Species Concept

  • Biological species concept: species is a group of populations that can breed and produce viable offspring
    • Defined by reproductive isolation and gene flow; basis for understanding macroevo.
  • Morphological species concept: distinguishes a species by body shape and other structural features.
  • Ecological species concept: defines a species in terms of its ecological niche
    • Species have diff adaptations to environment
  • Phylogenetic species concept: set of organisms with unique evolutionary history


  • Speciation: formation of new species
    • Reproductive isolation is main measure for speciation; can involve changes to 1 gene
  • In order for speciation to happen, there must be something that prevents interbreeding between closely related population or species → Reproductive Barriers
  • If two groups are reproductively isolated, they can accumulate allele differences and eventually diverge
  • Reinforcement: Natural selection for traits that prevent interbreeding among populations
    • Selected traits reinforce differences that evolved while the populations were isolated from one another
  • Fusion: two species become one
    • Likely to occur when an increasing number of viable, fertile hybrids are produced over the course of generations
    • Reduces species diversity

Methods of Speciation

  1. Allopatric Speciation: when a population is divided by a geographic barrier so that interbreeding between two populations is prevented
  • Barrier examples: rivers and regions that contain vital resources
  • Gene flow is interrupted when a population is geographically isolated → reproductive barriers form and maintain speciation → gene pool/allele frequencies in two populations can diverge bcuz of diff selective pressures
    • If gene pool diverges enough then interbreeding will not occur if barrier removed
  • Geographic barriers lead to reproductive isolation & barriers that prevent interbreeding:
    • Ex: Species that were seperated cannot breed when meet each other again
  1. Sympatric Speciation: formation of new species without presence of geographic barrier. Appearance of new species in the same area of the parent population. Can occur bcuz of…
  1. Sexual Selection:
  2. Habitat differentiation: When subpopulation exploits habitat not used by parent → natural selection can act
  3. Balanced Polymorphism among subpopulations may lead to speciation
  • Ex: a population of insects have polymorphism for color → each color provides camouflage to specific substrate → under these circumstances only insects with same color can associate and mate → similarly colored insects are reproductively isolated

Polyploidy: have more than the normal two sets of chromosome found in diploid (2n) cells

  • Occurs as result of nondisjunction of all chromosomes during meiosis                                               
    • Unlike animals, plants more tolerant to changes in chromosome sets
  • Ex: Tetraploid plant produces gamete (2n) that fuses with normal gamete from same species (n) → nonviable/infertile triploid plant
    • Bcuz tetraploid & diploid species cannot produce viable offspring = diff species
      • Thus speciation can occur over single generation
  1. Hybridization: two different species mate and produce offspring along geographic barrier called hybrid zone
  • Sometimes genetic variation of hybrids is greater than either parent so hybrid population can evolve adaptations to environmental conditions in hybrid zone beyond parent range
    • Hybrids become new species when exposed to different selection pressures or can only breed among themselves
  • Hybrid Zone: place where two different species meet and mate                                                                 
    • Form when two species do not have complete reproductive barriers
  1. Adaptive Radiation: rapid evolution of many species from single ancestor
  • Occurs when ancestral species is introduced to an area where diverse geographic conditions are available                 
  • Ex 1: 14 species of Darwin’s finches on galapagos islands evolved from single ancestor
  • Ex 2: adaptive radiation occurred after mass extinctions → many species go extinct → periods with ecological opportunities for species to colonize → colonization led to competition → promotes speciation

Maintaining Reproductive Isolation

  • Reproductive barriers: prevent interbreeding & maintain reproductive isolation when species are not physically separated by geographic barrier
  • Often single genes control phenotypic traits that can lead to reproductive isolation → speciation
    • Ex diff species of hummingbird prefer one type of coloured flower and thus only pollinate those
  1. Prezygotic Isolating Mechanisms: block fertilization from occurring
  • If populations do not attempt to breed, then is not considered prezygotic mechanism
  1. Genetic Incompatibility: can’t reproduce bcuz proteins or chromosomes incompatible
  • Sometimes when occurs in plants, they can self-pollinate and become new species
  1. Habitat isolation: occurs when species do not encounter one another
  2. Timing Isolation: occurs when species mate, flower, or are active during different times 
  • Ex: nocturnal and diurnal animals
  1. Behavioral Isolation: when species do not recognize another species as a mating partner because does not perform courtship rituals, release proper chemicals (scents, pheromones) or have appropriate appearance
  2. Mechanical/Anatomical isolation: when male and female genitalia are structurally incompatible or flower structures select diff pollinators
  3. Gametic Isolation: when male gametes do not survive females environment or failed recognition
  4. Postzygotic Isolating Mechanisms: mechanisms that prevent formation of viable progeny
  5. Hybrid Inviability: when zygote fails to develop properly and dies before reaching reproductive maturity
  6. Hybrid sterility: when hybrids grow to be adults but are sterile
  • Hybrid sterile bcuz chromosomes can’t pair up correctly during meiosis.
  1. Hybrid breakdown: when hybrids produce offspring with reduced viability or fertility

Directional, Disruptive, and Stabilizing Selection

  • Different ways environment/natural selection can act on a phenotype
  • Tip: If FRQ about natural selection involves changing phenotype will probs involve one of these vocab
  • Directional selection: when environment favors individuals with one extreme of a phenotypic
    • Ex: Moth case study; an increase number of large seeds over small seeds led to an increase in beak depth
  • Disruptive selection: when environment favors individuals at both extremes of a phenotypic range over individuals with intermediate phenotypes
    • Ex: Small-billed birds feed on small, soft seeds, large-billed birds feed on large, hard seeds; intermediate can’t eat either
    • Increases genetic variation
  • Stabilizing selection: favors intermediate variants and acts against both extreme phenotypes → reduces variation
    • Ex: human babies at intermediate range tend to be healthier and have higher survival rates

Patterns of Evolution

  1. Divergent Evolution: species that originate from a common ancestor become increasingly different over time
  • Might happen because of allopatric speciation or sympatric speciation
  1. Parallel Evolution: species that originate from a common ancestor have made similar evolutionary changes after divergence
  • Species from marsupial mammals and placental mammals have independently evolved similar adaptations
  1. Convergent Evolution:
  2. Coevolution:

Microevolution vs Macroevolution

  1. Microevolution: describes how the population of organisms change from generation to generation (how allele frequencies change)
  2. Macroevolution: describes general patterns of change in groups of related species that have occurred over broad geological time; evolution of new species
  • Different interpretations of fossil evidence have led to 2 contrasting theories for the pace of macroevolution

Patterns in Fossil Record

  1. Phyletic Gradualism: argues that evolution occurs by the gradual accumulation of small changes
  • Individual speciation events occur of long periods
  • Fossils then reveal only major changes in groups → intermediate stages of evolution not represented and shows incompleteness
  1. Punctuated equilibrium: argues that evolutionary history consists of long periods of with little evolution, then interrupted/punctuated by short periods of rapid evolution (new species)
  • Most new species accumulate their unique features rapidly as they come into existence, then change little for the rest of their duration as a species.
  • Most of species in first static event have become extinct or changed enough to be considered a new species