Descent With Modification

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adaptive change

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give the next generation a bigger proportion of better adapted alleles

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adaptive evolution

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devised by Fisher and is of general importance to population genetics. For almost any character there is likely to be a relation between the form of the character and its fitness, similar to that shown in the figure. The model considers both small and large mutations, in the form of uphill (positive for competition) or downhill (opposite). X axis is taken by the character, the animal, and the y is taken by fitness. A mutation is equally likely to be in any direction from where the species is; one is equally likely to be 'up' hill as 'down' hill. The important relation is between the size of a mutation and the chance that it is an improvement.

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assortive mating

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takes place when sexually reproducing organisms tend to mate with individuals that are like themselves in some respect (positive assortative mating) or dissimilar (negative assortative mating). In evolution, these two types of assortative mating have the effect of reducing and expanding the range of variation, respectively, when the assorting is cued on heritable traits. t is mirrored by selective fertilization in plants. Assortative mating has been invoked to explain sympatric speciation. For some populations there are two different resources for which different phenotypes are optimum. Intermediates between these two phenotypes are less favorable. It is then favourable if the organisms can recognize mates that are optimized for the same resources as they are themselves. If mutations that make such recognition possible appear, these will be selected for.

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balanced polymorphism

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efers to forms of natural selection which work to maintain genetic polymorphisms (or multiple alleles) within a population. Balancing selection is in contrast to directional selection which favor a single allele. A balanced polymorphism is a situation in which balancing selection within a population is able to maintain stable frequencies of two or more phenotypic forms. Evidence for balancing selection can be found by increased levels of genetic variation between alleles or haplotypes in a species. Note that balancing selection will not always result in an observable phenotypic difference because the genotype may not be one-to-one with the phenotype.

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Batesian mimicry

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amed after Henry Walter Bates, an English naturalist whose work on butterflies in the Amazon rainforest (including The Naturalist on the River Amazons) was pioneering in this field of study.[9] In this type of mimicry the mimic sends similar signals to model species, but does not share the attribute that makes it unprofitable to predators (e.g. unpalatability). Examples: The Ash Borer (a Batesian mimic of the Common wasp because it resembles the wasp, but is not capable of stinging. A predator that has learned to avoid the wasp would similarly avoid the Ash Borer.), The False Cobra (Malpolon moilensis) is a mildly venomous but harmless colubrid snake which mimics the characteristic "hood" of an Indian cobra's threat display. The Eastern Hognose Snake (Heterodon platirhinos) similarly mimics the threat display of poisonous snakes and the Octopuses of the genus Thaumoctopus (the Mimic Octopus and the "wunderpus") are able to intentionally alter their body shape and color so that they resemble dangerous sea snakes or lionfish.

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bottleneck effect

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An event in which a population's size is greatly reduced. When this happens, genetic drift may have a substantial effect on the population. In other words, when the population size is radically reduced, gene frequencies in the population are likely to change just by random chance and many genes may be lost from the population, reducing the population's genetic variation.

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directional selection

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occurs when natural selection favors a single allele and therefore allele frequency continuously shifts in one direction. Under directional selection, the advantageous allele will increase in frequency independently of its dominance relative to other alleles (i.e. even if the advantageous allele is recessive, it will eventually become fixed). Directional selection stands in contrast to balancing selection where selection may favor multiple alleles, or purifying selection which removes deleterious mutations from a population. Directional selection is a particular mode or mechanism of natural selection. A classic example is the evolution of the peppered moth, where the favored trait of moth color (light or dark) noticeably shifted to a darker shade in relation to the effects of human industrialization.

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diversifying selection

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a descriptive term used to describe changes in population genetics that simultaneously favor individuals at both extremes of the distribution. When disruptive selection operates, individuals at the extremes contribute more offspring than those in the center, producing two peaks in the distribution of a particular trait.

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fitness

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A genotype's success at reproducing (the more offspring the genotype leaves, the higher its fitness). Fitness describes how good a particular genotype is at leaving offspring in the next generation relative to other genotypes. Experiments and observations can allow researchers to estimate a genotype's fitness, assigning it a numerical value.

