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Section 25_2: Mechanisms of Speciation

Page history last edited by Melody Knight-Brown 11 years, 3 months ago

25.2 Mechanisms of Speciation


Learning Objectives

•   Define reinforcement in the context of reproductive isolation.

•   Explain the possible outcomes when two populations that are partially reproductively isolated become sympatric.

•   Compare and contrast sympatric and allopatric speciation.

•   Explain the conditions required for sympatric speciation to occur.


Quick Summary:

     Speciation is the formation of a new species from one original species. When this split of the original species occurs it is called cladogenesis and when it happens multiple times so there are many species stemming from that first original species, it is known as adaptive radiation. There are two kinds of speciation, allopatric speciation, which occurs when a population of the original species is geographically isolated while developing into a new species, and sympatric speciation, which occurs when the new species develops along side the original species. Allopatric speciation can lead to hybrid zones when two populations have incomplete geographical isolation, which means the two populations are able to interbreed. However for speciation to occur, the contact within these hybrid zones must be minimal. Sympatric speciation, which is less common than allopatric speciation, may occur in one of three ways. The first is polyploidy. Polyploidy may be due to complete chromosomal nondisjunction or through interspecies breeding; either way it creates an alloploid organism and leads to immediate reproductive isolation. Plants are more tolerant of polyploidy than animals. The second type of sympatric speciation is when organisms adapt to their different yet continuous local environments, and the third type of sympatric speciation is sexual selection in which members of a species selective choose mates to such a degree they eventually diverge into different species. 



In-Depth Summary:

     Speciation is the formation of new species due to an accumulation of genetic changes great enough to be deemed a separate species from the one in which it was originally derived. These genetic changes are often a result of random mutations which are then selected for or dismissed from the gene pool through natural selection. Other ways genetic changes may be brought about are interspecies mating, a change in the number of chromosomes, and horizontal gene transfer. Speciation is thought to be brought about by either abrupt changes which cause reproductive isolation, or more commonly, a series of gradual adaptations to a specific environmental niche. The term gradual adaptations must, however, be taken with a grain of salt as speciation will often rapidly occur especially when environments differ significantly. In such a case, natural selection will quickly alter the genetic composition in a way that is most favorable to that specific environment.


     Cladogenesis is the divergence of a population into two or more species. There are two ways for cladogenesis to occur, the first being Allopatric speciation (allos which is Greek for other) and Sympatric speciation (sym which is Greek for together). Allopatric speciation is the more prevalent of the two, and occurs when members of a given population occupy a habitat that is isolated from the rest of that population. This kind of separation may occur when a small population moves to a new location, e.g. a storm forcing a population of birds to an island. This separation may also be due to a geographical barrier e.g. an ocean, mountain range, desert, etc. Commonly, however, this geographical separation is incomplete which allows for the formation of Hybrid Zones in which the two populations are able to interact and interbreed. This is known as gene flow and is necessary to maintain cohesiveness throughout a species. This is not ideal for speciation, however, and in order for speciation to occur, gene flow must be minimal, until the species become reproductively incompatible. One way this may eventually happen is through mechanical isolation i.e. tall members of one population may not easily interbreed with small members of another population. Mechanical isolation is the reason why there are no chihuahua/great-dane mutts. Allopatric speciation is also responsible for Adaptive Radiation in which a single ancestral species evolves into a wide array of descendant species. For example, in Hawaii there are at least 54 different species of honeycreepers, all which originated from a single species of finch. 


     Sympatric speciation occurs when populations are in direct contact. This may be accomplished through polyploidy, adaptions to a local environment, or sexual selection. Polyploidy is a type of genetic change in which an organism has more than two sets of chromosomes. This results in immediate reproductive isolation. Polyploidy generally not tolerated in animal species, however it is the major method of plant speciation. 40-70% of ferns and flowering plants are polypoids whereas less than 1% of reptiles and amphibians are polypoids. 


     There are two ways polyploidy may occur. The first way is through complete chromosomal nondisjunction. This creates a tetraploid organism meaning it has four complete sets of chromosomes. When a tetraploid reproduces with a normal diploid organism, the resulting hybrid will have three sets of chromosomes. As three is an uneven number, these sets of chromosomes are unable to divide evenly, which results in sterility of the organism and reproductive isolation between the tetraploid and the diploid. The second mechanism that causes polyploidy is interspecies breeding. This happens when two organisms from different species, commonly evolutionary relatives and genetically similar, reproduce. This leads to the creation of an alloploid organism, who has at least one set of chromosomes from two or more different species. When the offspring only gains one set of chromosomes from each species, it is called allodiploid. Allodiploid organisms may then go through complete nondisjunction and this leads to an allotetraploid organism, who has four complete set of chromosomes. Allotetraploid is a form of the more general term allopolyploid, which is an individual who has two or more complete sets of chromosomes from two or more species. 


