Formation of Species

Biological Species Concept (BSC) 

A species is a group of natural populations that can actually or potentially interbreed and produce viable, fertile offspring, and that are reproductively isolated from other such groups

Reproductive Isolating Mechanisms: BSC 

• Speciation is the process of how new species arise

• Many types of isolating mechanisms to keep reproductively distinct: 

  • Seasonal isolation — breeding seasons do not correspond 

  • Sexual isolation — courtship behaviors 

  • Mechanical Isolation — incompatibility in the structure of male and female sexual organs 

  • Various post-mating mechanisms – sterility

• Recognition Species Concept 

• Emphasizes the role of mate recognition in the speciation process 

• Unique traits or behaviors that allow individual members of the same species to recognize each other for mating 

• Potential mates from different populations may not recognize each other 

• Coloration, calls, etc.

Major Modes of Speciation  (not either/or, both can exist)

• Cladogenesis

• Anagenesis 

Cladogenesis

• plitting of one lineage into two or more distinct lineages 

• These are branching events… but species 1 may continue to persist, even after the origin of the two daughter species

Anagenesis 

• One species evolves over time 

• Species 1 slowly and gradually evolved into species 2, and species 1 is no longer identifiable in the fossil record 

Processes of Speciation 

1. Allopatric 

2. Parapatric

3. Sympatric 

Allopatric 

1. Speciation occurring via complete geographic isolation 

   1. Divergence 

   2. Genetic drift 

   3. Mutation 

   4. Natural selection 

2. The population begin to look different and behave differently over time

Parapatric

1. Speciation involving only partial geographic isolation 

   1. Divergence 

   2. Mutation

   3. Natural selection 

   4. Selective breeding & reduced gene flow

2. Baboon variation (example)

Sympatric

1. Speciation occurring in the absence of geographic isolation

   1. Divergence 

   2. Mutation 

   3. Natural selection 

   4. Mate recognition 

   5. Selective breeding

Tempo of Speciation

• Punctuated Equilibrium

  • Evolutionary change proceeds through long periods of stasis punctuated by rapid periods of change  

• Gradualism 

  • Evolutionary change proceeds gradually through accumulated small – scale changes 

Patterns of Adaptive Change

• A niche is how a species interacts with its environment, with other species in its community, and how it utilizes resources in its habitat

  • Each species plays a unique role in its environment 

  • No two species have the same ecological niche

  • Species must have different niches in order to avoid direct competition 

Adaptive Radiation

• When a single kind of organism diversifies to fill different ecological niches 

• Original species with evolutionary novelty quickly yields descendant species with that novelty → think advantageous trait 

• Each descendant represents some new variation on that novelty

  • Exploit environment in different ways 

• Examples: lemurs in Madagascar

Diversification of Life Classification

• Species is only biologically-define category 

• Other categories are defined based on degree of relatedness

Criteria for Classification

physical similarities

• analogy

• homology

analogy

• Structures that are similar smong organisms in appearance or properties (function) but are inherited from different precursors 

• Separate adaptations independently produced by natural selection 

Homology

• Similarities that are shared between organisms because they were inherited from the same structures in a common ancestor 

• Testing for homology in the similarity of morphology and development 

• We use homologous traits to reconstruct evolutionary relationships

Ancestral homology

 characteristics that are shared through very remote or distant common ancestry

Derived homology

 is a feature that is diagnostic for a specific taxon, meaning that feature was acquired by the last common ancestor of that taxon, and is shared by all members of the group