evolution

🧬 Evolution Theories & Variation

Evolution — concise Grade 9 notes

Evolution is the gradual change in populations of organisms over generations that produces the diversity of life and new species.

Key terms

  • Species: group that can interbreed and produce fertile offspring.

  • Population: members of a species in the same area.

  • Variation: differences among individuals.

  • Adaptation: trait that improves survival or reproduction.

  • Natural selection: differential survival and reproduction of individuals with favourable traits.

Homologous and Analogous Structures

Homologous structures

  • Definition: Body parts in different species that share a common evolutionary origin.

  • Structure: Have similar internal anatomy or developmental patterns.

  • Function: May perform different functions in different species.

  • Evolutionary meaning: Indicate common ancestry.

  • Example: Human arm, whale flipper, and bat wing share the same basic bone arrangement.

Analogous structures

  • Definition: Body parts in different species that perform similar functions but do not share a common origin.

  • Structure: Have different internal anatomy and developmental origins.

  • Evolutionary meaning: Result from convergent evolution where similar environmental pressures produce similar solutions.

  • Example: Wings of birds and wings of insects both enable flight but evolved independently.

Lamarckism (1809 – Philosophie Zoologique)

Evolution occurs because organisms can willfully modify their bodies and pass those acquired improvements to offspring.

  • Three pillars

    1. Direct environmental effect

    2. Use & disuse of organs

    3. Inheritance of acquired characters

  • Classic tale: Short-necked giraffes repeatedly stretch for tree leaves; necks lengthen by effort and the longer neck is passed on – generation after generation → modern long-necked giraffe.

  • Doubts never answered
    – Can an organ be wilfully created?
    – Are all acquired traits truly inherited?

Darwinism (1859 – Origin of Species)

Evolution proceeds by natural selection acting on heritable variation, not on acquired characters.

Five postulates

  1. Enormous fertility: every species can over-reproduce (elephant pair → 1,900,000 descendants in 750 y if all calves survive).

  2. Struggle for existence: over-production ⇒ competition for food, shelter, mates.

  3. Variation & heredity: individuals differ; some differences are heritable.

  4. Natural selection: individuals whose variants fit the environment survive & reproduce disproportionately.

  5. Origin of new species: accumulation of favorable variants over geologic time splits one ancestral species into several.

Criticism

  • No explanation of why favorable variation arises in the first place.

  • Cannot account for persistence of harmful traits.

  • Over-emphasises natural selection; neglects other forces (e.g. mutation).

Variation vs Mutation

Aspect

Variation

Mutation

Definition

Continuous, small differences among individuals

Sudden, discontinuous chromosomal/gene change

Cause

Genetic recombination + environment

Chemicals, radiation → DNA change

Appearance

Quantitative (height, skin colour)

Qualitative (six fingers, split lip, two-headed calf)

Inheritance

Only heritable part passes on

Usually recessive but transmissible

Role in evolution

Raw material for selection

Creates brand-new alleles instantly

homologus

Fossil Evidences (preview)

  • Preserved in sedimentary rocks.

  • Show progressive complexity from unicellular → multicellular → specialised forms.

  • Transitional types (e.g. fish ↔ amphibian) document descent with modification.

Practical Tasks

  1. Survey neighbourhood for human/animal mutants (polydactyly, fused limbs); report causes & consequences.

  2. Interview elders, list 5 ancestral traits vs present generation – classify each change as variation or mutation.

  3. Collect labelled mutation images for classroom exhibit.