BIOL 20C Baldo (Week 3 Speciation)
BIOL 20C Baldo (Week 3 Speciation) 20C
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This 5 page Class Notes was uploaded by Holly Chen on Tuesday October 4, 2016. The Class Notes belongs to 20C at University of California - Santa Cruz taught by Baldo Marinovic in Fall 2016. Since its upload, it has received 2 views.
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Date Created: 10/04/16
Unit 3 - Speciation Introduction to Speciation • Species - the basic unit for classifying organisms, a group of organisms that share a common gene pool. - Similar morphological characteristics that can be observed and measured. - Ability to interbreed. - Genetically distinct from other species. - The result of speciation. - Evolutionary independent (smallest unit that can evolve is populations). • Speciation - the divergence of biological lineages and the emergence of reproductive isolation between those lineages. Species Concept • The term species is useful but common. It’s usages varies among biologists who are interested in different aspects of speciation. • The most important factor in the divergence of sexually reproducing lineages from one another is the evolution of reproductive isolation, a state in which two groups of organisms can no longer exchange genes. Biological Species: • Biological species - groups of actually or potentially interbreeding natural populations which are productively isolated from other such groups. - ‘Actually’says the individual live in the same area and interbreed with one another. - ‘Potentially’says that even though individuals do not live in the same area, and therefore do not interbreed, they would if they were able to get together. • Widely used. • Disadvantages in the fact that it does not apply to organisms that reproduce asexually, and is limited to a single point in evolutionary time so fossils are not taken into an account. Morphological Species: • Morphological species - a construct that assumes that a species comprises individuals that look alike and that individuals that do not look alike belong to different species. - Although Linnaeus, who developed the morphological species concept, did not know it at the time, groups he classified as species look alike because they share many alleles of the ones that code for their morphological features. • However, members of the same species do not always look alike (e.g males, females, and young individuals don’t always resembled one another closely). • Morphology is of little use in the case of cryptic species, species that are morphologically indistinguishable but do not interbreed. • Biologists now use an addition of behavioral and genetic data to differentiate morphological species. • An organism can belong to several morphological species, but only to one biological species. Phylogenetic Species: • Phylogenetic species (lineage species concept) - a way of putting organisms into species based on ancestral analysis. • Long term view of species. • Allows biologists to consider species over evolutionary time. • Limitations involve the fact that very view thorough phylogenies are available. Ecological Species: • Ecological species - a species concept defined by the environmental context (each species’s ecological niche).Advantages of this concept involve the fact that it avoids problems with morphologically similar, asexual reproducing and fossils. Disadvantages due to the fact that it’s difficult to get sufficient detail of every organism’s niche. Speciation • Speciation - the formation of a new species from ancestral species. For this to happen, gene flow must not occur. The process of speciation starts with genetic (or reproductive) isolation, then there must be a form of genetic divergence (adds a new branch to the tree of life/ evolution). - Allopatry: populations physically separated. - Sympatry: co-occuring population become reproductively isolated. • Genetic divergence - natural selection, gene flow, mutations. • Speciation has concentrated on geographic processes that can result in the division of an ancestral species. - Splitting of the geographic range of a species is one obvious way of achieving such division • Specific patterns in speciation include: - Anagenesis: one species gradually transforms into another species. There isn’t a new branch added to the tree of life, the species simply looks different. - Cladogenesis: one species give rise to two or more species.Anew branch is added. Genetic Isolation: • Prezygotic isolation - when a zygote is never formed. - The sperm of one species may not attach to the eggs of another species because the eggs do not release the appropriate attractive chemicals. - The sperm may be unable to penetrate the eggs because the two gambits are chemically incompatible. - Extremely important to for many aquatic species that spawn (releasing gametes directly into the environment). - Causes of prezygotic isolation are disruptions such as temporal, spatial, behavior barrier or gamete barriers (incompatibility to form a zygote). • Postzygotic isolation - zygote is formed. - Low hybrid zygote viability: hybrid zygotes may fail to mature normally, either dying during development or developing phenotypic abnormalities that prevent them from surviving. - Low hybrid adult viability: offspring may have low survivorship than non-hybrid offspring. - Hybrid infertility: hybrid offspring may mature into infertile adults (e.g mule). - Causes postzygotic isolation are when the result of sufficient genetic divergence of populations has occurred. Allopatric Speciation (Reproductive Isolation): • Allopatric