6 - Speciation Flashcards
MIcroevolution
- Evolution within populations, within species
- Changes in allele frequencies and associated traits within populations
Macroevolution
- Evolution of species, genera, families, and other higher order levels of classification
- Relevant to interpret the fossil record
Species
A group of interbreeding organisms that is reproductively isolated from other such groups
Reproductive isolation isolating mechanisms
- Geographical seperation
- Differences in behaviour
- Physical appearance
- Number of chromosomes
- Reproductive physiology
Hybridisation
- Genetic exchange between distinct populations
- Die or are sterile
- Creates a barrier to gene flow
Gene flow
- Movement of genetic material within or between populations
- Maintains similarities among members of the population
- Counters the effect of selection for different
phenotypes (if alleles are selected against in one part of the population, they may still reappear through migration)
No gene flow
- Migration not possible
- No gene flow
- Local adaptations favoured
- Phenotypes become distinct
- Species reproductively isolated
With gene flow
- Migration possible between habitats
- Gene flow swamps local
adaptation - Phenotype intermediate
Problems with the biological species concept
Many clearly distinct species are not reproductively isolated
Does speciation appear in the fossil record
- No, too rapid
- Too slow to observe in living populations
Two processes of speciation
- Anagenesis
- Cladogenesis
Allopatric speciation
- Two or more populations of a single species become isolated geographically and then diverge to form two
(or more) new species - Mechanism of reproductive isolation
- Subjected to different selection pressures
Two additional mechanisms that may increase reproductive isolation and facilitate speciation
- Character displacement
- Reinforcement
Character displacement
- Competition for
resources will promote greater morphological differences (niche
separation) - e.g. Competition from newly arrived big beaks causes small beaks to use wetter habitat (which then selects for even smaller beaks)
- When there is no geographic overlap
(hence no competition), the two species will have more similar diets/beaks
Reinforcement
- Parents initially adapted to different environments → hybrid offspring may have a mosaic of traits that make them less fit for either environment
- Hybrids selected against
Reinforcement vs character displacement
- In both cases, these mechanisms increase
reproductive isolation - Via competition and niche partitioning (CD)
- Via selection against hybrids (R)
- Not mutually exclusive
Allopatric speciation following recolonisation
- Reproductive isolation may be complete→speciation complete
- Isolation may be
amplified via reinforcement and/or
character displacement
Parapatric speciation
- Limited overlap in ranges (no true barrier)
- Groups in different parts of the range experience different environments
- At boundaries, hybrids selected against
- Reproductive isolation not complete, but natural selection keeps populations separate
Sympatric speciation
- Overlapping ranges
- Differences between sub-groups develop without geographic separation
- Selection against intermediate types will lead to two new species (disruptive selection)
- Different adaptations to SAME environment
Ecological niche
How a species exploits its habitat
Adaptive radiation
- Occurs when a species colonises a new habitat with many open niches
- With fewer competitors, species will differentiate, filling available niches relatively rapidly
Phyletic Gradualism
- Tiny changes accumulate gradually via microevolution
- Gradual transformation of one species into another
- Rates of evolution are constant and slow
- Anagenesis
Punctuated equilibrium
- Short periods of rapid change after long periods of
little or no change (stasis) - Rates vary
- New species arise through splitting (isolation)
- Can be fast
- Cladogenesis
Tempo and mode in evolution
- Room for both modes!
- Depends on many factors, but especially the environment
- Fossil record is difficult to interpret but adaptive radiations a good example of punctuated events