Mechanisms of Variation and Evolutionary Processes

Q: What are the mechanisms by which variation may occur in gametes?

Independent assortment - During meiosis I, chromosomes from the mother and father are randomly distributed into gametes, creating different combinations of alleles. Example: A gamete may receive chromosome 1 from the mother and chromosome 2 from the father. Crossing over - In prophase I of meiosis, homologous chromosomes exchange segments, creating new gene combinations. Example: A maternal chromosome may gain a segment from its paternal homolog. Random fertilisation - Any sperm can fuse with any egg, increasing the number of genetic combinations. Example: One egg could be fertilised by any of millions of sperm. Mutations - Changes in DNA sequences introduce new alleles. Example: A base substitution may result in a new protein variant.

Q: What are the mechanisms by which variation may occur in body (somatic) cells?

Mutation - DNA changes during mitosis can affect cell function and lead to diseases like cancer. Example: A mutation in a skin cell may result in melanoma. Environmental influences - External agents like UV radiation or chemicals can damage DNA. Example: Benzene exposure may cause mutations in bone marrow cells. Epigenetics - Environmental factors may switch genes on or off without changing DNA. Example: Stress may suppress genes related to immunity.

Q: What is the purpose of sexual reproduction?

Sexual reproduction increases genetic diversity, making a population more resilient to environmental challenges. Example: If a virus spreads, some individuals may carry resistance due to genetic variation, ensuring population survival.

Q: Define mutation.

A mutation is a permanent alteration in a DNA sequence. It can have beneficial, harmful, or no effects. Example: A mutation in the CFTR gene causes cystic fibrosis.

Q: What are the possible causes of mutation?

Spontaneous - Errors in DNA replication. Example: DNA polymerase inserting the wrong base. Induced - Exposure to mutagens like radiation (UV light, X-rays), chemicals (e.g. nicotine), or viruses. Example: UV light causes thymine dimers that distort DNA.

Q: What is a germ line mutation?

A mutation in sperm or egg cells. It is heritable and affects all cells in the offspring. Example: A mutation passed from parent to child causing Huntington's disease.

Q: What is a somatic mutation?

A mutation in non-reproductive body cells. It is not passed to offspring. Example: Lung cancer from cigarette smoke affects only the individual.

Q: What are the three possible outcomes of a mutation?

Positive - Provides an advantage. Example: Sickle-cell trait protects against malaria. Negative - Harms the organism. Example: Tay-Sachs disease. Neutral - No observable effect. Example: A silent mutation in a non-coding region.

Q: How can different mutation outcomes affect evolution?

Positive mutations increase an organism's fitness and can become more common through natural selection. Negative mutations may be eliminated over generations. Neutral mutations may accumulate and become useful if conditions change. Example: A neutral mutation could provide resistance in a future pandemic.

Q: Define evolution.

Evolution is the change in the genetic makeup of a population over generations. It occurs due to mechanisms like mutation, selection, gene flow, and drift.

Q: What is Darwin's theory of natural selection?

1. Variation exists among individuals in a population. 2. Some variations are inherited. 3. More offspring are born than can survive. 4. Individuals with advantageous traits are more likely to survive and reproduce. 5. Over generations, favorable traits increase in frequency.

Q: Define gene pool.

The gene pool includes all the alleles present in a population. Greater diversity means greater potential for adaptation.

Q: Define gene flow.

Gene flow is the transfer of alleles from one population to another through migration. Example: Pollen from one plant population fertilises another, introducing new alleles.

Q: What are the mechanisms that change a gene pool?

Mutation - introduces new alleles. Natural selection - increases/decreases allele frequency based on advantage. Genetic drift - random fluctuations in small populations. Gene flow - movement of genes between populations. Non-random mating - certain traits selected more often (e.g. peacock tails).

Q: What are types of selection pressures?

Environmental - climate, water availability. Predation - camouflage or defense traits help survival. Competition - for mates, food, space. Disease - immune resistance can be favored. Human impact - habitat destruction, pollution, antibiotics.

Q: How do selection pressures impact survival?

