Over time, all life forms evolve, and evolutionary biology focuses on how that evolution happens. Changes in an organism's observable qualities cause changes in its genetic makeup, which is how populations of living things evolve. Mutations in an organism's DNA are one type of genetic alteration produced by damage or replication errors. Because of the relative reproductive success of organisms with particular qualities, natural selection gradually increases or decreases the frequency of certain traits in a population's genetic variation over successive generations.
The Earth is estimated to be around 4.5 billion years old, and at least 3.5 billion years ago, the earliest undisputed evidence of life on Earth has been discovered. In contrast to abiogenesis, which explains the origin of life, evolution explains how early lifeforms developed into today's complex ecosystems. A last universal common ancestor (LUCA) from which all known species have diverged has been hypothesized to be the source of all life on Earth, based on the similarities between all living things.
Genetic material inherited from one's parents is passed down to one's descendants in DNA. As a result of mutations or the re-arrangement of genes during sexual reproduction, kids can have different genetic characteristics. Minor, haphazard differences separate the progeny from their parents, and it is more likely that the children will survive and reproduce if those differences are beneficial. Therefore, fewer people will have the same likelihood of reproducing in the future since their progeny will have more beneficial variation. As a result, features that help animals adapt to their environment become more prevalent in descendent populations. The population changes due to these variations, which gives rise to the world's wide variety of living organisms.
Charles Darwin's "On the Origin of Species", first published in 1859, ushered in a new era in evolutionary theory. In addition, the work of Gregor Mendel on plants aided in the explanation of hereditary genetic patterns. Research in paleontology, the study of populations, and an international scientific network have all contributed to a better understanding of evolution. Speciation has been empirically observed in the lab, and the wild, and scientists now have a decent grasp on how new species emerge (speciation). Biological biologists employ evolution as a primary scientific theory to comprehend life. It is used in many fields, from medicine to psychology, conservation biology, anthropology, and forensics.
According to evolution, the most important concepts can be summarized with some key points below:
Assembling natural history collections and arranging them in museums was popular in the 19th century. Naturalists were engaged by the European expansion and naval expeditions, while the curators of significant museums displayed preserved and live specimens of the variety of life in their collections. He was an English graduate who had been schooled and trained in the fields of natural history. These naturalists would study the vast collections of specimens held and controlled by museum curators. Darwin worked as a ship's naturalist on the HMS Beagle during a five-year study voyage around the globe. During his voyage, he examined and gathered a wide variety of species from the shores of South America and the Galápagos Islands, which he found fascinating.
According to Darwin, Orchids have evolved various pollination strategies developed from their essential floral elements. Darwin gathered and studied the natural history of organisms worldwide throughout his travels. According to his research, each species evolved from an ancestral line with many characteristics. Natural selection, which he coined in 1838, was the mechanism he proposed to explain this phenomenon. How much and how many resources are available to support a population determines the size of that population. There must be a balance between the growth of the population and the resources available for the population to remain stable year after year. For every generation, there are fewer and fewer individuals who are able to make it out to the other side. Resources that are necessary for survival must be contested. As a result, Darwin realized survival was not a matter of chance alone. An organism's likelihood of surviving and reproducing are influenced by the unique characteristics, or "traits," that each individual within that species possesses. It is more likely that well-adapted individuals will have more children than those who are less well-adapted. Over time, traits detrimental to one's survival and reproduction would fade away. Over time, traits that aid an organism's survival and reproduction would accumulate. Using the phrase natural selection, Darwin explained how the unequal capacity of individuals to live and reproduce might lead to progressive population changes.
The hypothesis of natural selection is based on observations of variation in animals and plants. If you look at the close interaction between orchids and insects that Darwin discovered, you'll see how important it is for the pollination of plants. Several features attract insects, allowing pollen from the flowers to adhere to their bodies. Insects carry pollen from a male orchid to a female orchid in this manner. Even though orchids appear to be highly decorated, these specialized portions are formed of the same essential components as other flowers. It is widely accepted that Darwin's theory that orchid blossoms evolved from pre-existing pieces through natural selection may be found in his book, Fertilization of Orchids (1862).
When Darwin got a letter from Alfred Russel Wallace outlining a theory very similar to his own on natural selection, he was still deep in the trenches of experimentation. As a result, both theories were immediately published in conjunction. According to Wallace and Darwin, every tree branch represents a common ancestor in the evolution of life. Modern species were symbolized by the tips of the limbs, whereas the branches depicted common forebears shared by many distinct species. According to Darwin, all living organisms are connected, meaning that all life must have descended from a few forms or even from a common ancestor. "Descent with modification" is what he dubbed this process.
