Evolution: Key Concepts for MCAT

What is evolution?

Evolution refers to the change in heritable characteristics of biological populations over successive generations. Genetic variation, mutation, natural selection, genetic drift, and gene flow are just a few variables that cause evolution. These processes can alter the physical characteristics, behavioral patterns, and genetic makeup of populations of organisms.

A brief history of evolution

The French biologist Jean-Baptiste Lamarck coined the word "evolution" for the first time in a biological context in the early 19th century. He put forth his theory of evolution, particularly in his 1809 book "Philosophie Zoologique." Lamarck's theory of evolution was one of the first scientific attempts to explain how species change over time. The theory was based on the notion that traits acquired during an individual’s lifetime might be passed on to its progeny. Lamarck's theory was influential at the time. However, the modern theory of evolution by natural selection was independently developed by Charles Darwin and Alfred Russel Wallace in the middle of the 19th century. This theory replaced Lamarck's idea and has become the dominant scientific theory explaining how organisms change over time.

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Natural selection

Darwin and Wallace first developed a comprehensive and detailed theory of natural selection as the primary process causing evolutionary change. Natural selection, which describes the process by which particular traits become more or less prevalent in a community over time, is a crucial mechanism of evolution. The fundamental idea behind natural selection is that individuals with advantageous traits are likelier to pass their genes to the next generation than individuals without them. This is why natural selection is also known as survival of the fittest. This process can result in the gradual accumulation of genetic alterations that give rise to the emergence of new species over many generations.

The theory of Natural selection revolves around four key concepts: variation, heritability, competition, and adaptability. Variability refers to the fact that individuals are naturally variable in any population. The source of variations is mutations. Individuals variate in size, color, physiology, and behavior.

Some of these variations are heritable and pass from parents to offspring. Natural selection can act only on those heritable traits (and amplify or diminish them) that vary between people in a population. Therefore, even if a trait is heritable, evolution by natural selection cannot occur if every member of a population shares that trait at a genetic level. Heritable traits influence not only the performance of organisms but also their offspring. For instance, an organism might possess a trait that helps its progeny avoid predators, find food, or cope with unpleasant physical circumstances. So, heritable characteristics contribute to an individual’s fitness by influencing their ability to survive and reproduce in a given environment.

Concept of fitness

Fitness is the central idea of the process of natural selection. Natural selection and the evolution of organisms depend on the fitness concept. Fitness is an organism’s ability to live and reproduce which depends on its characteristics and the selective pressures it must withstand in a specific environment. The degree to which an organism has adapted to its surroundings compared to other members of the same population determines its fitness. Higher-fitness individuals are more likely to survive, breed, and pass on their advantageous traits to the next generation. Fitness is either absolute fitness or relative. Absolute fitness and relative fitness are the two kinds of fitness. The total number of offspring an organism produces is its absolute fitness. Relative fitness, on the other hand, refers to how many offspring an organism produces in comparison to other organisms in the same population.

Selection by differential reproduction

Another essential element of natural selection is selection by differential reproduction. It states that organisms with advantageous traits are better at reproducing and passing their genes to the next generation than organisms with disadvantageous traits. Numerous elements, including genetic characteristics, environmental variables, and social interactions, may contribute to the varying reproductive success of organisms. Selection by differential reproduction drives the adaptability in populations. As advantageous traits predominate, organisms evolve in a manner that makes them better adapted to their environments. Adaptations include Physiological, behavioral, and morphological changes that enhance an individual’s ability to survive and reproduce in a specific environment. For instance, a population of birds living in an area with many predators may develop adaptations, such as improved camouflage or more effective flight, that increase their chances of survival and reproductive success.

Concepts of natural and group selection

The natural section may operate at the individual level or group level. At the individual level, it refers to the variation in individual survival and reproduction within a population. For instance, the peppered moth was primarily light-colored in the 19th century, which helped it fit in with the light-colored tree bark. However, the pollution in England during the industrial revolution made the forests darker. As a result, the population frequency of the darker moths increased due to their improved camouflage and higher survival rates. This shows how the natural section operates at the individual level.