     The other two ways sympatric speciation may be accomplished are less complicated, though no less important. The first, adaption to local environment occurs due different various albeit continuous geographic environments. For example, a colonial bentgrass diverged into two species with one being more tolerant of metal in the soil. Another example would be the North American apple maggot fly which originally fed on hawthorn fruit. However, as apple fruit matures more quickly than the hawthorn fruit, the apple-feeding population also began to developed more quickly and this resulted in partial temporal isolation, which essentially means that the difference species had different breeding seasons, e.g. one breed in the spring while the other breed in the fall. This is also an example of prezygotic reproductive isolation. 


     The final form of sympatric speciation is sexual selection, in which the members of a species selectively choose their mates based on things like, showy coats, bright colors, large horns, etc.  Peacocks, for example, have some of the showiest costumes with which to attract mates. Deer grow antlers. Sexual selection is not only limited to costumes; Some male birds attract mates through building homes, such as the male bower bird. Whatever the mechanism of attraction, over time, this selective breeding may separate the population into two distinct species.




Speciation - the formation of new species due to an accumulation of genetic changes great enough to be deemed a separate species from the one in which it was originally derived

Cladogenesis - the divergence of a population into two or more species

Allopatric speciation - cladogenesis that occurs when members of a given population occupy a habitat that is isolated from the rest of that population

Hybrid Zones - a place where two populations of the same species are able to interact and interbreed

Adaptive Radiation - when a single ancestral species evolves into a wide array of descendant species

Sympatric speciation - cladogenesis that occurs when populations are in direct contact

Polyploidy - a type of genetic change in which an organism has more than two sets of chromosomes

Alloploid - when an organism has one set of chromosomes from two or more different species


Useful Materials

  This video provided a nice overview of speciation. There was some overlap with a couple of the other sections. He also covered a couple topics which I did not such as types of reproductive isolation beginning at 4:10 to 6:30. The graphic at 7:25 (more specifically at 8:01) I felt really helped me visual how sympatric speciation occurs. He also explains more in-depth adaptive radiation. Stop listening at 12:45 because it begins a new section in the book. 
  Begin watching at 9:20. He does a good job at explaining allopatric speciation as well as provide several good examples such as the snapping shrimp.
I really enjoyed how she explained Polyploidy. It was pretty simple and easy to follow. Definitely watch this if you're having trouble understanding this topic. It helped me a lot.

Hank is amusing and lightweight while being informative. Everything before 3:00 goes a little into the previous section but I'd watch it anyway because it's still important and interesting.


Also if you didn't get the Sasha Baron Cohen comment go look him up, and take the hint on adolescent poetry.  

http://evolution.berkeley.edu/evosite/evo101/VCCausesSpeciation.shtml This site was helpful. I'd explore some of the tabs on the side to better understand some of the key topics. There is also a quick quiz on this site if you feel like testing yourself.
http://www.polyploidy.org/index.php/Information:How_are_polyploids_formed Polyploidy was one of the most difficult part of my section for me to learn and comprehend. This site was very helpful if you have any questions about how polyploids are formed. Additionally by clicking on some of the other tabs, this site is helpful in clearing up other aspects of polyploidy.
http://www.nature.com/scitable/topicpage/polyploidy-1552814 This site has a good overview of polyploidy as well as explained both its advantages and disadvantages. 



Primary Literature:


This is the study on the Heliconius butterflies I mentioned in the Virtual Lectures.  This was a study on butterflies pertaining to the genus of Heliconius which primarily live in South and Central America. Due to the relatively widespread geographical habitat the interaction between particular members of this genus is limited. This allows scientists to truely see the formation of hybrid zones, the continuum they represent, and ultimately the steps to eventual speciation.


The Heliconius erato for example has 20 different variations on wing coloration depending on that particular populations' geographical habitation. Additionally the scientists conducting this study took samples from 121 different butterflies mitochondrial DNA. When compared and cross referenced with the different color patterns of the wings, analysis shows that theses signs are indicative of littler gene flow and low frequencies of hybrid genotypes and phenotypes. What few cases of hybridization there is could be explained by a kind of geographical separation although the habitations of the different populations are close to each other.


Due to this unique circumstance of geographical separation without a barrier, this article then introduces two concept which were not discussed in the book section, the first being the idea of parapatric speciation and the second being the idea of bimodel hybrid zones. Parapatric specation is similar in ways to both allopatric and sympatric speciation and is generally regarded as the halfway point between these to forms of specation. Parapatric specation occurs when the populations do not co-inhabit the same region but are immediately adjacent to each other. 


The second idea was that of bimodel hybrid zones. This is the idea that although hybrid zones may overlap, the members of the populations within that region still maintain a distinct genetic lineage. 


This is an study on polyploidy in flowering plaints and the relation to self-fertilization. Polyploidy is widely accredited for the diversity in plants. It is also widely accepted that polyploidy plants are more likely to undergo self-fertilization. It is this second idea that this study was meant to prove or disprove. 


Using the data from 235 species of flowering plants it was discovered that polypoidy plants do indeed have a higher degree of self-fertilization than regular diploid plants. It is theorized that this may increase the successfulness of the population although the relationship between polyploidy and self-fertilization is still complex and has been shown to be dependent or many more factors and will continue to be of interest to scientists and potentially key to understanding how flowering plants evolve. 




Virtual Lectures:











Grading Sheet (Melody)

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