Selection favored variants that were better able to survive and reproduce e. Over many generations of random mutation and non-random selection, complex adaptations evolved. To learn more about random mutation , visit our article on DNA and mutations. Hence, evolutionary change is not always necessary for species to persist. Many taxa like some mosses, fungi, sharks, opossums, and crayfish have changed little physically over great expanses of time.
Mutation, migration , and genetic drift may cause populations to evolve in ways that are actually harmful overall or make them less suitable for their environments. Climates change, rivers shift course, new competitors invade — and an organism with traits that are beneficial in one situation may be poorly equipped for survival when the environment changes.
It is tempting to see evolution as a grand progressive ladder with Homo sapiens emerging at the top. But evolution produces a tree, not a ladder — and we are just one of many twigs on the tree.
Populations, not individual organisms, evolve. Changes in an individual over the course of its lifetime may be developmental e. New gene variants i. We have many examples of slow and steady evolution — for example, the gradual evolution of whales from their land-dwelling, mammalian ancestors, as documented in the fossil record.
But we also know of many cases in which evolution has occurred rapidly. For example, we have a detailed fossil record showing how some species of single-celled organism, called foraminiferans, evolved new body shapes in the blink of a geological eye, as shown here. And since humans often cause major changes in the environment, we are frequently the instigators of evolution in other organisms. Here are just a few examples of human-caused evolution for you to explore: — Several species have evolved in response to climate change.
Genetic drift occurs because, due to chance, the individuals that reproduce may not exactly represent the genetic makeup of the whole population. For example, in one generation of a population of captive mice, brown-furred individuals may reproduce more than white-furred individuals, causing the gene version that codes for brown fur to increase in the population — not because it improves survival, just because of chance.
The same process occurs in large populations: some individuals may get lucky and leave many copies of their genes in the next generation, while others may be unlucky and leave few copies. However, in large populations, the changes in gene frequency from generation to generation tend to be small, while in smaller populations, those shifts may be much larger.
Whether its impact is large or small, genetic drift occurs all the time, in all populations. To learn more about genetic drift , visit Evolution To learn more about population size as it relates to genetic drift , visit this advanced article. We have invented medical treatments, agricultural practices, and economic structures that significantly alter the challenges to reproduction and survival faced by modern humans.
So, for example, because we can now treat diabetes with insulin, the gene versions that contribute to juvenile diabetes are no longer strongly selected against in developed countries. However, this is not the case. Humans still face challenges to survival and reproduction, just not the same ones that we did 20, years ago. The direction, but not the fact of our evolution has changed. For example, modern humans living in densely populated areas face greater risks of epidemic diseases than did our hunter-gatherer ancestors who did not come into close contact with so many people on a daily basis — and this situation favors the spread of gene versions that protect against these diseases.
Scientists have uncovered many such cases of recent human evolution. That definition of a species might seem cut and dried — and for many organisms e. For example, many bacteria reproduce mainly asexually. How can the biological species concept be applied to them? Many plants and some animals form hybrids in nature, even if they largely mate within their own groups. Should groups that occasionally hybridize in selected areas be considered the same species or separate species? The concept of a species is a fuzzy one because humans invented the concept to help get a grasp on the diversity of the natural world.
It is difficult to apply because the term species reflects our attempts to give discrete names to different parts of the tree of life — which is not discrete at all, but a continuous web of life, connected from its roots to its leaves. To learn more about the biological species concept , visit Evolution To learn about other species concepts , visit this side trip. Natural selection naturally results from genetic variation in a population and the fact that some of those variants may be able to leave more offspring in the next generation than other variants.
Instead, resistance evolves because random mutation happens to generate some individuals that are better able to survive the antibiotic, and these individuals can reproduce more than other, leaving behind more resistant bacteria.
If a population happens to have genetic variation that allows some individuals to survive a challenge better than others or reproduce more than others, then those individuals will have more offspring in the next generation, and the population will evolve. If that genetic variation is not in the population, the population may survive anyway but not evolve via natural selection or it may die out.
Of course, some species may possess traits that allow them to thrive under conditions of environmental change caused by humans and so may be selected for, but others may not and so may go extinct. For example, as climate change causes the Arctic sea ice to thin and break up earlier and earlier, polar bears are finding it more difficult to obtain food. However, this impression is incorrect.
Natural selection has no foresight or intentions. Theoretically, in fact, a trait that is advantageous to the individual e. The more fertile offspring an organism leaves in the next generation, the fitter it is.
