Microtaxonomy

• Classification

  • Organisms are classified into a hierarchal classification that groups closely related organisms and progressively includes more and more organisms.
• • Darwin explored the Galápagos Islands

  • And discovered plants and animals found nowhere else on Earth.

  • The origin of new species, or speciation

  • Is at the focal point of evolutionary theory, because the appearance of new species is the source of biological diversity.

  • Evolutionary theory

  • Must explain how new species originate in addition to how populations evolve.

  • Macroevolution

  • Refers to evolutionary change above the species level.
• Two basic patterns of evolutionary change can be distinguished:

  • Anagenesis - Evolutionary change within lineage over time

  • Cladogenesis - Splitting of lineage over time
• Pace of Speciation

  • The rates of evolutionary change are not constant.

  • Gradualism
  • Punctuated Equilibrium
• Gradualism

  • New species evolves gradually over long spans of time.

  • Due to slow accumulation of many small genetic changes.
• What is a Species

  • The species is the basic biological unit around which classifications are based.
  • However, what constitutes a species can be difficult to define and there are multiple definitions of species used today.
• Species


  a population of individuals that interbreed and produce fertile offspring under natural conditions.
• Species Concepts:

  
  

  • Typological species concept - species are distinct unchanging entities defined by unique, morphological features. This concept has been abandoned by biologists.
• Problems with TSC

  • It ignores evolution: species are not “unchanging entities.”
  • Sexual Dimorphism: the sexes differ morphologically.
  • Alternative Morphs
  • Cline: a change in morphology along an environmental gradient
  • Cryptic species: some species are similar morphologically but differ in other important ways.
• Species Concepts:

  • Biological species concept - a species is a population or group of populations that can potentially interbreed and produce viable, fertile offspring, but that is reproductively isolated from other populations.
• The Biological Species Concept


  Defines a species as a population or group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring but are unable to produce viable fertile offspring with members of other populations.
• The Biological Species concept assumes:

  • Common Characteristics
  • Generally Compatible
  • Interbreed under Natural Conditions
  • Sexual Reproduction
• Drawbacks of BSC

  • Cannot be used with exclusively

  asexual organisms:

  • Prokaryotes
  • Amoeba & some protist
  • Some animals, plants and fungi.

  •Isolated Populations

  • How to test for interbreeding under natural conditions?
  • Some species look and behave differently.
  • Can interbreed and have viable, fertile offspring.
  • E.g. coyotes, wolves, dogs
• How does one species become two?

  • Reproductive isolating mechanisms
  • environmental, behavioral, mechanical, and physiological barriers
  • prevent individuals of two species from producing viable offspring
  • G.L. Stebbins
• Reproductive Isolation

  • Is the existence of biological factors that impede members of two species from producing viable, fertile hybrids
  • Is a combination of various reproductive barriers.
• Mechanisms of Reproductive Isolation:

  • Prezygotic barriers: prevent mating or fertilization.
  • Impede mating between species or hinder the fertilization of ova if members of different species attempt to mate.
  • Behavioral
• Prezygotic Mechanisms

  • Habitat/Residential
  • Seasonal or Temporal
  • Ethological (in animals only)/Sexual
  • Mechanical
  • Gametic
• Habitat/Residential


  The populations live in the same region but occupy different habitats
• Seasonal or Temporal


  The populations exist in the same region but are sexually mature at different times
• Ethological (in animals only)/Sexual


  The populations are isolated by incompatible premating behavior i.e. differences in behavior, physiology, or morphology
• Mechanical


  Cross-fertilization is prevented or restricted by incompatible differences in reproductive structures
• Gametic


  Gametic transfer takes place, but gametes fail to unite with each other. this can occur because the male and female gametes fail to attract because they are unable to fuse or because the male gametes are inviable in the female reproductive tract of another species
• Prezygotic Mechanisms - prevent fertilization and zygote formation
• Mechanisms of reproductive isolation:

