BIOL 4160

Evolution

Phil Ganter

301 Harned Hall

963-5782

Ripsalis is a cactus but one without stem succulence

02 - Species and Speciation

Email me

Link to a list of Specific Objectives for lectures

Back to:

Academic Page 
Tennessee State Home page 
BIOL4160 Page 
Ganter home page 

Species Concepts

Plurality of species concepts is persistent and may not be resolvable

Why are species important?

  • need for organization - artificial justification
  • natural groupings versus artificial grouping - natural grouping

Concepts

  • those that work only for sexually reproducing species
    • Biological Species Concept
    • Recognition Species Concept
  • those that work for sexually or asexually reproducing species
    • Ecological Species Concept (not in book, but could be the final winner!)
    • Phylogenetic Species Concept
    • Genealogical Species Concept
    • Cohesion Species Concept
    • Evolutionary & Phylogenetic Species Concept

Species are a part of the hierarchy of life (more properly, populations are)

The fascinating case of ring species

Reproductive Isolation

The BSC requires that gene pools be separate, which required some sort of reproductive isolation

Mechanisms of reproductive isolation

  • Prezygotic
    • Pre-mating
      • Ecological isolation (Temporal and Habitat)
      • Behavioral Isolation (Pollinators included)
    • Post-mating
      • Mechanical isolation (I include the book's copulatory isolation here)
      • Gametic isolation
  • Postzygotic
    • Extrinsic
      • Ecological Inviability - low fitness is context dependent
      • Behavioral Sterility
    • Intrinsic
      • Hybrid inviability
      • Hybrid sterility
      • Hybrid breakdown

Variation, both within and between species

Variation is a requisite for evolution and understanding patterns in variation is one key to understanding evolution

Species are the unit of biodiversity

  • Species are not always apparent to us

Species can be sub-divided by distance or geography

  • races, varieties, subspecies
  • Sibling species (animals)
  • Saccharomyces sensu stricto

Genetic distance and speciation

  • Percentage of loci that are similar (decreases)
  • DNA-DNA homology
  • Genomic Similarity
    • Nucleotide diversity
    • Synteny
    • Karyotype

Genes and Isolation

Drosophila Primer

  • belong to a large clade of flies that are detritus feeders (mostly the bacteria and fungi that are breaking down the detritus)
    • Called fruit flies (not the true fruit flies which are in the family Tephritidae and are agricultural pests, while Drosophila are just pests) or, better, pomace flies (also vinegar or wine flies)
  • Drosophila as currently described is Paraphyletic and has over 1,450 described species
    • Such diversity has lead to many subdivisions of the genus below the generic level and above the species
    • Largest subdivision is into Drosophila (~1,100 species) and Sophophora (~350 species)
      • D. melanogaster is in the Sophophora and should be written D. (S.) melanogaster
  • Chosen by Thomas Hunt Morgan as his model organism when he began using an experimental approach to biology
    • Easy to maintain and breed in laboratory
    • Short generation time
    • Adult cells are mostly post-mitotic and adults will spontaneously age and die off (many studies of senility)
    • Karyotype of only 4 chromosome pairs (3 autosomal pairs and one pair of sex chromosomes) - fourth chromosome is very small and is often ignored (you book does so, so that you might think there were only two pairs of autosomes)
      • Genome size is about 165 million base pairs and contains about 14,000 genes (the human genome has about 3,200 million base pairs and about 22,500 genes; Saccharomyces cerevisiae has about 5900 genes and about13 million base pairs)
    • Chromosomes in males do not recombine during meiosis
    • Giant polytene chromosomes in large, easy to dissect salivary glands
      • Banding patterns of polytene chromosomes clearly visible with light microscope
  • Speciose group of flies that occur worldwide with lots of variation in habitat and life history
    • Larvae live in rotting vegetation (including bark in moist forests) and in some other odd places (some live in the grooves that drain away the nitrogenous waste on the heads of land crabs!)
    • Easy to collect and maintain in lab (in most cases - there are notable exceptions)
    • Have undergone radiations - most well known is the spectacular radiation on the Hawai'ian Islands
      • Scaptomyza - related genus that we now know started when a Hawai'ian fly made it back to the mainland and started a new radiation there!
  • Drosophila have provided more insight into evolution than any other group of animals

Epistasis and Sterility

  • Effect of epistasis on viability is in accord with effect of epistasis on sterility but often even more loci are involved
  • Sterility can be the outcome of mismatches at specific loci but may be the cumulative effect at many loci
    • The many-loci scenario leads to the idea of species as Coadapted Gene Pools that differ in many ways from other, closely related species
  • Specific genetic problems may result in effective genetic barriers
    • Chromosomal mismatches can lead to cell division problems
    • During prophase I of meiosis, homologous chromosome regions synapse
    • Translocation of chromosome segments can make synapsis difficult in hybrids
    • Segregation during Anaphase I may result in aneuploidy
  • Overall picture of genetics of species separation are consistent with Darwinian gradualism in that lots of loci are involved (small steps)

