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“AD-ap-TAY-shun”
a feature produced by natural selection for its current function.
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“uh-DAP-tiv LAND-skayp”
graph that relates all the potential combinations of gene frequencies a population might have to the average fitness of the population. Peaks on the landscape are gene frequencies for which the population would have high fitnesses, and valleys correspond to frequencies with low fitnesses. The highest peak on the landscape is called the global fitness peak, and other, lower peaks are called local fitness peaks. Click here to read more about it (link to shifting balance).
“uh-DAP-tiv RAY-dee-AY-shun”
an event in which a lineage rapidly diversifies with the newly formed lineages evolving different adaptations.
Biologist who, along with Charles Sibley, used DNA-DNA hybridization to help untangle the relationships between different species of birds, and then applied the same technique to the great apes. Although some of their methodology was contested, their work suggested that humans and chimpanzees are each other’s closest relatives. Ahlquist and Sibley’s work on birds has fundamentally changed bird classification and our understanding of bird evolution.
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“uh-LEEL”
one of the versions of a gene that may exist at a locus. For example, the pea color locus may have either the yellow allele or the green allele. Different alleles of the same locus are often symbolized by capital and lowercase letters (e.g., the Y and y alleles).
“AL-oh-MEH-trik” growth
when some part of the organism grows at a rate different from the rest of the organism during development. For example, the neck vertebrae of fetal giraffes must grow at a faster rate than the rest of the body (in comparison to giraffe’s short-necked relatives).
“AL-oh-PA-trik SPEE-see-AY-shun”
speciation that depends on an external barrier to gene flow (such as geographic isolation) to begin or complete the process of speciation.
“uh-MEE-noh” acid
a building block of proteins. There are about 20 amino acids and protein-coding DNA tells the cellular machinery which amino acids to use to build a particular protein. See also protein.
“uh-NAL-uh-gus”
similar because of convergent evolution, and not because of common ancestry. Two characters are analogous if the two lineages evolved them independently. See also homologous, homoplasious.
“AN-throh-poh-SEN-trik”
centering on humans and considering all other things in relation to humans.
Greek philosopher and scientist who wrote on an encyclopedic range of topics. He recorded many accurate and important observations about the natural world, influencing later scholars, such as Galen. In relation to life and the history of Earth, Aristotle believed that the Earth was the center of the universe, that life could be spontaneously generated, and that life was organized into a ladder-like hierarchy, the scala naturae.
a pattern of co-evolution (usually in a predator/prey system) in which ever more extreme defensive and offensive abilities evolve. For example, a newt species evolves toxicity that defends it against predation. A snake species evolves resistance to the toxin and so can eat the newts. Because of this, the newt experiences selection and evolves greater toxicity. Because of that, the snake experiences selection and evolves greater resistance...and so on, until both the newt’s toxicity and the snake’s resistance are extreme.
“ARE-teh-FISH-ul seh-LEK-shun”
a process in which humans consciously select for or against particular features in organisms. For example, the human may allow only organisms with the desired feature to reproduce or may provide more resources to the organisms with the desired feature. This process causes evolutionary change in the organism and is analogous to natural selection, only with humans, not nature, doing the selecting.
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“ek-STINK-shun”
the extinction that lineages have “normally” experienced throughout life’s history. Even when the Earth is not experiencing a major catastrophe, lineages are constantly going extinct—and this is background extinction. Brief periods (often associated with major geologic or climactic changes) in which extinction rates have been elevated across many lineages are called mass extinctions and are not a part of background extinction.
the information coding part of DNA, the letters of the genetic code. The sequence of bases on a stretch of DNA (i.e., the sequence of As, Ts, Gs, and Cs) determines what the DNA does—if it codes for a protein, turns on a gene, or whatever. In protein-coding regions, three base pairs code for a single amino acid. For example, the base pair sequence ATG codes for the amino acid methionine. In a strand of DNA, bases are paired and are lined up across from one another: A pairs with T and G pairs with C.
Set of chemical reactions that occur within or are associated with living things.
“BY-oh-dy-VUR-sit-ee”
the variety and variability among organisms inhabiting a particular region. However, the term may be more specifically defined and measured in different ways. For example, sometimes biodiversity is used to refer to the number of species in a particular area, sometimes to the number of different ecological niches occupied by organisms in a particular area, and sometimes to the amount of genetic divergence that the organisms in a particular area have experienced.
“BY-oh-LODGE-eh-kul SPEE-seez” concept
the definition of a species that is most typically used, especially for communication to the general public. See species for this definition.
'; } else if (Word == 'Brongniart') { WordDef = 'Alexandre Brongniart (1770-1847)
French geologist and student of Cuvier who, along with his mentor, was one of the first to identify and cross-reference geologic strata using fossils, a methodological innovation credited to William Smith. Brongniart and Cuvier identified the same fossil layers all across the Paris region and showed that the regional fossil fauna had alternated between marine and freshwater forms over geologic time.
English geologist and teacher of Lyell. Buckland is known for his attempts to reconcile religion and geology and for being among the first to identify dinosaur fossils. As a natural theologist, he believed that new life forms were continually created. He also believed that the Earth had been shaped by a series of catastrophes and tried to find evidence that a worldwide flood—Noah’s biblical flood—was the most recent of these.
French naturalist whose views about the history of life and Earth foreshadowed an evolutionary perspective. Buffon argued that the Earth was much older than previous scientists believed and that, to a limited degree, new species could evolve. He did not believe that evolution was responsible for entirely new body plans (like evolving a dog from a cat ancestor), but he did believe that, for example, a cat ancestor could evolve into different cat species according to their habitats.
Scottish author who, before Darwin but after Lamarck, wrote about evolutionary change. He argued that “higher” forms had evolved from “lower” forms, over the course of Earth’s history.
a recognizable feature of an organism. Characters may be morphological, behavioral, physiological, or molecular. They are used to reconstruct phylogenies. See also character state.
one of the possible variants a character might have. For example, the character “wing color” might have the character states “green” or “brown.” A molecular character (a particular position in an organism’s DNA) might have the character states A, T, G, or C. See also character.