On the Origin of Species was published by Darwin in 1859, laying out his hypothesis of evolution through natural selection. According to his thesis, all life, including humanity, results from ongoing natural processes. Some religious organizations are offended by the notion that all life on Earth has a common ancestry. More than 99 percent of scientists currently embrace the hypothesis, which means their concerns are out of step with the scientific consensus.
Surviving to the strongest may be usually associated with Darwinian natural selection, but Herbert Spencer's Principles of Biology, published in 1864, is where this statement first appeared. According to Darwin, survival of the fittest misrepresents natural selection because it is not only about survival, and it's not always the fittest that survives in natural selection.
Darwin's studies and observations created the framework for current evolutionary theory, showing that creatures in groups differed, that some of these differences were inherited and that these differences could be influenced by natural selection. However, he was unable to explain these discrepancies. According to Darwin, traits that can be passed down from generation to generation result from an organism's use and depletion of those traits. There were plenty of examples he could draw from, such as huge ground feeders like ostriches, whose legs got stronger with exercise but whose wings got weaker with inactivity. Jean-Baptiste Lamarck proposed the notion of transmutation of species in 1809, and this misconception was referred to as the inheritance of acquired characteristics. Lamarckism, a term coined in the late 1800s, was the name given to this hypothesis. When Darwin attempted to explain how hereditary traits may be acquired, he coined the term "pangenesis." The tests of August Weismann in the late 1880s showed that changes in use and disuse could not be inherited, and Lamarckism began to lose popularity.
Gregor Mendel's groundbreaking work in genetics filled in the gaps left by previous scientists' explanations for how new traits could be passed down through generations. Research on pea plants by Mendel showed that hereditary traits are passed down by separating and rearranging genetic information during the creation of sperm and egg cells, then recombining that information during fertilization. An organism is given half its DNA from one parent and a half from the other in a random combination. Genes were initially referred to as information factors by Mendel. Genes are the fundamental elements of inheritance in living beings. Species' physical and behavioral development is governed by the information contained in these molecules.
DNA is the building block of genes. Nucleotides are the building blocks of DNA, a long molecule made up of individual molecules. In the same way, as the sequence of characters on a page encodes information, the sequence of nucleotides in DNA encodes genetic information. The DNA alphabet's "letters" form tiny instructions called genes. An organism's "instruction manual" is provided by the whole set of these genes when seen as a whole. However, mutations can alter the instructions in this DNA alphabet, affecting the instructions contained within genes. Chromosomes, which are DNA-carrying packets in cells, carry genes. In the offspring, the chromosomes are rearranged to produce new genetic combinations. Sexual reproduction can increase genetic diversity in populations even when no new mutations occur because genes interact during development to create unique combinations. People from different species interbreeding with each other can also expand the genetic diversity of a particular group of people, resulting in gene flow between populations. Genes that weren't previously present in a population can be introduced through this method.
The process of evolution is not a chance event. DNA mutations may be random, but natural selection does not rely on randomness: the environment controls the likelihood of successful reproduction. Evolution is inevitable if self-replicating, imperfectly copied organisms continue to reproduce over billions of years. Evolution does not produce a wholly formed organism in its current form. Instead, small changes accumulate over time. Ultimately, natural selection results in organisms better suited to their present habitats than their predecessors. In natural selection, no progress is made toward a goal. No matter how evolved, intelligent, or smart a living form may be, it is not the goal of evolution. Although snakes and lizards no longer need legs, they are descended from an ancestral scorpionfly with wings. For example, the flea (wingless parasite) descended from an ancestral scorpionfly with wings, and the snake (a lizard that no longer needs limbs) both evolved over time. In the end, organisms are nothing more than the result of a series of random mutations that either succeed or fail, depending on their current environment.
Extinction is frequently the result of abrupt alterations in the environment. One hundred and ninety-nine percent of Earth's species have vanished. Mass extinctions have occurred five times since the beginning of life on Earth, resulting in significant and rapid decreases in the number of species. Cretaceous–Paleogene extinction was the most recent, taking place 66 million years ago.