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founder effect

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Changes in gene frequencies that usually accompany starting a new population from a small number of individuals. The newly founded population is likely to have quite different gene frequencies than the source population because of sampling error (i.e., genetic drift). The newly founded population is also likely to have a less genetic variation than the source population.

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frequency-dependent selection

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an evolutionary process where the fitness of a phenotype is dependent on its frequency relative to other phenotypes in a given population. In positive frequency dependent selection, the fitness of a phenotype increases as it becomes more common. In negative frequency dependent selection, the fitness of a phenotype increases as it becomes less common. Frequency dependent selection is a particular mechanism of balancing selection. One example of negative frequency dependent selection is in the case of plant self-incompatibility alleles. When two plants share the same incompatibility allele, they are unable to mate. Thus, a plant with a new (and therefore, rare) allele has more success at mating, and its allele spreads quickly through the population.

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gene flow

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The movement of genes between populations. This may happen through the migration of organisms or the movement of gametes (such as pollen blown to a new location)

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gene pool

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All of the genes in a population. Any genes that could wind up in the same individual through sexual reproduction are in the same gene pool.

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geographical variation

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the quality or state of existing in or assuming different forms, such as the existence of a species in several forms independent of the variations of sex; existence of a gene in several allelic forms; also : a variation in a specific DNA sequence; existence of a molecule (as an enzyme) in several forms in a single species.

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Hardy-Weinberg equilibrium

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the Hardy–Weinberg principle is a relationship between the frequencies of alleles and the genotype of a population. The occurrence of a genotype, perhaps one associated with a disease, stays constant unless matings are non-random or inappropriate, or mutations accumulate. Therefore, the frequency of genotypes and the frequency of alleles are said to be at "genetic equilibrium". Genetic equilibrium is a basic principle of population genetics.

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heterozygote advantage

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the case in which the heterozygote genotype has a higher relative fitness than either the homozygote dominant or homozygote recessive genotype. This selection favoring the heterozygote is one of the mechanisms that maintain polymorphism and help to explain some kinds of genetic variability. There are several cases in which the heterozygote conveys certain advantages and some disadvantages while both versions of homozygotes are only at disadvantages. A well-established case of heterozygote advantage is that of the gene involved in sickle cell anaemia.

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inbreeding

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mating between relatives. Technically, this is defined as a pattern of mating in which mates are more closely related than two individuals selected at random from the population.

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macroevolution

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evolution above the species level. The adaptive radiation of a lineage into many different niches is an example of macroevolution. Since evolutionary change above the species level, means that populations and species must be evolving, macroevolutionary change entails microevolutionary change.

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microevolution

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evolution on a small scale—within a single population. That means narrowing our focus to one branch of the tree of life.

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natural selection

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Differential survival or reproduction of different genotypes in a population leading to changes in the gene frequencies of a population

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neutral variation

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The idea that most of the molecular variation within populations is not being selected for or against — it is just neutral variation "drifting" around. The neutral theory de-emphasizes the role of natural selection in explaining molecular variation and emphasizes the importance of mutation and genetic drift.

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nonadaptive genetic drift

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"It becomes hard to

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pleiotropy

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occurs when a single gene influences multiple phenotypic traits. Consequently, a new mutation in the gene will have an effect on all traits simultaneously. This can become a problem when selection on one trait favours one specific mutant, while the selection at the other trait favours another mutant. A classic example of pleiotropy is the human disease PKU (phenylketonuria). This disease can cause mental retardation and reduced hair and skin pigmentation, and can be caused by any of a large number of mutations in a single gene that codes for an enzyme (phenylalanine hydroxylase) that converts the amino acid phenylalanine to tyrosine, another amino acid. PKU is totally benign if a diet free from phenylalanine is maintained. Depending on the mutation involved, this results in reduced or zero conversion of phenylalanine to tyrosine, and phenylalanine concentrations increase to toxic levels, causing damage at several locations in the body.