“klayd”
a group of organisms that includes all the descendents of a common ancestor and that ancestor. For example, birds, dinosaurs, crocodiles and their extinct relatives form a clade. This is another word for a monophyletic group.
“KO-don”
a sequence of three bases in a stretch of DNA or RNA that code for an amino acid or terminate a protein.
“ko-EH-voh-LOO-shun”
a process in which two or more different species reciprocally effect each other’s evolution. For example, species A evolves, which causes species B to evolve, which causes species A to evolve, which causes species B to evolve, etc.
ancestral lineage shared by two or more descendent lineages — in other words, an ancestor that they have in common. For example, the common ancestors of two biological siblings include their parents and grandparents; the common ancestors of a coyote and a wolf include the first canine lineage and the first mammal lineage.
“kun-VER-jent EH-voh-LOO-shun”
the independent evolution of similar structures in species, often due to similar lifestyles and selection pressures.
“ko-SPEE-see-AY-shun”
a case of lineage splitting in which two or more lineages split in parallel with one another as a result of their close ecological association. If the association is very close, cospeciation may produce identical patterns of branching in the phylogenies of the two clades.
English molecular biologist who, along with James Watson, unraveled the structure of DNA, a discovery which paved the way for a genetic revolution in biology. For this work, he jointly received the Nobel Prize in Physiology or Medicine in 1962. More recently, Crick has studied mammal vision and human consciousness.
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“KUR-en-see”
current unit of exchange. Here, the term refers to a construct used to place values on ecosystems and their biodiversity.
influential French anatomist and paleontologist. He rejected classifying animals on a scale according to the “Great Chain of Being,” and instead divided them into several categories, with living and fossil organisms classified in the same system. Although an anti-evolutionist, Cuvier went against popular opinion of the day and established that many species had gone extinct. He argued that periodic catastrophes caused these extinctions and that new life forms were created after each catastrophe.
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Italian scholar and artist who studied a vast range of disciplines, including physics, math, geology, cosmology, anatomy, botany, and engineering. Although not generally considered a central figure in the history of evolutionary biology, Leonardo made some evolutionarily relevant observations, arguing—as did Steno—that fossils were the remains of living creatures and that the Earth must be much older than the few thousand years some literal interpretations of Bible suggested.
English naturalist who, along with Alfred Russel Wallace, originated the theory of evolution by natural selection. Darwin’s theory, carefully argued in The Origin of Species by Means of Natural Selection (1859), was both attacked and defended after its publication but is now accepted as one of the keystones of biology. Beyond natural selection, Darwin studied and wrote about a number of other topics including human evolution, sexual selection, coral reefs, volcanic islands, orchids, barnacles, and plant movement.
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“DEL-uh-TEER-ee-us uh-LEEL”
a version of a gene that, on average, decreases the fitness of the organism carrying it.
“DEL-uh-TEER-ee-us jeen”
'; } else if (Word == 'Deletion') { WordDef = 'Deletion
a mutation in which at least one base pair is removed from a DNA sequence.
the process through which one generation produces the next involving the inheritance of genetic material. One generation descends from the previous generation. Similarly, a lineage descends from some ancestral lineage in the sense that the lineage existing today is the genetic offspring of the ancestor (though you may have to go back many generations to see the relationship).
change in an organism over the course of its lifetime, the processes through which a zygote becomes an adult organism and eventually dies.
a tendency for evolutionary change to happen in some direction because the development of the lineage makes some phenotypes less likely to arise than others.
Dutch botanist famous for his contributions to genetics. He rediscovered the results first obtained by Mendel and described genetic changes in his plants. Based on his observations, DeVries argued that individual mutations had wide-ranging effects and could cause speciation in a single step; however, T. H. Morgan later discovered that many mutations seemed to have rather small effects. DeVries, it turns out, had observed changes in chromosome number, not the minor change in base pair sequence that are typical of mutation.
directed “myoo-TAY-shun”
the hypothesis that mutations that are useful under particular circumstances are more likely to happen if the organism is actually in those circumstances. In other words, the idea that mutation is directed by what the organism needs. There is little evidence to support this hypothesis.
deoxyribonucleic acid, the molecule that carries genetic information from generation to generation. More information is available from NIH’s Genetic Primer.
Ukrainian-American geneticist whose studies of fruit fly genetics showed—contrary to popular scientific opinion of the time—that populations were far from genetically uniform. Dobzhansky also worked out how reproductive isolation might transform a genetically variable population into two separate species. Dobzhansky is credited with the incisive observation that “Nothing in biology makes sense except in the light of evolution.”
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American microbiologist and biochemist who studies early cellular evolution using genetics. His lab was one of the first to discover that chloroplast DNA is relatively unrelated to the DNA in the nucleus of the same plant cell, a finding that supports Margulis’ endosymbiosis hypothesis.
Dutch paleontologist who traveled to Indonesia searching for hominid fossils to show that humans were related to other apes. He discovered Pithecanthropus erectus fossils, which he argued represented a “missing link” between apes and humans. Dubois’ argument was ridiculed by his contemporaries, although it was later widely accepted that P. erectus was indeed a relative of Homo sapiens.
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“en-DEM-ik”
organism native to a particular, restricted area and found only in that place.
“ep-ih-THEEL-ee-um”
a layer of tissue covering an organism’s internal or external surfaces.
“EH-voh-LOO-shun”
Evolution, simply put, is descent with modification. This definition encompasses small-scale evolution (changes in gene frequency in a population from one generation to the next) and large-scale evolution (the descent of different species from a common ancestor over many generations).
directional change within a single lineage or parallel change across lineages.
“EK-sap-TAY-shun”
a feature that performs a function but that did not arise through natural selection for its current use.
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a demonstration that something is indeed possible or does indeed exist.
“ek-STANT”
not extinct, existing.
