The Origin Of Species

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biological species concept

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-Ernst Mayr
-defines a species as a population or a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring, but can't produce vialb,e fertile offspring with members of OTHER populations

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reproductive isolation

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-the existence of biological factors (barriers) that impede members of two species from producing viable, fertile hybrids. Although a single barrier may not block all genetic exchange between species, a combination of several barriers can effectively isolate a species′ gene pool.

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prezygotic barries

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-(“before the zygote”) impede mating between species or hinder the fertilization of ova if members of different species attempt to mate. If a sperm cell from one species does overcome prezygotic barriers and fertilizes an ovum from another species, postzygotic barriers (“after the zygote”) often prevent the hybrid zygote from developing into a viable, fertile adult

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morphological species concept

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-characterizes a species by its body shape, size, and other structural features. The morphological species concept has advantages: It can be applied to asexual and sexual organisms, and it can be useful even without information on the extent of gene flow.
-One disadvantage, however, is that this definition relies on subjective criteria; researchers may disagree on which structural features distinguish a species.

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paleontological species concept

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focuses on morphologically discrete species known only from the fossil record. We are forced to distinguish many species in this way because there is little or no information about their mating capability.

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ecological species concept

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-views a species in terms of its ecological niche, its role in a biological community
-For example, two species of Galápagos finches may be similar in appearance but distinguishable based on what they eat. Unlike the biological species concept, this definition can accommodate asexual as well as sexual species.

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phylogenetic species concept

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-defines a species as a set of organisms with a unique genetic history—that is, as one branch on the tree of life. Biologists trace the phylogenetic history of a species by comparing its physical characteristics or its molecular sequences with those of other organisms.
-sometimes reveals the existence of “sibling species”: species that appear so similar that they cannot be distinguished on morphological grounds. Scientists can then apply the biological species concept to determine if the phylogenetic distinction is confirmed by reproductive incompatibility.

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allopatric speciation

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-a population forms a new species while geographically isolated from its parent population
-gene flow is interrupted when a population is divided into geographically isolated subpopulations
-can also occur without geologic remodeling, such as when individuals colonize a remote area, and their descendants become geographically isolated from the parent population

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sympatric speciation

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-a small population becomes a new species without geographic seperation
-speciation takes place in geographically overlapping populations
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polyploidy

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-Some plant species have their origins in accidents during cell division that result in extra sets of chromosomes, a mutational change that results in the condition called polyploidy

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autopolyploid

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-is an individual that has more than two chromosome sets, all derived from a single species. For example, a failure of cell division can double a cell′s chromosome number from the diploid number (2n ) to a tetraploid number (4n )
-This mutation prevents a tetraploid from successfully interbreeding with diploid plants of the original population—the triploid (3n) offspring of such unions are sterile because their unpaired chromosomes result in abnormal meiosis
-the tetraploid plants can still produce fertile tetraploid offspring by self–pollinating or mating with other tetraploids. Thus, in just one generation, autopolyploidy can generate reproductive isolation without any geographic separation.

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allopolyploid

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-various mechanisms can change a sterile hybrid into a fertile polyploid known as an allopolyploid
-fertile with each other but cannot interbreed with either parental species—thus they represent a new biological species.

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adaptive radiation

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-The evolution of many diversely adapted species from a common ancestor upon introduction to various new environmental opportunities and challenges is called adaptive radiation.
-typically occurs when a few organisms make their way to new, often distant areas or when environmental changes cause numerous extinctions, opening up ecological niches for the survivors

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macro-evolution

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-evolutionary change above the species level
-can accumulate through many speciation events

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micro-evolution

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-genetic changes with in a population over time, one gene pool

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Hardy–Weinberg theorem

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-states that the frequencies of alleles and genotypes in a population′s gene pool remain constant from generation to generation, provided that only Mendelian segregation and recombination of alleles are at work
-describes how Mendelian inheritance preserves genetic variation from one generation to the next in populations that are not evolving

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Hardy–Weinberg equilibrium

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-Not only will this population have the same allele frequencies from one generation to the next, but its genotype frequencies can be predicted from the allele frequencies
-random= all male-female matings are equally likely
-the equation for Hardy–Weinberg equilibrium states that at a locus with two alleles, the three genotypes will appear in the following proportions: (see equation)
*extremely large population size
*no gene flow
*no mutations
*random mating
*no natural selection

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mutations

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-changes in the nucleotide sequence of DNA. A mutation is like a shot in the dark—it is not possible to predict how it will alter DNA and what its effects will be. Most mutations occur in somatic cells and are lost when the individual dies. Only mutations in cell lines that produce gametes can be passed to offspring, and only a small fraction of these spread through populations.

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point mutations

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-A change of as little as one base in a gene—a “point mutation” can have a significant impact on phenotype, as in sickle–cell disease
-may have little effect because they do not alter the protein′s amino acid composition

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sexual recombination

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-more important than mutation on a generation–to–generation time scale in producing the variations that make adaptation possible. Nearly all phenotypic variations based on genetic differences result from recombinational shuffling of the existing alleles in the gene pool

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need to know:

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Recombination reshuffles alleles, but does not change their frequencies. Nonrandom mating can affect the relative frequencies of homozygous and heterozygous genotypes but usually has no effect on allele frequencies. The three major factors that alter allele frequencies and bring about most evolutionary change are natural selection, genetic drift, and gene flow.

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gene flow

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-genetic additions to and/or subtractions from a population resulting from the movement of fertile individuals or gametes

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phenotypic polymorphism

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-When individuals differ in a discrete character, the different forms are called morphs. A population is said to display phenotypic polymorphism for a character if two or more distinct morphs are each represented in high enough frequencies to be readily noticeable

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genetic polymorhpisms

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-The heritable component of height is the result of such genetic polymorphisms for alleles at the several loci that influence height.

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fitness

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-the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals.

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relative fitness

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-The contribution of one genotype to the next generation compared to that of alternative genotypes for the same locus.