Speciation

Speciation is the origin of new species caused by two interbreeding species. These species

 don't differ genetically from each other. If one population is able to successfully breed with

 another, the species start to diverge genetically from the population and create a new

 population. At one point, the two populations separate from each other and become

isolated from each other and can no longer reproduce with each other and prevent gene

flow between them. The genetic differences that cause reproductive isolation is called

reproductive isolating mechanisms. There are two mechanisms of isolation that occur

prezygotic barriers and postzgotic barriers. (1)

The biological species concept is a population of a species whose members have the ability

to interbreed and produce viable, fertile offspring but are unable to produce fertile offspring

 with other populations. Because of this reproductive incompatibility, reproductive

isolation, the biological factors or barriers that stop two members of different species from

producing viable, fertile hybrids. There are two barriers prezygotic and postzygotic

barriers(1).

Prezygotic barriers impede mating or hinder fertilization if mating does occur. There are five types of prezygotic barriers. The

first type of prezygotic barrier is habitat isolation. Habitat isolation is when two species occupy different habitats in the same

area but will rarely encounter each other if they do encounter each other at all. These species are not isolated by obvious

physical barriers like mountain ranges (2). An example of this is cheetahs, lions and leopards. Lions and cheetahs hunt during

the day and interaction between the two is minimal to none between them. Leopards hunt during the night not coming into

contact with lions or cheetahs.(3)

The second type is temporal isolation. This is when species breed/mate during different times of the year, day, or seasons

cannot mix their gametes(2). An example of this is cheetahs breed during the wet seasons while leopards only breed two times

a year(3).

The third example of isolation is behavioral isolation. In behavioral isolation courtship rituals attract mates of a specific species,

these are reproductive barriers(2). An example of this is when crickets mate the male plays a song to impress the female, if the

female and male song match they will reproduce(3).

The fourth example is mechanical isolation. During mechanical isolation morphological differences will prevent successful

mating between species(2). An example of this is bush babies, a group of small arboreal primates, are divided into several

species based on mechanical isolation. Each species has distinctly shaped genitalia that, like locks and keys, only fit with the

genitalia of its own species.

The fifth example is gametic isolation. Gametic isolation is when the sperm of one species is not able to fertilize the eggs of

another species because the sperm cannot penetrate the membrane of the egg. An example of this is the reproduction of

guppies of two different species. Guppies of different species can not reproduce together(6).

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Postzygotic barriers prevent a hybrid zygote from developing into a fertile adult. There are three types of

postzygotic

barriers. The first type of postzygotic barrier is reduced hybrid viability. In hybrid viability the genes of

different parent species

can interact and impair the hybrid's development(2) and the zygote does not live to reproduce(1).

An example of this is the sea urchins release their sperm and eggs into the surrounding water where they fuse and form zygotes.

The second type is reduced hybrid fertility. Here if hybrids are vigorous they maybe sterile

because the chromosomes of the

parents failed to produce normal gametes in the offspring. This causes genes to not flow freely between species(2). An

example of this is the liger which is a cross between a male lion and a female tiger(3). The third type is hybrid breakdown. In

hybrid breakdown the first generation is fertile but when they reproduce the next generation is sterile(2). An example of this is

frogs from the genus //Rana// can form hybrid tadpoles but the tadpoles die before becoming adults(5).

Both of these reproductive barriers can block genetic exchange between species by using several barriers(1).

There are two main ways that speciation can occur: allopatric speciation or sympatric

speciation. Allopatric speciation occurs when the gene flow is interrupted

<span style="color: #008080; font-family: Verdana,Geneva,sans-serif; font-size: 120%;">when a population becomes divided geographically into subpopulations. It can also occur

<span style="color: #008080; font-family: Verdana,Geneva,sans-serif; font-size: 120%;">not by geographical divide but by the organisms moving to a remote area. After the

<span style="color: #008080; font-family: Verdana,Geneva,sans-serif; font-size: 120%;">geographic seperation the gene pools diverge through any of the following mechanisms:

<span style="color: #008080; font-family: Verdana,Geneva,sans-serif; font-size: 120%;">different mutations arise, sexual selection, other selective pressures, and genetic drift alters

<span style="color: #008080; font-family: Verdana,Geneva,sans-serif; font-size: 120%;">allele frequencies. Populations can be separated by rivers, canyons, hills and mountains. An

<span style="color: #008080; font-family: Verdana,Geneva,sans-serif; font-size: 120%;">example of this is two species of gophers separated by a canyon(2).

