S

Saltation: A variation of large effect; also, a major mutation.
Satellite DNA: Highly repeated DNA sequence, which was originally detected as a “satellite” component with a density distinct from the rest of the genome.
Scat: The excrement of an animal.
Secondary contact: Contact between populations that have previously been geographically separate (i.e. allopatric). It contrasts with primary contact.
Segregating sites: Sites that are polymorphic in a sample of sequences.
Segregation: The movement of two homologous chromosomes during meiosis, one to each pole of the cell. Also, the production of different genotypes in the offspring of heterozygotes, as a result of this random meiotic process.
Segregation distortion: Deviation from the expected 1:1 segregation of alleles from a heterozygous at meiosis. It includes both meiotic drive and differential survival of the haploid products of meiosis.
Segregation load: The loss of mean fitness caused by the segregation of homozygotes, when polymorphism is maintained by heterozygous advantage.
Selection coefficient: Difference in relative fitness.
Selection differential: The difference in mean trait value between those that reproduce and the original population.
Selection, direct: A change in genotype frequency that is caused by the effects on fitness on the alleles themselves.
Selection response: The change in mean trait value over one generation.
Selective death: A failure to survive or reproduce; also, a loss of fitness attributable to differences in phenotype.
Selective sweep: Increase of neutral alleles by hitchhiking with a favorable mutation. This sweeps variation out of a region of the genome surrounding the favorable mutation.
Self-fertilization:  When a hermaphrodite organism mates with itself.
Self-incompatibility: When individuals cannot self-fertilize.
Selfish: A gene that increases its own transmission but reduces the fitness of the individual that carries it.
Selfish DNA: Sequences that replicate faster than the rest of the genome and that reduce the fitness of the individual carrying them.
Self-replication: Used to describe a molecule or other structure that can cause its own replication.
Semiconservative: Describes the replication of double-stranded DNA, where the two new molecules each carry one strand from their parent and a complementary strand that has been synthetized.
Senescence: aging
Sensory bias: An innate preference for particular male traits, which did not evolve as a result of the sexual selection caused by that preference.
Sex: Production of offspring that are a mixture between two different parental phenotypes.
Sex-biased inheritance: Transmission of genes through one or the other sex (e.g. maternal inheritance or sex linkage).
Sex chromosome: A chromosome that is inherited differently by the two sexes. In mammals, males carry an X and a Y chromosome, and females carry two X chromosomes. In birds and butterflies, males carry two Z chromosomes, and females carry a Z and z W chromosome.
Sex ratio: The ration of males to females in a population.
Sexual selection: Selection arising from variation in the ability to find a mate.
Shifting balance theory: A theory develop by Sewall Wright, in which species evolve toward the best among many alternative adaptive peaks.
Short interspersed nucleotide element (SINE): A class of transposable element.
Shotgun sequence: A method of sequencing genomes and environmental samples in which random fragments of DNA are sequenced and then computational methods are used to “reassemble” genomes from the sample.
Simple sequence repeats (SSR): Tandem repeats of a short sequence.
Simulated annealing: An optimization algorithm that chooses random changes that improve the desired trait.
Single-nucleotide polymorphism (SNP): A SNP occurs when members of a population vary in which base they carry at a single nucleotide site.
Sister chromatid: The two copies of a chromosome after it has been replicated.
Slime mold: Eukaryotes, from multiple phyla, that normally exist as single-celled amoeba-like organism but that sometimes gather into “slugs” which move together as a unit.
Slip-strand mispairing (SSM): A process in which a DNA polymerase adds one to many or one to few copies of a repetitive sequence during replication.
Small non-coding RNAs (sncRNAs): Short (20-30 nucleotides), non-coding RNAs that can be produced from various precursors and form various classes, including those involved in epigenetics and post transcriptional gene silencing such as microRNAs
Social Darwinism: The idea that, by analogy with natural selection, societies evolve through competition between individuals or groups.
Social evolution: The study of the evolutionary consequences of interaction between individuals.
Soma: Those parts of a multicellular organism that will not directly produce gametes. It contrasts with germline.
Speciation: The process by which new species are formed.
Species: Basic category of biological classification composed of a group of individuals that are evolutionary related and that when mating are able to produce viable and fertile progeny.
Species selection: Selection between species arising from differences in the rate of speciation and/or extinction of lineages.
Species tree: A phylogenetic tree showing the relationship among species. It is usually shown to contrast with gene trees, which may include events such as gene duplication and lateral gene transfer.
Specificity: Where individual molecules take up a stable conformation with specific biological functions.
Sphenopsid:  A member of a group of plants that include trees in the Carboniferous coal swamp forest as well as the living horsetail.
Sponges: Common name for members of the phylum Porifera, which are thought to be the earliest branching lineage of animals. Sponges feed by moving water through their bodies using specialized cells with cilia called choanocytes.
Sporangium: A structure containing spores.
Sporophyte: The diploid phase of the life cycle of plants that gives rise to the production of spores by means of meiosis. It contrasts with gametophyte.
Stabilizing selection: Selection that favors intermediate trait values.
Standard deviation: Standard deviation is a measure of dispersion; a large value of standard deviation indicates a greater variability and thus the values are more spread out.
Star genealogy: A genealogy in which all lineages coalesce in a common ancestor at the same time. It is produced by a population bottleneck or by a selective sweep.
Statistical power: The chance that the null hypothesis will be rejected when the data are generated by a different model.
Stem group: The series of extinct organisms within a clade of living and fossil organism that lie below the crown group.
Stem-loop structure: A hairpin structure in an RNA molecule that is maintained by complementary base pairing.
Steranes: Chemical derivatives of sterols that have been used as chemical fossils.
Stereoisomers: Molecules whose atoms are connected with each other in the same way but are arranged differently in space. This include enantiomers, which are mirror-image reflections of each other.
Sterol: Amphipathic molecules found in the membranes of many organisms, specially eukaryotes.
Stigma: The female reproductive organ in a flowering plant, which receive pollen.
Style: The long structure between the stigma and ovule in a flower. The pollen must grow through the style in order to fertilize the plant.
Styptic: Tendency to constrict; as to constrict blood vessel and stop bleeding.
Substitution: The replacement in a population of one nucleotide or amino acid by another.
Substitution load: The total loss of mean fitness caused because favorable alleles substitute gradually by selection rather than instantaneously.
Supercoiling: Higher-order twisting of DNA strands.
Supergene: A cluster of tightly linked genes, which allow distinct alternative morphs to be maintain as a polymorphism within one population.
Suppressor mutation: A secondary mutation that can cancel the effect of a primary mutation, resulting in a wild-type phenotype.
Survival: The act or fact of surviving under adverse or unusual circumstances.
Survive: To remain alive after the cessation of something or the occurrence of some event. To continue to live.
Surroundings: All that is around, environment.
Symbiosis: The system in which members of different species live in physical contact with one another.
Sympatric speciation: The separation of a single population into two or more reproductively isolated species in the absence of any geographical barriers.
Sympatry: Coexistence in the same place.
Synapsis: The lining up of homologous chromosomes in meiosis.
Synaptic plasticity: A process whereby synapses, the connections between two neurons, get strengthened by experience or practice.
Sync: Synchronization.  To harmonize. To match, to occur at the same time.
Syngamy: The union of two genomes, which leads to doubling the ploidy level.
Synonymous mutation: A point mutation in a protein-coding region that changes a codon such that it does not alter the resulting amino acid sequence of the protein.
Systematics: Taxonomy.