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SPECIES Is Latin for “Kind” Ernst Mayr ○ formalized this idea in what he called the biological species concept (BSC), defined as follows: “Species are groups of actual or potentially interbreeding populations, which are reproductively isolated fr...

SPECIES Is Latin for “Kind” Ernst Mayr ○ formalized this idea in what he called the biological species concept (BSC), defined as follows: “Species are groups of actual or potentially interbreeding populations, which are reproductively isolated from other such groups.” The biological species concept was developed partly to acknowledge variation, both within a single population (such as the color morphs of the eastern screech owl) and among different geographic populations, which often show evidence of interbreeding where they meet Biological Species Concept “Groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.” (Mayr 1963) “Systems of populations; the gene exchange between these systems is limited or prevented in nature by a reproductive isolating mechanism or by a combination of such mechanisms.” (Dobzhansky 1970) phylogenetic Species Concept an irreducible (basal) cluster of organisms diagnosably different from other such clusters, and within which there is a parental pattern of ancestry and descent Sibling Species almost identical in appearance and are often discovered by differences in ecology, behavior, chromosomes, or genetic markers. sister species two species descended from a single ancestral species, and are therefore one another’s closest relatives Reproductive Isolation any of several biological differences between the groups greatly reduce gene exchange between them, even if they are not geographically separated. Gene flow between biological species is prevented by biological differences called reproductive isolating barriers (RIBs), also referred to as isolating mechanisms. The total degree of reproductive isolation between two species may result from several RIBs that act in sequence—and some potential RIBs may not come into play. For example, the monkeyflower Mimulus lewisii is distributed in the Sierra Nevada of California at higher elevations than its close relative Mimulus cardinalis, although they both occur at intermediate elevations. Mimulus lewisii has pink flowers with a wide corolla and is pollinated by bees, whereas M. cardinalis has a narrow, red, tubular corolla and is pollinated by hummingbirds Prezygotic Barriers reduce the likelihood that hybrids are formed. These include such factors as separation of the species in different habitats, pollination by different animals, mating at different seasons, mating preferentially with conspecifics, and failure of gametes to unite even if mating occurs. Prezygotic isolation takes many forms, illustrated by some species that have been extensively studied. Modes of prezygotic isolation. (A) Seasonal isolation: the band-rumped storm-petrel (Oceanodroma castro) includes two genetically different populations that mate at different times of year. (B) Temporal isolation: related species of periodical cicadas (Magicicada) have either 17- or 13-year life cycles, and rarely emerge in the same year. (Different seasons) (C) Ecological isolation: closely related species of ladybird beetles (Henosepilachna) feed and mate on different species of plants. Gametic isolation occurs when gametes of different species fail to unite. This barrier is important in many externally fertilizing species of marine invertebrates that release eggs and sperm into the water. Because cell surface proteins determine whether or not sperm can adhere to and penetrate an egg, divergence in these proteins can result in gametic isolation. Examples of sexual isolation based on different sensory modalities. Female Physalaemus frogs respond almost exclusively to the calls of conspecific males. A calling male P. pustulosus is shown. In moths and many other animals, sexual isolation is based on different chemical signals. Two forms of the European corn borer (Ostrinia nubilalis) are strongly isolated by responses of males to different female sex pheromones. Males of Heliconius pachinus recognize conspecific females by their wing color pattern. Postzygotic Barriers Postzygotic barriers consist of reduced survival or reproductive rates of hybrid zygotes that would otherwise backcross to the parent populations. These barriers can be classified as either extrinsic or intrinsic, depending on whether or not their effect depends on the environment. Intrinsic Postzygotic Barriers Intrinsic isolation is based on interactions between genes from two populations, and is often more permanent than extrinsic isolation. hybrids suffer high mortality, or are partially or entirely sterile, irrespective of environment. The causes of intrinsic postzygotic isolation and its genetic bases are diverse.  Reduced hybrid viability is largely caused by incompatible interactions among genes from the two populations when they occur together in hybrids. Hybrid fertility may be reduced by incompatible genes or by differences in the number or structure of chromosomes. Bear in mind that the genetic differences that cause these effects may have evolved after prezygotic barriers, so we cannot assume that they were the cause of speciation. Extrinsic postzygotic isolation is often based on reduced survival because of ecological factors. In some cases, the parent species are adapted to different environments; the hybrid may be poorly adapted to both. A simple example is provided by hybrids between species of Heliconius butterflies that are distasteful to birds and have different patterns of warning coloration. Birds learn to associate common color patterns with distastefulness, but are likely to attack butterflies with rare, unfamiliar phenotypes, such as hybrids. A classification of isolating barriers I. Premating barriers: