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Chapter 5

BIOL 180 Chapter 5: Reading 5-3

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BIOL 180
H E R R O N, J O N

BIOL 180 - Reading 5-3: 24.1 - How are Species Defined and Identified?: Speciation - A splitting event that forms two or more distinct species from a single ancestral species- can occur abruptly or gradually There are things you need to define to understand when two species are actually different Although there is no single, universal answer for this, scientists do agree there is a distinction between the general definition of a species and the criteria used in the practical identification of species in particular cases Species - An evolutionary independent population or group of populations. Of the many criteria that can be used to identify species, the three most common ones are 1. The biological species concept 2. The morphospecies concept 3. Phylogenetic species concept The Biological Species Concept: Biological species concept - According to this concept, the main criterion for identifying species is reproductive isolation If two different populations do not interbreed in nature, or if they fail to produce viable and fertile offspring when matings take place, then they are considered distinct species The criterion of reproductive isolation offers clear evidence of evolutionary independence Reproductive isolation can result from a wide variety of events and processes To organize the various mechanisms that stop gene flow between populations, biologists distinguish between: Prezygotic isolation - Prevents individuals of different species from mating successfully Postzygotic isolation - The hybrid of matings between members o different species either do not survive or do not reproduce The criterion of reproductive isolation cannot be evaluated in fossils or in species that reproduce asexually, and is difficult to apply when closely related populations don’t overlap with each other geographically. In the case of the lack of geographical overlap, biologists are left to guess whether interbreeding and gene flow would occur if the populations happened to come info contact Reproductive isolation can also sometimes be a complex gradient rather than an all-or- nothing scenario. For example, if a male lion and female tiger produce a rare “liger” that survives and reproduces in captivity (the ranges of these cants do not overlap in nature), are lions and tiger necessarily the same species? Mechanisms of Reproductive Isolation: Prezygotic Isolation: Type Process Example Temp Populations are isolated because Bishop pines and Monterey pines release oral they breed at different times. their pollen at different times of the year. Habit Populations are isolated because Mainland mice and beach mice mate in at they breed in different habitats. different habitats. Populations do not interbreed Beha because they courtship display To attract females, male songbirds sing vioral differ. species-specific songs. Gam Differences in bindin protein determine etic Matings fail because eggs and which sea urchin sperm will penetrate barri sperm are incompatible. eggs.i er Mech Matings fail because male and Very large dogs and very small dogs anica female reproductive structures are cannot mate due to the incompatible l incompatible. sizes. Postzygotic Isolation: Type Process Example Hybri When ring-necked doves mate with rock d Hybrid offspring do not develop doves, less than 6 percent of the eggs viabili normally and die as embryos. hatch. ty Hybri Horses and donkeys can mate and d Hybrid offspring mature but are sterili sterile as adults. produce viable offspring, but their hybrid ty offspring (mules) are usually sterile. The Morphospecies Concept: Morphospecies concept - According to this concept, researchers identify evolutionarily independent lineages by differences in size, shape, or other morphological features The logic behind the morphospecies concept is that distinguishing features are most likely to arise if populations are independent and isolated from gene flow The morphospecies concept is compelling simply because it is so widely applicable It is a useful criterion when biologists have no data on the extend of gene flow, and it is equally applicable to sexual, asexual, or fossil species Its disadvantages are that 1. It may lead to the n
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