Species

  • What is a species

    • Biological species concept

      • A species is a population or a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring

      • Criteria: potential reproduction

    • Problems

      • Key — genetic isolation, impractical/impossible to speculate about interbreeding or conduct breeding experiments for all species

      • A & B cannot mate, B & C cannot mate

      • A & C can mate

  • Prezygotic isolation

    • Prezygotic barriers to fertilization between two species

      • Habitat isolation (ex. animal species in the desert opposed to other habitats)

      • Temporal isolation (ex. diurnal/nocturnal creatures)

      • Behavioral isolation (ex. morphological incompatibility)

      • Mechanical isolation

      • Gametic isolation

  • Postzygotic isolation

    • Postzygotic barriers between two species

      • Donkey x mule = sterile mule

      • Reduced hybrid viability

      • Reduced hybrid fertility

      • Hybrid breakdown

  • does reproduction isolation cause speciation

    • No, reproductive isolation & speciation are a by-product of genetic changes that occur during changes of populations for other reason

    • What drives speciation?

      • Genetic drift/mutation

  • Unifying theme of biology

    • Theodosius Dobzhansky “nothing in biology makes sense except in the light of evolution”

    • Nictitating membrane—vestigial in humans, useful to sharks & cats

  • What is evolution?

    • The unifying theme to explain the diversity of life on Earth

    • All species have descended—with modifications—from one/few common ancestors through the process of natural selection

  • Descent with modification

    • Evolution only occurs when there is a change in gene frequency within a population (group of organisms in the same species) over time

    • These genetic differences are heritable and can be passed on to next gen

    • Industrial revolution & Kettlewell’s Moth study—birds eat moths that are more conspicuous on trees

    • Soot from industrial pollution had turned trees sooty black—frequency of dark-colored moths increase by 95%

  • Modes of speciation

    • Allopatric speciation by variance — barrier removed or new species disperse oer it, re-establishing sympatry

    • Peripatric speciation—range expansion re-establishes sympatry

    • Parapatric speciation—range expansion leads too sympatry (different habitats right next to each other)

    • Sympatric speciation—genetic differences result in reproductive isolation

  • Allopatric speciation

    • Allopatric speciation: evolution of genetic reproductive barriers between populations that are geographically separated by a physical barrier

    • Most prevalent mode of speciation in animals

    • Populations that begin speciation process in allopatry come back into contact, and either interbreed freely at hybrid zones, or not at all

    • Clades: monophyletic groups

    • Freshwater fish speciation—western clades and eastern clades in spotted sunfish, warmouth sunfish, gluegill sunfish, redear sunfish, bowfin, mosquitofish

      • diverged 3-4 million years ago when sea level was higher, creating a barrier to gene flow

  • Homology: a character state shared by taxa without modification for a common ancestor

    • Characters can be morphological (shape of wing, number of bones); behavioral, biochemical, or involve DNA sequence data

    • Different forms of a character are character states

  • Homoplasy: a character state that has independently evolved two or more times, or similarity that is NOT from common ancestry (often convergent evolution)

  • Autapomorphy: derived character state present in a single lineage

    • ex. lack of strips in automorphy

  • Plesiomophy: ancestral character state (ex. gelatinous eggs)

  • Synapamorphy: homologous character state that supports the monophyly of a group through common ancestry (ex. amniotic eggs)

  • Genetic changes in closely related species

    • Small changes in sequences such as base changes and small deletions

    • Duplications of genes and DNA
      rearrangements on all scales

    • Acquisition of foreign DNA and genes

    • Loss of DNA and genes

  • Maximum parsimony: All mutations are considered equally likely

  • Long-branch attraction: Species with many autapomorphies will be chosen as sister groups, when in reality they are not

  • Model-based analyses

  • Transition mutations occur at a higher frequency than transversion

  • Maximum likelihood assigns probabilities of character change to each state and then calculates the likelihood of any potential of any potential phylogenetic tree, using complex models

  • Likelihood asks: what is the probability of the data, given the preferred

  • How we know this works: Mutagen used to mimic speciation in viruses, scientists could document mutation changes and “cladogenesis” events, phylogenetic analysis produced one true tree

  • Molecular clocks

  • Concept: DNA sequences mutate at a constant rate, so it is possible to correlate # of mutational changes to a node in a phylogenetic tree, this estimating the date of speciation

  • molecular clock estimates can be calibrated w/ fossils, by providing a minimal estimate of time since divergence

  • Problems: distantly rated taxa and different genes rarely have similar mutation rates, creating issues for molecular clocks. Fossils used in calibrations have inexact ages, making clock estimates inexact also