Genetic drift is a significant factor in a species' allelic frequency variation. Different gene variants are known as alleles. Genetic drift can't introduce new alleles to a population. Still, it can remove an allele from the gene pool and limit a population's diversity. The random sampling of alleles is the cause of genetic drift. To have a genuinely random sample, no outside influences can be at play throughout the selection process. It's like sifting through a brown paper bag, looking for marbles of the same size and weight but in different colors. It is entirely up to chance whether a person lives long enough to reproduce and pass on a sample of their ancestor's ancestry to the next generation through the alleles they inherit. A population's allelic frequency is the ratio of the number of alleles that share the same form to the total number of alleles in the population. Smaller populations are more susceptible to genetic drift than larger populations.
No change in the frequency of alleles over time is predicted by the Hardy–Weinberg principle under idealized conditions, including the lack of selection forces. Hardy–Weinberg equilibrium refers to a population that meets these criteria. For example, Hardy and Weinberg demonstrated that dominant and recessive alleles do not inevitably increase or decrease in frequency, as had previously been supposed.
No mutations, immigration, or emigration are allowed to maintain Hardy-Weinberg equilibrium, as these actions can alter allelic frequencies. As the last point, mating must be completely random, with each man (or female for that matter, depending on the species) considered equally desirable. This ensures that the alleles are mixed in a truly random manner. No matter how often you shuffle a population in Hardy–Weinberg equilibrium, no new cards are added, and no old cards are removed. The cards represent alleles in a population's gene pool in the deck.
In reality, no population can be perfectly in equilibrium with the Hardy-Weinberg model. Over time, the population's finite size and the processes of natural selection drive the allelic frequencies to shift.
Due to external factors, the population of an organism is substantially reduced during a short period. There is no advantage to any gene combination in an actual population bottleneck; survival is determined solely by chance. A population's genetic variety can be reduced or eliminated by a bottleneck. After a bottleneck event, subsequent drift episodes might reduce the population's genetic diversity. When the population lacks diversity, it is more susceptible to other selective forces.
The Northern elephant seal is a prominent example of a population bottleneck. The northern elephant seal population was reduced to as few as 30 animals due to over-hunting during the nineteenth century. They've fully recovered, with a population of around 100,000 people and rising. However, the bottleneck's impacts can be seen. Because the population is so homogeneous, sickness and genetic diseases are more prone to strike the seals.
Small new populations are formed with varying allele frequencies from the parent population due to the founder effect. One population splits into a new one due to geographic isolation and the founder effect. New populations may have a different allelic frequency than the original population, and this will modify how frequently specific alleles appear in the populations. The ancestors of a new population will significantly impact its genetic makeup and, thus, its long-term viability.
When the Amish arrived in Pennsylvania in 1744, they had a founder effect. The recessive allele for Ellis–van Creveld syndrome was detected in two of the colony's founders in Pennsylvania. Because the Amish live in religious isolation, they interbreed. As a result of generations of this practice, the incidence of Ellis–van Creveld syndrome in the Amish community is significantly higher than that of the general population.
It is the contemporary evolutionary theory that populations of organisms have a great deal of genetic variation due to mutation and recombination during sexual reproduction. Genetic drift, gene flow between sub-populations, and natural selection all contribute to the evolution of an organism's allelic frequencies. This emphasis on natural selection emphasizes the importance of the gradual accumulation of tiny evolutionary changes over lengthy periods.
An amalgamation of several scientific domains has led to a more comprehensive knowledge of evolutionary theory. A new field of study known as population genetics was born in the 1920s due to Ronald Fisher, J.B.S. Haldane and Sewall Wright combined Darwin's natural selection theory with statistical models based on Mendelian inheritance. An attempt was attempted to combine population genetic studies, field naturalists' observations of species and subspecies distributions, and fossil record analyses into a coherent explanatory model in the 1930s and 1940s
To better comprehend evolution, scientists like Ernst Mayr and Theodosius Dobzhansky applied genetic principles to naturally occurring populations. Genetics and the Origin of Species by Dobzhansky helped connect genetics and field biology by presenting the mathematical framework of population geneticists in a form more useful to field biologists and by demonstrating that wild populations had much more genetic variability with geographically isolated subspecies and reservoirs of genetic diversity in recessive genes than models of the early population geneticists had allowed for. Genes and field research helped Mayr develop the biological species idea that defined a species as a group of interbreeding or possibly interbreeding populations reproductively isolated from all other populations.
A new species' development depends on reproductively isolated subspecies, which Dobzhansky and Mayr both emphasize. George Gaylord Simpson's statistical examination of the fossil record helped integrate paleontology with a branching and non-directional process of evolution predicted by the modern synthesis into the fossil record.
