Chapter 18 Speciation

What is a Species?

• The morphological species concept expresses the following:– Species, in its simplest interpretation means

“kind”

• The biological species concept relies on reproduction to define relatedness of species– Ernst Mayer says, “Species are groups of

interbreeding natural populations that are reproductively isolated from other such groups.”

Continue…• 1. Linnaeus separated species based on morphology, i.e., their traits

differed; Darwin saw that similar species are related by common descent.2. Ernst Mayr (1942) developed the biological species concept: a species is a group of actually or potentially interbreeding populations that are reproductively isolated from other such groups.3. The biological definition of a species says that the members of one species interbreed and have a shared gene pool, and each species is reproductively isolated from every other species.4. Gene flow occurs between populations of one species but not between populations of different species.5. Biochemical genetics uses DNA hybridization techniques to determine relatedness of organisms; the phylogenetic species concept uses DNA/DNA comparisons.

Speciation

• Is the attainment of reproductive isolation, but genetic changes between population of the same species can be countered by gene flow.

• Genetic divergence: is the process whereby local units of a population become reproductively isolated from other units and thus experience changes in gene frequencies between them

Reproductively Isolating Mechanisms

• Any heritable features of body form, functioning, or behavior that prevent interbreeding between genetically divergent populations

Prezygotic mechanisms take effect before or after fertilization

• 1- Behavioral Isolation: patterns of courtship may be altered to the extent that sexual union is not achieved.

• 2- Temporal Isolation: Different groups may not be reproductively mature at the same season, or month, or year.

• 3- Mechanical Isolation: Two populations are mechanically isolated when differences in reproductive organs prevent successful interbreeding

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• 4- Ecological Isolation: Potential mates may be in the same general area by not in the same habitat where they are likely to meet

• 5- Gametic Mortality: Incompatibilities between egg and sperm prevent fertilization

Postzygotic Mechanisms: takes effect after fertilization

• Sometimes fertilization occurs between different species, but the hybrid embryo is weak and dies.

• In some instances the hybrids are vigorous but sterile – Example: Mule produced by a male donkey and

female horse

Allopatric Speciation Defined

• Model some physical barrier arises and prevents gene flow between populations

• Reproductive isolating mechanisms evolved in the genetically diverging populations and will result in complete speciation when the two species can no longer interbreed

• First postzygotic isolation occurs and then prezygotic isolation occurs.

Geographic Isolation

• Isolation may be sudden as in an earthquake that results in speciation of species

• Studies of enzymes from fishes on the Atlantic and Pacific sides of the Isthmus of Panama reveal molecular differeneces

Allopatric Speciation on Archipelagos

• Archipelagos is an island chain some distance away from a continent

• The finches of the Galapagos Islands are evidently ancestors of mainland finches that invaded first a few islands and then spread to others, diverging as time progressed

Sympatric Speciation

• Species may form within the home range of an existing species, in the absence of a physical barrier

• In two crater lakes of East Africa exist small fish called cichlids– The species in each lake are alike in their

mitochondrial DNA and unlike the species in neighboring lakes and streams

– The lakes are small so the fish must live in sympatry

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• Polyploidy is the inheritance of three or more of each type of chromosome due to improper separation of chromosomes during meiosis and mitosis.– Speciation is instantaneous for plants that are

polyploid– Polyploid animals are rare because of dosage

compensation which regulates the level of gene expression in chromosomes.

Parapatric Speciation

• Daughter species form from a small proportion of individuals along a common border between two populations

• Interbreeding individuals produce hybrid offspring in this region called a hybrid zone

Branching and Unbranching Evolution

• Cladogenesis: applies to populations that become isolated from one another and subsequently diverge in different directions

• Anagenesis: is a pattern of descent in which species form within a single, unbranched line

Evolutionary Trees and Rates and Change

• Evolutionary Trees- summarize information about the continuity of relationship among species

• Gradual model of speciation- is represented by tree diagrams with branches at slight angles to each other to show slow change over time

• Punctuation Model of speciation- drawn with short, horizontal branches that represent abrupt periods of speciation followed by stable periods

Adaptive Radiation

• A burst of microevolutionary activity that results in the formation of new species in a wide range of habitats.

• The presence of adaptive zones presents new ways of life by physical, evolutionary, or ecological access.

• The case of Darwin's finches illustrates the adaptive radiation of 13 species from one founder mainland finch.

Extinctions

• Extinction is the rather inevitable loss of species as local conditions change over periods (usually long) of time.

• Mass extinctions are abrupt disappearances dues to catastrophic, global events