speciation - the result when a population is divided by a physical barrier. - This is thought to be the dominant mode of speciation in most groups of organisms. • Populations must be geographically isolated (no gene flow) • Diverge genetically. • Over time, the two populations will separate.And if enough time has passed, they may become two separate biological species that can no longer interbreed. • The populations separated by such barriers are often large initially. • Once a barrier to gene flow is established, reproductive isolation will begin to develop through genetic divergence. • Over generations, differences accumulate in the isolated lineages, reducing the probability that individuals from each lineage will mate successfully with individuals in the other when they come back into contact. - Reproductive isolation can evolve as a by-product of the genetic changes in the two diverging lineages. • There are two basic type of allopatric speciation: - Disperse: when individuals disperse to the new habitat (founder’s effect increases the likelihood of genetic drift) and a sudden environmental change (physical barrier) causes them to be unable to return. Environmental difference between the two habitat will eventually cause selective pressure to change. This is most prevalent in islands. - Example of disperse allopatric speciation: Darwin’s finches. - Vicariance: large populations split into two or more subpopulations. This happens usually due to emerging geographic barriers (e.g river changes course) that results in no gene flow. - Example of vicariance allopatric speciation:Ancestors of modern day ratites that separates into ostrich, kiwis..etc Sympatric Speciation (Reproductive Isolation): • Sympatric speciation - speciation that occurs without the existence of a physical barrier. - Usually a gradual process. • Polyploidy - results from the duplication of sets of chromosomes within individuals. - This can arise either from chromosome duplication in a single species or from the combining of the chromosomes of two different species. - More than two homologous chromosomes. - This doesn’t always kill the organism but they may not be able to reproduce with other offsprings of their parent’s generation. - This often occurs in the zygote stage. - Autopolyploid: an individual originates when two accidentally unreduced diploid gambits (each with two sets of chromosomes) combine to form a tetraploid individual (with four sets of chromosomes). - Allopolyploid: when individuals from two different (but closely related) species interbreed. • In sympatric speciation, natural selection overwhelms gene flow. • Sympatric speciation may occur with some forms of disruptive selection if individuals with different genotypes have a preference for distinct microhabitats where mating takes place. Hybrids • Hybrids - the offspring of genetically dissimilar parents. They are formed when isolated populations reconnect. • Hybrid zones - areas of separated species (morphologically) interbreed - When hybrid zones first form, most hybrids are offspring of crosses between purebred individuals of the two hybridizing species. - Subsequent generations include a variety of individuals with varying proportions of their genes derived from the original two species. - Hybrid zones often contain recombinant individuals resulting from many generations of hybridization. • If separation is recent, they may be similar enough to produce viable offspring. • Lack of genetic divergence should allow gene flow. • If viable hybrid forms, fitness is usually lower than the parent species or higher than the parent species. It is possible that the hybrids outperform other offspring from their parent’s generation but it’s not common. • If the hybrid is weaker in fitness than either parent species, the hybrid zone is narrow. Hybridize Toads in Europe: • The fire-bellied toad lives in Eastern Europe, they are very closely related to the yellow-bellied toad. • The range of the two species overlap a long but very narrow hybrid zone (4800 kilometers). • Hybrids between the two species suffer from a range of defects, many of which are lethal. • Surviving hybrids have skeletal abnormalities, such as misshapen mouths, ribs that are fused to the vertebrae and a reduced number of vertebrae. • Investigators found that the hybrid toads, on average, is only half as fit as the purebred. • Hybrid zone remains narrow because there is strong selection against hybrids. - Adult toads do not move over long distances. - The zone has persisted for hundreds of years because individuals of both species continue to move short distances into it, continually replenishing the hybrid population. Isolating Mechanisms Preventing Hybridization: • Reproduction may be incomplete when incipient species come back into contact. • If hybrid individuals are less fit than non-hybrids, selection will favor parents that do not produce hybrid offspring. • Selection will result in reinforcement of mechanisms that prevent hybridization. • These mechanisms are also processes that lead to sympatric speciation. • Temporal isolation - when two closely related species breed at different times of the year (or different times of the day), the two species will not have an opportunity to hybridize. - Example: field crickets that breed in spring and fall. • Behavioral isolation - individuals may reject, or fail to recognize, individuals of other species as potential mating partners. • Habitat (spatial) isolation - when two closely related species evolve preferences for living or mating in different habitats, they may never encounter one another.