The Origin of Species

I.         The biological species concept emphasizes reproductive isolation. A species is defined as a population or group of populations whose members have the potential to breed with each other in nature to produce viable, fertile offspring, but who cannot produce viable, fertile offspring with members of other species. Species are based on interfertility, not physical similarity.

            A.        Because the distinction between biological species depends on reproductive incompatibility, the concept hinges on reproductive isolation, the existence of biological barriers that prevent members of two species from producing viable, fertile hybrids.

            B.        A single barrier may not block all genetic exchange between species, but a combination of several barriers can effectively isolate a species’ gene pool.

            C.        Typically, these barriers are intrinsic to the organisms, not due to simple geographic separation.

            D.        Reproductive isolation prevents populations belonging to different species from interbreeding, even if their ranges overlap.

II.        Reproductive barriers can be categorized as prezygotic or postzygotic, depending on whether they function before or after the formation of zygotes.

            A.        Prezygotic barriers impede mating between species or hinder fertilization of ova if members of different species attempt to mate.

            B.        These barriers include habitat isolation, behavioral isolation, temporal isolation, mechanical isolation, and gametic isolation.

                        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.

                        2.         Behavioral isolation. Many species use elaborate courtship behaviors unique to the species to attract mates.

                        3.         Temporal isolation. Two species that breed during different times of day, different seasons, or different years cannot mix gametes.

                        4.         Mechanical isolation. Closely related species may attempt to mate but fail because they are anatomically incompatible and transfer of sperm is not possible.

                        5.         Gametic isolation. The gametes of two species do not form a zygote because of incompatibilities preventing fertilization.

            C.        If a sperm from one species does fertilize the ovum of another, postzygotic barriers may prevent the hybrid zygote from developing into a viable, fertile adult.

                        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.

                        2.         Reduced hybrid fertility. Even if the hybrid offspring are vigorous, the hybrids may be infertile, and the hybrid cannot backbreed with either parental species.

                                    a.         This infertility may be due to problems in meiosis because of differences in chromosome number or structure.

                        3.         Hybrid breakdown occurs when first generation hybrids are viable and fertile but when they mate with either parent species or with each other, the next generation is feeble or sterile.

III.       While the biological species concept has had an important impact on evolutionary theory, it is limited when applied to species in nature.

            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 information on interbreeding needed 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.

IV.      Evolutionary biologists have proposed several alternative concepts of species that emphasize the processes that unite the members of a species.

            A.        The ecological species concept defines a species in terms of its ecological niche, the set of environmental resources that a species uses and its role in a biological community.

            B.        The paleontological species concept focuses on morphologically discrete species known only from the fossil record. There is little or no information about the mating capability of fossil species, and the biological species concept is not useful for them.

            C.        The phylogenetic species concept defines a species as a set of organisms with a unique genetic history. Biologists compare the physical characteristics or molecular sequences of species to those of other organisms to distinguish groups of individuals that are sufficiently different to be considered separate species.

                        1.         Sibling species are species that appear so similar that they cannot be distinguished on morphological grounds.

                        2.         Scientists apply the biological species concept to determine if the phylogenetic distinction is confirmed by reproductive incompatibility.

            D.        The morphological species concept, the oldest and still most practical, defines a species by a unique set of structural features.

                        1.         The morphological species concept has certain advantages. It can be applied to asexual and sexual species, and it can be useful even without information about the extent of gene flow.

                        2.         However, this definition relies on subjective criteria, and researchers sometimes disagree about which structural features identify a species. In practice, scientists use the morphological species concept to distinguish most species.

V.        Two general modes of speciation are distinguished by the way gene flow among populations is initially interrupted.

            A.        In allopatric speciation, geographic separation (mountain ranges, glaciers, land bridges, splintering of lakes) of populations restricts gene flow.

                        1.         A barrier may divide one population into isolated groups or some individuals may colonize a new, geographically remote area and become isolated from the parent population.