“ek-STINK-shun”
event in which the last members of a lineage, clade, or species die. A single species may go extinct when all members of that species die, or an entire clade may go extinct when all the species that comprise that clade go extinct.
a genotype’s success at reproducing (the more offspring the genotype leaves, the higher its fitness). Fitness describes how good a particular genotype is at leaving offspring in the next generation relative to other genotypes. Experiments and observations can allow researchers to estimate a genotype’s fitness, assigning it a numerical value.
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English statistician and geneticist. He developed basic statistical techniques that are now critical in medical and biological experiments. With regard to evolution, Fisher’s mathematical analyses showed how Mendel’s laws about discrete characters (like wrinkled or smooth peas) could be squared with the inheritance of continuous characters (like height). This reconciliation helped make clear that the theory of evolution by natural selection is actually supported, not contradicted, by our understanding of genetics.
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Any trace of a living creature (body, part of body, burrow, footprint, etc.) preserved over geologic time.
changes in gene frequencies that usually accompany starting a new population from a small number of individuals. The newly founded population is likely to have quite different gene frequencies than the source population because of sampling error (i.e., genetic drift). The newly founded population is also likely to have a less genetic variation than the source population.
French physicist and mathematician, most famous for creating the mathematical tools to study how heat flows through solids. His studies of heat led him to argue that Earth’s history had a direction, beginning warm and cooling through time—an idea at odds with Lyell’s view of Earth’s history as one of constant, but directionless, change.
a type of mutation that causes the cellular machinery to parse the DNA code differently and produce a different protein. Each sequence of three base pairs codes for an amino acid. If, for example, you add a single base pair in the middle of the sequence, it changes where each subsequent set of three base pairs begins and ends—and hence, changes the amino acid sequence produced. Frameshifts are caused by insertions and deletions where the number of base pairs added or removed is not a multiple of three.
Roman physician and the doctor to several emperors. He made detailed studies of anatomy and physiology, performing animal dissections and using those observations to help explain how the body operates. Despite its errors, his work was considered the authority on anatomy for centuries and was dutifully studied by later scholars, including Vesalius.
“jeen”
the unit of heredity. Generally, it means a region of DNA with a particular phenotypic effect. Technically, it may mean a stretch of DNA that includes a transcribed and regulatory region. See also locus.
the movement of genes between populations. This may happen through the migration of organisms or the movement of gametes (such as pollen blown to a new location).
(also called allele frequency) proportion of genes/alleles in a population that are of a particular type. For example, at a particular locus, pea plants may have either a “yellow pea” allele or a “green pea” allele—so a population of pea plants would have some frequency of yellow pea alleles ranging from zero to one (100%).
all of the genes in a population. Any genes that could wind up in the same individual through sexual reproduction are in the same gene pool.
“jeh-NEH-tik deh-VUR-jents”
the amount by which particular DNA sequences differ from one another or from an ancestral sequence. For example, the hemoglobin genes of dogs and cats have diverged genetically since the time of their common ancestor.
“jeh-NEH-tik” drift
random changes in the gene frequencies of a population from generation to generation. This happens as a result of sampling error—some genotypes just happen to reproduce more than other genotypes, not because they are “better,” but just because they got lucky. This process causes gene frequencies in a population to drift around over time. Some genes may even “drift out” of a population (i.e., just by chance, some gene may reach a frequency of zero). In general, genetic drift has the effect of decreasing genetic variation within a population.
“jeh-NEH-tik” variation
loosely, a measure of the genetic differences there are within populations or species. For example, a population with many different alleles at a locus may be said to have a lot of genetic variation at that locus. Genetic variation is essential for natural selection to operate since natural selection can only increase or decrease frequency of alleles already in the population.
“JEE-nohm”
all the genetic information an organism carries. Read more in the National Center for Biotechnology Information Primer.
“JEE-no-type”
the set of genes an organism has. Sometimes, genotype refers to the entire genome of an organism and sometimes it refers to the alleles carried at a particular locus. See also phenotype.
germ line “myoo-TAY-shun”
a mutation that occurs in reproductive cells and ends up being carried by gametes (e.g., eggs and sperm). See also somatic mutations.
American paleontologist best known for his theoretical contributions to evolutionary theory. Among other contributions, he jointly proposed the hypothesis of punctuated equilibrium, refocused biologists’ attention on the role of developmental changes in evolution, and encouraged biologists to look beyond natural selection in their search for evolutionary explanations. Gould was a speaker and activist against pseudo-scientific racism and the insinuation of creationist teaching into public schools. His award-winning science writing caught the attention of the public and helped popularize evolutionary biology.
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German naturalist and enthusiastic evolutionist, famous for his not-strictly-true “Biogenetic Law.” The Law proposed that, as an organism develops, it inevitably goes through stages corresponding to its evolutionary ancestors. Haeckal also performed important research in invertebrate zoology and was the first scientist to try to reconstruct the phylogeny of all animals. Haeckal believed in Social Darwinism and the evolutionary superiority of his own race.
British-Indian biologist and population geneticist whose work helped show that evolution by natural selection is compatible with our understanding of genetics. Haldane also helped map the X chromosome and made the first estimates of human mutation rates. A proclaimed communist and atheist, Haldane wrote about the social responsibilities of scientists and adopted Indian nationality in 1957.
the amount of time it takes for half of the unstable atoms in a sample to decay into a different material. For example, uranium 238 (atoms with 146 neutrons and 92 protons) has a half-life of about 4.5 billion years—meaning that half of the uranium 238 atoms in a sample will decay into thorium 234 (atoms with 144 neutrons and 90 protons) in 4.5 billion years. The process of half-life decay allows scientists to date material containing unstable atoms.
English physician most famous for discovering how blood circulates through the body and that the heart acts as a pump. His views on circulation were ridiculed when made public but now are known to be correct.
“HET-eh-roh-KROH-nee”
an evolutionary change in the timing of a developmental event. For example, relative to the lineage’s ancestor, the early maturation of sex organs is an example of heterochrony.
