Introduction
1. Macroevolution is the origin of new species or other taxonomic groups.
2. Speciation is the origin of new species.
3. Species is a Latin word meaning "kind" or "appearance."
4. Traditionally, morphological differences have been used to distinguish species.
5. Today, differences in body function, biochemistry, behavior, and genetic makeup are also used to differentiate species.
A. What Is a Species?
1. The biological species concept emphasizes reproductive isolation.
a. In 1942 Ernst Mayr enunciated the biological species concept to address biological diversity.
1. A species is a population or group of populations whose members have the potential to interbreed with each other in nature to produce viable, fertile offspring, but who cannot produce viable, fertile offspring with members of other species.
b. Species are based on interfertility, not physical similarity. (Fig. 24.2)
1. For example, the eastern and western meadowlarks may have similar shapes and coloration, but differences in song help prevent interbreeding between the two species.
2. In contrast, humans have considerable diversity, but we all belong to the same species because of our capacity to interbreed.
2. Prezygotic and postzygotic barriers isolate the gene pools of biological species.
a. Reproductive isolation prevents populations belonging to different species from interbreeding, even if their ranges overlap.
b. Reproductive barriers can be categorized as prezygotic or postzygotic, depending on whether they function before or after the formation of zygotes.
c. Prezygotic barriers impede mating between species or hinder fertilization of ova if members of different species attempt to mate. These barriers include habitat isolation, behavioral isolation, temporal isolation, mechanical isolation, and gametic isolation. (Fig. 24.5)
1. Habitat isolation. Two organisms that use different habitats even in the same geographic area are unlikely to encounter each other to even attempt mating.
a. This is exemplified by the two species of garter snakes, in the genus Thamnophis, that occur in the same areas but because one lives mainly in water and the other is primarily terrestrial, they rarely encounter each other.
2. Behavioral isolation. Many species use elaborate behaviors unique to a species to attract mates.
a. For example, female fireflies only flash back and attract males who first signaled to them with a species-specific rhythm of light signals.
b. In many species, elaborate courtship displays identify potential mates of the correct species. (Fig. 24.3)
3. Temporal isolation. Two species that breed during different times of day, different seasons, or different years cannot mix gametes.
a. For example, while the geographic ranges of the western spotted skunk and the eastern spotted skunk overlap, they do not interbreed because the former mates in late summer and the latter in late winter.
4. Mechanical isolation. Closely related species may attempt to mate but fail because they are anatomically incompatible and transfer of sperm is not possible.
a. Flowering plant anatomy is often adapted to the specific insects or other animals that pollinate them.
b. With many insects the male and female copulatory organs of closely related species do not fit together, preventing sperm transfer.
5. Gametic isolation occurs when gametes of two species do not form a zygote because of incompatibilities preventing fusion or other mechanisms.
a. In species with internal fertilization, the environment of the female reproductive tract may not be conducive to the survival of sperm from other species.
b. For species with external fertilization, gamete recognition may rely on the presence of specific molecules on the egg�s coat, which adhere only to specific molecules on sperm cells of the same species.
c. A similar molecular recognition mechanism enables a flower to discriminate between pollen of the same species and pollen of a different species.
d. Postzygotic barriers-if a sperm from one species does fertilize the ovum of another, postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult. These barriers include reduced hybrid viability, reduced hybrid fertility, and hybrid breakdown.
1. Reduced hybrid viability. Genetic incompatibility between the two species may abort the development of the hybrid at some embryonic stage or produce frail offspring.
a. This is true for the occasional hybrids between frogs in the genus Rana, which do not complete development and those that do are frail.
2. Reduced hybrid fertility. Even if the hybrid offspring are vigorous, the hybrids may be infertile and the hybrid cannot breed with either parental species.
a. This infertility may be due to problems in meiosis because of differences in chromosome number or structure.
b. For example, while a mule, the hybrid product of mating between a horse and donkey, is a robust organism, it cannot mate (except very rarely) with either horses or donkeys.
3. Hybrid breakdown. In some cases, first generation hybrids are viable and fertile.
a. However, when they mate with either parent species or with each other, the next generation is feeble or sterile.
b. Example: different cotton species can produce fertile hybrids, but breakdown occurs in the next generation when offspring of hybrids die as seeds or grow into weak and defective plants. (Fig. 24.5)
3. The biological species concept has some major limitations.
a. For example, one cannot test the reproductive isolation of morphologically similar fossils, which are separated into species based on morphology.
b. Even for living species, we often lack the information on interbreeding to apply the biological species concept.
c. In addition, many species (e.g., bacteria) reproduce entirely asexually and are assigned to species based mainly on structural and biochemical characteristics.