Naturalist Charles Darwin collected fossils, living specimens (and non-living) in South America during the second voyage of HMS Beagle. He discovered armor bits that resembled larger copies of the scales found on nearby present armadillos, according to Darwin. Anatomist Richard Owen showed him the fragments were from extinct glyptodons linked to the armadillos. This was one, amongst many, patterns of distribution that aided Darwin in his hypothesis of evolution. Fossils, homologous structures (traits that originated from shared ancestry, rather than independent origin), and molecular similarities between species' DNA are all examples of evidence supporting evolution.
Paleontology, the study of fossils, provides evidence that all living things are interconnected. Only through the preservation of fossils can we see the evolution of life as we know it today. Paleontologists can build a family tree for all of Earth's living forms by studying fossils, which disclose the anatomy of the organism and the links between contemporary and extinct species.
Georges Cuvier is credited with founding modern paleontology. Sedimentary rocks, according to Cuvier, have distinct fossil communities within each layer. Simpler living forms lived in the deeper levels, which he believed to be older. It was pointed out to him that many organisms from the past no longer exist today. To understand the fossil record, Cuvier made a significant contribution by establishing extinction as an actuality. Cuvier postulated catastrophism or "revolutions" to explain the extinction of enormous numbers of species, which he attributed to geological disasters throughout the Earth's history. When James Hutton and Charles Lyell suggested that the Earth's geological processes were steady and constant, they overthrew Cuvier's revolution idea. However, the fossil record shows that massive extinctions have occurred. Consequently, the overall concept of catastrophism has resurfaced as a viable possibility for explaining at least some of the fast changes in living forms that appear in the fossil record.
The number of fossils that have been found and recognized has exploded in recent years. As a record of evolution, these fossils are essential. Ancestral relationships between previous and current living forms can be traced back to transitional species seen in fossils. Archaeopteryx is a good example of a transitional fossil, as it has both the traits of a reptile and a bird (such as a long, bony tail and conical teeth) (such as feathers and a wishbone). According to evolutionary theory, the discovery suggests that current reptiles and birds share a common ancestry.
Morphology, or the study of how different components of an organism look compared to one another, has long been used to group closely related species. Comparing the structures of adult creatures from several species or the patterns of cell growth, division, and even migration during an organism's development can be a means of accomplishing this.
In taxonomy, we identify and classify all living organisms. Ancestry-based classification of organisms is made possible by using features such as morphology and genetics. For example, orangutans, chimpanzees, gorillas, and humans are all members of the Hominidae family, which is a taxonomic grouping. Because they have a common ancestor, these animals are grouped (called homology).
These structures are known as homologous structures because they are seen in different species that no longer perform the same activity yet share structural similarities. Mammal forelimbs exhibit this characteristic. There are notable similarities in the bone structure of the forelimbs of mammals such as humans, cats, whales, and bats. However, the forelimbs of each of these four species are used for a distinct purpose. The same bones are employed for both purposes when it comes to a bat's wings and a whale's flippers. If they are unrelated and built for a specific purpose, such a "design" doesn't make sense. According to evolutionary theory, all four animals share a common ancestor, and each has experienced evolution over many generations to achieve similar structures. The forelimbs that resulted from these structural alterations may now perform various functions.
Analogous anatomical comparisons can be misleading, as not all anatomical similarities mean that there is a tight relationship. Organisms that share comparable habitats will often develop similar physical traits, a process known as convergent evolution. Even though sharks and dolphins look alike, they are not closely related; sharks are a type of fish, whereas dolphins are marine mammals. Such resemblances are due to the same selective pressures experienced by both groups. Changes that help swimmers have been favored by both parties. As a result, even though they are not closely related, they have developed similar appearances over time (morphology).
Identifying a shared ancestral link between two or more species is possible based on anatomical analysis of embryonic features. The embryo's growth may obscure this similarity, and the structures may take on new roles. The presence of a tail and pharyngeal slits are a part of the classification of vertebrates, including humans. Both features can be seen in the developing embryo, although they may not be readily apparent in the adult form.
In the past, it was believed that embryos of various species re-enact their evolutionary history because of their morphological similarities during development. An amphibian and a reptilian stage were previously considered necessary for developing human embryos into mammals. In other words, such a re-enactment is unsupported by scientific evidence. However, the initial phases of development across large groupings of species are very similar. For example, in its earliest stages, all vertebrates look strikingly similar, yet they bear no resemblance to any ancestral species. From this basic structure, various features emerge as development continues. We know today that this idea (that ontology recapitulates phylogeny) is only a superficial similarity of developmental stages to evolutionary history.
