Topic 2 - Key Terms

Página 1

Additive - In relation to alleles, describes interaction between alleles in which their effects are summed. Additive alleles always affect the phenotype and so are always subject to selection. (T2P2)

Allele Frequency - The frequency or proportion of each allele at a given locus within a designated group, most commonly a population. (T2P1)

Anisogamy - Having gametes of different sizes. Usually individuals producing the larger gamete (egg) are defined as female, individuals producing the smaller gamete (sperm) as male. (T2P4)

Antagonistic Coevolution - In a sexual context, antagonistic coevolution is driven by conflict between the sexes in maximising reproductive success. Adaptations that benefit males at the females' expense select for counter-adaptations in females that mitigate male-imposed harm. Often referred to as an arms race between the sexes. Antagonistic coevolution can also occur between predators and prey, between parasites and their hosts and between parents and offspring. (T2P4)

Antagonistic Pleiotropy - Refers to a situation in which a single gene creates multiple opposing effects, such that beneficial effects of a trait created by the gene are offset by 'losses' in other traits. (T2P2)

Arbitrary Preference - In the context of female choice, an arbitrary preference selects for the elaboration of any male trait that confers a mating advantage on male offspring that express genes for that trait. There is not necessarily a relationship between the trait and the male's genetic quality. (T2P4)

Balancing Selection - A form of natural selection that maintains polymorphism at a locus within a population. (T2P2)

Census Population Size - The number of individuals that are in a particular population. Abbreviated Nc. (T2P1)

Coevolution - The process of reciprocal evolutionary change in two or more interacting species (genes, sexes) by natural selection. Each party in a coevolutionary relationship exerts selective pressures on the other so that they affect each other's evolution. (T2P4)

Cryptic Female Choice - Female behaviour, physiology or morphology that selectively biases paternity in favour of some conspecific males and against others after copulation has already begun. The female's choice is cryptic in that she may accept a male for copulation but reject him as a father. (T2P4)

Deterministic - Describes a model or system in which there is no randomness - for any given starting condition, the outcome is always the same. (T2P1)

Directional - Describes selection that favours one extreme phenotype over others, causing allele frequencies to shift over time in the direction of that phenotype. (T2P2)

Disruptive (or Diversifying) Selection - A type of selection that favours the extreme phenotypes and eliminates the intermediate between them. (T2P3)

Effective Population Size - The number of individuals in a population that contribute offspring to the next generation. Abbreviated Ne. (T2P1)

Fecundity - In the context of the reproductive rate of an organism, it is usally measured in the number of eggs, seeds or propagules (in the case of asexually reproducing organisms) that it can produce. (T2P4)

Female Mate Choice - Also known as intersexual selection, this is where males compete among themselves to be chosen by females. Females are selective about the male they choose to mate with. (T2P4)

Fitness - The success of an individual at surviving and reproducing and therefore contributing offspring to future generations. (T2P2)

Founder Effect - A situation where a small number of founding individuals carry a fraction of the alleles present in the larger population, which results in reduced genetic variation. (T2P1)

Frequency-Dependent Selection - A form of balancing selection that occurs when the relative fitness (survival and reproductive success) of a phenotype (and hence its underlying genotype) differs according to whether it is common or rare in a population. (T2P2)

Genetic Drift - A change in the frequency of alleles or genotypes within a population due to the random sampling of organisms & not as a result of natural selection. (T2P1)

Genetic Load - The burden imposed by the accumulation of deleterious mutations. The genetic load equals the chance that an average individual will die before reproducing because of the deleterious mutations it possesses. (T2P4)

Genotype Frequency - The frequency or proportion of genotypes in a population. Gives an indication of the genotypes that are more or less prevalent in the population. (T2P1)

Hardy–Weinberg equilibrium - A predictable ratio of genotype frequencies in a sexually reproducing population of infinite size with random mating and no selection, genetic drift or other evolutionary forces acting on it. It is represented in a one-locus, two-allele system as p2 + 2pq + q2 = 1. (T2P1)

Hardy–Weinberg principle - According to this principle, allele and genotype frequencies (i.e. genetic variation) in a population will remain constant from generation to generation provided that individuals in the population mate at random and there are no evolutionary mechanisms acting on that population. (T2P1)

heritability - Denoted by h2, the amount of the variance in a trait that is attributable to genetic factors. Often used in reference to the resemblance between parents and their offspring. (T2P3)

heterozygosity - The proportion of a population that has a heterozygous genotype. (T2P1)

inbreeding depression - A reduction in the fitness of offspring that results from breeding between close relatives; caused by the expression of deleterious recessive genes that were present in the parents as heterozygotes. (T2P2)

indirect benefits of choice - Factors that increase the genetic fitness of offspring. Indirect benefits include high quality genes for offspring (adaptive indirect benefits) or genes that make offspring more attractive (arbitrary indirect benefits). (T2P4)