“HET-eh-roh-ZY-goht” advantage
a selective regime that favors the heterozygote over either homozygote. For example, the “A” locus would demonstrate heterozygote advantage if Aa individuals were more fit than AA or aa individuals.
“HOM-ih-nid”
here, humans and their extinct relatives (i.e., organisms on the “human side” of the human/chimpanzee lineage split). However, some scientists use the term hominid to refer to a larger group: humans, other great apes (gorillas, chimpanzees, bonobos, and orangutans), and their extinct relatives. However you decide to name the groups, the important thing is how all these species are related to one another and not exactly what we decide to call each lineage. A cladogram from the Tree of Life Web Project illustrates a phylogeny of hominids.
“huh-MAHL-uh-gus”
inherited from a common ancestor. Human eyes and mouse eyes are homologous because we each inherited them from our common ancestor that also had the same sort of eyes. Contrast this with homoplasious and analogous.
“HOH-moh-PLAY-zee-us”
similar but not because of inheritance from a common ancestor. Homoplasious characters may be explained by convergent evolution in two different organisms or character reversals.
English, “jack-of-all-trades” scientist and part of the scientific milieu during the time of William Harvey and Steno. Though perhaps most famous as a physicist, his contributions to biology were substantial—he was the first to observe life through a magnifying device, discovering cells and the documenting the amazing complexity of even tiny organisms.
“HOKS jeens”
genes active in the development of all animals that regulate other genes. Although they do different things in different organisms, they are very important in laying out the body plan of the organism. For example, they help organize the head to tail differentiation in both insects and vertebrates.
Scottish farmer and geologist. In his travels around Britain, he made observations which suggested to him that the geologic processes that shaped the ancient Earth could be seen operating all the time, an idea which would later form the basis of Lyell’s uniformitarianism. Hutton used his observations and hypothesis to argue that the Earth must be extremely old. English biologist, who made contributions to paleontology and comparative anatomy but is best known for defending Darwin’s evolutionary views from religious attack; he coined the term “agnostic” to describe his own religious views. Huxley was also a supporter of human evolution and took on opponents who argued that humans could not have evolved from apes.
“HY-brid-uh-ZAY-shun”
the production of offspring from different parental forms. For example, if two recognizably different species of plant fertilized one another and produced viable, fertile offspring, the process would be called hybridization.
“HY-droh-THERM-ul” vent
an opening in the ocean floor that hot water comes out of. Some scientists hypothesize that life on Earth first began near a hydrothermal vent.
“hy-POTH-eh-sis”
concept or idea that can potentially be falsified with evidence. If a statement can’t be falsified by experimental results, observation, or some other means, then it is not a scientific hypothesis.
mating between relatives. Technically, this is defined as a pattern of mating in which mates are more closely related than two individuals selected at random from the population.
the lowered fitness of offspring produced by the mating of relatives.
“in-SIP-ee-ent SPEE-seez”
a group of organisms that is about to become a separate species from other, related individuals.
“in-SUR-shun”
a mutation in which at least one base pair is added somewhere in a DNA sequence.
The intelligent design (ID) movement promotes the idea that many aspects of life are too complex to have evolved without the intervention of a supernatural being — the intelligent designer. Because it relies on supernatural explanations, ID is not science. To learn more, read the Understanding Evolution brief on the intelligent design movement.
a partially assembled adaptation. Complex adaptations evolve in a series of smaller steps and these steps along the history of an adaptation’s evolution are called intermediate forms.
“in-VY-uh-bul”
inherently no capable of surviving for any length of time. Inviable fertilized eggs are those that will never reach the stage of an adult organism because of inherent (perhaps genetic) problems with the egg itself.
“ih-RID-ee-um”
a rare element that is found in relatively high concentrations in asteroids.
DNA that doesn’t code for proteins. The term “junk DNA” is a bit of a misnomer since some of this non-coding DNA performs important functions like helping to turn genes on and off.
an imagined story that describes how a feature might have evolved by natural selection, without providing any relevant evidence. The “just so” story is a reference to Rudyard Kipling’s stories like How the Camel Got His Hump and How the Leopard Got His Spots.
British physicist who made important contributions to electromagnetism and thermodynamics, laying out the law of conservation of energy and creating the idea of absolute zero. Based on his studies of heat, he argued, in opposition to Darwin, that the Earth must be relatively young. This debate was eventually settled with Claire Patterson’s radiometric dating of its rocks, which suggested that the Earth was 4.5 billion years old.
key “AD-ap-TAY-shun”
an adaptation that allows an organism to exploit a new niche or resource.
Yale biologist who studies how developmental changes contribute to macroevolution. His results support the view, championed by Stephen Jay Gould, that changes in the timing of developmental events—when they start, when they stop, and how fast they happen—led to major morphological shifts over the course of evolution.
French naturalist, who proposed an early theory of evolution. His theory included the critical evolutionary idea that new species can evolve from other species over time. However, the mechanisms of his theory, which included the inheritance of acquired characteristics and an inherent drive to evolve towards complexity, have been largely rejected by modern science. Lamarck’s theory also denied that species could go extinct and argued that fossil species not alive today were simply the ancestors of modern species.
“LEE-thul”
a mutation that results in the early death of the organism, fetus, or zygote carrying it. Organisms with lethal mutations never reproduce and so, do not pass that mutation on to any offspring.
broad patterns in the way that a group of organisms go about making a living and getting offspring into the next generation. For example, organisms that put resources into making a few high-quality offspring and organisms that put resources into making many offspring, each with a lower probability of survival, are said to have different life history strategies.
“LIN-ee-edge” splitting
an event in which a single historical lineage gives rise to two or more descendant lineages. Every node on a phylogeny is a lineage-splitting event. See also speciation.
