Recent Examples on the Web California's climate and the evolution of Jewish cooking has also led to a blending of Eastern European with Sephardic and Middle Eastern foods, reflected on Saul's menu. Smith, cleveland , 30 Oct. First Known Use of evolution , in the meaning defined at sense 6.
Learn More About evolution. Time Traveler for evolution The first known use of evolution was in See more words from the same year. From the Editors at Merriam-Webster. Style: MLA. Kids Definition of evolution. Medical Definition of evolution. Which traits are adaptive depends on environmental conditions and mating choices. Evolution that is driven by natural selection is called adaptive evolution.
Adaptive radiation is the selection of different sets of adaptive traits in populations or species living in different environmental conditions. Another mechanism that drives evolution is genetic drift. It produces random changes in the frequency of traits in a population.
Genetic drift is more common in small populations, since random changes have a greater effect in small populations. In addition to such founder effects , genetic drift can also occur in populations that have emerged from some catastrophic bottleneck. The very low genetic diversity in cheetahs has been attributed to this bottleneck effect. Evolutionary change has consequences at two levels: microevolution and macroevolution.
Microevolution refers to changes in the gene frequencies in a single population over relatively short periods of time. For example, changes in the population of finches on one of the Galapagos islands, Daphne major, have been tracked for many years by Peter and Rosemary Grant. The Grants have observed changes in the structure of the beak of these finches associated with changes in the type of seed available to the birds on the island. Macroevolution refers to changes at and above the level of speciation.
The critical event for speciation in eukaryotes is reproductive isolation — the inability of individuals from two different populations to produce viable and fertile progeny.
Reproductive isolation may begin with the simple geographic separation of two populations, but must at some point be reinforced by chromosomal changes that prevent the chromosome pairing required for normal meiosis. For example, horses and donkeys can mate to produce a viable hybrid — mules — but the mules almost never produce progeny. Mules have 63 chromosomes — 32 from horses and 31 from donkeys — and meiosis rarely produces functional gametes. Only 60 successful births from mules have been documented since Ref.
Reproductive isolation then allows further changes to accumulate in the species, e. Reproductive isolation is one of the ways to cause allopatric speciation.
Find out other mechanisms. More info here: Sympatric vs allopatric speciation. Join our Forum now! Evolutionary biology is a subfield in biology that focuses primarily on evolution. An expert in evolutionary biology is called an evolutionary biologist. Some of the major topics are natural selection , genetic drift , biodiversity , and speciation.
In , his theory was presented jointly with that of Alfred Russel Wallace — , a naturalist who independently formulated a similar theory. In brief, the theory posited that natural selection was the mechanism of evolutionary change and that all species were related in a branching pattern from a common ancestor.
Some think that genes are a very good place to start, while others insist that important concepts about evolution are not captured in allele-frequency definitions. However, when it comes down to the nature of the evolutionary process, much of this is a matter of semantics — what to spell out and what to leave implicit. Despite the superficial differences in these descriptions, the apparent disagreements do not usually entail differences of opinion about what happened in the course of evolution, at least not in broad outline.
They very often claim that there are barriers to changing beyond the "kind" an ill-defined term with no fixed meaning, which seems roughly equivalent to "species" — although such change has been observed many times as new species have been observed to evolve from old ones.
This is the lowest level of macroevolution, as we will see in a moment. Some anti-evolutionists even allow for evolution of one species into another, but deny that the emergence of "major" groups, such as families or orders in the Linnaean hierarchy, can be the result of microevolutionary change.
Often, this concession to microevolution is made only to accommodate the species diversity we see today from the necessarily restricted variation among the original "kinds" that are supposed to serve as the founding populations at the Creation or that were carried on the Ark.
Even among scientists, the term "macroevolution" is a vague concept. Many authors think that there is a qualitative difference between adaptive evolution and the origins of higher taxa or forms.
In the original formulation, Y'uri'i Filipchenko in used the term to mean origination of a novel species by splitting from an ancestral species — what we now call speciation or cladogenesis. Today it is more widely used to mean "large-scale" change, such as the evolution of novel "body-plans", "grades" of ecological niche specialization, or "key innovations".
Those who prefer the allele-frequency definition of evolution argue that every such novelty began as minor variations on a theme in the origination of a slightly different species and that large- scale changes are the result of continued evolution of this kind over large periods of time.
Often they think that evolution is always gradual anagenetic and that evolutionary trees phylogenies are just the additive sum of these gradual changes. Nobody denies either cladogenesis or anagenesis these days, but there is a fair bit of debate over the right mix see Figure 2.
I take the broadest definition of biological evolution to be: Transmutation descent with modification : This is the notion that new species emerge from existing species and that all existing species are the product of change in older ones. This view was common by the s, and Darwin did not invent the idea. A slightly narrower conception of evolution: Common ancestry : Related species have changed from a common ancestor species; that is, the reason that species are similar and are related in classification is because they have evolved from a shared ancestral species.
This is also called phylogenetic change , or more simply, phylogeny. In a limited way, both Lamarck and Erasmus Darwin proposed common ancestry, but the first complete account was propounded by Charles Darwin. Narrower still: Biogeographic distribution : Related species arise as geographic neighbors; this is the view that no new species arises except in close contact with its most related species.
This view was proposed by Alfred Wallace and Charles Darwin. Of course, the fact that new species arise as biogeographic neighbors is explained by common ancestry, but Wallace formulated this model before the common ancestry model was published. Evolutionary theory also has some strictly Darwinian elements : The struggle for existence : More individuals will be born than the environment can support, so not all organisms survive to reproduce. Variation among individuals : All organisms generate offspring that are slightly different from each other, so there is variety within all populations.