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point-mutation

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a type of mutation that causes the replacement of a single base nucleotide with another nucleotide. Often the term point mutation also includes insertions or deletions of a single base pair (which have more of an adverse effect on the synthesized protein due to nucleotides still being read in triplets, but in different frames- a mutation called a frameshift mutation). Mutagens can be physical, such as radiation from UV rays, X rays or extreme heat, or chemical (molecules that misplace base pairs or disrupt the helical shape of DNA).

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polygenic traits

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`these do not follow patterns of Mendelian inheritance (qualitative traits). Instead, their phenotypes typically vary along a continuous gradient depicted by a bell curve. An example of a polygenic trait is human skin color. Many genes factor into determining a person's natural skin color, so modifying only one of those genes changes the color only slightly. Many disorders with genetic components are polygenic, including autism, cancer, diabetes and numerous others. Most phenotypic characteristics are the result of the interaction of multiple genes.

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polymorphic

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the quality or state of existing in or assuming different forms: as a; existence of a species in several forms independent of the variations of sex. This term may be applied to a variation a specific DNA sequence or the existence of a molecule (as an enzyme) in several forms in a single speciesthe property of crystallizing in two or more forms with distinct structure

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polymorphism

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discontinuous variation in a single population—in other words, the occurrence of more than one form or type of individual (whether the differences are visible or solely biochemical). The most obvious example of polymorphism is the sexual dimorphism of most higher organisms. Other examples are melanic and non-melanic forms of the same species (such as the Peppered Moth or the Jaguars shown at right). Not all polymorphisms as defined above are genetic, but in genetics the term is reserved for variation in a population's DNA.

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sampling error

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a term from statistics, in evolutionary biology it is called a genetic drift. This drifting happens in populations of organisms. Due to many random factors, the genes in one generation do not wind up in identical ratios in the next generation, and this is evolution. While this is evolution, it is evolution due to chance, not selection. Genetic drift reduces genetic variation in populations, acts faster and has more drastic results in smaller populations and can contribute to speciation;

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selection coefficient

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the fitness deviation, measuring the intensity of natural selection acting on the genotypes in the population. It is often denoted by the letter s.

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sexual dimorphism

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the systematic difference in form between individuals of different sex in the same species. Examples include size, colour, and the presence or absence of parts of the body used in courtship displays or fights, such as ornamental feathers, horns, antlers or tusks.

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heterozygote advantage

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the case in which the heterozygote genotype has a higher relative fitness than either the homozygote dominant or homozygote recessive genotype. This selection favoring the heterozygote is one of the mechanisms that maintain polymorphism and help to explain some kinds of genetic variability. There are several cases in which the heterozygote conveys certain advantages and some disadvantages while both versions of homozygotes are only at disadvantages. A well-established case of heterozygote advantage is that of the gene involved in sickle cell anaemia.

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inbreeding

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mating between relatives. Technically, this is defined as a pattern of mating in which mates are more closely related than two individuals selected at random from the population.

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macroevolution

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evolution above the species level. The adaptive radiation of a lineage into many different niches is an example of macroevolution. Since evolutionary change above the species level, means that populations and species must be evolving, macroevolutionary change entails microevolutionary change.

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microevolution

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evolution on a small scale—within a single population. That means narrowing our focus to one branch of the tree of life.

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natural selection

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Differential survival or reproduction of different genotypes in a population leading to changes in the gene frequencies of a population

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neutral variation

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The idea that most of the molecular variation within populations is not being selected for or against — it is just neutral variation "drifting" around. The neutral theory de-emphasizes the role of natural selection in explaining molecular variation and emphasizes the importance of mutation and genetic drift.

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nonadaptive genetic drift

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"It becomes hard to

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pleiotropy

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occurs when a single gene influences multiple phenotypic traits. Consequently, a new mutation in the gene will have an effect on all traits simultaneously. This can become a problem when selection on one trait favours one specific mutant, while the selection at the other trait favours another mutant. A classic example of pleiotropy is the human disease PKU (phenylketonuria). This disease can cause mental retardation and reduced hair and skin pigmentation, and can be caused by any of a large number of mutations in a single gene that codes for an enzyme (phenylalanine hydroxylase) that converts the amino acid phenylalanine to tyrosine, another amino acid. PKU is totally benign if a diet free from phenylalanine is maintained. Depending on the mutation involved, this results in reduced or zero conversion of phenylalanine to tyrosine, and phenylalanine concentrations increase to toxic levels, causing damage at several locations in the body.

