Variation and Selection
Continuous and Discontinuous Variation
Variation Definition: The differences in characteristics that exist between individuals belonging to the same population or species.
Continuous Variation:
Exhibits many intermediate characteristics within a population.
Involves a wide range of individuals whose statistics produce a normal distribution curve when plotted on a line graph.
Characteristics are not easily distinguishable into discrete categories.
Environmental factors affect the way the genes express themselves.
Characteristics are brought about by the combined effects of many genes (polygenic inheritance).
Examples: Intelligence, height, weight, colour of skin.
Discontinuous Variation:
Features sharply contrasting characteristics with no intermediate characteristics.
Data is normally presented in a bar graph.
Characteristics can be distinguished easily into separate categories.
The environment has little or no effect on the way the genes express themselves.
Brought about by one or only a few genes.
Examples: ABO blood group, ability to roll tongue, fingerprints, eye colour.
Causes of Variation
Variation is caused by the combination of genes during sexual reproduction, mutations, and the immigration of individuals with new gene combinations.
Sexual Reproduction:
Random Assortment: Occurs during the formation of gametes by meiosis.
Random Fertilization: Random pairing of individual gametes during fertilization gives rise to new sets of gene combinations in offspring.
Note: Sexual reproduction can only produce new combinations of existing genes.
Crossing over
Immigration:
When new individuals enter a population, they introduce new genes.
When these individuals mate and reproduce with existing members of the population, new gene combinations occur.
Note: Immigration introduces new genes into a gene pool.
Mutations:
Spontaneous (sudden) changes in chromosomes or genes that cause coding for a different characteristic.
Note: Mutations introduce new genes into a gene pool.
Gene and Chromosome Mutations
Mechanisms and Mutagens:
Chromosomes carry genes that control protein synthesis. A change in the number, size of chromosomes, or chemical structure of a gene can result in a different protein or enzyme, causing a different characteristic to arise.
Mutations are naturally very rare.
The rate of mutation is greatly increased by mutagens:
Environmental Mutagens: Ultraviolet (UV) light and high-energy radiations.
Chemical Mutagens: Artificial sweeteners such as cyclamates (now banned) and formaldehyde at certain concentrations.
Gene Mutations:
Definition: A change in the sequence of base pairs in a DNA molecule, making it code for a different characteristic.
A gene may mutate to become dominant or recessive. The mutation may be inherited and persist within a species.
In Bacteria and Insects:
Gene mutations in bacteria and insects are generally beneficial to them and increase their chance of survival.
Antibiotic Resistance: Bacteria may mutate to become resistant to antibiotics.
Immunity Challenges: Humans are unable to maintain lifelong immunity against some bacterial infections. Primary bacterial infections result in the production of antibodies and memory cells, but memory cells may not be effective during a secondary infection due to new mutant strains of the bacteria.
Pesticide Resistance: Mutant forms of insects become resistant to chemical pesticides.
In Humans:
Gene mutations in humans may lead to severe genetic diseases.
Sickle Cell Anaemia:
Caused by a mutation in the gene for the blood pigment, haemoglobin.
When faulty haemoglobin is present in a red blood cell, it causes the cell to deform and become sickle-shaped, especially when oxygen levels in the blood become low.
Sickled red blood cells are rigid (not flexible like normal red blood cells) and more likely to become stuck in a capillary, preventing blood flow.
Sickled red blood cells have a short lifespan; every few days they break down, leading to a constant shortage of red blood cells.
Alleles for sickle cell anaemia display incomplete dominance:
— not anaemic
— sickle cell traits (carrier, non-anaemic)
— anaemic
Haemophilia:
Haemophilic blood lacks necessary blood clotting factors.
Caused by a mutant recessive allele found on the X chromosome.
Chromosome Mutations:
Definition: Involves the change in number and size of chromosomes, resulting in a different phenotype.
Caused by abnormal behavior of chromosomes during meiosis.
Down Syndrome:
Chromosome mutation occurs during the formation of an ovum.
Chromosome fails to separate during anaphase (non-disjunction).
Results in an extra chromosome in one of the ova.
When this mutant ovum is fertilized, the zygote has chromosomes instead of .
Individuals with Down syndrome are born with characteristic facial features, physical defects, and mental disability.
Mechanism of Natural Selection
Principles of Natural Selection:
Natural selection is the competition for existence between individuals in a population.
Individuals that are better adapted to their environment tend to have an increased chance of survival and a greater opportunity to reproduce, whereas poorly adapted individuals die off before reproducing.
Involves a struggle for survival where the fittest survive.
