Microevolution and Macroevolution: Speciation
According to the theory of natural selection, speciation is the creation of new species by genetic modifications of previously existing species, so the resulting organisms can no longer successfully mate and produce fertile offspring. Consequently, the most modern definition of species includes a retrieval of the genetic understanding from ancestral parents into a biological species concept, which states that a species is a population that can interbreed in nature and produce fertile offspring. New species have three principle mechanisms describing their formation, each of which involves reproductive isolation:
- Allotropic speciation
- Prezygotic isolation
- Postzygotic isolation
Suppose a volcano erupts and the overflow of lava blocks a stream to create a large lake. The resulting geographic separation may isolate organisms on either side of the lake as well as those above and below the dam. A physical separation that prohibits the gene migration between populations creates the opportunity for allotropic speciation for that subpopulation. When this happens, natural selection, mutation, and genetic drift act to genetically diversify the two populations so they are no longer capable of mating and producing fertile offspring. Geographic isolation presents the opportunity for the formation of a new species but cannot create a new species. True speciation only occurs when reproductive barriers prevent productive interbreeding. Two major types of reproductive barriers prevent a species from interbreeding even if they are in the same geographic area: prezygotic and postzygotic reproductive isolation.
Prezygotic Reproductive Isolation
There are five main types of prezygotic reproductive barriers that prevent intraspecies fertilization:
- Territorial. When two species are found in the same general area but not in the same habitat, they may not have an opportunity to mate. For instance, many species of plants live in the Amazon Basin rainforest; however, they are often stratified in their territories by competition and their need for light. Plants that require strong light grow only in the tree canopy, whereas low-light plants inhabit the shaded, forest floor, so the opportunity to interbreed never exists because of territorial separation.
- Seasonal. Species often have different mating seasons, which may prohibit mating opportunities. For instance, the wood frog breeds in late winter, whereas the common leopard frog breeds in early spring, and the common bullfrog breeds in early summer in most territories. Although sexually compatible in captivity, they do not interbreed in their natural setting because their reproductive seasons do not overlap!
- Behavioral. The “biological clock” in the natural state usually adjusts the breeding cycle to normal environmental pressures such as food production, availability of suitable nesting sites, mating opportunities, and predator activity. Different species often have different seasonal requirements that do not overlap.
- Structural. Species-specific behavior characteristics activate certain species without affecting the surrounding species. Different species often have various means by which the male and female of the species communicate their sexual readiness. The signals may be behavioral, such as the dance of the prairie chicken, or chemical, such as the scent or pheromone of your unspayed female dog. Even the vivid displays, such as the deep coloration change in tropical fish or the flagrant feather display of male peacocks, serve as specific readiness signals to potential mates. However, the mating ritual of one species may have no effect on a neighboring species even though their territories overlap. The inability of one species to recognize the mating signals of another species further increases their incompatibility and isolation. For instance, the ring-neck pheasant and the prairie chicken are similar birds that occupy similar habitats in the Midwest. However, the pheasant is not excited by the premating dance of the prairie chicken and vice versa.
- Structural limitations are prominent in certain flowering plants such that the actual plant structure, usually the flower, is designed in such a way that only a specific native pollinator has access to the pollen. A good example is the long, tubular shape of certain flowers that favor pollinators with long beaks. This mutualistic relationship ensures that the pollinators have diminished competition for that flower and the flower is assured that the specific pollinator will carry the pollen to like flowers to increase the likelihood of fertilization. This efficiency prevents wasted pollen and effort and prevents interspecies reproduction, which further isolates the plants. On a more visual level, a large dog may be reproductively attracted to a much smaller dog but find the structural orientation creates a barrier.
- Genetic. Even if the previous limitations are met, the genetics of a successful copulation do not always mean a successful fertilization. Even though sexual reproduction is designed to work the first time and every time thereafter, that rule applies to intraspecies mating. Successful interspecies copulation may produce gametes that are also incompatible and therefore do not unite to create a zygote, such as a hypothetical mating between a dog and a cat.
Postzygotic Reproductive Isolation
Three main barriers act on hybrid zygotes after interspecies fertilization:
- Mortality. Even if a hybrid zygote is formed, sometimes the inherited genes are incompatible, which then act to create weak or unhealthy offspring that do not survive to reproduce or are considered undesirable and not allowed to reproduce. In both cases, the genome for that organism fails to convey to the next generation and is lost. This represents a mortality barrier.
A whinny is the less-common, sterile product of a mating between a male horse and a female donkey.
- Infertility. In a similar scenario, the resulting hybrid may reach mature adult status, capable of sexual reproduction, but is sterile. This represents an infertility barrier. Again, the inability to pass on the genes is a terminal genetic destination as exemplified by mules, the sterile product of a male donkey and a female horse. Because of their inability to breed and produce fertile offspring, horses and donkeys remain two separate species. A zebroid is the sterile offspring of a cross between a zebra and a horse. The zebra and the horse are in the same genus but not the same species.
- Longevity. Finally, in some cases, first-generation hybrid offspring are viable and do successfully interbreed and produce a second-generation offspring. Unfortunately, the second-generation offspring are inviable, either reproductively infertile or too unhealthy to reproduce. Regardless of the reason, in all cases, the longevity barriers promote genetic isolation and inhibit gene migration.
Sympatric speciation is the opposite of allopatric speciation because organisms, predominately plants, often create new species without the requisite geographic isolation. Plant-seed dispersal mechanisms often prohibit reproductive isolation, leaving sympatric speciation as the only major evolutionary cause agent for plants. Typically, a mutation occurs that prevents the offspring from successfully mating with a parent, but still allows viable reproduction with other individuals who inherited the same mutation. The most common mechanism is the chromosomal mutation that occurs because of a meiotic failure during gamete formation, when the chromosomes divide mitotically instead. When this happens, the duplicated chromosomes do not segregate and migrate into separate sex cells. Instead, they remain duplicated in the same sex cell, creating an overload of genes in certain gametes, which then become diploid, and deficient in others. It is possible then for the diploid gametes to unite with other diploid gametes to produce a polyploid individual, which contains more than the normal diploid complement of chromosomes. In plants, this occurs most frequently because of self-fertilization. The polyploid offspring can no longer successfully interbreed with the parent or any other similar-species organism that did not inherit the extra set of genes. Sympatric speciation is the reproductive isolation created by genetic abnormalities not as a result of geographic isolation. Although not widespread among animals, sympatric speciation has been significant in plant variation. Hugo de Vries, a Dutch botanist, is credited with identifying polyploidy as an agent of sympatric speciation. Through self-pollination, he created a large flowering polyploid evening primrose with 28 chromosomes instead of the normal diploid number of 14 chromosomes.
The speed by which new species are created depends upon the genetic makeup of the species, their ability to adapt to environmental changes, and the speed and severity of the environmental changes. In earlier times, it was thought that speciation occurred slowly over long periods of time. This gradualistic theory has recently given way to the punctuated equilibrium model that defines speciation as occurring in jumps or sudden shifts of speciation interspersed within long periods of inactivity. Emerging evidence from fossils lends support to the punctuated equilibrium model. This discussion is continued in the next section.
Excerpted from The Complete Idiot's Guide to Biology © 2004 by Glen E. Moulton, Ed.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.