Understanding Evolution Terminology
Part of the Evolution For Dummies Cheat Sheet
Evolution is the process by which populations and species change over time and the principles of evolution explain why life on Earth is so diverse and why organisms are the way they are. You need to understand evolution because it is the key scientific principle in biology (the study of living things), so study these evolution fundamentals:
Adaptations: Changes resulting from natural selection.
Allele (plural alleles): The specific DNA sequence found at a given locus in an individual. A haploid individual has one allele at every locus while a diploid individual has two alleles at each locus (one on each set of chromosomes), which can be the same or different.
Artificial selection: The process of selection when people control which characters are favored—for example selectively breeding cows that make the most milk to produce the next generation of dairy cows.
Chromosome: The cellular structures that contain DNA. Humans, a diploid organism, have 23 pairs of chromosomes.
Diploid genome: The genome of an organism that has of two sets of chromosomes. In sexually reproducing organisms, diploid parents each contribute one set of chromosomes to offspring, producing a new diploid individual whose genome is a combination of some of the DNA from each parent. Examples of diploid organisms include mammals, birds, many plants, and so on.
DNA (deoxyribonucleic acid): A long molecule made up of four different subunits (or nucleotides, which you can think of as a four-letter alphabet). The sequence of the four different nucleotides governs the specific details of traits. While almost all organisms have DNA as the genetic material, a few (some viruses) use a slightly different molecule (RNA, ribonucleic acid) but the process is otherwise the same.
DNA sequence: The exact arrangement of the four nucleotides in a specific individual. The sequence information can be for the entire genome or just some location of interest.
Evolution: A change in the percentage of inherited (heritable) traits in a group of organisms over time. For evolution, time is measured in generations, which is one of the reasons that bacteria evolve faster than elephants.
Evolutionary theory: The field of scientific investigation that works to understand what processes are responsible for the evolutionary changes we observe and what the consequences of those changes are.
Fitness: A measure of an organism's ability to contribute offspring to the next generation.
Gene: The classic unit of heredity that governs the traits that are passed from parent to offspring. The term predates an understanding of how the process of heredity actually works, which involves DNA. Therefore, in science articles, gene primarily serves as an easy-to-understand, if not exactly precise stand in for locus and allele, which more precisely identify the exact units of heredity.
Genetic drift: Random factors—volcanoes erupting, trees falling, or airplanes crashing, for example—that impact the gene frequency in subsequent populations.
Genome: The sum total of all of an organism's DNA.
Genotype: The specific combination of alleles that an individual organism has. Genotype does not map directly to phenotype (or physical traits) because of the effect of environmental factors.
Haploid genome: The genome of an organism with a single set of chromosomes. Examples of haploid organisms include bacteria and fungi which produce asexually (new individuals simply divide from existing ones). Note: Diploid individuals produce haploid gametes (sperm and egg).
Locus (plural loci ): A particular location in an organism's genome where the information for a particular trait resides. The eye color locus, for example, is the place in an individual organism's genome that has the DNA sequence controlling eye color.
Mutations: Changes in the DNA sequence caused by errors in DNA replication or such factors (like radiation) that can cause DNA damage.
Natural selection: The process of selection when the natural environment is the selective force.
Neutral evolution: Evolution as the result of genetic drift. When two different alleles are selectively neutral—that is, they don't differ in fitness—changes in their relative frequencies can only be caused by random events.
Phenotype: The physical traits that the organism has, including things like body structure, wing span, running speed, and so on. Phenotype is a product of both the genotype and the effects of the environment.
Selection: When a particular character is favored such that organisms that possess that character are more likely to contribute offspring to the next generation. If the character under selection is heritable, then the frequency of that character in future generations increases. Selection acts on phenotypes rather than genotypes.
Speciation: When a group of individuals in a species evolves differently from the rest of the species, leading to the accumulation of enough genetic differences to prevent the two groups from interbreeding.