Patterns in Macroevolution

Fossils

  • Trace of any organism that lived in the past
  • 99% extinct, used as evidence for evolution
  • Law of Succession: resemblance between living and fossil forms in the same region

Relative Dating

  • Fossils formed in sedimentary rock in which the sediments form layers that differ in colour, thickness and composition
  • Principle of superposition:  Younger rocks are deposited on older rocks
  • Relative place of fossils determines relative age of fossils
  • Radiometric dating gives absolute age
  • Limitations to record (fossilization depends on the nature of the specimen, soft things are less durable, better fossilizations occurs when there is less oxygen)

Modes of Evolution

Anagenesis

  • Accumulation of changes in a lineage as it adapts to changing environments
  • If changes are significant, may warrant naming a new species
  • Does not increase number if species over time

Cladogenesis

  • Evolution of two or more descendent species from a common ancestor
  • Should increase number of species over time
  • Evidence by coexisting or overlapping fossil forms

Tempo of Evolution

  • Gradualism: Large differences in species represent the culmination of many small changes (should see transitional forms in fossil record).
  • Punctuated Equilibrium: Species diverge in spurts of rapid change, followed by long periods of ‘stasis’ (really rapid, huge change followed by nothing).

Sexual Selection

Advantages of Sexual ReproductionDisadvantages of Sexual Reproduction
1. Reversing Muller’s Ratchet

-Recombination generates offspring with possibly fewer mutations than parent (could also be more or same)

2. The Lottery Model

-Higher probability favourable trait will be selected & offspring will survive

1.Energy wasted on mating rituals

2.Can’t produce offspring without mate

3.Only pass half of your genes

Sexual Selection: Differential reproductive success due to variation among individuals in success at getting mates

i) Intrasexual selection -> Males monopolize access to females

  • Direct control of females
  • Control of a resource important to females
  • Types of competition:
  • Combat (size dimorphism, aggression, weaponry, complex songs, agility)
  • Sperm competition (copulatory plugs, hooked heads, guard females post cop.)
  • Infanticide (kill babies of other males)

ii) Intersexual selection -> Females choose mates based on displays

  • Leads to elaborate displays/calls, ornamentation
  • Females are choosier because mothers make a larger parental investment in offspring than fathers
  • Males fitness limited by mates
  • Females motivated by quality not quantity

Speciation

Microevolution: Changes in allele frequencies

Macroevolution: The origin of new taxonomic groups

Morphological Species Concept: If they look the same, then are most likely interbreed with each

Biological Species Concept: groups of potentially or actually interbreeding organisms, reproductively isolated from one another by gene pool

Reproductive Isolation

Pre-zygotic

  • Ecological – living in different habitats
  • Temporal – ‘active’ at different times
  • Behaviour – have unique courtship
  • Mechanical – incompatible reproductive structures
  • Gametic – incompatible sperm and eggs

Post-zygotic

  • Hybrid inviability – embryo fails to develop
  • Hybrid sterility – offspring are infertile (mule)
  • Hybrid Breakdown – F2 generation infertile

Ring Species

  • Ring shaped distribution due to inhabitable terrain in the centre
  • Gene flow between distant populations through intermediary populations

Phylogenetic Species Concept: the smallest group of organisms that share a unique set of traits NOT shared by any other groups (‘monophyletic’)

  • Traits: morphological, behavioral, genetic…
  • Populations must have been evolutionary independent long enough

Mechanisms of Speciation

Allopatric Speciation: A physical barrier subdivides a population

a) Long distance dispersal (and founder affect)

b) Vicariance – formation of a geographic barrier

Secondary Contact: Opportunity to determine if divergence was sufficient to cause reproductive isolation

Reinforcement: Natural selection may favour isolation if hybrid has a lower fitness

Parapatric Speciation: Population spreads across a heterogeneous landscape; Different selection in adjacent habitats

Sympatric Speciation: Divergence within a homogeneous environment

a) Polyploidy – errors in mitosis causing extra sets of chromosomes

b) Disruptive Selection with Assortative mating – (ex: a species of birds dimorphic for beak size mate selectively with similar morphs)

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