  • Prezygotic barriers: prevent mating or fertilization.
  • Postzygotic barriers: prevent hybrid from developing into a viable, fertile adult.
• Other Definitions of Species

  • The morphological species concept
  • Characterizes a species in terms of its body shape, size, and other structural features.
  • The paleontological species concept
  • Focuses on morphologically discrete species known only from the fossil record.
  • The ecological species concept
  • Views a species in terms of its ecological niche
  • The phylogenetic species concept
  • Defines a species as a set of organisms with a unique genetic history.
• Morphological Species Concept

  • Group of individuals that share common characteristics.
  • Used for:
  • Fossils
  • Exclusively asexual
  • Traditional method for sexual organisms
  • Even within the same species there are variations
• The Phylogenetic Species Concept

  • Species are defined as the smallest diagnosable monophyletic group.
  • Any population that forms an independent branch on the phylogeny is recognized as a species.
  • To be recognized as a species, populations must have been evolutionary independent for a long enough time for diagnostic traits to emerge.
• Strengths and Weaknesses: Phylogenetic Species Concept

  Strengths:

  • It contains a historical component
  • It provides specific criteria that can be diagnosed in natural populations.

  Weakness:

  • It is somewhat vague. What exactly is meant by "irreducible cluster" and "diagnosably different?"
  • Taken literally to the extreme, it is absurd.
• Speciation can occur in two ways:

  • Allopatric speciation
  • A population forms a new species while geographically isolated from its parent population.
  • Sympatric speciation
  • A small population becomes a new species without geographic separation.
• Allopatric (“Other Country”) Speciation

  • In allopatric speciation
  • Gene flow is interrupted or reduced when a population is divided into two or more geographically isolated subpopulations.
  • Once geographic separation has occurred.
  • One or both populations may undergo evolutionary change during the period of separation.
• Allopatry

  Mechanisms:

  1. Geographic subdivision of large populations via geological processes.
  2. Founder Effect - occurs when a small group migrates to a new location that is geographically separated from the population.

  • The founding population on the island may evolve into a new species.
• Allopatry

  • Two evolutionary mechanisms:
  • Genetic Drift - may quickly lead to the random fixation of certain alleles and the elimination of other alleles from this smaller population.
  • Natural Selection - the environment on an island may differ significantly from the mainland environment, in which case natural selection on the island may favor certain alleles over others.
• In order to determine if allopatric speciation has

  occurred

  Reproductive isolation must have been established.
• Sympatric (“Same Country”) Speciation

  • In sympatric speciation
  • Speciation takes place in geographically overlapping populations.
  • Occurs when a new species is in the same geographic area as the species from which it was derived.
  • Mechanism: formation of polyploids in plants which can abruptly lead to reproductive isolation.
• Polyploidy

  • Is the presence of extra sets of chromosomes in cells due to accidents during cell division.
  • Has caused the evolution of some plant species.
• An autopolyploid

  • Is an individual that has more than two chromosome sets, all derived from a single species.
• An allopolyploid

  • Is a species with multiple sets of chromosomes derived from different species.
• Habitat Differentiation and Sexual Selection

  • Sympatric speciation
  • Can also result from the appearance of new ecological niches.
  • When members of a population occupy different local environments that are continuous with each other.
  • In cichlid fish
  • Sympatric speciation has resulted from nonrandom mating due to sexual selection.
• Allopatric and Sympatric Speciation: A Summary

  • In allopatric speciation
  • A new species forms while geographically isolated from its parent population
  • In sympatric speciation
  • The emergence of a reproductive barrier isolates a subset of a population without geographic separation from the parent species.
• Parapatry

  • Occurs when members of a species are partially separated, not complete geographic isolation.
  • Mechanisms:
  • Members of a given species may invade a new ecological niche at the periphery of an existing population.
  • A mountain may divide a species into two populations, but breaks occur in the range.
  • In a very sedentary species even though geographic isolation exists.