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).
READ:
Speciation: Prezygotic and Postzygotic Barriers

Sexual Selection

Advantages of Sexual Reproduction Disadvantages 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)

READ:
What is a Test Cross: Why is it used (Biology)
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