BIOLOGY 1A: VARIATION

Why are offspring resemble their parents?

• Parents pass on information to their offspring through genes (hereditary unit)

• Genes contain sequences of DNA that code for different enzymes and proteins, producing traits (eg. eye colour)

• Genes are passed on by packages known as chromosomes

• Each species has a characteristic number of chromosomes (eg. humans have 46, fruit flies have 8 and sheep have 54)

• Chromosomes are passed on through DNA replication, which produces copies of genes that can be passed down from parents to offspring

When are parents and offspring genetically identical?

• When:

  • Reproduction is asexual

  • A single individual is the sole parent

• The offspring will inherit all of the parent’s genes through mitotic division (mitosis)

• The parent and offspring may not be phenotypically identical because factors such as environment can influence both of their phenotypes

Draw (with labels) a diagram showing the process of mitosis

When does genetic variation arise in asexual reproduction?

• Through mutations

Does sexual reproduction generate new genetic variation?

• No

  • It rearranges pre-existing genetic variation to produce new combinations of genes

  • Only mutations introduce new genetic variation

What are the 3 mechanisms of genetic variation in sexual reproduction?

• Independent assortment of chromosomes (during meiosis)

  • Chromosomes are assorted randomly into gametes, resulting in the gametes having a random mix of maternal and paternal chromosomes

• Crossing-over (during meiosis)

  • Chromosomes cross over, resulting in them having a mix of maternal and paternal genetic material

• Random fertilization (during fertilization)

  • The gametes from each parent that fuse during fertilization is random

Draw (with labels) a diagram for the process of independent assortment of chromosomes

Draw (with labels) a diagram for the process of crossing-over

What is the Mendelian law of segregation?

• States that: during gamete formation in each parent, their alleles segregate and end up in different gametes

• 2 random gametes (one from each parent) fuses during fertilization

What is dependent assortment of alleles?

• The inheritance of an allele from one gene is dependent on the inheritance of an allele from another gene (the alleles pf the genes are inherited together)

• The loci are close to each other on the same chromosome

• Phenotypic ratio: 3:1

What is independent assortment of alleles?

• The inheritance of alleles of different genes are independent of each other (alleles can be inherited separately)

• The loci are far apart on the same chromosomes or on different chromosomes

• Phenotypic ratio: 9:3:3:1

What are the probability laws that govern Mendelian inheritance?

• Probability of 2 independent events occurring = probability of event 1 occurring X probability of event 2 occurring

• Probability of 2 mutually exclusive events occurring = probability of event 1 occurring + probability of event 2 occurring

What is Hardy-Weinburg equilibirum?

• States that: if a population is not evolving, genotype and allele frequency will remain constant through the generations

• p + q = 1

  • Where p = frequency of dominant alleles and q = frequency of recessive alleles

What is the Hardy-Weinburg equation?

• p² + 2pq + q² = 1

  • Where p² is the frequency of homozygous dominant alleles, 2pq is the frequency of heterozygous alleles and q² is the frequency of homozygous recessive alleles

What are the conditions for Hardy-Weinburg equilibrium?

• No mutations

• Mating is random

• No selection

• Extremely large population size

• No gene flow

What are complex traits?

• Known as quantitative traits

• Traits that are dependent on more than one gene, where no allele is completely dominant over another allele

What is polygenic inheritance?

• The inheritance of quantitative or complex traits

• Gives rise to a wider variety of phenotypes

What is heritability?

• Proportion (from 0 to 1) that indicates how much of phenotypic variation is due to genetic differences

• Can be calculated by:

  • h² = additive genetic variation (Va)/phenotypic variation (Vp)

  • h² = 2(slope) of a parent-offspring regression

What is a parent-offspring regression?

• Shows the relationship between variation in a parent’s trait (x-axis) and variation in offsprings’ trait (y-axis)

• Steeper slow indicates that the trait is more heritable and genes are more important in determining the variation of a trait

• A near-horizontal slope indicates that the trait is less heritable and variation of the trait is mostly due to environment

What is natural selection?

• Production of more individuals than an environment can support leads to a struggle for existence between the individuals

• Individuals with traits better suited for the environment are more likely to survive and reproduce, passing on their favourable alleles to their offspring

• Over generations, the more favourable traits become more common and the less useful traits may disappear

• Natural selection only acts on complex traits

Conditions for natural selection

• Variation in traits within a population

• Some of the variation in traits has to be heritable and based on genetic differences

• The trait must influence the ability to survive and reproduce, affecting the genetic contribution to the next generation

What is homology?

