TR

Macroevolution

Morphological Traits

  • Morphological traits: physical characteristics of an organism
  • Drawbacks for determining species
    • How many traits to consider
    • Traits may vary in a continuous way
    • What degree of dissimilarity to use
    • Members of the same species can look very different
    • Members of a different species can look very similar

The Biological Species Concept

  • Biological species concept: states that a species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring; they do not breed successfully with other populations
  • Gene flow between populations holds the phenotype of a population together

Reproductive Isolation

  • Reproductive isolation: prevents one species from successfully interbreeding with other species
  • Four main problems for determining species
    • may be difficult to determine in nature
    • can interbreed and yet do not
    • does not apply to asexual species
    • cannot be applied to extinct species

Molecular Features

  • Compare features to identify similarities and differences among different populations
    • DNA sequences within genes
    • gene order along chromosomes
    • chromosomes structure
    • chromosome number
  • May be difficult to draw the line when separating groups

Ecological Factors

  • Variety of factors related to an organism’s habitat can be used to distinguish one species from another
  • Many bacterial species have been categorized as distinct species based on ecological factors
    • Drawback – different groups of bacteria sometimes display very similar growth characteristics, and even the same species may show great variation in the growth conditions it will tolerate

Species Concepts

  • Way to define the concept of a species and/or provide an approach to distinguish one species from another
  • Biological species concept: species is a group of individuals whose members have the potential to interbreed with one another in nature to produce viable, fertile offspring, but cannot successfully interbreed with members of other species
    • Limitations:
    • The biological species concept cannot be applied to fossils or asexual organisms (including all prokaryotes)
    • The biological species concept emphasizes absence of gene flow
    • However, gene flow can occur between distinct species
  • Evolutionary lineage concept: species should be defined based on the separate evolution of lineages
  • Ecological species concept: each species occupies an ecological niche– the unique set of habitat resources that a species requires, as well as its influence on the environment and other species

Cladogenetic Speciation

  • Cladogenetic speciation: occurs when a population is different enough from its ancestral species so that no genetic exchange can occur between them
    • The appearance of a reproductive isolating mechanism produces 2 or more species where there was one
  • Two modes
    • Allopatric speciation
    • Sympatric speciation

Sympatric Speciation

  • Sympatric speciation: occurs when members of a species that are within the same range diverge into two or more different species even though there are no physical barriers to interbreeding
  • Mechanisms include 
    • Polyploidy
    • Adaptation to local environments
    • Sexual selection
  • Adaptation to local environments
    • Geographic area may have variation so that some members of a population may diverge and occupy different local environments that are continuous with each other
  • Sexual selection
    • Certain females prefer males with one color pattern, while other females prefer males with a different color pattern
  • Disruptive selection
    • Animals diverge mostly due to reproductive isolation. 
    • Reproductive isolation is a result of genetic factors that cause offspring to rely upon resources not used by previous generations. 

Hybrid Zones

  • Hybrid zones reveal factors that cause reproductive isolation
  • Hybrid zone: a region in which members of different species mate and produce hybrids
  • Hybrids: the result of mating between species with incomplete reproductive barriers

Reinforcement Fusion and Stability

  • When hybrids are less fit than parent species, reinforcement of reproductive barriers may occur through strong selection for prezygotic barriers
    • Over time, the rate of hybridization decreases
  • Fusion: if hybrids are as fit as parents, there can be substantial gene flow between species
    • If gene flow is great enough, reproductive barriers weaken and the parent species can fuse into a single species
  • Stability: extensive gene flow from outside the hybrid zone can overwhelm selection for increased reproductive isolation inside the hybrid zone

Adaptive Radiation

  • Adaptive radiation: produced from a combination of allopatric and sympatric speciation; the rapid evolution of diversely adapted species from a common ancestor

    • Separation from ancestral species allopatrically
    • Presence of a variety of open ecological niches provides opportunity for sympatric speciation by disruptive selection

  • Adaptive radiations may follow

    • Mass extinction

    • The evolution of novel characteristics

    • The colonization of new regions

    • Adaptive radiations can occur when organisms colonize new environments with little competition