SPECIATION Mr Tsigaridis Species Concepts Biological Species are
SPECIATION Mr. Tsigaridis
Species Concepts Biological “Species are groups of actually or potentially interbreeding individuals that can mate and produce fertile offspring”-idea was promoted by Ernst Mayr, an evolutionist who worked on birds. ○ In practice, this applies to most species, but in many cases, it is simply impossible to test whether two species have the potential to interbreed. Phylogenetic A species is a lineage, separate from other such lineages, perpetuated ancestor to descendant, over time. Morphological criteria are used to define species.
Speciation The theory of evolution must explain the origin of new organisms. From the beginning, the origin of species has been a focal point of evolutionary theory. Ironically, Darwin was wrong about the origin of species. He assumed that, given enough time, natural selection would inevitably produce them. This is not actually the case. It takes reproductive isolation. Macroevolution is the origin of new taxonomic groups. Speciation is the origin of new species. With extinction, it is one of two keystone processes of macroevolution.
Cladogenesis, the origin of lineages, is the budding of a new species from a parent species that continues to exist. Cladogenesis promotes biological diversity by increasing the number of species. Although it culminates over thousands or millions of years, cladogenesis is a real event. New species originate by cladogeneis, which is ultimately responsible for the origin of every major group of animals.
Speciation Types Allopatric Speciation: Involves a geographic barrier. Sympatric Speciation: Does not involve a geographic barrier.
Sympatric Speciation Sympatric speciation results from intrinsic factors, such as chromosomal changes and nonrandom mating. Sympatric populations become genetically isolated even though their ranges overlap.
Why Do We Have Separate Species At All? Sympatric species live in the same place. Without some mechanism preventing allele and gene exchange among sympatric species, distinct species would be impossible, we would probably see a continuum from one form of life to another. Barriers to allele flow are called isolation mechanisms. Isolation mechanisms allow sympatric species to exist. Without isolation mechanisms, closely related species would hybridize: allele flow and recombination would eventually transform them into a single, polymorphic species.
Evolution of New Species The Evolution of New Species Results From (and also causes) Barriers to Allele Flow. At the time when a genetic or behavioral mechanism evolves that keeps populations of a species from interbreeding, two new species are formed. As we shall see, this is frequently the result of a geographic barrier, although it may be the result of a chromosomal change or habitat preference.
Isolation Mechanisms Presygotic isolation mechanisms prevent mating, so that gametes of sympatric species never form hybrid zygotes. Postzygotic isolating mechanisms act after a mating has occurred, to prevent fertilization or to prevent potential hybrids from passing on their genes.
Prezygotic Isolation Mechanisms Habitat Isolation Temporal Isolation Behavioral Isolation Mechanical Isolation Gametes Die
Habitat Isolation Habitat isolation occurs because sympatric species meet due to differences in their habitat preference. Examples of habitat isolation: Sympatric species of spadefoot toads (Scaphiopus) seldom meet because they prefer different soil types. Many species of closely-related parasites, such as bird lice, never meet because they live and mate on different hosts.
Temporal Isolation: Occurs because species mate at different times. Examples: Different species of plants frequently have differing flowering seasons. Closely related species of fireflies frequently mate at different times of night.
Behavioral Isolation Behavioral, or Ethological isolation mechanisms include differences in courtship behavior, differences in chemical signals or vocalizations, and differences in color or morphology that allow individuals to recognize their own species. They are a very common mechanism keeping closely-related sympatric animals from interbreeding
Examples: Female fireflies respond only to the light pattern emitted by their own species. Sympatric species of fireflies emit different light patterns.
Mechanical Isolation Mechanical isolation occurs because the sexual organs of closely-related sympatric species are incompatible: they do not fit together. This is thought to be an important isolation mechanism in arthropods, particularly insects and millipedes.
Sperm of various mammals Gametic Mortality Gametes are frequently very specialized cells, which can only perform well in the reproductive tract of the opposite sex of the same species. In many angiosperms, for instance, pollen transferred to the stigma of another species will not germinate, or if they do, will not form a pollen tube.
Postzygotic Isolation Mechanisms Zygote dies after fertilization Hybrid Inviability Hybrid Sterility Low Hybrid Fitness
Hybrid Inviability Some species that do not ordinarily interbreed occasionally do so. Frequently, the progeny of these interspecific matings die at some point during their development. Example: Hybrids between the frogs Rana pipiens and Rana sylvatica do not survive more than a day or so.
