Evolution Terms

Biogeography

The geographic distribution of living organisms

Common Ancestry

The concept that if you trace back the lineages of living species far enough in time, those species will converge to a shared ancestor

Evolution

The change in frequency of genetic variants in a population

Homology

Traits whose similarities are explained by common ancestry

Natural Selection

The tendency for genetic variants that enhance fitness to go to fixation and variants that reduce fitness to be lost from populations (survival of the fittest)

Nested Hierarchy

A pattern of groups nested within groups (without overlaps) as seen in taxonomies and classification systems

Taxonomies

Systems for classifying organisms based on shared characteristics and evolutionary relationships.

Polymorphism

The existence of multiple variants within a population

Transitional Fossil

Fossil taxa that have some, but not all, of the derived traits of a living group

Allele Frequency

The proportion of all alleles at a locus that are of a particular type

Allele

A variant of a gene

Diploid

Having two sets of chromosomes

Evolution

Change in the genetic composition of a population over time (change in allele frequencies)

Fixed

An allele that has a frequency of 1.0 in a population

Genotype

Genetic makeup of an organism

Haploid

The trait of having one set of chromosomes in an organism

Hardy Weinberg Law

The law used to predict the genotype frequencies in a population for the next generation, based on the allele frequencies in the current generation

Locus

A place in the genome where alleles reside; in diploids, each individual has two alleles per locus

Phenotype

The physical and behavioral characteristics of an organism that result from the interaction of the organism’s genotype and its environment

Polymorphic

When more than one allele is found at a given locus in a population

Populations

A group of inbreeding organisms

Selfing

When an organism reproduces with itself, providing both the egg and sperm components

Beneficial mutation

A new allele that ENHANCES the fitness of an organism

Deleterious mutation

A new allele that DECREASES the fitness of an organism

Directional selection

Selection that arises when one allele consistently raises fitness, eventually leading to fixation of that beneficial allele

Mutation

A change in a gene sequence

Neutral mutation

A new allele with neither a beneficial nor a deleterious effect

Relative fitness

The fitness of a given genotype divided by the fitness of a reference genotype that is assigned to be 1.0

Genetic bottleneck

The phenomenon in which a population lineage shrinks to a small size for a period of time, causing that population to lose genetic variation

Genetic drift

RANDOM changes in allele frequencies in a population over time

Genetic load

The frequency of deleterious alleles that have accumulated in a population

Fitness

The expected reproductive output of a genotype

Genetic Disorders

Diseases caused by changes at a single genetic locus that reduce fitness

Branch

The lines that make up a tree diagram that represent population lineages

Clade

All the descendants of a single ancestral lineage

Lineage splitting

A splitting of a population into genetically separate populations that no longer have gene flow

Node

Branching points of tree diagrams that represent lineage splitting

Phylogenic tree

A branching diagram used to represent evolutionary relationships and relatedness between different organisms based on their common ancestry

Pruning

Removing tips/taxa or clades from a tree without changing the topology

Root

The base of the tree that represents the common ancestral lineage of all taxa in the tree

Speciation

Lineage splitting that leads to taxa being classified as separate species

Taxon (plural = taxa)

A named group of biological organisms at the tips of a tree

Tree thinking

The ability to use the metaphor of a phylogenetic tree to convey accurate evolutionary information

Tree topology

A list of all the clades that a tree contains

Convergent Evolution

The phenomenon observed in phylogenetic trees where the same trait evolves separately in more than one lineage (not homologous)

Homologous

A trait shared in separate species that was inherited from the common ancestor

Principle of parsimony

The idea that if you have multiple alternative hypotheses, the most likely hypothesis is the one that makes the fewest assumptions. In the case of phylogeny, parsimony favors the scenario that invokes the fewest evolutionary changes

Reversal

The phenomenon in which an ancestral trait was lost and then re-evolved along a lineage

Separate ancestry

The alternative hypothesis to common ancestry, which proposes that each living taxon has an independent origin

Allopatric speciation

Speciation driven by geographic isolation

Assortative mating

The phenomenon in which individual organisms tend to mate with other organisms with trait values like theirs

Biological Species Concept

The view that species are defined by the ability of their members to reproduce with one another and to be unable to reproduce with other species

