Mechanisms of Evolution Macroevolution Speciation MICROEVOLUTION - A change in the frequency of alleles. Review population genetics Ch. 23. MACROEVOLUTION - Speciation (or emergence of higher taxonomic levels) Speciation, or the origin of new species, is the central process of macroevolution because any higher taxon originates with a new species novel enough to be the first member of that taxon. 1
Darwin explored the Galápagos Islands And discovered plants and animals found nowhere else on Earth Figure 24.1 The origin of new species, or speciation Is at the focal point of evolutionary theory, because the appearance of new species is the source of biological diversity Evolutionary theory Must explain how new species originate in addition to how populations evolve Macroevolution Refers to evolutionary change above the species level There are two patterns of speciation in the fossil record: anagenesis and cladogenesis Anagenesis (phyletic evolution) - The transformation of an unbranched lineage of organisms, sometimes to a state different enough from the ancestral population to justify renaming it as a new species. Cladogenesis (branching evolution) - The budding of one or more new species from a parent species that continues to exist; is more important than anagenesis. It is more common and can promote biological diversity. 2
Two basic patterns of evolutionary change Anagenesis Cladogenesis Figure 24.2 (a) Anagenesis (b) Cladogenesis Defining a Species Species - Latin term meaning kind or appearance Linnaeus (founder of modern taxonomy) - described species in terms of their physical form (morphology). Morphology is still the most common method used for describing species. Modern taxonomists also take into account genetic makeup and functional and behavioral features when describing species. The Biological Species Concept The biological species concept relies on reproductive isolation (proposed by Ernst Mayr, 1942) Biological species - A population or group of populations whose members have the potential to interbreed with one another in nature and to produce viable, fertile offspring, but cannot produce viable, fertile offspring with members of other species. 1. Largest unit of population in which gene flow is possible 2. Defined by reproductive isolation from other species in natural environments (hybrids may be possible between two species in the laboratory or in zoos) 3
Species (a) Similarity between different species. The eastern meadowlark (Sturnella magna, left) and the western meadowlark (Sturnella neglecta, right) have similar body shapes and colorations. Nevertheless, they are distinct biological species because their songs and other behaviors are different enough to prevent interbreeding should they meet in the wild. (b) Diversity within a species. As diverse as we may be in appearance, all humans belong to a single biological species (Homo sapiens), defined by our capacity to interbreed. Figure 24.3 A, B Reproductive Isolation Reproductive isolation Is the existence of biological factors that impede members of two species from producing viable, fertile hybrids Is a combination of various reproductive barriers Prezygotic and postzygotic barriers to reproduction The gene pools of different species are isolated from those of others by more than one type of reproductive barrier. The barriers that isolate gene pools are either prezygotic or postzygotic, depending on whether they occur before or after fertilization. 4
Prezygotic barriers Impede mating between species or hinder the fertilization of ova if members of different species attempt to mate Postzygotic barriers Often prevent the hybrid zygote from developing into a viable, fertile adult Prezygotic Barriers 1. Habitat isolation Two species living in different habitats in the same area encounter each other rarely, even though they are not technically geographically isolated. Example: two species of garter snakes occur in the same areas but one species lives mainly in water and the other is mainly terrestrial and they seldom come into contact. 2. Behavioral isolation Species-specific signals and behaviors that attract mates are barriers among closely related species. Example: Male fireflies of different species signal to females of the same species by blinking their lights in a characteristic pattern; females discriminate among the different signals and respond only to flashes of their own species. Includes behavioral responses to chemical attractants, courtship rituals, bird and insect song, etc. 3. Temporal isolation Two species that breed at different times of day, seasons, or years don t mix their gametes. Example: brown trout and rainbow trout cohabit the same streams, but brown trout breed in the fall and rainbow trout breed in the spring. ATTEMPT TO MATE 4. Mechanical isolation Morphological differences prevent mating. Example: Male dragonflies use a pair of special appendages to clasp females during copulation. The male s clasping appendages do not fit the form of the females of other species. Prezygotic barriers impede mating or hinder fertilization if mating does occur Habitat Temporal Behavioral Prezygotic isolation and isolation postzygotic isolation barriers Mechanical isolation Individuals of different species Mating attempt HABITAT ISOLATION TEMPORAL ISOLATION BEHAVIORAL ISOLATION MECHANICAL ISOLATION (g) (b) (d) (a) (c) (e) (f) Figure 24.4 5