intersexual selection - See female mate choice. (T2P4)

intrasexual selection - Members of the same sex (usually males) within a species compete with one another for access to members of the opposite sex. Usually results in the evolution of physical or behavioural features that determine the outcome of aggressive encounters, e.g. weapons, large body size. (T2P4)

male–male competition - Often used synonymously with intrasexual selection but unlike intrasexual selection where competition occurs between members of the same sex, be they male or female, male-male competition refers to only those situations where males compete with each other for access to females. (T2P4)

mechanisms of evolution - Mechanisms of evolutionary change include natural selection, genetic drift, mutation and migration (or gene flow). All these can result in a change in allele frequencies in populations. (T2P1)

negative FDS - A mode of natural selection in which the fitness of a genotype in a population is greater when it is rare compared to when it is common. (T2P2)

operational sex ratio (OSR) - The ratio of sexually active males (those ready to mate) to sexually fertilisable females (those ready to mate). Differs from the sex ratio which takes into account inactive or non-competitive individuals. (T2P4)

overdominant - Describes selection in which, at a given locus, heterozygous individuals have a higher fitness than homozygous individuals; can also be described as heterozygote advantage. (T2P2)

parthenogenesis - Also known as 'virgin birth'; the development of offspring from an unfertilised egg. (T2P4)

phenotypic plasticity - Sensitivity of a phenotype produced by a single genotype to differences in the environment. (T2P3)

pleiotropic - Describes a cellular process, signal or gene that has more than one effect, or more than one phenotypic outcome. At the genetic level, describes a situation where a single mutation affects two or more apparently unrelated phenotypic traits. (T2, T6)

population bottleneck - An event that drastically reduces the size of a population. (T2P1)

population genetics - The study of distributions and changes in allele frequency in populations and genetic variation within and among species. (T2P1)

positive FDS - A mode of natural selection in which the fitness of a genotype in a population is greater when it is common compared to when it is rare. (T2P2)

potential reproductive rate - The offspring production per unit time that each sex would achieve if unconstrained by mate availability. Because females generally produce large eggs and care for offspring, their potential reproductive rate is lower than that for males. (T2P4)

reaction norm - The pattern of phenotypic expression of a single genotype across a range of environments. (T2P3)

relative fitness value - The relative fitness of a genotype, w, is calculated by comparing the fitness of all individuals in a population with the fitness of the most successful genotype in the population (often set at 1.0). (T2P2)

response to selection, R - The change in the mean value of a character over one or more generations in response to selection. (T2P3)

runaway theory of sexual selection - A mechanism of sexual selection proposed by R.A. Fisher to explain evolution of exaggerated male ornamentation. According to Fisher's theory, the initial female preference for the male trait is an arbitrary preference; there is no necessity for there to be a relationship between the trait expression and the male's inherent genetic quality. (T2P4)

sampling error - Error associated with estimates that are based on samples; occurs because the proportions of different kinds of items (e.g. alleles) in a sample are likely to differ, by chance, from the proportions in the set of items from which the sample is drawn (e.g. population). (T2P1)

selection coefficient - A measure of the intensity of selection against a genotype; the difference between the mean relative fitness of individuals of a given genotype and that of a reference genotype. For each genotype: s = 1−w, where w is relative fitness. (T2P2)

selection differential (S) - The difference between the mean value of the trait for the population and the mean value of the trait for those individuals of that population that reproduce. (T2P3)

sexual conflict - Conflict that arises between the sexes because strategies that maximise the fitness in one sex can reduce the fitness of the other sex. Can lead to antagonistic coevolution and sexual 'arms races'. (T2P4)

sexual selection - A form of natural selection that is associated with success in mating. Differential reproductive success resulting from competition for fertilisation, that can occur through competition among the same sex (intrasexual selection) or through attraction to the opposite sex (intersexual selection). (T2P4)

sperm competition - The physical competition between the sperm of different males to fertilise the egg(s) of a single female. (T2P4)

stabilising selection - Selection against phenotypes at the extremes of the distribution of phenotypes. (T2P3)

trade-off - When traits that are advantageous in one environment are disadvantageous in other environments. (T2P2)

underdominant - Selection in which, at a given locus, heterozygotes have a lower fitness than both homozygotes. (T2P2)

variance - A quantifiable measure of the dispersion or spread of a trait about the mean (i.e. the amount of variation). (T2P3)

Zahavi’s handicap principle - Proposes that females prefer to mate with males with handicaps (mating/display characteristics that reduce survival chances) because handicaps are indicators of heritable male quality. Only individuals that are of high quality can 'afford' to produce extravagant displays and so they are, therefore, honest signals of quality. (T2P4)