Vestigial structures are a special subset of homology that comprises various shared components. By "vestigial," we mean body components with no functional use for the organism in which they are found. In earlier forms, these seemingly irrational features were vestiges of organs that were critical. They appear to be leg bones from their predecessors who were able to walk on land, as is the case with whales. We still have wisdom teeth, tail bone, hair on our bodies (including goose bumps), and the semilunar fold at the corner of our eyes, to name a few.
The geographical distribution of species is the subject of biogeography, a branch of biology. Darwin and Alfred Russel Wallace were persuaded by evidence from biogeography, particularly the biogeography of oceanic islands, that species evolved in a branching pattern of common descent. Although endemic species are unique to an island and cannot be found anywhere else, many endemic species on an island may be found on the nearby continent. As a result, islands are often home to various closely related species with diverse ecological niches or means of living in the natural world.
When a single ancestral species colonizes an island with a range of open ecological niches, it diversifies by evolving into multiple species adapted to occupy those available niches. A few well-known examples include Darwin's finches, a group of 13 endemic finch species on the Galápagos Islands, and Hawaiian honeycreepers, a group of birds that once numbered 60 species and all descended from a single finch-like ancestor that arrived on the Hawaiian Islands around 4 million years ago and serves a variety of ecological roles. As another illustration, there's the Silversword alliance, an endemic group of perennial plant species found only in the Hawaiian Islands. This alliance includes a wide range of plant types, from trees to shrubs to ground-hugging mats, and is capable of hybridizing with other species and tarweeds found on the west coast of North America.
Genomic information is encoded in DNA molecules in all living organisms (except for RNA viruses). This information is encoded in an organism's DNA by genes. In general, one's looks and behavior are influenced by one's genes. There will be striking similarities in the DNA sequences of closely related organisms. When two organisms are far apart in terms of evolutionary distance, they will have less in common than those with close distance. There is a big difference in DNA between cousins and brothers since cousins are genetically far from each other, but brothers are genetically close and share comparable DNA. Similarities in DNA are used to show similarities between individuals and animals similarly to determine relationships.
When chimpanzees, gorillas, and humans are compared, the DNA of humans and chimps is shown to be up to 96% similar. Genome-to-genome DNA comparisons show that human and chimpanzee species share more in common than gorillas. Measuring the similarities between molecules and utilizing this data to figure out how various kinds of creatures are related throughout evolution is the focus of the science of molecular systematics. As a result of these comparisons, biologists have been able to build a tree of life's evolution on Earth. They've even permitted scientists to uncover the relationships between species that are so distantly related that their appearances bear no resemblance to those of their distant cousins.
Domesticated plants and animals can be bred by artificial selection. People control which animals and plants reproduce and which of their progeny survive so that they can influence future generations' genetic makeup. The development of domestic animals has been dramatically influenced by artificial selection. People, for example, have bred dogs to develop a variety of breeds. The Great Dane and the Chihuahua differ in size because of selective breeding. They and all other dogs are descended from a small number of wolves domesticated by humans less than 15,000 years ago in what is now China.
Broad diversity of plants has arisen as a result of artificial selection. Recent genomic data reveals that maize (corn) was domesticated in central Mexico 10,000 years ago, and the wild form's edible component was small and difficult to obtain before domestication. Since its inception, the Maize Genetics Cooperation • Stock Center has collected and cataloged more than 10,000 varieties of maize, each resulting from the combination of natural selection and mutation.
In artificial selection, the new breed or variation that emerges is the one with random mutations appealing to humans. Still, in natural selection, the surviving species have random mutations that benefit them in their non-human habitat. In natural and artificial selection, random mutations are the primary source of variation, and the underlying genetic mechanisms are nearly identical. Darwin's arguments in favor of natural selection were based on his meticulous observation of the results of artificial selection in animals and plants. These observations on the many types of domestic pigeons resulting from artificial selection occupied a significant portion of Darwin's book On the Origin of Species. For millions of years, Darwin theorized that natural selection could develop the distinctions seen today in living things if humans could make significant changes in domestic animals over short periods of time.
Two or more species coevolve when they impact the evolution of one another. Coevolution shows that genetically determined traits in each species directly result from the interaction between the two organisms, but the life around them influences all organisms.
The interaction between the acacia plant, which the ant uses for food and shelter, and the Pseudomyrmex ant is a well-documented example of coevolution. Since the two are so closely intertwined, new structures and behaviors have arisen in both organisms. The ant protects the acacia from herbivores and removes seeds from competing plants from the forest floor. Ant-eating floral parts and enlarged thorns on the plant's response to the ants' predation. To say that ants and trees coevolve does not mean that these two organisms are acting in an altruistic manner. As a result, both ant and tree populations profited from minor genetic modifications. The benefit increased the likelihood of the trait being handed to future generations by a small margin. The connection we see today was formed through a series of mutations that occurred throughout time.
