Theory of evolution by Natural selection

What is evolution?

• Evolution is a change in the inherited traits of a population over time via the process of natural selection that might result in the creation of new species.
• The theory of evolution is a short form of the word “theory of evolution by natural selection,” which was introduced by Charles Darwin and Alfred Russel Wallace in the nineteenth century.
• In the theory of natural selection, species generate more offspring in their environment that are able to survive.
• Those who are better suited physically to live, they evolve and grow to maturity.
• In the other hand, those who lack such fitness either do not reach an age where they can reproduce or produce less offspring than their equivalents.
• Natural selection is often summed up as “survival of the fittest” since the “fittest” species are the ones that reproduce more efficiently and are more likely to pass on their characteristics to the next generation, the most appropriate for their environment.
• This implies that as an environment changes, the characteristics that boost survival will also gradually change or develop in that environment.
• In explaining the evolution of organisms, natural selection was such a strong concept that it was set up as a scientific theory.
• Numerous examples of natural selection shaping evolution have since been observed by biologists.
• It is understood today that it is only one of the pathways by which life evolves.
• A process known as genetic drift, for example, may also cause organisms to evolve.
• In genetic drift, some species produce more descendants than would be predicted, simply by chance.
• These organisms are not essentially the fittest of their species, but they are passed on to the coming generation by their genes.

History of Darwin’s theory of evolution:

• When Darwin was on his youthful voyage as naturalist on the survey ship Beagle, he was inspired by observations made during that time.
• On the Galapagos Island, he found out subtle variations that made tortoises recognizably distinct from other islands.
• The famous “Darwin’s finches,” which displayed minor variations from island to island, also observed a whole array of unique finches.
• Moreover, all of them seemed to resemble, but vary from the common finch on Ecuador’s mainland, 600 miles to the east.
• Patterns in the distribution and similarity of species had a significant impact in Darwin’s thinking.

Darwin’s theory of evolution:

• As Darwin started his study, the first three ideas were already under discussion among earlier and scholarly naturalists researching on the “species problem”.
• The mechanism of natural selection and copious quantities of evidence for evolutionary change from several sources were Darwin’s original contributions.
• For our understanding of the origin of life and contemporary biological diversity, he also offered thoughtful descriptions of the implications of evolution.
• The following basic principles are included in Darwin’s theory of evolution.
• Species are evolving over time and space (populations of interbreeding organisms).
  • Representatives of today’s organisms vary from those of the recent past, and communities of various geographical regions today differ marginally in shape or behavior.
  • These differences reach into the fossil record, which provides this argument with sufficient evidence.
• Each organism shares common ancestors with the other organisms.
  • Populations can split into separate species over time, which share a common ancestral population.
  • Any pair of species shares a common ancestor far enough back in time.
  • For instance, humans shared a common ancestor about eight million years ago with chimpanzees, about 60 million years ago with whales, and over 100 million years ago with kangaroos.
  • The similarities of species that are grouped together are explained by mutual ancestry: their similarities reflect the inheritance of traits from a common ancestor.
• In Darwin’s view, evolutionary changes are progressive and slow.
  • The long episodes of gradual change in organisms in the fossil record and the fact that no naturalist had witnessed the sudden rise of a new species in Darwin’s time supported this claim.
  • The long episodes of gradual change in organisms in the fossil record and the fact that no naturalist had witnessed the sudden rise of a new species in Darwin’s time supported this claim.

What is natural selection?

• Natural selection is a mechanism that will sustain and replicate species that are best suited to an environment.
• This suggests that this variant organism’s beneficial alleles are passed on to offspring.
• The mechanism of natural selection over several centuries contributes to evolution.

