If so, it will become more prevalent in the next generation and spread throughout the population. In this way, natural selection guides the evolutionary process, preserving and adding up the beneficial mutations and rejecting the bad ones. But natural selection isn't the only mechanism by which organisms evolve, she said. For example, genes can be transferred from one population to another when organisms migrate or immigrate — a process known as gene flow.
And the frequency of certain genes can also change at random, which is called genetic drift. The reason Lamarck's theory of evolution is generally wrong is that acquired characteristics don't affect the DNA of sperm and eggs. A giraffe's gametes, for example, aren't affected by whether it stretches its neck; they simply reflect the genes the giraffe inherited from its parents. But as Quanta reported , some aspects of evolution are Lamarckian. For example, a Swedish study published in in the European Journal of Human Genetics found that the grandchildren of men who starved as children during a famine passed on better cardiovascular health to their grandchildren.
Researchers hypothesize that although experiences such as food deprivation don't change the DNA sequences in the gametes, they may result in external modifications to DNA that turn genes "on" or "off. For instance, a chemical modification called methylation can affect which genes are turned on or off. Such epigenetic changes can be passed down to offspring. In this way, a person's experiences could affect the DNA he or she passes down, analogous to the way Lamarck thought a giraffe craning its neck would affect the neck length of its offspring.
Even though scientists could predict what early whales should look like, they lacked the fossil evidence to back up their claim. Creationists viewed this absence, not just with regard to whale evolution but more generally, as proof that evolution didn't occur, as pointed out in a Scientific American article.
But since the early s, scientists have found evidence from paleontology, developmental biology and genetics to support the idea that whales evolved from land mammals. These same lines of evidence support the theory of evolution as a whole. The critical piece of evidence was discovered in , when paleontologists found the fossilized remains of Ambulocetus natans , which means "swimming-walking whale," according to a review published in the journal Evolution: Education and Outreach.
Its forelimbs had fingers and small hooves, but its hind feet were enormous relative to its size. The animal was clearly adapted for swimming, but it was also capable of moving clumsily on land, much like a seal.
When it swam, the ancient creature moved like an otter, pushing back with its hind feet and undulating its spine and tail. Modern whales propel themselves through the water with powerful beats of their horizontal tail flukes, but A. In recent years, more and more of these transitional species, or "missing links," have been discovered, lending further support to Darwin's theory. For example, in , a geologist discovered the fossil of an extinct aquatic mammal, called Indohyus , that was about the size of a cat and had hooves and a long tail.
Scientists think the animal belonged to a group related to cetaceans such as Ambulocetus natans. This creature is considered a "missing link" between artiodactyls — a group of hoofed mammals even-toed ungulates that includes hippos, pigs, and cows — and whales, according to the National Science Foundation.
Researchers knew that whales were related to artiodactyls, but until the discovery of this fossil, there were no known artiodactyls that shared physical characteristics with whales. After all, hippos, thought to be cetaceans' closest living relatives, are very different from whales. Indohyus , on the other hand, was an artiodactyl, indicated by the structure of its hooves and ankles, and it also had some similarities to whales, in the structure of its ears, for example.
Genetic evidence also supports the idea that whales evolved from land mammals and provides information about the exact branching of the evolutionary tree. For instance, in , researchers reported in the journal Proceedings of the National Academy of Sciences that according to genetic analysis of " jumping gene " sequences, which copy and paste themselves into genomes, hippos were whales' closest living relatives.
Before , researchers thought pigs were more closely related to whales, but this study overturned that idea, as the Associated Press reported. In , researchers reported in the journal Science Advances about which genes within the whale genome were inactivated during the process of the creature's evolution from land mammals, as Science Friday reported. Take us inside this top-secret facility and talk about the strange case of NCTC 1.
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See More. This conclusion is not generally accepted today. The contemporary view is that species arise not from genetically distinct races, but from local and isolated populations that may initially be much the same genetically as the main populations of a species. However, Darwin did not use the word "evolution" in the first edition of the Origin although he used "evolving" in the sense of "unfolding" in the final paragraph.
Only later, in the Descent of Man ; see Gould , was Darwin forced to adopt the term "evolution", partly because it was in common use, and also because his associate Herbert Spencer had used the term two years before Darwin went public on evolution. The fact that the ideas he outlined are often included in the general understanding of "evolution" causes some confusion, which can be alleviated by carefully distinguishing among the components of Darwin's theory.
The main components of Darwin's theory are that species change transmutation : Darwin's preferred term was "descent with modification" ; that related species are descended from a common ancestor common descent ; that the main mechanism by which species become distinct from one another is natural selection ; and that species arise geographically near to their ancestor biogeography. His theory had several other components, some of which are now rejected his model of inheritance is the main one , but they are minor components of his evolutionary theory and can be ignored here Wilkins In the book that effectively crystallized the Modern Synthesis of genetics and Darwinism, Ronald Fisher began by saying Natural Selection is not Evolution.
Yet, ever since the two words have been in common use, the theory of Natural Selection has been employed as a convenient abbreviation for the theory of Evolution by means of Natural Selection, put forward by Darwin and Wallace Fisher xi.
Fisher may be historically inexact, but there is a solid point here: natural selection is what many people mean by "evolution". This impression is reinforced by the writings of such luminaries as Richard Dawkins, who makes selection the core of his view of evolution. But natural selection is also the mechanism by which species do not change.
Selection only forces change if a population is not well-suited to competing for resources or overcoming risks in its local environment. If a species is well-adapted, then selection will inhibit change; this is called balancing selection.