For example, a puny male bird with bright tail feathers might leave behind more offspring than a stronger, duller male, and a spindly plant with big seed pods may leave behind more offspring than a larger specimen — meaning that the puny bird and the spindly plant have higher evolutionary fitness than their stronger, larger counterparts.
To learn more about evolutionary fitness , visit Evolution In most populations, organisms with many different genetic variations survive, reproduce, and leave offspring carrying their genes in the next generation. This is apparent in the populations around us: for example, a plant may not have the genes to flourish in a drought, or a predator may not be quite fast enough to catch her prey every time she is hungry.
For example, living things are made up of traits resulting from a complicated set of trade-offs — changing one feature for the better may mean changing another for the worse e. And of course, because organisms have arisen through complex evolutionary histories not a design process , their future evolution is often constrained by traits they have already evolved. For example, even if it were advantageous for an insect to grow in some way other than molting, this switch simply could not happen because molting is embedded in the genetic makeup of insects at many levels.
To learn more about the limitations of natural selection , visit our module on misconceptions about natural selection and adaptation. Some may be the chance results of history. There is nothing special about the relationship between GGC and glycine. Others traits may be by-products of another characteristic.
For example, the color of blood is not adaptive. To read more about explanations for traits that are not adaptive , visit our module on misconceptions about natural selection and adaptation. To learn more about what traits are adaptations , visit another page in the same module. Organisms that share a more recent branching point i. For example, on the tree here, taxon A is adjacent to B and more distant from C and D. However, taxon A is equally closely related to taxa B, C, and D.
Similarly, in the tree below, taxon B is adjacent to taxon A, but taxon B is actually more closely related to taxon D. The following phylogenies are all equivalent. Even though each phylogeny below has a different order of taxa at the tips of the tree, each portrays the same pattern of branching. These are value judgments that have no place in science. One form of a trait may be ancestral to another more derived form, but to say that one is primitive and the other advanced implies that evolution entails progress — which is not the case.
For more details , visit our misconception on this topic. For example, on the tree below, taxon D may be more or less specialized than taxa A, B, and C.
Unfortunately, students may assume that all traits follow this simple model, and that is not the case. This theory documents the change in the genetic makeup of a biological population over time. Evolution helps us understand the development of antibiotic resistance in bacteria and other parasitic organisms. While evolution is easiest to see in bacteria due to their short life cycles, every living population experiences evolution of one kind or another.
Skip to main content. Module Theory of Evolution. Frequently Asked Questions About Evolution. Biological evolution refers to the cumulative changes that occur in a population over time. Sometimes, individuals inherit new characteristics that give them a survival and reproductive advantage in their local environments; these characteristics tend to increase in frequency in the population, while those that are disadvantageous decrease in frequency. This process of differential survival and reproduction is known as natural selection.
Non-genetic changes that occur during an organism's life span, such as increases in muscle mass due to exercise and diet, cannot be passed on to the next generation and are not examples of evolution. Learn More Evolution of Diversity. In science, a theory is a rigorously tested statement of general principles that explains observable and recorded aspects of the world. A scientific theory therefore describes a higher level of understanding that ties "facts" together.
A scientific theory stands until proven wrong -- it is never proven correct. The Darwinian theory of evolution has withstood the test of time and thousands of scientific experiments; nothing has disproved it since Darwin first proposed it more than years ago. Indeed, many scientific advances, in a range of scientific disciplines including physics, geology, chemistry, and molecular biology, have supported, refined, and expanded evolutionary theory far beyond anything Darwin could have imagined.
Learn More Evolution Revolution. Just as the tree of life illustrates, all organisms, both living and extinct, are related. Every branch of the tree represents a species, and every fork separating one species from another represents the common ancestor shared by these species.
While the tree's countless forks and far-reaching branches clearly show that relatedness among species varies greatly, it is also easy to see that every pair of species share a common ancestor from some point in evolutionary history.
For example, scientists estimate that the common ancestor shared by humans and chimpanzees lived some 5 to 8 million years ago. Humans and bacteria obviously share a much more distant common ancestor, but our relationship to these single-celled organisms is no less real. Indeed, DNA analyses show that although humans share far more genetic material with our fellow primates than we do with single-celled organisms, we still have more than genes in common with bacteria.
It is important to realize that describing organisms as relatives does not mean that one of those organisms is an ancestor of the other, or, for that matter, that any living species is the ancestor of any other living species.
A person may be related to blood relatives, such as cousins, aunts, and uncles, because she shares with them one or more common ancestors, such as a grandparent, or great-grandparent. But those cousins, aunts, and uncles are not her ancestors. In the same way, humans and other living primates are related, but none of these living relatives is a human ancestor.
Learn More All in the Family.
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