Examples of natural selection:

1. Peppered moths:

• Most peppered moths were of the pale variety till the industrial revolution in Britain in the early 1800s.
• This indicated that they were camouflaged against the pale birch trees where they rested.
• Moths with a mutant black coloring were quickly detected by birds and eaten.
• This gave an advantage to the white variety, and they were more likely to continue to reproduce.
• Airborne emissions in manufacturing areas blackened the birch tree bark with soot during the second half of the 1800s.
• This indicated that they were now camouflaged by the mutant black moths, while the white variety became more susceptible to predators.
• This offered a benefit for the black type, and they were more likely to survive and replicate.
• The dark moths passed on the black wing color alleles that led to offspring with the phenotype of black wing color.
• Over time, in urban areas, black peppered moths have become much more common than the pale type.
• It should be noted that the shift in phenotype was not due to the moths being darker by pollution.
• The dark variety still existed, but when the climate changed, it was the better suited version.
• It took several years until the moth population was predominantly black in colour.

2. Galapagos finches:

• It is popular example of natural selection from Darwin’s voyage.
• Every Galapagos island visited by Darwin had its own kind of finch (14 in all), found nowhere else in the world.
• Some had beaks adapted for feeding large seeds, some for small seeds, some for feeding on buds and fruits had parrot-like beaks, and some for feeding on small insects had slender beaks.
• As some woodpeckers do, one used a thorn to test for insect larvae in wood.
• Eight of them were tree finches and six of them were ground dwellers.
• It seemed that each was slightly altered from an original colonist, most likely the finch on South America’s mainland, some 600 miles to the east.
• Adaptive radiation is likely to contribute to the creation of so many species because other birds were few or absent, leaving empty niches to fill; and because there were enough opportunities for geographical isolation in the various Galapagos islands.

The process of natural selection:

• There are four components in the Darwin’s process of natural selection:
• Variation:
  • In appearance and behavior, species (within populations) exhibit individual variation.
  • Body size, hair color, facial markings, speech properties, or number of offspring can be included in these variants.
  • On the other hand, some characteristics show little or no difference among individuals, such as the number of vertebrate eyes.
• Inheritance:
  • Some characteristics are routinely passed on from parent to offspring.
  • These characteristics are heritable, while other traits are highly affected by environmental factors and demonstrate poor heritability.
• Increasing population growth rate:
  • Every year, most populations have more offspring than local resources can sustain, leading to a battle for resources.
  • Considerable mortality is faced by each generation.
• Variability in survival and reproduction:
  • Individuals with characteristics well suited to the fight for local resources will bring more offspring to the next generation.
• Evolution as a genetic function:

The concept of natural selection:

• With the presence of genetic variation, the core argument of Darwin’s theory of evolution begins.
• It was demonstrated to Darwin that the experiences with the plant and animal breeding led to variations that could be significant for man.
• Thus, he concluded that variations that are favorable or important for organism to survive should exist.
• These favorable variations enhanced the chances for the living and procreation.
• Those beneficial variations were conserved and passed on to generations.
• This process is actually known as natural selection.
• An organism that is well adapted to its environment is the outcome of the process, and evolution also occurs as a consequence.
• Natural selection can then be characterized as the differential reproduction of alternative hereditary variants, defined by the fact that certain variants increase the probability of survival and reproduction of the organisms more effectively than organisms that carry alternative variants.
• Selection may occur by differences in survival, fertility, rate of development, success in mating, or any other aspect of the life cycle.
• All of these differences can be integrated under the term differential reproduction since all result in natural selection to the point that they affect the number of progenies an organism leave.
• It is possible to see evolution as a two-step process.
• First, hereditary variation takes place; second, certain genetic variations are chosen that will be passed on to the subsequent generations most effectively.
• Hereditary variation also contains two mechanisms—the spontaneous mutation of one variant into another and the sexual method that recombines those varieties (see recombination) to form a wide range of variations.
• The variants that occur from mutation or recombination are not equally passed from one generation to another.
• Others will occur more often because they are beneficial to the organism; events of chance, called genetic drift, may decide the frequency of others.

