Speciation Hyla chrysoscelis Hyla versicolor

Using the (older) typological definition of species, these two groups of frogs were classified as one. Upon further investigation, it was discovered that there are two groups that differ only in their mating calls. They call for mates at different frequencies.

A biological concept of a species is a population or group of populations that are able to interbreed, under natural conditions to produce fertile offspring. The selective mechanisms that favour beneficial traits, natural selection, are also responsible for speciation (the formation of an entirely new species). Evolutionary changes that occur at the species level are known as microevolution.

Reproductive Isolating Mechanisms Biologists rely on morphological features from fossil records to distinguish thousands of different species. For living populations that are morphologically similar, behavioral or other biological methods are needed to distinguish species. One method used is to identify the species’ reproductive isolating mechanisms (any behavioral, structural or biochemical traits that prevent individuals of different species from reproducing successfully together). There are two types of isolating mechanisms prezygotic isolating mechanisms postzygotic isolating mechanisms.

Prezygotic isolating mechanisms Prezygotic mechanisms prevent interspecies mating and fertilization. There are four types of isolation that prevent mating from occurring, thus maintaining species isolation. ecological isolation temporal isolation behavioural isolation mechanical isolation gametic isolation Prezygotic isolating mechanisms

Example – ground squirrel species occupy different habitats. Ecological isolation is when species occupy separate habitats or niches and do not encounter one another to reproduce due to some geographic or ecological barrier. Example – ground squirrel species occupy different habitats. Marmots live in the Rocky Mountains at high elevations Woodchucks live in fields at low elevation

Ecological isolation Example – Atlantic blue-headed wrasse and the Pacific Cortez rainbow wrasse

Temporal isolation is when two species are found in the same area, but are incapable of mating due to different reproductive cycles for flowering or mating. Example – Red and black sea urchins live in the same location, but release their gametes at different times of the year.

Behavioural isolation is when distinct mating rituals by one species may prevent members of another species from recognizing or selecting a mate. Example: male jumping spiders dance (shake their legs and wave their palps). Females of different species do not respond to the dance. Example: Different species of fireflies do not recognize each others' mating signals, and as a result do not generally interbreed.

Mechanical isolation is when structural differences in reproductive organs prevent successful fertilization. Example - This is especially true in flowering plants that have evolved specific structures adapted to certain pollinators. Nectar-feeding bats searching for flowers are guided by their echolocation system. Therefore, plants which depend on these bats as pollinators, have evolved acoustically conspicuous flowers that assist in detection.

Gametic isolation – may prevent reproduction at a molecular level. Example – in coral reefs, many species with external fertilization may release gametes simultaneously, so trillions of sperm and eggs may be in the shallow water at one time. Sperm and eggs of the same species recognize each other by molecular markers. This can also be the reason pollen from one species will not be able to form a pollen tube if it lands on the stigma of a different species.

Postzygotic Isolating mechanisms Postzygotic mechanisms prevent the hybrid zygote from developing into healthy and fertile adults. There are three likely cases that will occur to ensure that the hybrid does not reproduce: Zygote mortality is a result of chromosomal incompatibility. Hybrid inviability is when the embryo does develop but the hybrid experiences reduced fitness and often an early death. Hybrid infertility occurs when a hybrid develops into a mature adult but is unable to undergo successful meiotic division, and is unable to produce offspring. (seen in donkey-horse hybrids: mules) Postzygotic Isolating mechanisms

Modes of Speciation Allopatric Speciation Populations begin to diverge when gene flow between them is restricted. Geographic isolation is often the first step in allopatric speciation. Examples: Galapagos finches and tortoises, Asian elephants, and the following experiment by Diane Dodd in 1989.

Sympatric Speciation Sympatric speciation happens when members of a population develop some genetic difference that prevents them from reproducing with the parent type. This mechanism has occurred several times in plants, where failure to reduce chromosome number results in polyploid plants that reproduce successfully only with other polyploids. This causes instant speciation due to non-disjunction. Sympatric speciation has also occurred to cichlid fish in Lake Victoria in Africa.

Parapatric Speciation Two separate regions occur with a zone of hybridization where the two species overlap Examples Bullocks Orioles and Baltimore Orioles Ensatina salamanders