Prezygotic Barriers ATTEMPT TO MATE 5. Gametic isolation Gametes of different species that do meet rarely complete fertilization (do not form a zygote). For animals that use internal fertilization the sperm of one species may not be able to survive the internal environment of the female reproductive tract of a different species. Cross-specific fertilization is also uncommon for animals that utilize external fertilization due to a lack of gamete recognition. Gamete recognition is based on specific molecules on the coats of the egg that adhere only to complementary molecules on sperm of the same species. Gametic isolation Fertilization Reduce hybrid viability Reduce hybrid fertility Hybrid breakdown Viable fertile offspring GAMETIC ISOLATION REDUCED HYBRID VIABILITY REDUCED HYBRID FERTILITY (k) HYBRID BREAKDOWN (j) (m) (l) (h) (i) Postzygotic Barriers When prezygotic barriers fail and a hybrid zygote forms, postzygotic barriers prevent development of a viable, fertile hybrid. 1. Reduced hybrid viability (hybrid inviability) Genetic incompatibility may cause the abortion of the hybrid at an embryonic stage. Hybrids generally do not complete development, and those that do are frail and soon die. 2. Reduced hybrid fertility (hybrid sterility) If two species mate and produce viable hybrid offspring, reproductive isolation is maintained if the hybrids are sterile. This prevents gene flow between the parent species. One cause of this barrier is that if chromosomes of the two parent species differ in number or structure, meiosis cannot produce normal gametes in the hybrid. Mules. 3. Hybrid breakdown In some crosses, the first generation hybrids are viable and fertile, but when these hybrids mate with each other, or with either parent species, the next generation is feeble or sterile. Example: Different cotton species can produce fertile hybrids, breakdown occurs in the next generation when progeny of the hybrids die in their seeds or grow into weak defective plants. 6
Gametic isolation Fertilization Reduce hybrid viability Reduce hybrid fertility Hybrid breakdown Viable fertile offspring GAMETIC ISOLATION REDUCED HYBRID VIABILITY REDUCED HYBRID FERTILITY (k) HYBRID BREAKDOWN (j) (m) (l) (h) (i) Limitations of the Biol. Species Concept The biological species concept cannot be applied to: 1. Organisms that are completely asexual. Some protists and fungi, some commercial plants (bananas), and many bacteria are exclusively asexual. 2. Extinct organisms represented by fossils - must be classified by morphology. 3. Sexual organisms about which little is known. The species problem may never be completely resolved. It is unlikely that a single definition will ever apply in all cases. Other Definitions of Species The morphological species concept Characterizes a species in terms of its body shape, size, and other structural features; useful in the field; sometimes difficult to apply. The paleontological species concept Focuses on morphologically discrete species known only from the fossil record The ecological species concept Views a species in terms of its ecological niche The phylogenetic species concept Defines a species as a set of organisms with a unique genetic history 7
Modes of Speciation The evolution of reproductive barriers that keep species separate is the key biological event in the origin of new species. An essential episode in the origin of a species occurs when the gene pool of a population is separated from other populations of the parent species. This genetically isolated splinter group can then follow its own evolutionary course as changes in allele frequencies caused by selection, genetic drift, and mutations occur undiluted by gene flow from other populations. There are two general modes of speciation: allopatric speciation and sympatric speciation. Speciation can take place with or without geographic separation Allopatric speciation Sympatric speciation Figure 24.5 A, B (a) Allopatric speciation. A (b) Sympatric speciation. A small population forms a new population becomes a new species species while geographically without geographic separation. isolated from its parent population. Allopatric ( Other Country ) Speciation In allopatric speciation Gene flow is interrupted or reduced when a population is divided into two or more geographically isolated subpopulations 8