Evolution can lead to the creation of new species under the appropriate conditions and with enough time. For a long time, scientists could not come up with a definition of species that was both exact and comprehensive. According to Ernst Mayr, a species is a population or a set of populations whose individuals are capable of spontaneously breeding with one another to create viable, fruitful children. There are no viable, fruitful children between one species and members of another species. Asexually reproducing creatures like bacteria do not fit Mayr's concept, which is widely accepted among biologists.
The separation of a single ancestral population into two distinct species is known as speciation. Allopatric speciation is a common method of speciation. When a population splits up due to distance, allopatric speciation begins. Separate populations can be formed due to geological processes like mountain range formation, canyon formation, or sea-level rise flooding land bridges. For speciation to occur, the genetic exchange between the two groups must be entirely disrupted by a significant separation. As a result of their genetic segregation, the genetically distinct populations have evolved in distinct ways. Various mutations will accumulate in each group, and different selective pressures will be applied to each group. If populations are reunited, they may no longer be able to interbreed due to the accumulated genetic alterations.
Prezygotic and postzygotic barriers to interbreeding can inhibit mating or fertilization (barriers that occur after fertilization). They'll be classified as separate species if there's no more way for them to cross-pollinate. The diversity of life on Earth is the product of four billion years of evolution, with an estimated 1.75 million distinct species now existing.
Speciation usually takes place over extended periods of time, making direct observations by humans scarce. On the other hand, speciation has been witnessed in living animals, and fossil records of previous speciation occurrences attest to this. Five new species of cichlid fish have been discovered in Lake Nagubago; all descended from a single common ancestor isolated from the parent population less than 5,000 years ago. The morphology (physical appearance) and lack of natural interbreeding provided evidence for speciation in this case. It is impossible to interbreed these fish because of their unique mating rituals and various colorations, which were altered somewhat when they were introduced into the new species.
All of biology's subfields are connected by the idea of evolution, which scientists universally accept. Evolving biology has a strong scientific foundation. According to Theodore Dobzhansky, "nothing in biology makes sense until it is seen in the context of evolution." However, evolution is not a static hypothesis. The scientific community engages in a great deal of debate over how evolution works. For instance, the rate of evolution is still up for debate. In addition, there are differing views on whether the organism or the gene is the main unit of evolutionary change.
For Darwin and his colleagues, evolution was a slow and steady process that would occur over time. Many tiny changes accumulated over extended periods of time lead to major changes in species, according to evolutionary trees. Scientists James Hutton and Charles Lyell laid the groundwork for gradualism. Geological change, according to Hutton, is a gradual process rather than a one-time event, as is the case with catastrophism, which advocates the belief that rapid changes have reasons that can no longer be seen in action. For biological changes, a uniformitarian perspective was adopted. The fossil record often shows evidence of new species arising unexpectedly and remaining in their current form over lengthy periods, which would seem to contradict such a perspective. While evolution may seem sluggish to us, paleontologists Niles Eldredge and Stephen Jay Gould created a theory in the 1970s suggesting periods of rapid change (between 50,000 and 100,000 years) alternated with extended intervals of relative stability, which they published in their seminal work. Punctuated equilibrium explains the fossil record without undermining Darwin's theory.
An organism is a common unit of selection in evolution. Natural selection occurs when an individual's reproductive success is improved or lowered by an inherited trait, and the number of an individual's surviving offspring is used as a metric of this success. Many scientists and philosophers have argued against the organism view. Richard Dawkins argues that looking at evolution from the gene's perspective can provide a wealth of information and that natural selection affects both genes and organisms when acting as an evolutionary mechanism. He explains this in The Selfish Gene, published in 1976.
Individuals are temporary and unreliable. After being dealt, chromosomes are shuffled into nothingness like cards in a deck. The cards, on the other hand, are unaffected by the shuffle. The cards are a person's genetic code. By crossing across, the genes don't die. They only change partners and keep marching on. They'll keep marching, of course. That's their prerogative. They are the cloners, and we are the ones who keep them alive. We are discarded after we have served our purpose. Genes, on the other hand, are eternal residents of geological time. A hierarchical viewpoint on selection, for example, was advocated by Stephen Jay Gould in the 1980s.