Clinal variation

Gradual changes in traits as a function of geographical separation

Discrete variation

Genetic variation among geographically separated populations, where each population contains genetically similar individuals

Extrinsic reproductive isolation

The phenomenon in which two organisms are unable to reproduce due to geographic separation

Intrinsic reproductive isolation

The phenomenon in which two organisms are no longer able to reproduce, even when they are not isolated

Phylogenetic Species Concept

The view that species are clades that biologists have chosen to assign to the species rank for practical reasons

Speciation

The splitting of an ancestral species into descendant species

Sympatric speciation

Speciation without geographical separation, driven by assortative mating within a population

Continuous trait

A trait that is characterized by values on a continuous scale, rather than being controlled by a single locus

Disruptive selection

Selection that favors trait values at both extremes of the trait value distribution

Heritability (h²)

The fraction of the variation in a population that can be explained by genetics

Response to selection (r)

The amount the mean trait value in a population changes after one generation

Stabilizing selection

Selection that disfavors extreme trait values and favors trait values towards the center of the trait distribution

Standard deviation

Square root of the variance

Variance

A measure of the spread of a distribution of a trait values in a population

Altruistic behavior

Actions that result in the organism exhibiting the behavior to lower its own fitness while increasing the fitness of other organisms within its population

Exaggerated secondary sexual characteristics

Dramatic traits in an organism that lower the organism’s viability but evolve because they are favored by sexual selection

Exaptation

The phenomenon in which a trait that evolved for one function is currently used for a different function

Polygamy

The phenomenon in which one male can mate with many females

Runaway sexual selection

The phenomenon in which secondary sexual characteristics become exaggerated due to feedback between male traits and female preferences

Anoxygenic photosynthesis

The reduction of carbon dioxide to organic molecules using light energy in which something other than water is the electron donor and oxygen gas is not released

Autogenous hypothesis for eukaryotes

The theory that mitochondria and nucleus both evolved within the same eukaryotic lineage

Cyanobacteria

A clade of bacteria characterized by its ability to perform oxygenic photosynthesis

Endosymbiosis

The phenomenon in which a prokaryotic cell comes to live and divide within a host cell

Endosymbiotic hypothesis for eukaryotes

The theory that mitochondria are derived from endosymbiotic bacteria taken up by the host cell whose genome is found in the eukaryotic nucleus

Eukaryotic cells

A category of cells characterized by an outer membrane and many internal membrane-bound organelles

Inside-out model

A theory for the development of the endomembrane system in eukaryotes that suggests that the outer plasma membrane of a prokaryotic ancestor was pushed outward and ultimately fused to create the cytoplasm and plasma membrane of eukaryotes

Last universal common ancestor (LUCA)

The most recent common ancestor of all known life

Organelle

Membrane-bound compartment in eukaryotic cells

Outside-in model

A theory for the development of the endomembrane system that starts with the production of vesicles within the cytoplasm by internalization

Oxygenic photosynthesis

The reduction of carbon dioxide to organic molecules using light energy in which water is the electron donor and oxygen gas is released

Prokaryotic cells

A category of cells characterized by an outer membrane containing no internal membrane structures

Adaptive landscapes

Visualizations of how selection acts on lineages over time

Adaptive radiations

Rapid expansions of clades to give rise to many diverse species adapted to different ecological niches

Amniotes

A clade of tetrapods characterized by an egg that can persist on dry land, complex lungs, and protective skin

Arthropods

A clade characterized by exoskeletons

Baleen

Specialized filter-feeding structures found in whales

Bilateral symmetry

The trait of having a single plan of symmetry

Cambrian period

The first period of the Paleozoic era; a time characterized by rapid radiation of bilaterians

Evolutionary arms race

A cycle of measures and countermeasures between interacting lineages

Key innovation

A trait that allows a lineage to occupy a new niche

Opisthokont clade

A clade of eukaryotes characterized by a flagellum

Pre-adaptations

Traits in a lineage that existed prior to a transition that give lineage some advantage in a new ecological niche

Radiations

Rapid expansion of clades to give rise to many species

Red Queen Principle

The idea that populations must keep adapting via directional selection

Tetrapods

A clade of vertebrates characterized by four limbs and lungs

Admixture

The mating of two individuals from genetically distinct groups

Arboreal

Tree-dwelling

Diurnal

The characteristic of being active during the day