Science Y10: Natural Selection and Evolution

Biology

Natural Selection

Natural Selection - is the process in which a population of living things change as the environment selects favourable characteristics; the organism with favourable characteristicts survive and the ones without die, before reproducing. 

• Individuals with beneficial traits survive and reproduce.

• Individuals without these traits are less likely to survive and reproduce.

Steps to Natural Selection

Steps of Natural Selection

1. Variation

   • Individuals within a population are genetically different.

   • These differences lead to variation in traits (e.g., fur color, beak shape).

2. Overpopulation

   • Organisms produce more offspring than can survive.

   • Leads to competition for limited resources (food, space, mates).

3. Survival of the Fittest

   • Individuals with advantageous traits are more likely to survive.

   • “Fittest” = best adapted to the environment, not necessarily the strongest.

4. Reproduction

   • Survivors pass on their favorable genes to the next generation.

   • Over many generations, advantageous traits become more common in the population.

Environmental Change

Environmental Changes

• A changing environment can introduce new selective pressures.

• This may:

  • Cause certain traits to become more or less favorable.

  • Lead to adaptation over generations.

  • In extreme cases, cause extinction if a population cannot adapt.

Homologous Structures

Homologous Structures

• Structures that originate from a common ancestor.

• May have different functions but share a similar underlying anatomy.

• Formed due to divergent evolution — the same structure adapts differently in different environments.

• Example: Human arm, bat wing, whale flipper — same bones, different functions.

• Significance: Supports the theory of evolution and natural selection.

Analagous Structures

Analogous Structures

• Structures in unrelated species that perform similar functions.

• Formed due to similar environmental pressures, not common ancestry.

• Known as convergent evolution.

• Example: Wings of birds and insects — both for flying, but different structures.

• Significance: Shows adaptation to environment but does NOT indicate common ancestry.

Evolution

The genetic change in species over many generations resulting in new species. Evolution can also bring about a change witin a species. Occurs through the process of natural selection. 

Convergent Evolution

Convergent Evolution

• The process by which unrelated species evolve similar traits because they face similar environmental pressures.

• Often results in analogous structures.

• Example: Dolphins (mammals) and sharks (fish) — similar streamlined body shape for swimming.

Darwin’s theory of evolution

Charles Darwin proposed the theory of evolution. He proposed that life on earth has changed over a long period of time and different species developed from a common ancestor.

Sexual Selection

• A special case of natural selection.

• Acts on an organism’s ability to obtain or successfully reproduce with a mate.

• Can result in traits that seem disadvantageous for survival but increase reproductive success.

• Examples:

  • Fruit flies performing elaborate courtship dances.

  • Male elephant seals fighting over territories for access to females.

• Leads to traits that are often highly exaggerated (bright colors, large size, elaborate displays).

Selective Pressure

• Environmental factors that favor certain traits and disadvantage others.

• Can be biotic (living) or abiotic (non-living).

• Examples: temperature changes, light levels, availability of food.

• Drives evolution by increasing the survival and reproduction of individuals with advantageous traits.

Artificial Selection

• Definition: Humans deliberately breed plants and animals for desirable traits.

• Purpose: Increase productivity, enhance specific traits (e.g., larger fruits, faster growth).

• Types:

  1. Crossbreeding: Breeding two different species together.

  2. Inbreeding: Breeding closely related organisms to speed up selective breeding.

     • Can increase risk of genetic disorders.

Speciation

Speciation: The formation of a new species.

• Occurs when one species splits into two or more species over time due to genetic changes and reproductive isolation.

Determining Species

• Cannot rely on anatomy alone.

• Tests include:

  1. Breeding test:

     • Can two organisms interbreed and produce fertile offspring? → same species

     • If offspring are infertile → different species

  2. DNA analysis / Protein comparison:

     • Similar DNA or amino acid sequences indicate closely related species.

Biodiversity

Biodiversity: The number and range of different species on Earth.

• High biodiversity indicates healthy ecosystems.

• Important for resilience, ecological stability, and evolution.

Half - Life

The time it takes for half of the nuclei in a radioactive substance to decay or transform into a more stable form.

• Example Calculations:

  • Substance = 40 mg, Half-life = 2 days

    • After 2 days → 20 mg left

    • After 4 days → 10 mg left

    • After 6 days → 5 mg left

• Tip: Just keep halving for each half-life period.

Fossil

Preserved evidence in rocks or soils of organisms that once existed on Earth.

Fossil Records

• Lists all species found as fossils, their location, and relative age.

• Supports evolutionary studies and understanding of extinct organisms.

Steps of Fossil Formation

• Death – organism dies.

• Decomposition & Sedimentation – soft tissues decay, hard parts buried by sediment.

• Composition & Fossilisation – minerals replace organic material or preserve impressions.

• Uplift – geological movements bring fossil-bearing rocks closer to the surface.

• Erosion & Exposure – natural processes reveal the fossils.

Replacement

3D impressions of an organism.

• Mold: hollow impression in sediment.

• Cast: filled-up mineralized impression.

• Example: Dinosaur footprint.

Molds & Casts

Organism completely preserved, often in amber or other protective material.

• Example: Insects in amber.

Preserved Remains

Preservation in Peat

Soft tissues preserved in deep layers with low oxygen and bacteria.

• Example: Bog bodies.

Preservation in Dry Air

Organism dries out completely; skin & organs preserved.

• Example: Mummified humans.

Preservation in Tar

Animals trapped in sticky tar pits, preserving soft and hard tissues.

• Example: Bison & mammoths in tar pits.

Carbonisation

All material decays except carbon, leaving a thin carbon film.

• Example: Fossil leaves.

Relative Dating

Determines the sequence of events by comparing rock layers (strata) or artifacts.

• Order of events known, not exact age.

• Example: Layer A is below Layer B → Layer A is older.

Absolute Dating

Determines the exact age of rocks, fossils, or artifacts using radiometric dating (decay of radioactive isotopes).

• Example: “I got my doll on my 5th birthday” → exact age known.

Law of Superposition

• In undisturbed rock sequences, the oldest layers are at the bottom, youngest at the top.

  • Example: Sandstone layer contains granite fragments → granite is older than sandstone.

Cross-Cutting Relationships

A rock or feature that cuts across another rock layer is younger than the layer it cuts.

Geological Time Scale & Stratigraphy

• Geological Time Scale (GTS): chronological dating relating rock strata to time.

• Stratigraphy: study of rock layers; used for relative dating to determine the order of past events.

Index Fossils

• Fossils used to determine relative age of rock layers. Must be:

  1. Widely distributed → found across many areas

  2. Existed for a short period of time → precise time indicator