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point-mutation

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a type of mutation that causes the replacement of a single base nucleotide with another nucleotide. Often the term point mutation also includes insertions or deletions of a single base pair (which have more of an adverse effect on the synthesized protein due to nucleotides still being read in triplets, but in different frames- a mutation called a frameshift mutation). Mutagens can be physical, such as radiation from UV rays, X rays or extreme heat, or chemical (molecules that misplace base pairs or disrupt the helical shape of DNA).

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polygenic traits

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`these do not follow patterns of Mendelian inheritance (qualitative traits). Instead, their phenotypes typically vary along a continuous gradient depicted by a bell curve. An example of a polygenic trait is human skin color. Many genes factor into determining a person's natural skin color, so modifying only one of those genes changes the color only slightly. Many disorders with genetic components are polygenic, including autism, cancer, diabetes and numerous others. Most phenotypic characteristics are the result of the interaction of multiple genes.

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polymorphic

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the quality or state of existing in or assuming different forms: as a; existence of a species in several forms independent of the variations of sex. This term may be applied to a variation a specific DNA sequence or the existence of a molecule (as an enzyme) in several forms in a single speciesthe property of crystallizing in two or more forms with distinct structure

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polymorphism

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discontinuous variation in a single population—in other words, the occurrence of more than one form or type of individual (whether the differences are visible or solely biochemical). The most obvious example of polymorphism is the sexual dimorphism of most higher organisms. Other examples are melanic and non-melanic forms of the same species (such as the Peppered Moth or the Jaguars shown at right). Not all polymorphisms as defined above are genetic, but in genetics the term is reserved for variation in a population's DNA.

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sampling error

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a term from statistics, in evolutionary biology it is called a genetic drift. This drifting happens in populations of organisms. Due to many random factors, the genes in one generation do not wind up in identical ratios in the next generation, and this is evolution. While this is evolution, it is evolution due to chance, not selection. Genetic drift reduces genetic variation in populations, acts faster and has more drastic results in smaller populations and can contribute to speciation;

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selection coefficient

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the fitness deviation, measuring the intensity of natural selection acting on the genotypes in the population. It is often denoted by the letter s.

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sexual dimorphism

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the systematic difference in form between individuals of different sex in the same species. Examples include size, colour, and the presence or absence of parts of the body used in courtship displays or fights, such as ornamental feathers, horns, antlers or tusks.

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nonadaptive genetic drift

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"It becomes hard to

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nonadaptive genetic drift

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"It becomes hard to explain why species experience evolutionary stasis (equilibrium)

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nonadaptive genetic drift

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"It becomes hard to explain why species experience evolutionary stasis(equilibrium) for millions of years, especially when the environment is changing at the same time. Evolutionists have proposed that organisms experience genetic corrections, called “stabilizing selection,” where any new versions of genes and phenotypes are quickly eliminated because they are nonadaptive."

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sexual recombination

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the process by which a strand of DNA is broken and then joined to the end of a different DNA molecule. This shuffling of genes is thought to have many advantages, including that of allowing sexually reproducing organisms to avoid Muller's ratchet. However, a recombination pathway in DNA is any way by which a broken DNA molecule is reconnected to form a whole DNA strand.

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sexual selection

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Selection acting on an organism's ability to obtain or successfully copulate with a mate. This process may produce traits that seem to decrease an organism's chance of survival, while increasing its chances of mating.

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speciation

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the process by which a species is generated.

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species

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Members of populations that actually or potentially interbreed. In this sense, a species is the largest gene pool possible under natural conditions.

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stabilizing selection

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also referred to as purifying selection, is a type of natural selection in which genetic diversity decreases as the population stabilizes on a particular trait value. Put another way, extreme values of the character are selected against. This is probably the most common mechanism of action for natural selection.