Swedish physician, naturalist, and botanist. He conceived a hierarchical classification system that he first applied to plants and then extended to include all life. This classification system and its Latin names were standards of biology for centuries. Biologists, however, have recently begun to modify the system to better reflect the evolutionary process, of which Linnaeus had no knowledge.
“lin-NAY-in” system of classification
the standard system of classification in which every organism is assigned a kingdom, phylum, class, order, family, genus, and species. This system groups organisms into ever smaller and smaller groups (like a series of boxes within boxes, called a nested hierarchy). See also phylogenetic classification, rank.
living “FAHS-ul”
an extant species that appears to be very similar to its ancient, fossil ancestor. Living fossils have experienced morphological stasis—that is, they have changed very little morphologically, although their DNA may have evolved substantially. The classic example of a living fossil is the coelacanth. Modern coelacanths appear to be extremely similar to their 80 million year old ancestors, known from the fossil record.
“LOH-kus”
the place in the DNA where a gene is located. For example, the pea color locus is the place in a pea plant’s DNA that determines what the color of the peas will be. The pea color locus may contain DNA that makes the peas yellow or DNA that makes the peas green—these are called the yellow and green alleles.
“LOO-see”
the name given to a particular female hominid (of the species Australopithecus afarensis) who lived in what is now Ethiopia about three million years ago. “Lucy” is famous because she left behind a very complete fossilized skeleton found in 1974.
Scottish geologist who studied the rock formations of Europe and the U.S. In the influential book, Principles of Geology, Lyell used his observations to bolster his theory of uniformitarianism, which argued that the processes that shaped the Earth have been slowly but continually operating over long periods of history and, in fact, can still be observed today. Although Lyell argued that the Earth must be very old, he accepted evolution only at the end of his career. Even then, he denied that humans evolved.
“MAK-roh-EV-uh-LOO-shun”
evolution above the species level. The adaptive radiation of a lineage into many different niches is an example of macroevolution. Since evolutionary change above the species level, means that populations and species must be evolving, macroevolutionary change entails microevolutionary change.
“MAK-roh-myoo-TAY-shun”
a single mutation of large phenotypic effect producing an individual morphologically distinct from its parent. Although many mutations certainly cause large phenotypic effects, it is not clear that these mutations are often adaptive. However, it is possible that single mutations that affect development have large phenotypic effects and have played an important role in the history of life.
English economist who made groundbreaking analyses of population growth. He showed that populations have the potential to grow at a pace that would outstrip their food supply. Both Darwin and Wallace cited this analysis as influential in their conceptualizations of natural selection.
American biologist who studies cellular evolution. Margulis provided evidence to support her unconventional hypothesis: that endosymbiosis has been a major force in cellular evolution and that, in the course of history, some cells engulfed others and formed symbiotic relationships so close that their evolutionary lineages merged. Her hypothesis has become the widely accepted explanation for several cellular organelles.
mass “ek-STINK-shun”
event in which many different lineages go extinct around the same time. Mass extinctions involved higher rates of extinction than the usual rate of background extinction that is going on all the time.
German-American zoologist who began his career studying birds in the Pacific Islands. He went on to make important theoretical advances about the process of speciation. Mayr proposed that geographic isolation plays a prominent role in species formation, especially when the isolated population is small and peripheral to the main population. Mayr also writes about the history and philosophy of biology.
Austrian botanist and monk. His experiments crossing pea plants suggested that genetic material is inherited in discrete units from each parent, and that these units are inherited independently of one another although it was later shown that only genes on separate chromosomes are inherited independently. Mendel’s work did not receive the scientific attention it deserved until his results were rediscovered decades later by Hugo DeVries and others.
“MY-croh-EV-uh-LOO-shun”
evolution within a species or population. An increase in a particular gene’s frequency in a population due to natural selection is an example of microevolution.
“MOD-jool”
a morphological unit that can be duplicated and further adapted, for example the body segments of arthropods.
“moh-LEK-yoo-lur”
in evolutionary biology, having to do with DNA sequences or the amino acid sequences of proteins.
“moh-LEK-yoo-lur” clock
the idea that changes in a stretch of DNA happen in clock-like way (e.g., on average, one change per 200,000 years). Some stretches of DNA do seem to evolve according to a molecular clock and others do not. The existence of molecular clocks let scientists estimate the dates of some past events by comparing DNA sequences of different organisms.
“MON-oh-fy-LEH-tik” group
a group consisting of all the descendents of an ancestor and that ancestor. See also clade.
American geneticist whose studies of fruit flies showed that genes are linked together on chromosomes, which sometimes “cross over” and exchange genetic material. This discovery helped his lab create the first chromosome map. Morgan was awarded the Nobel Prize in Physiology or Medicine in 1933.
“mor-FAHL-uh-jee”
the study of the form and structure of organisms. For example, comparing the shape of the femur in different grazing mammals is a morphological study.
“myoo-TAY-shun”
a change in a DNA sequence, usually occurring because of errors in replication or repair. Mutation is the ultimate source of genetic variation. Changes in the composition of genome due to recombination alone are not considered mutations since recombination alone just changes which genes are united in the same genome but does not alter the sequence of those genes.
“MYOO-choo-ul-IZM”
a species interaction in which both of the interacting species profit from the interaction.
A process in which some individuals have genetically-based traits that improve survival or reproduction and and thus have more offspring surviving to reproductive age than other individuals. Because the offspring also carry the genes for these traits, this process causes the genes for advantageous traits to become more common in populations and the genes for disadvantageous traits to become less common in populations.
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“NOO-trul” theory
the idea that most of the molecular variation within populations is not being selected for or against—it is just neutral variation “drifting” around. The neutral theory de-emphasizes the role of natural selection in explaining molecular variation and emphasizes the importance of mutation and genetic drift.
English physicist and mathematician, most famous for his contributions to optics, calculus, and mechanics. Newton’s laws of motion (F=ma, etc.) were used by Buffon to argue that the Earth could have naturally formed from pieces of matter that had broken off of the sun.
a point on a phylogeny where a single ancestral lineage breaks into two or more descendent lineages.