Organisms with traits suited to the environment are more likely to survive and pass on genes. Example: Antibiotic resistance in bacteria spreads rapidly.

Q: What is stabilising selection?

Selects for the average phenotype; extremes are disadvantageous. Example: Human birth weight - very small or large babies have higher mortality.

Q: What is directional selection?

Favors one extreme trait. Example: Longer giraffe necks helped access food, so this trait became dominant.

Q: What is disruptive selection?

Favors both extremes, eliminates intermediates. Example: Dark and light limpets survive on rocky shores, while medium colors are eaten.

Q: How do different types of selection affect allele frequency?

Stabilising - reduces variation, favors average traits. Directional - shifts allele frequency toward one trait. Disruptive - maintains or increases variation, may lead to new species.

Q: How can selection pressures lead to extinction?

If a population can't adapt to changing conditions (e.g. climate change), its numbers may decline until extinction. Example: Dodo couldn't adapt to human hunting and habitat change.

Q: Give a plant and animal example of 'survival of the fittest' with reproductive advantage.

Plant: Dandelions with deep roots survive drought and produce more seeds. Animal: Male peacocks with vibrant tails attract more mates and have more offspring.

Q: Give a plant and animal example of 'survival of the fittest' with survival advantage.

Plant: Cacti store water and tolerate arid conditions. Animal: Arctic foxes with white fur blend into snow, avoiding predators.

Q: Why did the peppered moth change color during the Industrial Revolution?

Industrial soot darkened trees. Dark moths became camouflaged, survived better, and reproduced more. Lighter moths became easier targets for birds and declined.

Q: What are the three main ideas in Darwin's theory of evolution?

1. Variation exists. 2. More offspring are born than can survive. 3. Traits that help survival get passed on more often.

Q: What are the different types of evidence for evolution?

Fossils - show gradual change over time. Comparative anatomy - similar structures suggest common ancestry. DNA/protein similarity - closer species have more similarities. Biogeography - island species often resemble nearby mainland species. Vestigial structures - remnants of once-functional organs. Embryology - similar early development stages among vertebrates.

Q: How does DNA evidence support evolution?

Species with a recent common ancestor have similar DNA. Example: Humans and chimps share ~98% DNA, indicating close evolutionary relationship.

Q: How do comparative proteins support evolution?

Proteins like cytochrome c are nearly identical in related species. Example: Human and chimpanzee cytochrome c differ by only one amino acid.

Q: How does comparative anatomy support evolution?

Homologous structures (e.g. forelimbs of bats, whales, and humans) indicate shared ancestry. Vestigial structures (e.g. wisdom teeth, tailbone) suggest evolutionary changes.

Q: What is a micropipette used for?

To accurately measure and transfer small liquid volumes (microlitres), often for DNA or enzyme reactions in labs.

Q: What is gel electrophoresis used for?

To separate DNA fragments based on size using an electric field. Smaller fragments move faster through the gel.

Q: How are genes identified using gel electrophoresis?

DNA fragments form bands on gel. Matching band patterns between samples suggest common genes or ancestry.

Q: What is a DNA fingerprint used for?

To identify individuals by comparing unique patterns of DNA fragments. Used in forensics, paternity tests, and biodiversity studies.

Q: What is genetic engineering?

The process of modifying an organism's DNA by inserting or removing genes. Example: Bt corn contains a gene from bacteria that produces pest-killing proteins.

Q: What is a restriction enzyme?

A protein that cuts DNA at specific base sequences. Example: EcoRI cuts between G and A in GAATTC sequence.

Q: Why are restriction enzymes useful in genetic engineering?

They allow precise cutting of DNA for gene insertion. Example: Inserting the insulin gene into bacterial DNA to produce insulin.

Q: What is a recombinant organism?

An organism that contains DNA from another species. Example: A genetically modified bacterium that produces human insulin.

Q: How could recombinant DNA affect evolution?

It introduces new traits artificially, potentially accelerating evolutionary changes. Example: Crops modified to resist pests may outcompete natural varieties.