Hybrid Sterility occurs when the hybrid of an interspecific mating is unable to reproduce. Examples: Mules are the hybrid of a horse and a donkey, they do not form normal sperm. Hybrids between Drosophila melanogaster and Drospphila pseudoobscura have atrophied testes and are sterile,
Low Hybrid Fitness If interspecific hybrids do survive, they often have very low fitness, this effectively keeps them from spreading genes from one of their parent species to the other parent species. Example: Dog-Wolf hybrids are perfectly viable, but they are considered to be unsuitable pets in most areas. Wild wolf populations do not accept hybrids, they are killed on sight.
Allopatric Speciation Allopatric speciation involves a geographic barrier that physically isolates populations of a species and blocks gene flow. Once isolated, allopatric populations (living in different places) accumulate genetic differences due to natural selection, genetic drift, and new mutations.
Geographic Barriers If the Geographic Barrier is Removed, the Two Species May: 1) meld together by allele flow and recombination to once again form a single species. 2) remain reproductively isolated. The fate of the new, incipient species, depends upon whether isolation mechanisms have evolved during the period of isolation. These isolation mechanisms may be premating or postmating. Premating isolation mechanisms may evolve in incipient species that have postmating isolation, to reduce the probability of incorrect matings and the subsequent loss of fitness.
Example of geographic isolation and possible allopatric speciation: Two closely-related species of antelope squirrels live on opposite sides of the grand canyon. On the South rim is Ammospermophilus harrisi, on the North rim is Ammospermophlus leucurus. Birds, and other species that can cross the canyon, have not diverged into different species on opposite sides.
Example: the Drongo The Drongo is a black bird with a crest of feathers, it is highly variable in behavior and appearance throughout its range. Each semi-isolated population has its own appearance.
Adaptive Radiation on Islands ·Island chains frequently produce many new species. ·Islands chains provide barriers that facilitate invasion and re-invasion by different species. ·This is the probable mechanism for the proliferation of Darwin’s finches on the Galapagos. ·The Hawaiian islands once supported thousands of unique Drosophila flies that probably evolved by a similar mechanism
How Long Does Allopatric Speciation Take? Nobody knows for sure, and it depends upon the group. Mc. Cune and Lovejoy, based on a study of reproductive isolation in 40 pairs of allopatric fishes, estimated that, for fishes, it takes between. 8 and 2. 4 million years for reproductive isolation to evolve.
http: //www. nativefish. org Based on molecular evidence, the two species/subspecies have been isolated for between one and two million years. Mc. Cune and Lovejoy found that males of each species prefer to mate with females of their own species, but given a choice, they will hybridize.
Phylogeny, Taxonomy, and Systematics Phylogeny: The phylogeny of a group is a “family tree” describing how species are related. The branching pattern of different groups of organisms is caused by repeated cladogenesis. Systematics is the study of phylogeny. Taxonomy: Is the process of describing and naming organisms. Our modern process of taxonomy is based on phylogeny, so an understanding of phylogenetic relationships-names reflect relatedness.
The Taxonomic Heiriarchy Every species has a place in the taxonomic heirarchy: Human Mud-Dauber Species Homo sapiens Trypoxylon politum Genus Homo Trypoxylon Family Hominidae Sphecidae Order Primates Hymenoptera Class Mammalia Insecta Phylum Chordata Arthropoda Kingdom Anamalia Domain Eukarya
Inferring Phylogeny The branching pattern of a phylogenetic tree reflects ideal place in our taxonomic hierarchy. Classification schemes are hypotheses of past history based on the available evidence. Like all hypotheses, they make predictions that can be tested by future study.
Inferring Phylogeny The phylogeny of a group of organisms can be inferred from the following lines of evidence Shared characteristics passed down from an ancestor, called homologies. These are essential in inferring a phylogeny. ○ Morphology and DNA sequences are very useful places to look for homologies Biogeography The Fossil Record
Homology A character state is homologous in two species when it is inherited by both from a common ancestor. The most widely accepted school of systematics today is called cladistics. Cladistics infers the pattern of phylogeny based on homologies. Groups are constructed based on shared characteristics inherited from a common ancestor, that no other group has.
Example of a Homology which is a Synapomorphy
Homoplasy If a character has evolved more than once, if possessed by two species but not present in the common ancestor, it is called a homoplasy. One form of homoplasy is called convergent evolution, it is quite common because different species are often subject to similar selective pressures. Homoplasy, when mistaken for homology, can obscure the pattern of evolutionary history.
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