“NOO-klee-oh-tyde”
the building blocks of DNA. A chain of nucleotides forms DNA. Nucleotides are made of a sugar, a phosphate, and a base. See also base.
“on-TAHJ-uh-nee”
see development.
“or-GAN-ik MAHL-uh-KYOOL”
a molecule containing carbon generally associated with living things. Organic molecules make up living things and living things produce organic molecules.
mating between very distantly related individuals.
English anatomist and student of Cuvier. Owen reconstructed the skeletons of many extinct animals, even working on some of Darwin’s specimens. He was, nonetheless, an early opponent of Darwin, arguing that God created new species by modifying a basic anatomical idea—an “archetype.” Later he modified his own views to accept a kind of “divine” evolution. Owen is also known for overstating the differences between the human brain and those of other apes in his struggle to place humans on a kind of pedestal, apart from the rest of the animal kingdom.
“PEE-doh-MOR-foh-sis”
having some features of the ancestral juvenile stage, but being an adult (with a mature reproductive system). This word means “child form,” and a paedomorphic change is any evolutionary change in the development of an organism that generates an adult with a “child’s form.”
British natural philosopher who helped popularize the argument that since organisms are like intricate and well-designed machines, they must have been created by a designer. Although Paley was neither the first nor the last to make this argument, he is famous for linking it to the alluring and much-cited “watch” analogy. If you found a watch lying in the middle of field, it would suggest to you the existence of a watch designer somewhere. So too, Paley suggested, a “well-designed” organism, must imply the existence of an intelligent creator. Darwin and Wallace would later show that natural selection could also explain the existence of “well-designed” organisms.
“PAIR-uh-PA-trik SPEE-see-AY-shun”
speciation within a continuously distributed population where the organisms within this population are not mating randomly (and are probably more likely to mate with those near them) but still constitute a single gene pool.
“PAR-sih-MOH-nee”
principle stating that the simplest explanation accounting for the observations is the preferred explanation. When reconstructing the evolutionary relationships among lineages, the principle of parsimony implies that we should prefer the phylogeny that requires the fewest evolutionary changes.
American geologist who, in 1956, used radiometric dating of meteorites to estimate that the solar system and the Earth are 4.5 billion years old, an estimate that is still considered relatively accurate. After studying the radioactive decay of lead, Patterson crusaded against the release of lead into the environment. His research led to the regulation of leaded paint and gasoline.
“PEH-ree-PAH-trik SPEE-see-AY-shun”
a special version of allopatric speciation in which one of the incipient species has a small population size and, because of this small population size, is strongly affected by genetic drift which contributes to the speciation process.
“feh-NET-ik SPEE-seez” concept
a definition that states that a species is a set of organisms that are phenotypically similar (i.e., they all look alike) and that look different from other sets of organisms.
“FEE-noh-TYPE”
the physical features of an organism. Phenotype may refer to any aspect of an organism’s morphology, behavior, or physiology. An organism’s phenotype is affected by its genotype and by its environment.
“fy-LED-ik GRA-joo-wuh-LIZ-um&”
the slow and steady evolution of a lineage with or without lineage-splitting. If for example two sister taxa were evolving according to phyletic gradualism, they would slowly but steadily evolve in different directions. There are well-documented cases in which lineages evolve in this manner. Contrast this with punctuated equilibrium.
“FY-loh-jeh-NEH-tik KLASS-ih-fih-KAY-shun”
a system of classification that names groups of organisms according to their evolutionary history. Like Linnaean classification, phylogenetic classification produces a nested hierarchy where an organism is assigned a series of names that more and more specifically locate it within the hierarchy. However, unlike Linnaean classification, phylogenetic classification only names clades and does not assign ranks to hierarchical levels.
“FY-loh-jeh-NEH-tik SPEE-seez” concept
a definition that states that a species is the smallest set of organisms that share an ancestor and can be distinguished from other such sets. In other words, a species is the smallest “tip” that you can get on a phylogeny.
“FY-loh-jeh-NEH-tiks”
the field of biology that deals with the relationships between organisms. It includes the discovery of these relationships, and the study of the causes behind this pattern.
“fy-LAH-juh-nee”
the evolutionary relationships among organisms; the patterns of lineage branching produced by the true evolutionary history of the organisms being considered. Many of the phylogenies you encounter are the “family trees” of groups of closely related species, but we can also use a phylogeny to depict the relationships between all life forms.
“PLOY-dee”
the number of copies of each chromosome an organism carries. For example, humans are diploid (i.e., we have a ploidy of two) because we carry two copies of each chromosome.
“PAH-lee-TOH-mee”
a node on a phylogeny where more than two lineages descend from a single ancestral lineage. A polytomy may indicate either that we don’t know how the descendent lineages are related or that we think that the descendent lineages speciated simultaneously.
generally, a group of organisms living close to one another that interbreed with one another and do not breed with other similar groups; a gene pool. Depending on the organism, populations may occupy greater or smaller geographic regions.
an event in which a population’s size is greatly reduced. When this happens, genetic drift may have a substantial effect on the population. In other words, when the population size is radically reduced, gene frequencies in the population are likely to change just by random chance and many genes may be lost from the population, reducing the population’s genetic variation.
a molecule made of a string of amino acids. Proteins are coded for by DNA and are essential molecules for life. See also amino acid.
“PUNK-shoo-ay-tid EE-kwih-LIB-ree-um”
hypothesis that suggests that much of evolutionary history is characterized by stasis punctuated by relatively rapid character change as a lineage splits. Available fossil evidence has highlighted several lineages that seem to follow this pattern of evolution. Click here for these examples and a complete description of the processes hypothesized to underlie this pattern (link to punc eq descript).
A method of determining the date at which an igneous rock solidified based upon the rate of decay of radioactive atoms within the rock.