                        2.         How significant a barrier must be to limit gene exchange depends on the ability of organisms to move about. A geological feature that is only a minor hindrance to one species may be an impassible barrier to another.

                        3.         Once geographic separation is established, the separated gene pools may begin to diverge through a number of mechanisms.

                                    a.         Mutations arise.

                                    b.         Sexual selection favors different traits in the two populations.

                                    c.         Different selective pressures in differing environments act on the two populations.

                                    d.         Genetic drift alters allele frequencies. A small, isolated population is more likely to have its gene pool changed substantially over a short period of time by genetic drift and natural selection.

                        4.         To confirm that allopatric speciation has occurred, it is necessary to determine whether the separated populations have become different enough that they can no longer interbreed and produce fertile offspring when they come back in contact.

            B.        In sympatric speciation, speciation occurs in geographically overlapping populations when biological factors, such as chromosomal changes and nonrandom mating, reduce gene flow.

                        1.         In this case, reproductive barriers must evolve between sympatric populations.

                        2.         In plants, sympatric speciation can result from accidents during cell division that result in extra sets of chromosomes, a mutant condition known as polyploidy. The origin of polyploid plant species is common and rapid enough that scientists have documented several such speciations in historical times.

                        3.         In animals, it may result from gene-based shifts in habitat or mate preference.

            C.        The evolution of many diversely adapted species from a common ancestor when new environmental opportunities arise is called adaptive radiation.

                        1.         Adaptive radiation occurs when a few organisms make their way into new areas or when extinction opens up ecological niches for the survivors.

VI.      In the fossil record, many species appear as new forms rather suddenly (in geologic terms), persist essentially unchanged, and then disappear from the fossil record.

VII.     Darwin noted this when he remarked that species appeared to undergo modifications during relatively short periods of their total existence and then remained essentially unchanged.

            A.        The term punctuated equilibrium describes these periods of apparent stasis punctuated by sudden change.

                        1.         This observation could be a result of species changing rapidly (relative to geologic time) and the changes in their speciation not being captured in the fossil record.

                        2.         The result would be that the species would seem to have appeared suddenly and then lingered with little or no change before becoming extinct.

                        3.         Even though the emergence of this species actually took tens of thousands of years, this period of change left no fossil record.

            B.        Stasis can also be explained.

                        1.         All species continue to adapt after they arise, but often by changes that do not leave a fossil record, such as small biochemical modifications.

                        2.         Paleontologists base hypotheses of descent almost entirely on external morphology.

                        3.         During periods of apparent equilibrium, changes in behavior, internal anatomy, and physiology may not leave a fossil record.

                        4.         If the environment changes, the stasis will be broken by punctuations that leave visible traces in the fossil record.

VIII.    Speciation is at the boundary between microevolution and macroevolution.

            A.        Microevolution is a change over 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.

IX.      Most evolutionary novelties are modified versions of older structures.

            A.        It may be difficult to believe that a complex organ like the human eye could be the product of gradual evolution, rather than a finished design created specially for humans. However, the key is to remember is that a very simple eye can be very useful to an animal.

                        1.         The simplest eyes are just clusters of photoreceptors, light-sensitive pigmented cells. These simple eyes appear to have had a single evolutionary origin. They are now found in a variety of animals, including limpets.

                        2.         These simple eyes have no lenses and cannot focus an image, but they do allow the animal to distinguish light from dark. Limpets cling tightly to their rocks when a shadow falls on them, reducing their risk of predation.

                        3.         Complex eyes have evolved several times independently in the animal kingdom.

                        4.         Examples of various levels of complexity, from clusters of photoreceptors to camera-like eyes, can be seen in molluscs.

                        5.         The most complex types did not evolve in one quantum leap, but by incremental adaptation of organs that benefited their owners at each stage.

            B.        Evolutionary novelties can also arise by gradual refinement of existing structures for new functions. Structures that evolve in one context, but become co-opted for another function, are exaptations.