• Similarities in different species due to a shared common ancestor

• Types of homology:

  • Developmental homology: many different species have similar early developmental stages

  • Molecular homology: the core of metabolism for all organisms is the same, though the details may differ

  • Homology in the genetic code: all organisms (except a few viruses) uses DNA as genetic information

What is fitness?

• Measure of how much a genotype contributes to the next generation

What is relative fitness (W)?

• The fitness of a genotype relative to the fittest genotype in the population

What is coefficient of selection (s)?

• Measure of how strongly natural selection acts on a genotype

• Calculated by: 

  • s = 1 - W

• If s = 0, the genotype has the highest fitness or has the same fitness as the fittest genotype

How does differences in fitness between genotypes affect evolutionary change?

• Bigger difference in phenotypes = more rapid evolutionary change

• If the fitness of a genotype decreases, the coefficient of selection increases

• Natural selection acts more strongly against the less-fit genotype and more strongly for the fitter genotype

• The fitter genotype contributes more of its alleles to the next generation (increase in allele frequency)

• Due to selection pressure, the less-fit genotypes contribute less of their alleles and diminish over time

What determines how the speed of alleles spreading throughout a population?

• The relative fitness of the genotypes in the population

• Whether the allele is dominant or recessive

What are continuous traits?

• Known as complex, quantitative or polygenic traits

• Traits that depend on more than one gene and the environment

What are the types of natural selection?

• Directional selection

  • Favours one extreme and acts against the other extreme

• Stabilizing selection

  • Favours the intermediate or average values and acts against the 2 extremes

  • Keeps the intermediate or average values the same but reduces variation

• Disruptive selection

  • Favours the 2 extremes and acts against the intermediate or average values

  • Results in the population having different morphs

What is the relationship between natural selection and the parent-offspring regression?

• The less steep the slope, the less heritable the trait and the less the trait will respond to natural selection

• An entirely horizontal graph indicates that the trait is not heritable at all and natural selection cannot occur 

  • Due to one of the conditions for natural selection: some of the variation must be heritable and based on the differences in genes

How does mutation occur?

• It is a random process

• Whether a particular mutation occurs is not dependent on how useful it would be in the environment

  • Organisms do not produce mutations just because they need them

What is the significance of mutations in evolution?

• Mutations are the source of all genetic variation

• When there are no mutations, there is no genetic variation in a population and natural selection cannot occur, meaning that there is no evolution

What are the rate of mutations dependent on?

• Species

• Environment (eg. some bacteria increases their rate of mutations in response to stress)

What are the effects of mutations on phenotype?

• Mutations in non-coding regions of the genome may have no effect

  • Mutations in introns do not get translated into proteins

  • Splicing gets rid of the introns (unnecessary parts of the mRNA) before the mRNA is translated into proteins

• Mutations in exons may not affect the protein being produced

  • Multiple codons code for the same amino acid

  • The 3rd base of the codon is redundant and any mutation occurring there does not affect the amino acid

  • Known as synonymous mutations

• Mutations in the exons may also affect the protein being produced

  • If the mutations occur at the 2nd base of the codon, the amino acid will be affected

  • Known as non-synonymous mutations

• Mutations that affect the protein sequence have phenotypic effects that are hard to predict

What are deleterious mutations?

• Mutations that affect an a individual’s phenotype and has a negative effect on fitness

What are the factors that affect the effects of mutations on fitness?

• Can be displayed on a phenotype space

• How well adapted a population is

  • Well adapted (old environment): a greater proportion of the mutations will bring the fitness level of the population away from the optimal fitness (deleterious mutations)

  • Not well adapted (new environment): a greater proportion of the mutations will bring the fitness level of the population close to the optimal fitness (positive effect)

• How big the effect of the mutations are

  • Bigger effect: more likely to bring the fitness of the population away from the optimal fitness (deleterious mutations)

  • Smaller effect: more likely to bring the fitness of the population closer to the optimal fitness (positive effect)

    • Most adaptations in a population is due to mutations of small effect

Why is there so much genetic variation?