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unpredictable in some way. Mutations are “random” in the sense that the sort of mutation that occurs cannot generally be predicted based upon the needs of the organism. However, this does not imply that all mutations are equally likely to occur or that mutations happen without any physical cause. Indeed, some regions of the genome are more likely to sustain mutations than others, and various physical causes (e.g., radiation) are known to cause particular types of mutations.
a level of a hierarchy. For example, in Linnaean classification, all orders (whether they are orders of mammals, orders of insects, or orders of flowering plants) are of equivalent rank. However, note that there is no reason to think that these groups of different organisms are equivalent in any way except in the way they were named.
“RATCH-ut”
a tool with teeth on the rim of a wheel that allows movement in only one direction. In evolution, a ratchet is used as a metaphor for changes that can occur in only one direction—once things have moved in that direction, they can’t go back.
rate of “dih-VURS-ih-fih-KAY-shun”
rate at which a lineage generates new descendant lineages. For example, a single species might split into 6 separate lineages over 6 million years, so the average rate of diversification would be one new species origin per million years.
rate of “ek-STINK-shun”
rate at which species in a clade go extinct (e.g., one per million years). The balance between a lineage’s rate of extinction and diversification determine whether or not the entire lineage (i.e., all its members) will go extinct.
“REK-ug-NISH-un SPEE-seez” concept
a definition that states that a species is a set of organisms that can recognize each other as potential mates.
“REE-kahm-bih-NAY-shun”
a process in which pairs of chromosomes swap DNA with one another. This happens during gamete formation. A single parent cell (containing two sets of chromosomes) will form four daughter cells (with one complete set of chromosomes each). In the process of forming these daughter cells, recombination happens so that the chromosomes the daughter cells have are “mosaic,” composed of different pieces of the parent cells’ chromosomes. Recombination is important for evolution because it brings new combinations of genes together—a source of variation for natural selection to act upon.
red “kween hy-POTH-eh-sis”
a hypothesis that states that as a species evolves, it is not less likely to go extinct because, as it evolves, its competitors, prey, and predators also evolve, making its chance of avoiding extinction constant over time. Like the Red Queen and Alice, each species is “evolving as fast as it can” just to stay in the same place. This hypothesis specifically deals with co-evolution and species longevity; however, the analogy to the Red Queen and Alice is so vivid that many people use the term to describe any case of a co-evolutionary arms race, whether they are interested in species longevity or not.
“reh-FYOO-jee-uh”
places where organisms can escape environmental changes. Examples of refugia include places that were free of ice during glacial periods, natural areas that are free of disturbances caused by humans, and, in agriculture, fields without pesticides or genetically modified crops located near fields planted with pesticide-producing crops. In each of these cases, the area provides a refuge for wildlife from changes going on in surrounding regions.
“REG-yoo-luh-TOR-ee jeen”
a gene that controls when protein-coding genes are turned on or off.
ribonucleic acid, a molecule similar to DNA involved in carrying information and producing proteins in cells. Some viruses carry RNA as their genetic material instead of DNA.
“RUN-uh-WAY seh-LEK-shun”
a hypothesized evolutionary process in which sexual selection causes organisms to have features that decrease their chances of survival. The key idea in runaway selection is that this process runs out of control until, at some point in the selection process, the feature (perhaps an extra long tail) indicates nothing about the organism except its ability to get a mate (i.e., having an extra long tail is not correlated with good genes or parasite resistance or any other feature that might increase offspring’s chances of survival).
American biochemist and physical anthropologist. His and Allan Wilson’s work on immunological proteins in the 1960s suggested that humans, gorillas, and chimpanzees share a much more recent common ancestor than was commonly believed at the time. Sarich’s later work has focused on the evolutionary basis of variation in human populations.
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German naturalist who described the skull of the first-discovered human fossil in 1858. Schaaffhausen argued that the skull was essentially the remains of a relatively recent, modern human, while T.H. Huxley argued that the skull was old, different than modern humans, and evidence of human evolution.
English geologist who studied the fossils in different geologic strata and helped give the strata (and corresponding time periods) the names we use today—Cambrian, Devonian, etc. Although he accepted naturalistic explanations for geologic events and studied them using the biostratigraphic methods of William Smith, Sedgwick rejected Darwin’s naturalistic explanation for the origin of species and argued that God created new forms of life at the beginning each geologic period.
“SEH-greh-GAY-shun”
the process in which pairs of chromosomes separate and are shuttled off to different gametic daughter cells. When gametes are formed, a single parent cell (containing two sets of chromosomes) will form four daughter cells (with one complete set of chromosomes each). In the process, the paired chromosomes of the parent cell separate into different daughter cells. This process is segregation. See also recombination.
“SEK-shoo-ul seh-LEK-shun”
selection acting on an organism’s ability to obtain or successfully copulate with a mate. This process may produce traits that seem to decrease an organism’s chance of survival, while increasing its chances of mating.
a model which suggests that genetic drift may play an important role in helping populations reach their maximum fitness. Click here for a description of how the model works (link to shifting balance).
crystals with a pattern of fracturing that can be caused by the intense pressure and heat of events such as asteroid impacts.
American ornithologist and pioneer in using molecular techniques to study evolutionary relationships. He and Jon Ahlquist used DNA-DNA hybridization to help untangle the relationships between different species of birds, and then applied the same technique to the great apes. Although some of their methodology was contested, their work suggested that humans and chimpanzees are each other’s closest relatives. Sibley studied bird relationships throughout his career and his work has fundamentally changed bird classification and our understanding of bird evolution.
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“SIK-ul SEL uh-NEE-mee-uh”
a genetically caused disease that generally results in the death of the person with it unless medical interventions are available. Sickle cell anemia is a popular topic for biology courses because it is one the few, well-worked out examples of heterozygote advantage that we have. People carrying two copies of the sickle cell allele have the disease, people with no copies of the sickle cell allele are normal, but people carrying just one copy of the sickle cell allele are resistant to malaria (thought they may occasionally have symptoms of sickles cell). So, if you live in a region where malaria is common, you are at an advantage if you are a heterozygote (i.e., if you carry one sickle cell allele and one normal allele).