This is one of Darwin's original contributions to biology, although he was influenced by de Candolle. Natural selection : The local environment is more favorable to organisms with a particular variety or combination of traits within a species. Those so favored survive longer and reproduce more, resulting in that variety's becoming more common in subsequent generations than other varieties in the species.
Darwin was not the first to recognize natural selection, but he was the first to use it as a mechanism of evolution. Sexual selection : In sexually reproducing species, mate choice sustains display traits — the sex that needs to compete for mates will show variations in the characteristics associated with this competition. Darwin is the sole author of this mechanism of evolutionary change. The Neo-Darwinian elements are: Random mutation , or blind variation : Changes arise in genes at random, without respect to the survival needs of organisms and species.
Weismannism : Information from the somatic cells of the body is not inherited. This principle was proposed by August Weismann in The synthetic elements are: Drift : Many changes occur that are not selected for, due to sampling accidents in population. Models of drift were sketched in the 19th century by Darwin, George Romanes, and Moritz Wagner, but they came into their own with the advent of the Synthesis. There are different and not necessarily incompatible models of drift.
One is genetic drift after Sewall Wright. This includes genetic neutralism, which involves selectively neutral mutations becoming fixed in a population after Mootoo Kimura. This includes "founder effect" models of speciation, formulated by Mayr and Hampton Carson. Canalization : Developmental processes are robust and resist change Models of canalization were proposed by Conrad Waddington and Ivan Ivanovich Schmalhausen.
This notion is also sometimes called the "developmental constraints" model of evolution see Schlichting and Pigliucci Mendelian genetics : Heredity is the passing on of discrete units of genetic material, which recombine in certain ways and frequencies, and which are either dominant or recessive. Mendelian genetics was incorporated into the synthesis by Fisher and Haldane. Postsynthetic elements include, but are not restricted to: Rate variability : Evolution occurs at many different rates, from the instantaneous to the gradual, all of which are gradual and continuous at some scale.
For example, GG Simpson , Mayr , and more recently, Niles Eldredge and Stephen Jay Gould in their famous punctuated equilibria model, examine how the pace of evolutionary change can vary under different circumstances. Process structuralism : Some changes are biased by their structural relations and form.
The foundation of process structuralism was laid by D'Arcy Thompson and recently revived by Brian Goodwin , and others. Complex structures and systems are not free to vary independent of their relationships with other components of the complex. The long inferential chain It is very hard to work out the implications of the effects of processes of local variation on large-scale ones. If gene frequency changes in local populations are the foundation for all evolution, it still may not be evident that a new species will arise, that there will be some trends in evolution, or that the patterns of life at any one time will reflect a process of speciation and retention of novelties in large groups.
In part, this is because the chain of inference from population-level genetics to macroevolution is a very long one. Even if genes determined everything about species, we would not be able to generalize and elaborate these facts with any clarity or detail. We are limited in our ability to take all these things into account and lack the time and skills to work it all out fully.
Reconciling the conservative character of genetic transmission with the tremendous potential for evolutionary change that is inherent in the genetic variation among individuals is particularly hard for those who lack a full appreciation of the way genes recombine and affect the development of organisms. This is especially true of secondary and post-secondary students and the general populace. The allele-frequency definition, if adequate, would leave us unsatisfied that evolution really had been explained.
Geneticists have observed in small scale a general resistance of the molecular components of the genome to change from the "norm" or "wild type". Evolution can also occur through genetic drift, mutation, or migration. Evolutionary theory, then, can be taken to be the study including, but not limited to, mathematical models of these and other modes of evolution.
With natural selection, the frequency of alleles that confer greater fitness would tend to increase over those which confer lesser fitness. Sexual selection would be the same, but with fitness understood strictly in terms of mating ability.
With genetic drift, a form of evolution that involves chance see the entry on genetic drift for explanation , there could be an increase in the frequency of alleles that confer greater fitness, an increase in the frequency of alleles that confer lesser fitness, or an increase in the frequency of alleles whose manifestation if any was neutral.
If organisms migrate from one population to another, it is likely that there will be a change in the frequency of alleles in both populations. And if there is a mutation from one allele to another, then the frequency of alleles in the population will likewise change, albeit by a small amount.
Distinguishing these different modes of evolution allows biologists to track the various factors that are relevant to evolutionary changes in a population. The careful reader may have noted that the previous paragraph invoked probabilistic language: what tends to happen, what could happen, what is likely to happen.
Indeed, mathematical evolutionary models today see the entry on population genetics are typically statistical models. This fact about evolutionary models has given rise to a debate in the philosophy of evolution over whether natural selection and genetic drift should be be understood as causes of evolution, as most biologists conceive them, or as mere statistical summaries of lower-level causes: births, deaths, etc.
The natural selection and genetic drift entries give more information about this debate. Although there is widespread agreement that there are multiple modes of evolution, much contemporary work in biology and philosophy of biology has been focused on natural selection.
Whether this focus is a good thing or not is in part what the debate over adaptationism is about. That is, do we have reason to think that natural selection is the most prevalent or most important mode of evolution?
Should scientific methodologies be geared toward testing natural selection hypotheses or toward a variety of possible evolutionary modes? What fitness means, what entities it applies to genes, organisms, groups, individuals, types , what sort of probabilities it invokes, if any, and how it should be calculated, are all under philosophical dispute.
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