• Even though natural selection seeks to reduce genetic variation in a population by increasing the frequency of alleles with high fitness (to fixation) and removing the alleles with low fitness, there is still genetic variation 

• Reasons include:

  • Neutral mutations

    • Neutral mutations have no effect on phenotype and fitness

    • Not affected by natural selection and instead by random processes such as genetic drift

    • Neutral alleles can persist in a population for long periods of time without being fixed

    • Variation is maintained because natural selection does not act to remove the neutral alleles

  • Mildly deleterious mutations

    • Results in alleles that reduce fitness by a very small amount

    • Natural selection is weak when the mutations have small effects

    • Variation is maintained because of mutation selection balance, where mildly deleterious mutations are constantly being introduced but natural selection is inefficient at removing these mutations

  • Disruptive selection

    • Selection favours 2 extremes, leading to diverse morphs within a population

  • Heterozygous advantage

    • Heterozygous alleles are fitter than either the homozygous dominant or homozygous recessive alleles

    • Since both alleles are required to produce the fittest genotype (eg. Aa), natural selection does not act against either one

    • This causes both alleles to remain at an intermediate equilibrium, with neither going to fixation

  • Frequency dependent selection

    • The fitness of each phenotype depends on their frequency

    • None of the alleles are able to fully replace others and all the alleles will be maintained

Why is there genetic variation between populations?

• Local adaptation

  • The environment varies in different populations, causing different phenotypes to be favourable in different populations

  • Variation occurs when different alleles are fixed in different populations

• Random divergence

  • The frequency of neutral alleles are determined by random processes such as genetic drift instead of natural selection

  • Random divergence will operate independently in each population, causing the frequency of alleles to differ among populations

  • The alleles will start off with relatively similar frequencies but they will diverge as time passes (alleles in some populations may be fixed while those in other populations may be lost)

  • Most important for neutral alleles and may affect non-neutral alleles

    • Effects on non-neutral alleles will be very weak unless the alleles have very small effects on fitness

• Migration

  • The movement of individuals from one population to another, introducing gene flow between populations

  • Homogenises populations genetically, making different populations more genetically similar than they were before

    • Reduces genetic diversity between populations

    • May increase genetic diversity within a population

  • The more migration occurs, the more similar the frequency of alleles are between the populations, with the trends following a similar trajectory

What is the impact of migration on local adaptation?

• Alleles that are favoured in one population will flow to the other population, where they may be less fit due to the differing environments of both populations

What is a species?

• Defined as a group of organisms that commonly interbreed in the wild (and do not interbreed with other similar species and are reproductively isolated from other species)

How do we determine if a group of organisms are from the same species or different species?

• Can use different concepts

• Biological species concept

  • Same species:

    • Fertilization can occur

    • Offspring are viable and fertile

  • Different species:

    • Fertilization cannot occur

    • Offspring are not viable and sterile

• Morphological species concept

  • Same species::

    • Have the same physical characteristics (eg. size, shape)

  • Different species:

    • Have different physical characteristics

• Physiological species concept

  • Same species:

    • Have the same bodily systems or enzyme functions

  • Different species:

    • Have different bodily systems or enzyme functions

• Ecological species concept

  • Same species:

    • Lives in the same environmental and uses the same resources

  • Different species:

    • Lives in different environments and uses different resources

• Phylogenic species concept

  • Same species:

    • Clustered together on a phylogenic tree

  • Different species:

    • Clustered apart from each other on a phylogenic tree

  • The phylogenic tree can be generated by extracting genetic information from a group of individuals and observing their similarities and differences

What are the 2 ways that 2 species can be reproductively isolated from one another?

• Pre-zygotic (before fertilization)

  • Prevents mating or fertilization from occurring

  • Eg. different species have different mating dances and live in different habitats

• Post-zygotic (after fertilization)

  • Prevents the hybrid offspring from developing properly (hybrid inviability) or reproducing (hybrid sterility)

What are the limitations of the biological species concept?

• Only applies to sexual organisms

• Requires a substantial amount of work to demonstrate

• Does not always work

What are the 2 kinds of evolutionary change?

• Anagenesis

  • Selection acting within a species and causes it to change

• Cladogenesis

  • The ancestral species splits into 2 daughter species

What are the different process that allow reproductive isolation to occur?

• Geographical isolation (allopatric isolation)

  • Main mechanism of selection in animals

  • Can stop gene flow for long enough for reproductive isolation to evolve

    • The population is geographically split into 2 different populations

    • Each population evolves independently and there is no gene flow

    • The populations are geographically brought back together but now they are different (eg. difference in mating displays, morphology)

    • The population is now diverged with individuals with different characteristics, increasing biodiversity

• No geographical isolation (sympatric isolation)

  • Can occur through polyploidy in plants but are very rare in animals

    • The diploid cell, which has 2 copies of each gene, becomes a polyploid cell, which has 4 copies of each gene

    • This results in a diploid gamete that is able to self-pollinate (does not need another parent)

    • This stops gene flow, reducing biodiversity