“SY-lent myoo-TAY-shun”
a mutation that does not change the amino acid sequence of the protein product.
sister “TAK-suh”
two clades that are each others’ closest relatives. If you look at any node on a phylogeny, the two clades that immediately descend from that ancestor are sister taxa. Sister taxa are important for phylogenetic studies because you can make comparisons between them—if two taxa are sister taxa, they have had the same amount of time to evolve. Click here for an example of how sister taxa comparisons are used to study evolutionary history (link to beetle radiation).
English civil engineer and geologist. He observed stratified rock layers during a construction project and went on to create the first geological map—one that included all of England. In the process of creating the map, he became one of the first scientists to identify different rock strata using fossils.
“soh-MAT-ik myoo-TAY-shunz”
mutations occurring in cells that do not form gametes, mutations that do not end up being carried by eggs or sperm. For example, mutations in your skin, muscle, or liver tissue are somatic mutations. See also germ line mutation.
“SPEE-see-AY-shun”
the process by which species form. This involves the reproductive isolation of different parts of an ancestral species so that they form distinct descendent species.
“SPEE-seez”
members of populations that actually or potentially interbreed (the biological species concept). In this sense, a species is the largest gene pool possible under natural conditions. See also recognition species concept, phenetic species concept, and phylogenetic species concept for other definitions of species.
Danish physician and naturalist who made scientific contributions in several fields, including anatomy, geology, and crystallography. With regard to evolution, Steno was among the first to cogently argue that the sediment layers one might observe in an exposed rock face were slowly laid down in former seas, and that the fossils contained in the strata were snapshots of life at those moments in history.
French naturalist who, before Darwin but after Lamarck, wrote about evolutionary change. He documented basic similarities among different species such as the similar structure of fish fins and land-dwelling vertebrates. These similarities supported his belief that new species could evolve from other species. His views were contested by Cuvier, who argued against evolution.
lack of substantial evolutionary change. If a lineage did not change much, diversify, or experience extinction for a long period of time, you would say that it experienced stasis. The hypothesis of punctuated equilibrium predicts that lineage will experience long periods of stasis punctuated by short periods of speciation and evolutionary change.
“stra-TIG-ruh-fee”
the study of rock layers. Stratigraphic analyses allow scientists to date rock layers relative to one another—in general, lower rock layers were deposited before higher rock layers.
“sim-PA-trik SPEE-see-AY-shun”
speciation within a randomly mating population. This is generally thought to be a rare mode of speciation because random mating tends to break down genetic differences between species.
“SUB-stuh-TOO-shun”
a mutation in which a single base is exchanged for another one.
“TAKS-on”
(pl. taxa) a named group of organisms, such as Mammalia or Hominidae. In phylogenetic classification, all taxa are clades—that is, they are named, monophyletic groups of organisms.
“TET-ruh-pawd”
the animal clade containing vertebrates with sturdy legs (as opposed to fins)
“tran-ZISH-un-ul” forms
fossils or organisms that show the transformation from an ancestral form to descendant species’ form. For example, there is a well-documented fossil record of transitional forms for the evolution of whales from their amphibious ancestor.
Flemish anatomist whose work set new standards of clarity and accuracy in anatomy. He was the first scientist to describe several organs and was one of the first to dissect human cadavers. He is also known for discovering flaws in the anatomical descriptions of Galen, which had been accepted as indisputable for centuries.
“veh-STIH-jee-ul” structure
a feature that an organism inherited from its ancestor but that is now functionless and usually less elaborate than in the ancestor. Usually, vestigial structures are formed when a lineage experiences a different set of selective pressures than its ancestors, and selection to maintain the elaboration and function of the feature ends.
Estonian-born German naturalist and pioneer in the field of developmental biology. He showed that mammal reproduction requires the presence of an egg. Although not an evolutionist, his observation that different species develop similarly early on and differentiate at later stages of development was used by Darwin to support the theory of evolution.
Welsh naturalist who, along with Charles Darwin, originated the theory of evolution by natural selection. Unlike Darwin, Wallace never fully accepted human evolution. Through his travels, Wallace was the first to describe many tropical species and may be considered the father of biogeography for his work linking geological changes to species distributions.
American biologist and biochemist who, along with Francis Crick, unraveled the structure of DNA, a discovery which paved the way for a genetic revolution in biology. For this work, he jointly received the Nobel Prize in Physiology or Medicine in 1962. More recently, Watson has headed initiatives to study how viruses cause cancer and has directed the Human Genome Project.
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German meteorologist and geophysicist who, in 1915, hypothesized that the super-continent, Pangaea, existed and then broke apart as the continents drifted to their current locations. Wegener supported his hypothesis with evidence from geology and paleontology, but since no mechanism for continental movement was known at the time, his hypothesis was controversial until the 1960’s when scientists mapped the ocean floor. Their observations suggested that continents are positioned on tectonic plates that slide past one another on the Earth’s surface.
New Zealand-born molecular biologist whose pioneering use of molecular techniques helped elucidate the origin of humans. His and Vincent Sarich’s work on immunological proteins in the 1960s suggested that humans, gorillas, and chimpanzees share a much more recent common ancestor than was commonly believed. Some of Wilson’s other work has documented the remarkable degree of genetic similarity between humans and chimpanzees and has traced human mitochondrial genes back to their common ancestor in Africa.
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American microbiologist who studies bacteria and other ancient life forms in order to understand the early history of life on Earth. His lab was one of the first to discover that chloroplast DNA is relatively unrelated to the DNA in the nucleus of the same plant cell, a finding that supports Margulis’ endosymbiosis hypothesis.
American geneticist whose mathematical descriptions of evolution helped show that evolution by natural selection is compatible with our understanding of genetics. Wright also conceptualized an influential hypothesis, called the shifting balance theory, that described how genetic drift and natural selection might interact and allow populations to be better adapted to their environments.
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