Types of natural selection:

1. Stabilizing selection:

• Through studying its effects on changing gene frequencies, natural selection can be studied, but it can also be studied by examining its effects on the observable characteristics or phenotypes of individuals in a population.
• Distribution scales of phenotypic traits such as height, weight, progeny number, or longevity usually display higher numbers of people with intermediate values and the so-called natural distribution is less and less towards the extremes.
• The selection is said to be stabilizing when individuals with intermediate phenotypes are preferred and extreme phenotypes are chosen against.
• Then the range and dispersion of phenotypes remains roughly the same from one generation to another.
• Stabilizing selection is quite usual.
• Those who have intermediate phenotypic values are the ones that live and reproduce more effectively.
• For example, mortality among newborn infants is greatest when they are either very small or very large; intermediate-size infants are more likely to survive.
• Stabilizing selection is usually noticeable after artificial selection.
• As a consequence of stabilizing selection, populations frequently maintain a steady genetic constitution in relation to many traits.
• This characteristic of populations is called genetic homeostasis.

2. Directional selection:

• In a population, the distribution of phenotypes often systematically shifts in a specific direction.
• The physical and biological aspects of the environment are evolving constantly, and the changes can be important over long periods of time.
• The environment and even the structure of the land or waters differ continuously.
• In biotic environments, that is, in the other species present, whether predators, prey, parasites, or rivals, changes often take place.
• As a result, genetic changes occur as the genotypic fitnesses can shift so that different sets of alleles are preferred.
• When species colonize new habitats where the conditions are different from those of their original habitat, the potential for directional selection often occurs.
• Furthermore, as the new genetic constitution replaces the preexisting one the emergence of a new desirable allele or a new genetic combination may prompt directional changes.
• The procedure of directional selection occurs in spurts.
• The substitution of one genetic constitution with another alters the genotypic fitnesses at other sites, which then modify their allelic frequencies, stimulating further modifications, and so on in a cascade of consequences.
• Directional selection is only possible if genetic variation occurs with regard to the phenotypic characteristics under selection.
• There are vast stores of genetic variation in natural populations, and these are constantly replenished by additional new variations that emerge by mutation.
• Artificial selection’s almost universal success and natural populations’ rapid response to new environmental challenges show that the current variety provides the materials needed for directional selection.
• Directional selection contributes to significant changes in morphology and ways of life over geologic time.
• Evolutionary changes that remain in a more or less consistent fashion over extended periods of time are defined as evolutionary trends.
• From the tiny brain of Australopithecus, human ancestors three million years ago, which was less than 500 cc in volume, to a brain about three times as large in modern humans, lateral evolutionary improvements expanded the cranial ability of the human lineage.
• Another well-studied example of directional selection is the evolution of the horse from more than 50 million years ago to modern times.

3. Diversifying selection:

• Diversifying selection, much like directional selection, drives the population towards the extremes of the trait.
• This type of selection is also termed disruptive selection.
• Diversifying selection moves the trait both directions, in contrast to directional selection.
• This can happen in a number of ways, but since species can become so distinct, it also leads to speciation.
• However, if only diversified for short periods, the selection will lead to a variety of characteristics that can be shared by one species.
• By diversifying selection, two or more divergent phenotypes in an environment may be preferred simultaneously.
• No natural environment is homogeneous; instead that the environment of any plant or animal population is a mosaic comprised of more or less distinct sub-environments.
• Also, the heterogeneity could be temporal, with change occurring over time, along with spatial.
• Species cope in different ways with environmental heterogeneity.
• One of the strategies is the selection of a generalistic genotype i.e called genetic monomorphism, that is well suited to all of the species’ sub-environments.
• Genetic polymorphism, the selection of a diversified gene pool that yields various genotypes, each suited to a particular sub-environment, is another strategy.
• In conditions in which populations living a short distance apart have been genetically distinct, the efficiency of diversifying natural selection is very evident.
• In one example, on heaps of mining waste heavily polluted with metals such as lead and copper, populations of bent grass can be found growing.
• The soil has been so polluted that it is poisonous to most plants, but it has been shown that the thick stands of bent grass growing over these refuse heaps have genes that make them resistant to high lead and copper concentrations.
• But bent grass plants that are not resistant to these metals can be found only a few metres from the polluted soil.
• Bent grasses reproduce mainly by cross-pollination, so that wind-borne pollen from the neighboring non-resistant plants is collected by the resistant grass.
• Since non-resistant seedlings are unable to grow in the polluted soil and the non-resistant seedlings outgrow the resistant ones in the surrounding uncontaminated soil, they retain their genetic differentiation.
• The evolution of these resistant strains has occurred in the lesser than 400 years since the mines were first opened.