B. Modes of Speciation
1. Definitions-Two general modes of speciation are distinguished by the mechanism by which gene flow among populations is initially interrupted.
a. In allopatric speciation, geographic separation of populations restricts gene flow. (Fig. 24.6)
b. In sympatric speciation, speciation occurs in geographically overlapping populations when biological factors, such as chromosomal changes and nonrandom mating, reduce gene flow.
2. Allopatric speciation: Geographic barriers can lead to the origin of species.
a. Conditions for allopatric speciation
1. Several geological processes can fragment a population into two or more isolated populations.
a. Mountain ranges, glaciers, land bridges, or splintering of lakes may divide one population into isolated groups.
b. Alternatively, some individuals may colonize a new, geographically remote area and become isolated from the parent population.
1. For example, mainland organisms that colonized the Galapagos Islands were isolated from mainland populations.
2. How significant a barrier must be to limit gene exchange depends on the ability of organisms to move about.
a. A geological feature that is only a minor hindrance to one species may be an impassible barrier to another.
b. The valley of the Grand Canyon is a significant barrier for ground squirrels that have speciated on opposite sides, but birds that can move freely have no barrier. (Fig. 24.7)
3. The likelihood of allopatric speciation increases when a population is both small and isolated.
a. A small, isolated population is more likely to have its gene pool changed substantially by genetic drift and natural selection.
b. For example, less than 2 million years ago, small populations of stray plants and animals from the South American mainland colonized the Galapagos Islands and produced the species that now inhabit the islands.
c. However, very few small, isolated populations will develop into new species; most will simply perish in their new environment.
4. Allopatric speciation has only occurred if the separated populations have become different enough that they can no longer interbreed and produce fertile offspring when they come back in contact. (Fig. 24.8)
b. Adaptive radiation and island chains
1. Flurries of allopatric speciation occur on island chains where organisms that were dispersed from parent populations have founded new populations in isolation.
a. Organisms may be carried to these new habitats by their own locomotion, through the movements of other organisms, or through physical forces such as ocean currents or winds.
b. In many cases, individuals of one island species may reach neighboring islands, permitting other speciation episodes.
1. For example: a single dispersal event may have carried a small population of mainland finches to one Galapagos Island.
2. Later, individuals may have reached neighboring islands, where geographic isolation permitted additional speciation episodes.
2. The rapid evolution of many diversely adapted species from a common ancestor is called an adaptive radiation. (Fig. 24.11)
c. How do reproductive barriers evolve?
1. While geographic isolation does prevent interbreeding between allopatric populations, it does not by itself constitute reproductive isolation.
a. True reproductive barriers are intrinsic to the species and prevent interbreeding, even in the absence of geographic isolation.
b. Reproductive isolation is probably a product of a series of changes not directly related to reproduction.
1. If 2 populations of the same species separate into different environments they may accumulate differences that cause them to evolve away from each other in ways that affect reproduction only indirectly.
3. Sympatric speciation: A new species can originate in the geographic midst of the parent species
a. Sympatric speciation requires the emergence of some reproductive barrier that isolates a subset of the population without geographic separation from the parent population.
b. Sympatric speciation in plants
1. In plants, the most common mechanisms are hybridization between species or errors in cell division that lead to polyploid individuals.
2. An individual can have more than two sets of chromosomes if a failure in meiosis results in a tetraploid (4n) individual.
a. This polyploid mutant can reproduce with itself (self-pollination) or with other tetraploids.
b. It cannot mate with diploids from the original population, because of abnormal meiosis by the triploid hybrids.
3. Another mechanism of sympatric speciation in plants occurs when individuals are produced by the matings of two different species.
a. While the hybrids are usually sterile, they may be quite vigorous and propagate asexually.
b. These hybrids are infertile with each other and cannot interbreed with either parent species.
c. Sympatric speciation in animals
a. In animals, sympatric speciation may occur when a subset of the population is reproductively isolated by a switch in resources or mating preferences.
b. These may include switches from one breeding habitat to another or changes that produce different mate preferences.
c. Sympatric speciation is one mechanism that has been proposed for the explosive adaptive radiation of almost 200 species of cichlid fishes in Lake Victoria, Africa.
1. These species are clearly specialized for exploiting different food resources and other resources.
2. Individuals of two closely related sympatric cichlid species will not mate because females have specific color preferences and males differ in color. (Fig. 24.16)
C. From Speciation To Macroevolution
1. Speciation is at the boundary between microevolution and macroevolution.
a. Microevolution is a change over the generations in a population�s allele frequencies, mainly by genetic drift and natural selection.
b. Speciation occurs when a population�s genetic divergence from its ancestral population results in reproductive isolation.
c. While the changes after any speciation event may be subtle, the cumulative change over millions of speciation episodes must account for macroevolution, the scale of changes seen in the fossil record.