24.1 How Are Species Defined and Identified?

• A species is defined as an evolutionarily independent population or group of populations.

• The three most common criteria that can used to identify species:

  • The biological species concept

  • The morphospecies concept

  • The phylogenetic species concept

• According to the biological species concept, the main criterion for identifying species is reproductive isolation.

• To organize the various mechanisms that stop gene flow between populations, biologists distinguish between

  • prezygotic (“before-zygote”) isolation, which prevents individuals of different species from mating successfully

  • postzygotic （“ er-zygote”） isolation, in which the hybrid offspring of matings between members of different species either do not survive or do not reproduce.

• Under the morphospecies (“form-species”) concept, researchers identify evolutionarily independent lineages by differences in size, shape, or other morphological features.

• The phylogenetic species concept identifies species based on the evolutionary history of populations.

• A monophyletic group, also called a clade or lineage, consists of an ancestral population, all of its descendants, and only those descendants.

• A synapomorphy is a trait that is found in certain groups of organisms and their common ancestor but is missing in more distant ancestors.

• Systematics is the discipline of biology that characterizes and classifies the relationships among all organisms on Earth.

• Taxonomy is the practice of describing, naming, and classifying individual species.

24.2 Isolation and Divergence in Allopatry

• Populations that are geographically separated are said to be in allopatry.

• Speciation that begins with geographic isolation is known as allopatric speciation.

• Allopatric speciation starts in one of two ways:

  • Dispersal is the movement of individuals from one place to another

  • Vicariance is the physical splitting of a habitat.

• Biogeography is the study of how species and populations are distributed geographically

24.3 Isolation and Divergence in Sympatry

• When populations or species live in the same geographic area, or close enough to one another to make interbreeding possible, biologists say that they live in sympatry

• Sympatric speciation is the speciation that occurs even though populations live within the same geographical area

• Two types of factors can initiate the process of sympatric speciation:

  • extrinsic (external) factors, such as disruptive selection for extreme phenotypes based on different ecological niches and/or mate preferences

  • intrinsic (internal) factors, such as chromosomal mutations.

• Polyploidy, the condition of possessing more than two complete sets of chromosomes, is caused by a massive error in mitosis or meiosis.

• There are two types of polyploids:

  • Autopolyploid (“same-many-form”) individuals are produced when a mutation results in a doubling of chromosome number and the chromosomes all come from the same species

  • Allopolyploid (“different many-form”)  individuals are created when parents of different species mate and then an error in mitosis occurs, resulting in viable, non sterile offspring with two full sets of chromosome

• Speciation by polyploidization is driven by chromosome-level mutations and occurs in sympatry.

24.4 What Happens When Isolated Populations Come Into Contact?

• Natural selection for traits that prevent interbreeding among populations is called reinforcement.

• A hybrid zone is a geographic area where interbreeding occurs and hybrid offspring are common.