<span style="color: #000080; font-family: Georgia,serif; font-size: 120%;">In sympatic speciation, speciation takes place in geographically overlapping populations. Two

<span style="color: #000080; font-family: Georgia,serif; font-size: 120%;">mechanisms of speciation are chromosomal changes and nonrandom mating which reduces gene flow. In

<span style="color: #000080; font-family: Georgia,serif; font-size: 120%;">some plant species have their origins in accident during cell division and an extra set of chromosomes is

<span style="color: #000080; font-family: Georgia,serif; font-size: 120%;">the result. This condition is called polyploidy.



<span style="color: #000080; font-family: Georgia,serif; font-size: 120%;">An individual that has more than two chromosomes sets and is from single species is an autopolyploid.

<span style="color: #000080; font-family: Georgia,serif; font-size: 120%;">Various mechanisms can change a sterile hybrid individual into a fertile polyploid individual in a process

<span style="color: #000080; font-family: Georgia,serif; font-size: 120%;">called allopolyploid. These individuals are fertile amongst themselves but are sterile when they attempt to

<span style="color: #000080; font-family: Georgia,serif; font-size: 120%;">interbreed with the paternal species thus representing a "new biological species"(2).

<span style="color: #046f8f; font-family: 'Trebuchet MS',Helvetica,sans-serif; font-size: 120%;">There are two two theories for the tempo of speciation. The first theory is the gradualism model. In

<span style="color: #046f8f; font-family: 'Trebuchet MS',Helvetica,sans-serif; font-size: 120%;">the gradualism theory, species descend from a common ancestor and slowly diverge from each other as

<span style="color: #046f8f; font-family: 'Trebuchet MS',Helvetica,sans-serif; font-size: 120%;">they acquire unique adaptations(2). Gradualism is hard to notice over a short period of time. The

<span style="color: #046f8f; font-family: 'Trebuchet MS',Helvetica,sans-serif; font-size: 120%;">small variations that occur help the organism fit to its environment. The organisms with better traits

<span style="color: #046f8f; font-family: 'Trebuchet MS',Helvetica,sans-serif; font-size: 120%;">survive and are passed on to the nest generation. Eventually, the population slowly changes over

<span style="color: #046f8f; font-family: 'Trebuchet MS',Helvetica,sans-serif; font-size: 120%;">time(8).

<span style="color: #046f8f; font-family: 'Trebuchet MS',Helvetica,sans-serif; font-size: 120%;">The second theory is punctuated equilibrium model. In the punctuated equilibrium mode,l a species a

<span style="color: #046f8f; font-family: 'Trebuchet MS',Helvetica,sans-serif; font-size: 120%;"> diverge in spurts over time(2). There is a period of time where there is little or no change in the

<span style="color: #046f8f; font-family: 'Trebuchet MS',Helvetica,sans-serif; font-size: 120%;"> organism, then a huge or a few changes occur, usually through mutations in genes(8).

(1). http://evoled.dbs.umt.edu/lessons/speciation.htm

(2). AP Biology Book Page 474-475

(3). My own examples

(4). VideoYoutube

(5). http://www.bioedonline.org/slides/slide01.cfm?q=hybrids&dpg=5

(6). http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1634901/

(7). http://www.bioedonline.org/slides/slide01.cfm?q=%22hybrids%22&dpg=1

(8). http://necsi.org/projects/evolution/evolution/grad+punct/evolution_grad+punct.html

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