Genetic Variation and Evolution - Chapter 4.4 Study Notes
Mutations
Mutations are permanent changes in the sequence of nucleotides of DNA and are the ultimate source of genetic variation in a species.
They produce new forms of a gene (alleles) in populations.
Types of mutations include: base substitution, insertion or deletion mutations to large-scale chromosomal mutations (Chapter 1.6).
Mutations in somatic cells can cause disease but are not passed to future generations.
Mutations in germline cells (cells that give rise to sperm or eggs) can be passed to the next generation and appear in all new cells produced in the embryo.
Mutations can be lethal, disadvantageous, neutral or beneficial.
Lethal Mutations
A zygote inherits half of its genetic material from its father (paternal) and half from its mother (maternal).
Genotype = the alleles of a gene inherited by an individual.
Phenotype = the appearance of an organism that is determined by genotype and the environment.
Genes can act in a recessive or dominant manner.
A characteristic is expressed when at least one dominant gene is in the genotype, or when two copies of a recessive gene are present (recessive expression).
Individuals have 2 alleles for each gene; they will be either:
homozygous (2 dominant alleles or 2 recessive alleles), or
heterozygous (one dominant allele and one recessive allele).
Cystic Fibrosis
Cystic Fibrosis (CF) is a genetic disease caused by a faulty recessive allele of a gene (Chapter 1.6).
The normal allele (C) is dominant over the recessive mutant allele (c).
Genotypes and phenotypes:
CC: Normal (does not carry the CF allele)
Cc: Normal (but carrier, one CF allele)
cc: Cystic Fibrosis sufferer (two CF alleles)
Inheritance: If an individual inherits two faulty alleles (cc), one from each parent, the child will suffer from the disease.
Sickle-Cell Anaemia
Cause: a change of one base leading to a protein with one amino acid changed (Chapter 1.6).
The specific change: an A changes to a T in a substitution mutation in the codon for the 6th amino acid in the beta chain. The codon changes from GAG to GTG, so glutamic acid is replaced by valine.
Normal haemoglobin HbA and mutant (HbS).
Genotypes:
HbA HbA (normal)
HbS HbS (sickle-cell)
HbA HbS (heterozygous)
Homozygous individuals: HbA HbA or HbS HbS.
Heterozygous individuals: HbA HbS.
Sickle-Cell Anaemia: Co-dominance
HbA and HbS exhibit a form of co-dominance where the heterozygous form (HbA HbS) expresses features from both alleles (normal and abnormal haemoglobin are both produced).
In mutant haemoglobin, red blood cells change from normal bi-concave shape to a sickle-cell shape.
The disease is called sickle-cell anaemia and is often fatal in homozygous individuals.
Source: https://www.youtube.com/watch?v=R4-c3hUhhyc
Disadvantageous Mutations
Some mutations are disadvantageous; they cause disease in those with the mutant gene, but are not generally fatal.
Osteoporosis is a disease affecting over 1 million people in Australia.
In osteoporosis, brittle bones are prone to fracture; the gene ‘low-density lipoprotein receptor related protein 5’ (LRP5) is involved in bone density, and mutations in this gene can cause osteoporosis.
Affected bones tend to lose minerals (e.g., calcium) into the blood, leading to low bone density.
Neutral Mutations
A neutral mutation is described as having no effect on appearance (phenotype) or function.
It is thought that the majority of mutations are neutral.
Some scientists question the neutral definition, arguing that mutations alter genotype and can be passed to future generations through the germline.
Changes, perhaps in non-coding regions of DNA, may cause unknown changes to function.
Beneficial Mutations
Heterozygotes for sickle-cell anaemia (HbA HbS) have some abnormal haemoglobin but more normal haemoglobin, and suffer from mild anaemia; the carrier state provides an advantage in malaria-infested areas because Plasmodium cannot reproduce in red blood cells containing HbS).
Thus, HbA HbS carriers have a reduced chance of contracting malaria.
Mutations in the LRP5 gene can give rise to osteoporosis; in a real-world anecdote, a boy involved in a serious car crash had no broken bones due to a different mutation in the same LRP5 gene that increased bone density and resistance to fractures.
Sexual Reproduction
Species that reproduce sexually and use meiotic division to produce sperm and eggs introduce variation so offspring do not look identical to their parents.
Variation is expressed as different genotypes (allele pairs) and consequently different phenotypes (appearance).
Mutations are the only source of new alleles of genes.
Sexual reproduction re-shuffles alleles from parents to produce new allele combinations in offspring.
Sexual Reproduction: Mechanisms of Variation
Meiosis:
Crossing Over: a process that introduces new combinations of alleles from maternal and paternal genes.
Independent Assortment of Chromosomes: a process that produces new combinations of maternal and paternal chromosomes in gametes.
