial DNA so the base sequence can only change by mutation
Analysing mitochondrial DNA
-The lack of crossing over in mtDNA has allowed scientists to research the origins of species, genetic drift and migration events; it has even been possible to estimate how long ago the first human lived and where
-Mitochondrial Eve is thought to have lived in Africa ~200,000 years ago
-The estimation of this date relies on the molecular clock theory which assumes there is a constant rate of mutation over time
-The greater the number of differences there are between nucleotide sequences, the longer ago the common ancestor of both species existed
-The molecular clock is calibrated by using fossils and carbon dating than a fossil of a known species is carbon-dated to estimate how long ago that organism lived
-This mtDNA of this species is then used as a baseline for comparison with the mtDNA of other species
Evolution causes
speciation
speciation
the formation of new species from pre-existing species over time, as a result of changes to gene pools from generation to generation
The two different situations when speciation can take place
-Two groups of a species are separated by a geographic barrier
-Two groups of species are reproductively isolated but still living in the same area (experiencing similar environmental selection pressures)
Allopatric speciation
-occurs as a result of geographical isolation
-is the most common type of speciation
-A species population splits into one or more groups which then become separated from each other by geographical barriers
-barrier could be natural like a body of water, or a mountain range or man-made (like a motorway)
How geographical isolation leads to speciation
-This separation creates two populations of the same species who are isolated from each other, and as a result, no genetic exchange can occur between them
-If there is sufficient selection pressure or genetic drift acting to change the gene pools within both populations then eventually these populations will diverge and form separate species
-The changes in the alleles/genes of each population will affect the phenotypes present in both populations
-Over time, the two populations may begin to differ physiologically, behaviourally and morphologically (structurally)
Sympatric speciation
-Takes place with no geographical barrier
-A group of the same species could be living in the same place but in order for speciation to take place there must exist two populations within that group and no gene flow occurs between them
-Something has to happen that splits or separates the population:
~Ecological separation: Populations are separated because they live in different environments within the same area [For example, soil pH can differ greatly in different areas. Soil pH has a major effect on plant growth and flowering]
~Behavioural separation: Populations are separated because they have different behaviours [For example differences in feeding, communication or social behaviour]
