Speciation

Two patterns

  1. Gradualism (Anagenesis)

    1. A slow, gradual accumulation of heritable changes (adaptations) due to many small episodes of natural selection.
    2. One species changes slowly until it looks different and is called a new species

  2. Branching (cladogenesis)

    1. Rapid splitting of one or more new species from an original species that may or may not exist

How one species evolves into 2 or more

Two-step process

  1. Geographic isolation
  2. Evolution of reproductive barriers

Geographic isolation

  • Gene flow between two populations is interrupted (or reduced) between two populations because they are geographically separated
  • Why?
    • Geological change
    • New lava flows
    • Gradual formation of a river valley, or mountain range
    • The slow movement of tectonic plates
    • The colonisation of a new area
    • Separation of a small “founding” population from the main population
    • Combination of natural selection and genetic drift
      • Increased likelihood that the population will change over time
      • Less and less similar to the main population
  • @@Geographic isolation is not sufficient to lead to speciation, only occurs when changes in gene pools result in reproductive barriers.@@

Reproductive barriers

  • Any morphological, physiological, or behavioural trait that prevents different organisms from successfully interbreeding
  • Either pre-zygotic or post-zygotic

Pre-zygotic

5 types

  1. Habitat isolation
  2. Temporal Isolation (matter of timing)
  3. Behavioural Isolation
  4. Mechanical Isolation
  5. Gametic incompatibility

Habitat isolation

  • Two species that occupy two different habitats within the same geographical area may never interact because they never (or rarely) encounter each other
  • Eg
    • Two species of garter snakes live in the same area but one is aquatic and one is terrestrial

Temporal Isolation

  • Species that breed during different times of day, seasons, or years and cannot mate
  • eg
    • Western skunks and Eastern skunks live in similar ranges but the Eastern mates in late winter and the Western in fall.

Behavioural Isolation

  • Little or no sexual attraction between males and females of different species due to courtship behaviours
  • eg
    • Eastern and Western meadowlarks look almost identical but their courtship behaviours differ

Mechanical Isolation

  • Reproductive structures are physically incompatible
  • eg
    • Different shapes penises of closely-related insects
  • In plants, the pollinators may be different. They attract different types of pollinates
    • Cross-pollination extremely rare

Gametic Incompatibility

  • Sperm from one species is unable to fertilize the eggs of another species
  • eg
    • Two closely-related species of sea urchins may breed at the same time but their gametes are not compatible

Post-zygotic reproductive barriers

2 Types

  1. Reduced hybrid viability
  2. Reduced hybrid fertility

Reduced hybrid viability

  • A hybrid zygote fails to survive embryonic or juvenile development
  • eg
    • Interbred salamanders do not survive, those who do are weak and frail

Reduced hybrid fertility

  • Even if hybrids are healthy, they may be sterile
  • If chromosomes of the 2 parents differ in structure or number, meiosis in the hybrid may fail to produce normal gametes
  • eg
    • Mules are healthy but sterile