Hybridization

  • Mating between organisms from different species was once treated as an exception that proved the rule and mostly ignored as an important evolutionary phenomenon but that has changed
    • hybrids common in fungi and plants, somewhat rarer in animals (perhaps due to behavioral isolation??)
  • Often, hybrids are common enough that they form Hybrid Zones where the distribution of two groups (sibling species or subspecies) overlap
    • Primary (ecologically determined) hybrid zones vs Secondary (overlap of previously separated populations or species) hybrid zones
    • Coincident rapid changes in allele frequency at many loci is the genetic marker for hybrid zones
      • Primary zones should have a matching change in some relevant habitat variable
      • Secondary zones may not have any habitat referent and the clines are maintained by Epistatic Incompatibility through heterozygote disadvantage
    • Hybrid zones may persist, collapse as the two species or subspecies coalesce into one, the two groups may become reproductively isolated or the hybrids may become isolated from either parental group and three species emerge
  • Introgression
  • WGD

Modes of Speciation

Speciation is:

  • promoted by local adaptive differences such that phenotypes from one population are selectively disadvantaged when in the other habitat
  • promoted by low fitness of  hybrids (or heterozygotes when working with individual loci)
  • prevented by gene flow between the two populations that might become isolated
    • gene flow reduces genetic distance between the populations and prevents reproductive isolation

So, models of speciation concentrate on situations that might reduce gene flow and promote habitat and genetic isolation

Modes of Speciation:

  • Allopatric
    • separate geographic ranges is the classic case, some now allow microgeographic habitat differences to count as allopatry (I do not in most cases) and habitat preference then is supposed to have the same effect as separation by an ocean, which it usually does not
    • Vicariant allopatry - disjoint geographic distributions (the classic cases of allopatry)
    • Peripatry - founder effect is important here
      • Darwin's finches are mostly cases of peripatry as only a few founders made it from South America to the Galapagos
      • Mayr theorized that peripatric speciation was likely to be so rapid that transitional morphologies would be very rare in the fossil record
      • rapidity depends on founding flies having unusual subset of alleles and epistatic interactions with other alleles at different loci altering their selective advantage almost instantly and, finally, rapid allelic sorting for those epistatic combinations favored in the new environment (both the genetic and physical environment!!!)
  • Parapatric
    • Isolation by distance is a part of the scheme as individuals may only cover a very small part of the species distribution in a lifetime
      • Classic case of House Sparrow (Passer domesticus) - 100 introduced in New York from Europe in mid 1800's, spread across the continent and the east coast birds are distinct from the west coast birds, happened in less than 100 years
  • Sympatric
    • controversial
    • theory says gene flow and recombination will trump genetic drift and selection and keep potential species from becoming isolated
      • sets of genes ideal for one set of environmental conditions can't be kept together (recombination will produce lots of new combinations each generation) unless there is very exacting positive selective mating (leading to homozygosity at the affected loci)
    • part of controversy is the advocates of allopatry keep redefining allopatry to make sympatry rare, so sympatric speciation is relegated to rarity!)
    • however, theory never trumps data and there are now several cases of sympatric speciation known, mostly among insects
  • Polyploid
  • Cacti: haploid chromosome numbers found in cacti are 11, 22, 33, 44, 55, 66, 77, 88, and 99
  • Saccharomyces sensu stricto
  • Recombinational

Geographic separation allows populations to diverge (phenotypically and genetically)

  • Divergence can be based on genetic drift (chance) or natural selection
  • Speciation may be either:
    • the by-produce of the divergence (epistatic incompatibility)
    • the result of natural selection to enforce the separation of the two (reinforcement of differences)
    • these are not mutually exclusive alternatives

Sexual selection is, perhaps, a common mechanism of population differentiation and reproductive isolation and also can act as reinforcement

  • Drosophila mojavensis is found on Baja Peninsula and across the Sea of Cortez as a smaller population in Sonora, Mexico
    • D. arizonae is very closely related and overlaps with D. mojavensis in Sonora
  • D. arizonae females almost always reject D. mojavensis males, no matter the origin of either fly
  • D. mojavensis females from Baja (allopatric to D. arizonae), who rarely interact with D. arizonae males, will sometimes make a mistake and choose D. arizonae males when males of both species are present but D. mojavensis females from Sonora (sympatric with D. arizonae), who regularly interact with D. arizonae males, are far better at rejecting D. arizonae males based on the sexually selected physical and behavioral traits important for mate choice in these flies
    • Complicated by effect of host cactus chemistry because the hydrocarbons found in the cuticle (the waxy outer layer of the exoskeleton) are important to mate recognition and choice
      • The composition of the cuticular hydrocarbons depends on the lipids present in the larval food and D. mojavensis and D. arizonae larvae occur in different cactus hosts
      • The mating tests were conducted with flies reared on standard laboratory food and, for that reason, the outcome of the mate choice experiments discussed here are not affected by ecological differences found in wild flies

Consequences of Speciation

  • Speciation, by isolating gene pools, makes their fates independent of one another
    • Within as species, genetic variation may be limited by gene interactions
      • Some combination of genes not favored because of epistatic interactions
    • When isolation occurs, some alleles that prevented variation at other loci may not be favored by selection in new species in new environment but the situation is unchanged in the other new species, living in the ancestral environment
      • So, the new species in the new situation can fix gene combinations that are not favored in the other species in the ancestral environment
  • This freedom may mean that speciation is associated with morphological increased speed of morphological change
    • This possibility gave rise to the idea of Punctuated Equilibrium, where species are in morphological stasis until speciation, where they rapidly diverge in morphology and then re-enter morphological stasis (no change in morphology) once again.
    • Some feel that there is no great burst of morphological change during or shortly after speciation, that Darwinian gradualism prevails and you see species' morphologies diverging over time but at a steady rate (and one comparable to the rate of divergence of populations not yet separated by complete geographic or genetic barriers)
Last updated January 19, 2009