Geographic Barriers 1. Geological processes can fragment a population into two or more allopatric populations (having separate ranges). This can include emergence of mountain ranges, movement of glaciers, formation of land bridges, subsidence of large lakes. 2. Small populations may become geographically isolated when individuals from the parent population travel to a new location. (splinter populations) 3. The extent of geographical isolation necessary to separate two populations depends on the ability of the organisms to disperse due to the mobility of animals or the dispersibility of spores, pollen and seeds of plants. Example: the Grand Canyon is an impassable barrier to small rodents, but is easily crossed by birds. As a result, the same bird species populate both rims of the canyon, but each rim has several unique species of rodents. Once geographic separation has occurred One or both populations may undergo evolutionary change during the period of separation A. harrisi A. leucurus Figure 24.6 In order to determine if allopatric speciation has occurred Reproductive isolation must have been established EXPERIMENT Diane Dodd, of Yale University, divided a fruit-fly population, raising some populations on a starch medium and others on a maltose medium. After many generations, natural selection resulted in divergent evolution: Populations raised on starch digested starch more efficiently, while those raised on maltose digested maltose more efficiently. Dodd then put flies from the same or different populations in mating cages and measured mating frequencies. Initial population of fruit flies (Drosphila Pseudoobscura) Some flies raised on starch medium Figure 24.7 Mating experiments after several generations Some flies raised on maltose medium 9
RESULTS When flies from starch populations were mixed with flies from maltose populations, the flies tended to mate with like partners. In the control group, flies taken from different populations that were adapted to the same medium were about as likely to mate with each other as with flies from their own populations. Male Maltose Starch Female Starch Maltose 22 9 8 20 Mating frequencies in experimental group Male Different Same populations population Female Same Different population populations 18 15 12 15 Mating frequencies in control group CONCLUSION The strong preference of starch flies and maltose flies to mate with like-adapted flies, even if they were from different populations, indicates that a reproductive barrier is forming between the divergent populations of flies. The barrier is not absolute (some mating between starch flies and maltose flies did occur) but appears to be under way after several generations of divergence resulting from the separation of these allopatric populations into different environments. Conditions Favoring Allopatric Speciation When populations are separated, speciation can occur as isolated gene pools accumulate differences by microevolution. These differences may cause a divergence in phenotype between the isolated populations. 1. A small isolated population is more likely to change substantially enough to become a new species than is a large isolated population. There is more effect from genetic drift. 2. The geographic isolation of a small population usually occurs at the fringe of the parent population's range (peripheral isolate). As long as the gene pool is isolated from the parent population, a peripheral isolate is a good candidate for speciation for three reasons: a. The gene pool of the peripheral isolate probably differs from that of the parent population from the outset. Fringe inhabiters usually represent the extremes of any genotypic and phenotypic clines in an original sympatric population. With a small peripheral isolate, there will be a founder effect with chance resulting in a gene pool that is not representative of the gene pool of the parental population. b. Genetic drift will continue to cause chance changes in the gene pool of the small peripheral isolate until a large population is formed. New mutations or combinations of alleles that are neutral in adaptive value may become fixed in the population by chance alone, causing phenotypic divergence from the parent population. c. Evolution caused by selection is likely to be different in the peripheral isolate than in the parent population. The peripheral isolate inhabits a frontier with a somewhat different environment, and it will probably be exposed to different selection pressures than the parent population. Because of the severity of a fringe environment (It s already at the edge of the species range.), most peripheral isolates do not survive long enough to undergo speciation. Though most peripheral isolates go extinct, a small population can accumulate enough genetic change to become a new species in only hundreds to thousands of generations. NOTE: Geographic barriers by themselves are NOT biological mechanisms of reproductive isolation and do not define species. 10