Chapter 24- Speciation
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.