                        1.         It is important to recognize that natural selection can only improve a structure in the context of its current utility, not in anticipation of the future.

                        2.         An example of an exaptation is the changing function of lightweight, honeycombed bones of birds.

                        3.         The fossil record indicates that light bones predated flight. Therefore, they must have had some function on the ground, perhaps as a light frame for agile, bipedal dinosaurs.

                        4.         Once flight became an advantage, natural selection would have remodeled the skeleton to better fit their additional function.

                        5.         The wing-like forelimbs and feathers that increased the surface area of these forelimbs were co-opted for flight after functioning in some other capacity, such as courtship, thermoregulation, or camouflage.

            C.        Genes that control development play a major role in evolution.

                        1.         “Evo-devo” is a field of interdisciplinary research that examines how slight genetic divergences can become magnified into major morphological differences between species.

                        2.         A particular focus is on genes that program development by controlling the rate, timing, and spatial pattern of changes in form as an organism develops from a zygote to an adult.

                        3.         Heterochrony, an evolutionary change in the rate or timing of developmental events, has led to many striking evolutionary transformations.

                                    a.         Allometric growth tracks how proportions of structures change due to different growth rates during development. Change relative rates of growth even slightly, and you can change the adult form substantially.

                                    b.         Different allometric patterns contribute to the contrast of adult skull shapes between humans and chimpanzees, which both developed from fairly similar fetal skulls.

                        4.         Heterochrony appears to be responsible for differences in the feet of tree-dwelling versus ground-dwelling salamanders.

                                    a.         The feet of the tree-dwellers are adapted for climbing vertically, with shorter digits and more webbing.

                                    b.         This modification may have evolved due to mutations in the alleles that control the timing of foot development.

                                    c.         Stunted feet may have resulted if regulatory genes switched off foot growth early.

                                    d.         In this way, a relatively small genetic change can be amplified into substantial morphological change.

            D.        Macroevolution can also result from changes in genes that control the placement and spatial organization of body parts.

                        1.         For example, genes called homeotic genes determine such basic features as where a pair of wings and a pair of legs will develop on a bird or how a plant’s flower parts are arranged.

                                    a.         The products of one class of homeotic genes, the Hox genes, provide positional information in an animal embryo. This information prompts cells to develop into structures appropriate for a particular location.

                        2.         One major transition in the evolution of vertebrates is the development of the walking legs of tetrapods from the fins of fishes.

                                    a.         A fish fin that lacks external skeletal support evolved into a tetrapod limb that extends skeletal supports (digits) to the tip of the limb.

                                    b.         This may be the result of changes in the positional information provided by Hox genes during limb development, determining how far digits and other bones should extend from the limb.

X.        Evolution is not goal oriented.

            A.        The fossil record shows apparent evolutionary trends.

                        1.         For example, the evolution of the modern horse can be interpreted to have been a steady series of changes from a small, browsing ancestor (Hyracotherium) with four toes to modern horses (Equus) with only one toe per foot and teeth modified for grazing on grasses.

                        2.         It is possible to arrange a succession of animals intermediate between Hyracotherium and modern horses to show trends toward increased size, reduced number of toes, and modifications of teeth for grazing.

                        3.         If we look at all fossil horses, the illusion of coherent, progressive evolution leading directly to modern horses vanishes. Equus is the only surviving twig of an evolutionary bush that included several adaptive radiations among both grazers and browsers.

            B.        Major evolutionary trends result as some species are more successful than others and give rise to more species than others.

                        1.         For a species, speciation is their birth, extinction is their death, and new species are their offspring.

                        2.         Just as individual organisms undergo natural selection, species undergo species selection.

                        3.         The species that endure the longest and generate the greatest number of new species determine the direction of major evolutionary trends.

                        4.         The species selection model suggests that “differential speciation success” plays a role in macroevolution similar to the role of differential reproductive success in microevolution.

                        5.         Remember that evolution is a response to interactions between organisms and their current environments, so changes in evolutionary trends can result as conditions change.