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

\