Competition among members of the same species for resources (e.g., food, water, space) and environmental pressure force weaker members of the population to die off or become limited in number.
Role in Speciation and Evolution:
Depends on variation within every species. Some variations help an organism survive in the struggle for survival.
Organisms with advantageous variations survive, reproduce, and pass favourable characteristics onto offspring.
Mutations give rise to beneficial variations, allowing species to adapt to environmental changes and preventing extinction.
On-going process promoting long-term changes in a species over many generations; serves as a primary mechanism for evolution.
Charles Darwin's Theory of Evolution: Proposes that existing species colonising different and isolated habitats develop specific characteristics adapted to these environments via natural selection over millions of years, eventually leading to the formation of a new species.
Proposed Chain of Events in Darwin's Theory:
1. Struggle for Survival & Variation: Organisms reproduce at a faster rate than food production rate, leading to competition for food, shelter, and mates. Discontinuous variation is caused by mutation, while continuous variation is driven by new gene combinations.
2. Natural Selection: Advantageous characteristics or variations are selected for.
3. Survival of the Fittest: Organisms with the best-suited characteristics predominate in the population.
4. Inheritance of Beneficial Variations: Favourable characteristics pass onto offspring over many generations.
5. New Species: Formation of genetically distinct species.
Examples of Natural Selection:
The Dark Peppered Moth:
Before the Industrial Revolution in England, light-coloured peppered moths resting on tree trunks were camouflaged by light-coloured lichen on the bark, protecting them from bird predation.
Sooty environments after the Industrial Revolution allowed the dark variation of the peppered moth to increase in number, as they survived bird predation against soot-blackened tree trunks.
Dark moths were selected for, and light-coloured moths were selected against due to adaptation to the new environment.
Sickle-Cell Anaemia:
The mutant gene for sickle-cell anaemia is common in areas where malaria is endemic.
Gives a selective advantage to individuals with the sickle-cell trait (), as these individuals possess a slight immunity to malaria.
Penicillin-Resistant Bacteria:
Disease-causing bacteria become resistant to penicillin due to a mutant allele that prevents cell destruction by penicillin.
Provides a selective advantage to the bacteria, which is passed to offspring by natural selection, allowing multiplication despite penicillin treatment.
Insecticide-Resistant Insects:
Mutant forms of insects are resistant to the pesticide DDT.
Continued use of DDT causes mutant insect strains to multiply and replace susceptible insect populations.
Artificial Selection and Breeding Methods
Definition and Process:
Selective breeding (artificial selection) is the process of choosing to breed the best plants or animals so that genes for desired characteristics remain in the population.
Humans deliberately select and interbreed individual plants or animals to produce new varieties with desirable characteristics.
Speeds up natural selection, allowing farmers and breeders to improve agricultural livestock and crop plants.
Applications:
Crop Plants: New varieties of wheat, barley, and potatoes are bred for higher yield, better nutritional value, greater disease resistance, and greater tolerance to environmental extremes.
Fruit Trees: Results in larger and better quality fruits with improved taste.
Livestock Animals: Cattle are selected for higher meat and milk production; sheep are selected for wool quality, coat thickness, and coat length.
Domestic Animals: Horses and dogs are bred for hunting, racing, and appearance. Dogs were originally domesticated from wolves.
The Five Steps in Selective Breeding:
Decide what characteristics are desirable.
Select organisms with these characteristics from a mixed population.
Breed these selected individuals together.
Select and breed the best offspring with the desired characteristics together.
Repeat this process for many generations until all offspring display the desired characteristic.
Types of Artificial Selection:
In-Breeding:
Involves the crossing of closely related individuals within a species.
In crop plants: self-pollination and self-fertilisation maintain desirable parent traits.
In animals: related offspring from the same parents are mated (e.g., highly bred pedigree dogs).
Out-Breeding:
Involves the crossing of unrelated species or strains.
Example: Crossing a wild species of grass with a strain of wheat produces a hybrid variety with increased resistance to diseases and adverse environmental conditions.
Disadvantages of Artificial Selection:
Can lead to the accumulation of harmful recessive genes in offspring, resulting in physical or mental defects that reduce survival chances if returned to the wild.
Reduces the Gene Pool:
Inbreeding reduces the number of alleles or versions of a gene within a species.
Decreased population variation lowers the chance that organisms will possess resistance to new diseases, risking population collapse or extinction.
Increases Genetic Disease and Defects:
Inbred white tigers are frequently cross-eyed.
Pugs, bred for squashed faces and short snouts, suffer severe breathing problems.
High risk of inheriting genetic diseases or defects through inbreeding raises significant ethical concerns.