features that impede transfer of gametes to members of other species A. Ecological isolation: potential mates do not meet Temporal isolation: species breed at different seasons or times of day Habitat isolation: species mate and breed in different habitats Immigrants between divergent populations do not survive long enough to interbreed B. Potential mates meet but do not mate Sexual isolation in animals: individuals prefer mating with members of their own species Pollinator isolation in plants: pollinators do not transfer pollen between species II. Postmating prezygotic barriers: mating occurs, but zygotes are not formed A. Mechanical isolation: reproductive structures of the sexes do not fit B. Copulatory isolation: female is not stimulated by males of the other species C. Gametic isolation: failure of fertilization III. Postzygotic barriers: hybrids are formed but have reduced fitness A. Extrinsic: hybrids have low fitness for environmental reasons Ecological inviability: hybrids are poorly adapted to both of the parental habitats Behavioral sterility: hybrids are less successful in obtaining mates B. Intrinsic: low hybrid fitness is independent of environmental context Hybrid inviability: reduced survival is due to genetic incompatibility Hybrid sterility: reduced production of viable gamate How fast does reproductive isolation evolve? Prezygotic isolation evolves faster than postzygotic isolation in flies and fishes. (A) The strength of prezygotic and postzygotic reproductive isolation between pairs of populations and species of Drosophila increases with the amount of time since the lineages split. The time is estimated by the genetic distance between each pair. The strength of prezygotic isolation was measured by observing mating between flies in the laboratory. The strength of postzygotic isolation was measured by survival and fertility of hybrid individuals. Comparison of the upper left part of the two graphs reveals that strong prezygotic isolation evolves shortly after isolation (at small genetic distances), while strong postzygotic isolation evolves only later. (B) Similar patterns are seen in a genus of freshwater fishes, the darters (Etheostoma). Thirteen pairs of allopatric species were tested for both sexual isolation and the survival of artificially produced hybrids. For both indices, a value of 0 indicates that the pairs are no more isolated than conspecific individuals, and a value of 1 indicates complete reproductive isolation. (A after ; B after.) The Causes of Speciation 1. Ecological Speciation a. the process by which new species form as a consequence of divergent natural selection between contrasting ecological environments. 2. SPECIATION BY GENETIC CONFLICT a. occurs when an allele increases its own transmission to the detriment of other alleles at the same or other loci i. For example, some X chromosomes kill Y chromosome sperm, so all offspring inherit the driving X, and all offspring are female. 3. SPECIATION BY SEXUAL SELECTION a. when a parallel change in mate preference and secondary sexual traits within a population leads to prezygotic isolation between populations, and when this is the primary cause of reproductive isolation. 4. REINFORCEMENT OF REPRODUCTIVE ISOLATIONS a. reinforcement can directly generate reproductive isolation between sympatric and allopatric populations. Specifically, reinforcement can generate the evolution of reproductive traits in sympatry that render reproduction between sympatric and allopatric conspecifics less likely. 5. SPECIATION BY POLYPLOIDY a. The polyploid offspring is autopolyploid if both unreduced gametes come from the same diploid species, and allopolyploid if they come from different diploid species. If similar events happen in tetraploid species, offspring with even more sets of chromosomes result. i. rare in animals, but it is quite common in some groups of plants. 6. HYBRID SPECIATION a. the hybridization between two or more distinct lineages that contributes to the origin of a new species. More specifically, hybridization must result in a hybrid population that is at least partially reproductively isolated from the parental species.  7. SPECIATION BY RANDOM GENETIC DRIFT a. One hypothesis for how random genetic drift might trigger the origin of new species is called founder effect or peripatric speciation. Drift can be particularly strong when a new population is founded by a small number of individuals The Geography of Speciation The most common way for speciation to begin is with the appearance of a geographic barrier that partly or completely blocks genetic exchange between two populations Allopatric speciation is the evolution of genetic barriers between populations that are geographically separated by a physical barrier (for example, a mountain range). Allopatry is defined by a severe reduction of movement of individuals or their gametes, not by geographic distance. Allopatric populations can originate in two ways. Vicariance which is when a barrier appears and divides a population that was occupying a larger region. Dispersal when individuals from one population colonize another region. Sympatric speciation -the most extreme case of speciation with gene flow which occurs when an ancestral population splits into two species without any geographic isolation Sympatric speciation is controversial because interbreeding between the populations causes genetic mixing that can prevent the populations from diverging parapatric speciation, in which neighboring populations diverge while they continue to interbreed. We expect parapatric speciation to be more common than sympatric speciation because it involves less gene flow between the diverging populations. The Genomics of Speciation Genomics can help us determine the number, identities, and genomic locations of speciation genes, the loci that contributed to the evolution of reproductive isolation