Fertilisation: the random fusion of an ovum and a sperm, each with their own unique set of chromosomes and alleles.
Genotypic and phenotypic variation within a species is important for natural selection and evolution. Populations of sexually reproducing species show variation.
Gene Pool
A population is a group of individuals of the same species that breed in a particular habitat.
The gene pool of a population is the sum of all alleles (and thus all genetic information) of all individuals.
The genotype of an organism, along with environmental effects, determines its phenotype.
Allele frequency refers to the relative proportion of a particular allele in the population (usually expressed as a fraction or percentage).
Populations with large gene pools and diverse alleles are more robust and more likely to survive changing or harsh environmental conditions.
Populations with reduced genetic diversity are at greater risk of extinction.
Allele Frequency (Example: Flower Colour)
Phenotypes: Red, White, Pink
Phenotype frequencies (per 500 members):
Red: 320
White: 20
Pink: 160
Genotypes:
CRCR
CWCW
CRCW
Genotype frequencies:
CRCR:
CWCW:
CRCW:
Alleles:
CR and CW
Allele numbers:
Allele frequencies:
Figure 4.31: Table describing allele frequency of flower colour in wildflowers.
Changes In The Gene Pool
The gene pool of populations changes over time.
Major factors that alter allele frequency and bring about evolutionary change are:
Mutations
Natural selection
Genetic drift
Gene flow
Natural Selection
Charles Darwin provided the first explanation for evolution.
In 1831, Darwin left England on the Beagle to tour the coast of South America.
In the Galapagos Islands, he observed variation within a population and between related species.
Darwin hypothesized that for each population, more individuals are born and survive to reproduce, and environmental factors regulate populations.
Darwin and finches illustrate how different environments lead to different beak shapes as adaptations for available food.
Natural selection is the mechanism by which variation providing reproductive advantage leads to more offspring, driving evolution.
Natural Selection: Survival Of The Fittest
Natural selection is the process whereby environmental or selective pressures act on the phenotypes of individuals in a population and lead to changes in allele frequency.
“Survival of the fittest” means alleles of favored traits increase in frequency over time, while alleles selected against decrease.
Natural Selection: Selective Pressures
Biotic factors:
Predators
Disease-causing organisms
Competition within or between species
Symbiotic relationships
Human activities
Abiotic factors:
Rainfall
Temperature
Nutrient levels
Light intensity
Medical drugs (e.g., antibiotics) or chemicals (pesticides)
A famous example: industrial melanism in peppered moths (case study on page 402 of the textbook).
Peppered moths: a video resource is linked: https://www.youtube.com/watch?v=3E4XZB6lKOE
Changes In The Gene Pool: Mutations, Gene Flow, Genetic Drift
Mutations: Permanent changes to DNA sequences are the only way new alleles appear.
Gene Flow: Movement of alleles between populations of the same species; can change allele frequencies in the gene pool.
Increased gene flow reduces differences between populations; limited gene flow promotes divergence and localized evolution.
Genetic Drift: Random changes in allele frequencies; more pronounced in small populations.
The bottleneck effect: random factors (e.g., bushfires) reduce population size, reducing genetic variation for many generations.
When bottlenecks occur, genetic diversity is low; drift and bottlenecks may contribute to this.
Founder effect: a special case of drift when a new population is started by a small number of individuals.
YouTube resources provide visual explanations: https://www.youtube.com/watch?v=-UfrN11V9SM
Summary of Evolution By Natural Selection
Those best adapted to environmental conditions are more likely to survive to reproductive age and pass on favorable alleles.
A population experiences a struggle for survival due to biotic or abiotic selective pressures.
Individuals show phenotypic variation due to environmental effects and genetic differences, which are inherited.
Summary of Evolution By Natural Selection (Continued)
Environmental selective pressures determine reproductive success; populations evolve over many generations with changes in the gene pool.
The effects of natural selection can be measured by changes in allele frequencies, providing evidence of evolution.
It is populations or species that evolve, not individual organisms.
Key Concepts
Mutations are permanent changes to DNA sequences; if they occur in germline cells, they can be passed to the next generation. They are the only source of new alleles in populations and can be lethal, disadvantageous, neutral or beneficial.
Offspring of sexually reproducing species differ from their parents in genotype and phenotype.
Sexually reproducing species introduce variation via meiosis (crossing over and independent assortment) and random fertilization.
Key Concepts (Gene Pool and Natural Selection)
A gene pool for a population comprises all of the alleles found in the population.
Populations with greater biodiversity in their gene pools are more likely to survive selective pressures.
Natural selection is essentially ‘survival of the fittest’, where better-adapted organisms are more likely to survive and pass on their genes.
Natural selection changes allele frequencies in populations, providing evidence of evolution.