Understanding Evolution: How Natural Selection Shapes Life

Evolution

A change in the heritable traits of a population over many generations.

Population

A group of individuals of the same species living in the same area at the same time.

Species

A group of organisms that can interbreed in nature and produce fertile offspring (working definition for this course).

Trait

Any observable feature of an organism, such as eye color, height, or beak shape.

Heritable trait

A trait that can be passed from parents to offspring through genes (DNA).

Variation

Differences in traits among individuals in a population.

Evolution (biological)

A change in the **genetic composition** or trait frequencies of a **population** over **generations**.

Natural selection

A process in which **heritable variation** in traits, combined with **differences in survival and reproduction** in a particular environment, leads to some traits becoming more common in a population over time.

Variation

Differences among individuals in a population in traits such as morphology, behavior, or physiology; some of this variation is **heritable**.

Fitness (evolutionary)

An individual's **relative reproductive success**: how many **viable, fertile offspring** it leaves compared with others in the same population and environment.

Adaptation

A **heritable trait** that increases an organism's **fitness in a specific environment**, and that became common in the population through **natural selection**.

Heritable trait

A trait that is influenced by genes and can be **passed from parents to offspring**, allowing natural selection to change its frequency over generations.

DNA

Deoxyribonucleic acid; the molecule that stores hereditary information in almost all living organisms. Its sequence of bases (A, T, C, G) encodes genetic instructions.

Gene

A specific segment of DNA that contains instructions for making a functional product (usually a protein), influencing one or more traits.

Allele

A different version of the same gene, with a slightly different DNA sequence that can produce variation in a trait.

Mutation

A change in the DNA sequence. Mutations create new alleles and are the ultimate source of new genetic variation.

Recombination

The process during sexual reproduction where paired chromosomes exchange segments, creating new combinations of alleles in offspring.

Heritable (Hereditary)

Describes a trait or variation that can be passed from parents to offspring through genes.

Fossil record

The ordered collection of fossils in rock layers, showing changes in life forms and the appearance and extinction of species over geological time.

Transitional fossil

A fossil that shows a mix of traits from older and newer groups in a lineage, illustrating step-by-step evolutionary change.

Homologous structures

Body parts that share a common evolutionary origin and underlying structure, even if they now have different functions (e.g., vertebrate forelimbs).

Analogous structures

Body parts with similar functions that evolved independently in different lineages, not from a common ancestral structure (e.g., bird wings and insect wings).

Convergent evolution

The independent evolution of similar traits in unrelated lineages due to similar environmental pressures or lifestyles.

Vestigial structure

A reduced or remnant structure that has lost much of its original function but remains as evidence of an organism’s evolutionary history (e.g., human tailbone).

Speciation

The process by which one ancestral species splits into two or more new species, usually after populations become reproductively isolated and diverge over time.

Reproductive isolation

A situation in which populations do not exchange genes (or do so very rarely) because of barriers such as geography, behavior, timing, or incompatibility, preventing successful interbreeding.

Common ancestor

An ancestral organism or population from which two or more descendant species evolved.

Phylogenetic tree

A branching diagram that represents hypotheses about evolutionary relationships among species or groups, based on evidence such as fossils, anatomy, and DNA.

Tree of Life

The overall phylogenetic tree that represents the evolutionary relationships among all living and extinct organisms.

Allopatric speciation

Speciation that occurs when populations are geographically separated and evolve independently until they become distinct species.

Natural selection

A process where individuals with traits better suited to their environment tend to survive and reproduce more, causing those traits to become more common over generations.

Selection pressure

Any factor in the environment that affects which individuals survive and reproduce, such as antibiotics, drought, predators, or pollution.

Antibiotic resistance

The ability of bacteria to survive and grow in the presence of an antibiotic dose that used to kill them, often due to specific genetic changes.

Pesticide resistance

An inherited ability of some insects (or other pests) to survive doses of a pesticide that would normally be lethal, leading to reduced effectiveness of the chemical.

Trait

Any observable characteristic of an organism, such as beak size, color, or enzyme structure, which can be influenced by genes and the environment.

Rapid evolution

Noticeable evolutionary change occurring over relatively few generations, often observed in organisms with short generation times or under strong selection pressures.

Common ancestor

An earlier species from which two or more later species evolved. Humans and chimpanzees share a common ancestor that is now extinct.

Primates

An order of mammals that includes lemurs, monkeys, apes, and humans, usually with forward-facing eyes, grasping hands, and relatively large brains.

Apes (Hominoids)

A group of primates that includes gibbons, orangutans, gorillas, chimpanzees, bonobos, and humans. Apes generally have no tails and larger brains for their body size.

Hominins

The group that includes modern humans and all species more closely related to us than to chimpanzees, such as Australopithecus and earlier Homo species.

Bipedalism

Walking on two legs as the main way of moving. A key feature of hominins, appearing in fossils millions of years before very large brains.

Neanderthals

An extinct human species (*Homo neanderthalensis*) that lived in Eurasia and interbred with modern humans. Many people today carry small amounts of Neanderthal DNA.

Modern evolutionary synthesis

The integrated framework that combines Darwin’s natural selection with Mendelian genetics and later population genetics, explaining evolution as changes in allele frequencies driven by selection, mutation, drift, and gene flow.

Allele

A different version of a gene. Evolution tracks changes in the frequencies of alleles in a population over generations.

Natural selection

The non-random process where individuals with heritable traits better suited to their environment tend to survive and reproduce more, causing those traits (and the alleles behind them) to become more common.

Genetic drift

Random changes in allele frequencies, especially strong in small populations, caused by chance events (like which individuals happen to reproduce or survive a disaster).

Mutation

A change in DNA sequence. Mutations introduce new genetic variation; they occur without regard to whether they are beneficial, harmful, or neutral.

Gene flow

The movement of alleles between populations, usually through migration and interbreeding.