Sympatric Speciation


Related Posts


Aneuploidy results when chromosomes fail to separate correctly during meiosis. As a result, one gamete has one too many chromosomes, shown as n plus 1, and the other has one too few, shown as n minus 1. When the n + 1 gamete fuses with a normal gamete, the resulting zygote has 2 n + 1 chromosomes. When the n minus 1 gamete fuses with a normal gamete, the resulting zygote has 2 n minus 1 chromosomes.
Aneuploidy results when the gametes have too many or too few chromosomes due to nondisjunction during meiosis. In this example, the resulting offspring will have 2n+1 or 2n-1 chromosomes. Source: OpenStax Biology 2e

OpenStax Biology 2e

Can divergence occur if no physical barriers are in place to separate individuals who continue to live and reproduce in the same habitat? The answer is yes. We call the process of speciation within the same space sympatric. The prefix “sym” means same, so “sympatric” means “same homeland” in contrast to “allopatric” meaning “other homeland.” Scientists have proposed and studied many mechanisms.

One form of sympatric speciation can begin with a serious chromosomal error during cell division. In a normal cell division event chromosomes replicate, pair up, and then separate so that each new cell has the same number of chromosomes. However, sometimes the pairs separate and the end cell product has too many or too few individual chromosomes in a condition that we call aneuploidy.

Polyploidy is a condition in which a cell or organism has an extra set, or sets, of chromosomes. Scientists have identified two main types of polyploidy that can lead to reproductive isolation of an individual in the polyploidy state. Reproductive isolation is the inability to interbreed. In some cases, a polyploid individual will have two or more complete sets of chromosomes from its own species in a condition that we call autopolyploidy. The prefix “auto-” means “self,” so the term means multiple chromosomes from one’s own species. Polyploidy results from an error in meiosis in which all of the chromosomes move into one cell instead of separating.

Autopolyploidy results in offspring with two sets of chromosomes. In the example shown, a diploid parent, 2 n produces polyploid offspring 4 n.
Autopolyploidy results when mitosis is not followed by cytokinesis. Source: OpenStax Biology 2e

For example, if a plant species with 2n = 6 produces autopolyploid gametes that are also diploid (2n = 6, when they should be n = 3), the gametes now have twice as many chromosomes as they should have. These new gametes will be incompatible with the normal gametes that this plant species produces. However, they could either self-pollinate or reproduce with other autopolyploid plants with gametes having the same diploid number. In this way, sympatric speciation can occur quickly by forming offspring with 4n that we call a tetraploid. These individuals would immediately be able to reproduce only with those of this new kind and not those of the ancestral species.

The other form of polyploidy occurs when individuals of two different species reproduce to form a viable offspring that we call an allopolyploid. The prefix “allo-” means “other” (recall from allopatric): therefore, an allopolyploid occurs when gametes from two different species combine. The image below illustrates one possible way an allopolyploid can form. Notice how it takes two generations, or two reproductive acts, before the viable fertile hybrid results.

Alloploidy results from viable matings between two species with different numbers of chromosomes. In the example shown, species one has three pairs of chromosomes, and species two has two pairs of chromosomes. When a normal gamete from species one (with three chromosomes) fuses with a polyploidy gamete from species two (with two pairs of chromosomes), a zygote with seven chromosomes results. An offspring from this mating produces a polyploid gamete, with seven chromosomes. If this polyploid gamete fuses with a normal gamete from species one, with three chromosomes, the resulting offspring will have ten viable chromosomes.
Alloploidy results when two species mate to produce viable offspring. In this example, a normal gamete from one species fuses with a polyploidy gamete from another. Two matings are necessary to produce viable offspring. Source: OpenStax Biology 2e

The cultivated forms of wheat, cotton, and tobacco plants are all allopolyploids. Although polyploidy occurs occasionally in animals, it takes place most commonly in plants. (Animals with any of the types of chromosomal aberrations that we describe here are unlikely to survive and produce normal offspring.) Scientists have discovered more than half of all plant species studied relate back to a species evolved through polyploidy. With such a high rate of polyploidy in plants, some scientists hypothesize that this mechanism takes place more as an adaptation than as an error.

Source:

Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at: https://openstax.org/details/books/biology-2e


Leave a Reply

Your email address will not be published.