Lecture 24

• emergent complexity

  process whereby larger entities, patterns, and regularities arise through interactions among smaller or simpler entities that themselves do not exhibit properties
• life on earth can be thought of as emergent complexity over 4.3 billion years
• emergent complexity is continuing, both at the organismal level and broader

  ex: humans are increasingly integrated/ organized
• charles darwin

  first to theorize that existing species evolved from preexisting ones

  influences that helped develop the theory

  • geology
  • experimental observations
  • thomas malthus' essay on the principle of population
• theories of geology - charles darwin


  earth is very old

  slow geological processes change the earths' characteristics
• darwins own experimental observations

  made a voyage abroad the HMS beagle

  carefully examined many different species

  • noted similarities and differences that enabled various species to adapt to different environmental conditions
• Essay on the principle of population

  Malthus asserted that resources cannot keep up with the reproductive potential of humans

  argued that famine, war, and disease will limit population growth, especially among the poor
• alfred wallace

  naturalist working in the east indies

  independently proposed same ideas as Darwin
• On the origin of species by means of natural selection

  1859, book by Darwin

  expounded his ideas in greater detail with experimental support
• genetic variation

  consistent feature of most natural populations
• species

  group of individuals whose members can interbreed to produce viable, fertile offspring

  single species may exist in two or more distinct populations that are slowly evolving into different species
• ecological niche

  describes how an organism or population responds to and interacts with its environment

  how an organism or populations responds to available resources, competitors, and challenges

  species tend to occupy one or a few niches
• reproductive isolation

  cannot successfully interbreed with other species

  can be achieved by:

  • prezygotic mechanisms
  • postzygotic mechanisms
• prezygotic mechanisms

  something prevents the formation of a zygote (fertilized egg)
• postzygotic mechanisms

  something prevents the development of a viable or fertile after fertilization has taken place individual

  ex: lions and tigers can mate to produce healthy offspring, but they are sterile
• speciation

  formation of new species via evolution

  two different patterns: anagenesis and cladogenesis
• anagenesis

  single species is transformed into different species over the course of many generations

  new species better adapted to survive in original or new environment
• cladogenesis

  single species gets divided into two or more species

  most common form of speciation

  each new species develops characteristics that prevent it from interbreeding with the original one
• allopatric speciation - cladogenesis

  species gets split into two populations by geographic barriers and the populations diverge from each other
• parapatric speciation - cladogenesis

  occurs when members of a species are not completely geographically isolated

  accomplished by two different mechanisms

  1. partial separation by geological processes
  2. sedentary species
• partial separation by geological processes - parapatric speciation

  mountain range may divide a species into two populations

  breaks in the range allow (infrequent) breeding
• sympatric speciation - cladogenesis

  occurs when a species splits into two despite occupying the same habitats
• sympatric speciation - plants

  occurs in the formation of polyploids

  major form of speciation
• sympatric speciation - animals

  polyploidy is far less common

  two groups may start to fall into different niches, and their physical characteristics then diverge due to differing selection
• speciation can be fast or slow

  gradualism (slow)

  punctuated equilibrium (fast)
• gradualism

  each new species evolves continuously over long spans of time
• punctuated equilibrium

  species exist relatively unchanged for many generations

  these long periods of equilibrium with the environment are punctuated by short periods where evolution occurs at a rapid rate
• phylogeny

  sequence of events involved in the evolutionary development of a species or a group of species
• phylogenetic tree

  diagram that describes the phylogeny of a species

  constructed using:

  • physiology, biochemistry, behavior - IN THE OLD DAYS
  • nucleotide and amino acid sequence data is NOW being used

  nodes or branch points of the tree illustrate when a species diverges into two or more species (cladogenesis)
• three main evolutionary branches

  bacteria, archaea, eukaryotes
• horizontal gene transfer

  widespread during early stages of evolution

  remains prevalent in prokaryotes

  less common among eukaryotic species

  • multicellularity and sexual reproduction may be barriers
• example of phylogenetic tree with horizontal gene transfer

  bacterium that gave rise to mitochondria

  bacterium that gave rise to chloroplasts
• supergroups of eukaryotic species

  plants are with algae

  animals and fungi are in the same group
• molecular clocks

  change in sequence of neutral DNA over time is thought to be relatively constant

  mitochondrial: change is popular molecular clock bc it is not subject to meiotic recombination
• analysis of ancient DNA

  1984, researchers first succeeded in determining DNA sequences from extinct species

  organism was quagga, zebralike species that became extinct in 1883

  also called molecular paleontology
• it is occasionally possible to obtain DNA sequence info from species that have lived in the past

  • can use tissue samples from museum specimens
• hox gene evolution

  first hox gene arose >600 million years ago

  • sponge hox gene descended from this primordial gene

  gene duplications produced a cluster about 600 mya

  cluster duplication occurred about 520 mya

  body plan diversification of invertebrates about 525 mya

  second hox cluster duplication about 420 mya

  tetrapods - vertebrates with four limbs occurred about 417-354 mya
• animal body plans related to changes in hox gene

  found in all animals, more hox genes appear to allow more complex body form

  sponge: 1

  jellyfish: 2

  anenomes: 4

  insects: 9

  chordates:

  • acorn: 1
  • amphioxus: 2
  • lamprey: 3

  mammals: 4
• evolutionary developmental biology (evo-devo)

  branch of biology that compares the developmental processes of different organisms to infer the ancestral relationships between them and how developmental processes evolved

  variation in expression of developmental genes may be commonly involved in acquisition of new traits that are basis of new species
• evo-devo - chicken and duck feet

  ducks have webbed feet

  chickens do not

  may be caused by differential expression of two genes

  • bone morphogenetic protein 4 (BMP4)
  • gremlin