4. Sexual selection:

• A significant factor in reproduction is mutual attraction between the sexes.
• Except for the reproductive organs and secondary sexual features, such as the breasts of female mammals, the males and females of many animal species are identical in size and form.
• However, there are species in which striking dimorphism is displayed by the sexes.
• Males are often larger and heavier, more brightly colored, or endowed with conspicuous adornments, especially in birds and mammals.
• However, bright colors make animals more conspicuous to predators-in the best of situations, the long plumage of male peacocks and paradise birds and the large antlers of aged male deer are bulky tons.
• Darwin knew that natural selection could not be predicted to favour the evolution of undesirable traits, and he was capable of offering a solution to this problem.
• He indicated that such characteristics occur by “sexual selection,” which does not depend on a fight for existence in relation to other organic beings or external conditions.”
• Other things being equal, species with greater fitness are more proficient in securing partners.
• There are two general conditions that lead to sexual selection.
• One is the choice displayed one sex (often females) for individuals of the other sex that display certain traits.
• The other is enhanced strength (usually among males) that produces greater success in attracting mates. The existence of a specific attribute among members of one sex can make them more attractive to the opposite sex in some way.
• In all kinds of species, from vinegar flies to pigeons, rats, dogs, and rhesus monkeys, this form of “sex appeal” has been experimentally illustrated.
• For example, when Drosophila flies are put together, some with yellow bodies as a result of random mutation and others with regular yellowish gray pigmentation, normal males are preferred over yellow males by females with either body color.

5. Kin selection and reciprocal altruism:

• Like other examples of sexual selection, the apparent altruistic behavior of many species is a characteristic that initially appears incompatible with the theory of natural selection.
• Altruism is a type of behavior that favors other people at the cost of the one who performs the action; the altruist’s fitness is decreased by his behavior, whereas people who behave selfishly benefit from it at no cost to themselves.
• Accordingly, natural selection may be expected to encourage the production of selfish behaviour and eradicate altruism.
• This outcome is not so convincing when it is realised that the beneficiaries of altruistic behaviour are usually relatives.
• Many of them bear the same genes, including those that foster altruistic behaviour.
• Altruism can grow through the selection of kin, which is simply a form of natural selection in which relatives are taken into account when determining the fitness of an individual.
• Natural selection favors genes that increase their carriers’ reproductive success, but it is not mandatory for reproductive success to be greater for all individuals that share a given genotype.
• On average, it is necessary for carriers of the genotype to replicate more effectively than those with alternative genotypes.
• A parent shares half of its genes with each progeny, so if the cost of the behavior to the parent is less than half of its average benefits to the progeny, a gene that promotes parental altruism is preferred by selection.
• Over the generations, such a gene is more likely to increase in frequency than an alternative gene that does not support altruistic behavior.
• Therefore, parental care is a type of altruism readily explained by the selection of kin.
• As it promotes the reproductive success of the parent’s genes, the parent spends some energy caring for the progeny.
• Kin selection goes beyond the association between parents and their offspring.
• It promotes the development of altruistic behavior when an individual’s energy invested, or the risk incurred, is compensated in excess by the benefits that follow through relatives.
• The finer the relationship between the beneficiaries and the altruist, and the higher the number of beneficiaries, the greater is the altruist’s risks and efforts.
• Individuals who live together in a herd or troop are generally related and often act in this way towards each other.
• For instance, adult zebras, instead of fleeing to protect themselves will turn towards an attacking predator to protect the young in the herd.
• Altruism often happens when the action is reciprocal among unrelated people and the cost of the altruist is smaller than the gain to the recipient.
• This reciprocal altruism is noticed in the mutual grooming of chimpanzees and other primates as they scrub each other of lice and other pests.