Speciation

Allopolyploidy

Allopolyploidy is the production of a polyploid species from the mating of two different species to produce hybrid offspring. A hybrid is possible even if the chromosome numbers of each species differ. The mating of two different organisms can produce a sympatric species that cannot reproduce with the parental populations. Fertile populations can form later with other hybrid mating events or as a result of errors in cell division.

Describe the differences between autopolyploidy and allopolyploidy.

Autopolyploidy arises from fusion of gametes of the same species (cell division errors, etc.). In this process, polyploid individuals develop as a result of cell division errors that double the DNA content of gametes in individuals from the same species or a self-fertilizing individual. When these double-dose gametes fuse, the result is an organism with double the chromosome number of its parent species. Allopolyploidy occurs when the gametes from two species fuse to form a polyploid generation and hybrid offspring.

Autopolyploidy

Autopolyploidy results from errors in cell division, either in mitosis or meiosis. If homologues or sister chromatids fail to separate during meiosis I or II, respectively, half or a quarter of the resulting gametes will have double chromosomes of the usual gamete for that species.

Fusion

Fusion occurs when two species that have recently diverged start exchanging genes again through gene flow, bringing the species back together into one population. Fusion can weaken reproductive barriers. It can happen naturally but can also be induced by human interference

sympatric speciation

He discussed a controversial hypothesis called sympatric speciation: the speciation of organisms from a single ancestor living in the same geographic region. Poulton derived the term from 'sym' meaning one and 'patric' meaning fatherland. At first, scientists could not come up with any methods that would cause a randomly mating population to reduce gene flow among members of that population. Today, scientists realize that certain phenotypic changes can cause members of a species to become reproductively isolated from other members in the same area.

Sexual selection

In the cichlid fish, sympatric speciation resulted from a reproductive isolation strategy called sexual selection. Sexual selection is a form of natural selection in which mate preference in part determines the phenotypic characteristics of the next generation. All cichlid species can interbreed; however, mate selection barriers keep them reproductively isolated.

Discuss factors that may strengthen, weaken, or stabilize reproductive barriers in hybrid zones!!!!

Overlap in the geographic ranges of sympatric species can lead to hybridization between two species in hybrid zones. Reproductive barriers in hybrid zones can be weakened by fusion, in which two species produce fertile, successful offspring. The barriers can be strengthened by reinforcement, when hybrids are less fit than either of the parent species. Finally, the reproductive barriers can remain stable when hybrids are produced but do not outcompete either of the parent species.

Polyploidy

Polyploidy is a phenomenon in which an organism has more than two full sets (diploid, 2n) of chromosomes. Because polyploid offspring often have unique phenotypic characteristics that distinguish them from the diploid population and do not successfully produce offspring with their diploid relatives, they are considered a NEW SPECIES. Sympatric speciation can arise from two types of polyploidy: autopolyploidy and allopolyploidy ('auto' means self and 'allo' means other)

Reinforcement

Reinforcement occurs when organisms do hybridize, but the hybrids are less fit and are selected against (or 'weeded out' of the population) in a process that 'reinforces' reproductive barriers.

cichlids and sexual selection

Researchers have learned that mate preference in cichlids is a result of their ability to see color ( they live in Lake Victoria) . The fish instinctually select mates based on color patterns, and the preferred patterns differ among populations. Mutations that affect color patterns could prevent a cichlid from being chosen by a mate. Similarly, mutations can also alter the response of individuals to these patterns so that the lineage develops a different color preference. These mutations become prezygotic reproductive barriers that prevent the mutated phenotype from perpetuation in the wild-type population. New populations form when cichlids with mutations that result in mutually complementary mating interactions select only each other for mating.

habitat differentiation

Scientists hypothesize that much of the diversity of tropical rainforest plants results from sympatric speciation driven by habitat differentiation, a process of speciation in which mutations in a population lead individuals to exploit different conditions within the same environment. Two factors control habitat differentiation: the diversity of conditions in the environment and the amount of genetic variation in a population of organisms.

Explain one major difference and one major similarity between allopatric and sympatric speciation.

The primary difference between the two types of speciation is that allopatric speciation occurs in geographically separated populations while sympatric speciation occurs in populations that could share a geographical area within the range of the ancestral population. The types of speciation are similar in that they both involve some kind of reproductive barrier.

allopatric speciation

allopatric speciation, which occurs after populations become physically isolated by geographical changes, like a river forming between two forests ('allo' = different). Sympatric speciation happens when geography does not isolate organisms from each other. reproductive barriers are at the root of the differences that arise between populations (pop. same species in same area).

Errors in Cell Division (like genetic isolation) May Result in Sympatric Speciation

errors in cell division can lead to sympatric speciation. For example, an error might occur in meiosis that produces gametes with double the DNA (Figure 4). For a diploid organism, these gametes are diploid instead of haploid.

allopolyploidy example

many crop plants came about via allopolyploidy. Domesticated wheat is a prime example of sympatric speciation of a fertile population. The wheat pollen can escape farmlands and form sympatric hybrids with local wild plants in the same genus. Domesticated crops like this can extend beyond the farm and form sympatric species by hybridizing with nearby, closely-related wild plants. The new species can become invasive and replace the native plants.