3.1D Genetic Diversity and Reproductive Isolation (HL) Notes

Genetic Diversity

1. Genetic diversity refers to the variety of genetic material within a population or species.
2. It is the foundation of evolution and adaptation, enabling species to survive in changing environments.

Mutation

1. Mutation is a random, spontaneous change in the DNA sequence of an organism’s genome.
2. It occurs naturally during DNA replication or due to environmental factors such as radiation, chemicals, or viruses.
3. Mutations alter the sequence of nucleotide bases (A, T, C, G) in a gene.
4. This change can produce a new version of a gene, known as an allele.
5. Different alleles may result in new traits, some beneficial, others neutral or harmful.

Types of Mutations

1. Point mutations: A change in a single base pair (e.g., substitution).
2. Insertion or deletion: Addition or removal of one or more bases, often causing a frameshift.
3. Chromosomal mutations: Large-scale changes such as duplication, inversion, or translocation.

Sources of Mutations

1. DNA replication errors: random copying mistakes.
2. Radiation: ultraviolet or ionizing radiation can cause base changes.
3. Chemical mutagens: such as tobacco smoke or pesticides.
4. Viruses: can insert foreign DNA into the host genome.

Sexual Reproduction and Genetic Recombination

1. While mutation creates new alleles, sexual reproduction reshuffles existing alleles into new combinations, creating genetic recombination.
2. This process occurs through three mechanisms:
   1. Independent assortment: During meiosis, chromosomes are randomly distributed to gametes.
   2. Crossing over: Homologous chromosomes exchange segments of DNA, creating unique allele combinations.
   3. Random fertilization: Any sperm can fuse with any egg, further increasing variation.

Advantages of Genetic Diversity

1. Increased adaptability: Diverse populations survive better under environmental stress.
2. Reduced inbreeding depression: Genetic variation prevents harmful alleles from accumulating.
3. Disease resistance: Some individuals carry alleles providing immunity to emerging diseases.

Reproductive Isolation and Speciation

1. Reproductive isolation occurs when two populations of the same species can no longer interbreed to produce fertile offspring.
2. Once gene flow stops, natural selection and mutations act independently, leading to speciation, the formation of new species.

Types of Reproductive Isolation

1. Geographical Isolation

1. Occurs when physical barriers (mountains, rivers, oceans, deserts) separate populations, preventing interbreeding.
2. Each population experiences different environmental pressures.
3. Mutations and natural selection drive independent evolution.
4. Over time, genetic differences become so great that interbreeding is impossible.

Example

• The Galápagos Islands provide classic examples of speciation due to geographical isolation.
• Isolated finch populations on different islands evolved unique beak shapes suited to distinct food sources.

2. Ecological or Behavioral Isolation

1. This occurs without physical barriers.
2. Occurs when populations in the same area diverge due to behavioral, ecological, or reproductive differences.
   1. Behavioral isolation: Changes in mating calls, rituals, or courtship prevent interbreeding.
   2. Temporal isolation: Different breeding times or seasons.
   3. Ecological isolation: Populations exploit different niches within the same habitat.
   4. Mechanical isolation: Anatomical differences prevent mating.

High Rates of Endemism on Islands

1. Endemism occurs when species are found only in one particular geographic area.
2. Islands often exhibit high endemism because of:
   1. Geographical isolation: Limited gene flow with mainland populations.
   2. Unique selection pressures: New climates, predators, and food sources.
   3. Small founder populations: Genetic drift and mutation quickly create distinct gene pools.
   4. Adaptive radiation: A single ancestral species evolves into many new species to fill ecological niches.

Tectonic Movements and Isolation

1. Plate tectonics have historically shaped biodiversity by creating physical barriers and new habitats by:
   1. Formation of mountain ranges (e.g., the Himalayas).
   2. Continental drift leading to island formation (e.g., Australia, Madagascar).
   3. Rising sea levels isolating land masses (e.g., Sundaland islands).

Case study

Birds of Paradise - Isolation and Diversification

Birds of paradise are a striking example of both geographical and behavioural isolation:

• Originated from a common crow-like ancestor in Australia.
• As land bridges disappeared, populations in New Guinea and surrounding islands became isolated.
• Over millions of years, behavioural adaptations such as courtship dances and feather displays evolved, leading to 39 distinct species.