Thus, we have to distinguish between the mechanism that is sometimes an agent for change from the process of change itself see Figure 1. Before Darwin, the prevailing view was that species are types. A type was both a means of identifying and classifying an organism and also a force that caused it to exhibit certain physical characteristics. The type made that organism what it was. Organisms that varied too far from the type were "monsters", degradations of the type for example, a French biologist contemporary of Linnaeus, Georges Buffon, thought that evolution was a degradation of a number of created types; Lovejoy Entire species were called monsters, because they were too far removed from the central type of a family or genus of organisms.
Such central types were called the archetype by Richard Owen. Darwin imagined the archetype as an actual historical ancestor — that the variation present in an ancestral population could be the basis for descendant populations both to share important identifying characteristics and to differ significantly in some ways from the ancestor. Furthermore, he saw that neither the archetype nor the type of a single species exerted any influence on the subsequent history of a lineage. For Darwin, types were just the most common form of a species or genus.
The type might remain stable, or it might change. In modern terms, we would say that the type is just the mode of the distribution of species' characters. What causes the type to remain stable or to change is another matter. Whether the type changes or remains stable, the cause might be selection or some other process, such as random drift. Evolution after the discovery of the gene Modern definitions of evolution are based on the fact that all organisms living in breeding populations are generally quite similar, but no two individuals are exactly alike.
They have differences due to their heredity, their upbringing, and their life histories. Some of these differences are of evolutionary significance: all of them involve their genes in some way; their dispositions to grow in particular ways, their ability to react to and make use of their environments, and even their abilities to deal with disease and injuries are genetically influenced, but not necessarily determined.
Selection acts at base on genetic differences among organisms, not on an individual gene. Moreover, these differences have to occur in populations by virtue of the normal processes of genetic recombination in reproduction. Darwin's theory was extended by the so-called "biometrics" movement which is the foundation of both statistics and population genetics and melded together with Mendelian genetics beginning about For the architects of the Synthesis, it was natural to use the powerful set of theoretical and analytic techniques of genetics to define evolution see references and discussion in Bowler Thus, Huxley, summarizing the views developed by Dobzhansky and others in the development of the Synthesis wrote: Mendelism is now seen as an essential part of the theory of evolution.
Mendelian analysis does not merely explain the distributive hereditary mechanism: it also, together with selection, explains the progressive mechanism of evolution 26 and [W]hat evolves is the gene-complex; and it can do so in a series of small if irregular steps, so finely graded as to constitute a continuous ramp The "allele-frequency" definition of evolution has survived to become the "standard" definition in textbooks and discussions about the nature of evolution.
Here is a more-or-less random collection of quotations from various sources to illustrate how different views have developed based on this initial insight.
Biological evolution The ontogeny of an individual is not considered evolution; individual organisms do not evolve.
The changes in populations that are considered evolutionary are those that are inheritable via the genetic material from one generation to the next. Biological evolution may be slight or substantial; it embraces everything from slight changes in the proportion of different alleles within a population such as those determining blood types to the successive alterations that led from the earliest proto-organism to snails, bees, giraffes, and dandelions Futuyma 7.
The fundamental evolutionary event is a change in the frequency of genes and chromosome configurations in a population Wilson On the simplest perspective of all, biological evolution is analyzed initially as changes in allelic frequencies at a single locus.
More complicated phenomena must be explained by means of combinations of these minimal units Hull Natural selection deals with frequency changes brought about by differences in ecology among heritable phenotypes; evolution includes this as well as random effects and the origin of these variants Endler Since evolution may be defined as cumulative change in the genetic makeup of a population resulting in increased adaptation to the environment, the fundamental process in evolution is change in allele frequency Hartl Notice that some say that observable change in the frequencies of alleles is sufficient to define evolution, while others, such as Futuyma, think it necessary to go into more detail.
For the purpose at hand, the dispute is unimportant. A more important controversy, however, is between the proponents of the allele-frequency definition and those who reject it altogether as too narrow: I pointed out more than a decade ago that the reductionist explanation, so widely adopted in recent decades — evolution is a change in gene frequencies in populations — is not only not explanatory, but is in fact misleading.
Far more revealing is the definition: "Evolution is change in the adaptation and in the diversity of populations of organisms" Mayr Evolution may be defined as any net directional change or any cumulative change in the characteristics of organisms or populations over many generations — in other words, descent with modification It explicitly includes the origin as well as the spread of alleles, variants, trait values, or character states.
Evolution may occur as a result of natural selection, genetic drift, or both; the minimum requirements are those for either process. Natural selection does not necessarily give rise to evolution, and the same is true for genetic drift Endler 5. Population geneticists use a different definition of evolution: a change in allele frequencies among generations.
This meaning is quite different from the original; it now includes random as well as directional changes It is roughly equivalent to microevolution subspecific evolution; macroevolution involves major trends, or trans-specific evolution Unfortunately, the use of the population genetics definition often results in an overemphasis on changes in allele frequencies and an underemphasis on or no consideration of the origin of the different alleles and their properties.
Both are important in evolution An additional problem is that, for quantitative genetic traits, the frequencies of alleles at many contributing loci can change while the overall mean and variance of the trait remain roughly constant Endler And even those who stress the genetic character of evolution sometimes take a broader view: Evolution is a directional and essentially irreversible process occurring in time, which in its course gives rise to an increase of variety and an increasingly high level of organization in its products Julian Huxley cited in Newman These examples illustrate that there is a wide range of approaches to defining evolution and that "experts" disagree over what to emphasize in their definitions.
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.
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