Comprehensive Genetics and Evolution: Natural Selection, Gene Variations, and Population Dynamics

Natural selection

The differential success (survival and reproduction) of individuals within the population that results from their interaction with their environment.

Conditions required for natural selection

(1) That variation occurs among individuals within a population in some heritable characteristic, and (2) that this variation results in differences among individuals in their survival and reproduction.

Fitness

Measured by the proportionate contribution an individual makes to future generations.

Adaptation

Any heritable behavioral, morphological, or physiological trait of an organism that has evolved over a period of time by natural selection such that it maintains or increases the fitness of an organism under a given set of environmental conditions.

Evolution

Changes in the properties of populations of organisms over the course of generations.

Phenotypic evolution

A change in the mean or variance of a phenotypic trait across generations as a result of changes in allele frequencies.

Tenebrionid beetles (Stenocara spp.)

Beetles in the Namib Desert that collect water by standing on their heads so fog droplets collect on their wing case (elytra) and roll into their mouths.

Gene

A stretch of DNA coding for a functional product (RNA), which ultimately results in the synthesis of a protein.

DNA (Deoxyribonucleic acid)

Molecule containing the information at the root of all similarities and differences among organisms.

Genome

All of the DNA in a cell collectively.

Chromosomes

Microscopic, threadlike bodies along which genes are arranged in linear order.

Locus

The position occupied by a gene on the chromosome.

Homologous chromosomes

The two copies of each type of chromosome contained in each individual cell of most multicellular organisms.

Alleles

The alternate forms of a gene.

Genotype

The pair of alleles present at a given locus.

Homozygous

An individual where the two copies of the gene (alleles) are the same at a given locus.

Heterozygous

An individual where the two alleles at the locus are different.

Phenotype

The outward appearance or the external, observable expression of the genotype for a given characteristic.

Dominant allele

The allele that is expressed when an individual is heterozygous, masking the expression of the other allele.

Recessive allele

The allele that is masked in a heterozygous individual; it is only expressed if the individual is homozygous for that allele (homozygous recessive).

Incomplete dominance

When the physical expression of a heterozygous individual is intermediate between those of the homozygotes.

Qualitative traits

Phenotypic characteristics that fall into a limited number of discrete categories (e.g., flower color controlled by a single locus).

Quantitative traits

Phenotypic traits that have a continuous distribution (e.g., height or weight).

Reasons for continuous phenotypic distribution

(1) Most traits are affected by more than one gene locus, and (2) most traits are affected by the environment.

Phenotypic plasticity

The ability of a genotype to give rise to a range of phenotypic expressions under different environmental conditions.

Norm of reaction

The set or range of phenotypes expressed by a single genotype across a range of environmental conditions.

Developmental plasticity

Irreversible phenotypic plasticity that occurs during the growth and development of the individual (e.g., plant leaf size/branching based on initial light environment).

Acclimation

Reversible phenotypic changes in an individual organism in response to changing environmental conditions (e.g., seasonal shifts in temperature tolerance in fish).

Genetic differentiation

When genetic variation occurs among subpopulations of the same species.

Gene pool

The sum of genetic information (alleles) across all individuals in the population, representing the total genetic variation within a population.

Allele frequency

The proportion of a given allele among all the alleles present at the locus in the population.

Genotype frequency

The proportion of a given genotype among all the genotypes present at the locus in the population.

Target of selection

The phenotypic trait that natural selection acts directly upon (e.g., beak size in finches).

Selective agent

The environmental cause of fitness differences among organisms with different phenotypes (e.g., the change in food resources/seed size distribution during the finch drought).

Directional selection

A type of natural selection in which the mean value of the phenotypic trait is shifted toward one extreme over another.

Stabilizing selection

A type of natural selection that favors individuals near the population mean at the expense of the two extremes.

Disruptive selection

A type of natural selection that favors both extremes simultaneously, potentially resulting in a bimodal distribution of the characteristic(s) in the population.

Mutation

Heritable changes in a gene or a chromosome; the ultimate source of genetic variation.

Genetic drift

A change in allele frequencies as a result of random chance; the evolutionary equivalent of sampling error.

Migration

The movement of individuals between local populations.

Gene flow

The movement of genes between populations.

Hardy-Weinberg principle

States that both allele and genotype frequencies remain constant in successive generations of a sexually reproducing population if mating is random, mutations do not occur, the population is large (no genetic drift), there is no migration, and natural selection does not occur.

Assortative mating

When individuals choose mates nonrandomly with respect to their genotype, typically by selecting mates based on some phenotypic trait.

Positive assortative mating

Occurs when mates are phenotypically more similar to each other than expected by chance, leading to an increase in the frequency of homozygotes.

Negative assortative mating

Occurs when mates are phenotypically less similar to each other than expected by chance, leading to an increase in the frequency of heterozygotes.

Inbreeding

The mating of individuals in the population that are more closely related than expected by random chance; increases homozygosity at all loci.

Inbreeding depression

Consequences of inbreeding, such as decreased fertility, loss of vigor, or death, resulting from inheriting rare, recessive, deleterious genes.

Cline

A measurable, gradual change over a geographic region in the average of some phenotypic character, usually associated with an environmental gradient that varies continuously (e.g., temperature).

Ecotype

A population adapted to its unique local environmental conditions, resulting from marked discontinuities (step clines) in phenotypic characteristics.

Common garden experiment

An experimental approach where individuals (genotypes) from different populations are grown under controlled, uniform environmental conditions to determine if phenotypic differences are due to genetic differences (genetic differentiation) or phenotypic plasticity.

Geographic isolates (or subspecies)

Semi-isolated populations where an extrinsic barrier (such as rivers or mountain ridges) prevents the free flow of genes.

Trade-offs and Constraints

The principle that characteristics maximizing an individual's fitness under one set of environmental conditions generally limit its fitness under a different set of conditions.

Adaptive radiation

The process in which one species gives rise to multiple species that exploit different features of the environment (e.g., Darwin's finches).

Artificial selection

The process of selective breeding by humans, analogous to natural selection, where humans function as the agent of selection.

Limitation of selective breeding

The array of characteristics that can be selected for is limited to the genetic variation (alleles) that exists within the population.

Genetic engineering

The process of directly altering an organism's genome.

Genetic recombination

The primary technology used in genetic engineering, involving the combining of genetic material (DNA) from one organism into the genome of another organism.

Recombinant DNA (rDNA)

The resulting DNA produced from combining genetic material from different organisms.

Transgene

The modified gene resulting from genetic recombination.

Transgenic organism

The recipient of the recombinant DNA.

Transformation (in genetic engineering)

The process in which changes in a cell or organism are brought about through the introduction of new DNA.

Promoter

A region of DNA that initiates the transcription of a particular gene.

Terminator

A section of genetic sequence that marks the end of a gene.

Genetically Modified Organisms (GMOs)

Organisms whose genetic material has been altered using genetic engineering techniques.

Ecological concern of GMOs (Brassica example)

The potential transfer of herbicide-tolerant traits by pollen to weedy relatives through cross-pollination.

Bt corn

A widely grown genetically modified crop plant engineered to incorporate genes from the soil bacterium Bacillus thuringiensis (Bt) encoding proteins toxic to certain insect pests.

Ecological concerns of Bt corn

Potential impacts on nontarget insect species or predators that feed on those insects.

Alternative ecological risk of GMOs

If viral resistance genes are transferred to wild plants, it could lead to the natural development of new plant viruses of increased severity.