Contrarily, group selection holds that natural selection can affect whole groups of organisms and individuals. This process results in the evolution of traits that promote sociality and collaboration because groups that cooperate or exhibit other advantageous social behaviors may have a higher survival rate and produce more offspring. The organization being chosen is typically a close social group in which interactions between individuals tend to be altruistic. Cooperation in rearing young, as seen in elephants, and cooperative hunting, as observed in lions and other social carnivores, are a few behaviors that appear to affect group selection.

Evolutionary success as an increase in percentage representation in the gene pool of the next generation

A population's frequency of a specific genetic trait may rise over time because individuals with that trait have more successful offspring than those without, which is a sign of evolutionary success. For instance, if a population of birds with longer beaks has a higher rate of successful reproduction than a population of birds with shorter beaks, then the prevalence of longer beaks in the population is likely to increase over time, showing that longer beaks are advantageous and, therefore, have a successful evolutionary history. In this way, natural selection proves to be a strong force for evolutionary success. However, measuring evolutionary progress can be complicated. For example, some species can flourish in one habitat but not in another, or they might survive temporarily but not permanently. Additionally, elements like genetic drift and bottlenecks can significantly affect a population's gene pool, possibly resulting in the loss of beneficial traits and a decline in evolutionary success.

Speciation: new species

The process by which new species develop from an original ancestral population is known as speciation. When a group of organisms loses their ability to interbreed and create fertile offspring, they become reproductively isolated from the rest of the community. Speciation is a crucial evolutionary process, promoting biodiversity and enabling organisms to adjust to various environments.

Types of speciation

Speciation can occur in four ways, including allopatric, sympatric, peripatric, and parapatric. In allopatric speciation, any geographic barrier, such as a river, mountain, etc., physically separates a small group from the rest of the population. Physical separation restricts the exchange of genetic information between the divided populations, which over time, causes genetic divergence. Geographic isolation (ocean) separated Darwin's finches on the Galapagos Islands, and they underwent allopatric speciation. Over millions of years, each species of finch developed a unique beak that is especially adapted to the kinds of food that it eats. The birds have evolved into different species with particular traits due to not breeding with one another due to their isolation. The finches on each island have adapted to specific conditions of the islands, leading to the evolution of numerous species of finches.

Sympatric speciation occurs when a single species population evolves into two or more distinct species while existing in the same geographic region with no physical barrier. The apple maggot fly is a good illustration of sympatric speciation. On the apple fruit's surface, apple maggot flies deposit their eggs, and the larvae burrow inside to feed on the fruit. When European apple maggot flies were brought to North America in the middle of the nineteenth century, they eventually split into the hawthorn and apple fly.

Due to variations in their host plants, the two fly types have developed. The apple fly deposits its eggs on the surface of apple fruit, whereas the hawthorn fly does so on the hawthorn fruit. The two populations gradually developed distinct genetic and phenotypic characteristics that improved their adaptability to their host plants.

In peripatric speciation, a small group splits from the main population due to geographical isolation. In contrast to allopatric speciation, peripatric speciation creates new species from a much smaller group of individuals. The isolated population develops distinct genetic and phenotypic traits as it evolves separately. For instance, Drosophila, or Hawaiian fruit flies, are a species of fly that can only be found in the Hawaiian Islands. The fruit flies are thought to have originated from a single species that became geographically separated on the various Hawaiian Islands. Hawaiian fruit flies have evolved into multiple species due to the isolated populations' long-term development of distinctive genetic and phenotypic characteristics that set them apart from the original population.

When environmental differences rather than physical barriers separate species, this is known as parapatric speciation. Despite the possibility of interbreeding, individuals adopt distinct traits and lifestyles. They only mate within their geographical area. So, a continuously dispersed population gave rise to new species. For example, populations of the grass Agrostis tenuis can be found in normal soil and mine waste. Some members of these populations have acquired a heavy metal tolerance, a heritable trait that enables them to survive in contaminated soil but to struggle in uncontaminated soil. Others need non-contaminated soil because they are intolerant to heavy metals. There is a significant selection for those with the proper heavy metal tolerance due to the differences in selective pressure across the two soil types. As a result, the tolerant and intolerant individuals become more genetically distinct over time, which causes a divergence in the genetic makeup of the two subpopulations.

Polymorphism

The occurrence of numerous forms or variations of a gene, trait, or phenotype in a population is polymorphism. Human blood type, which can be divided into four main types: A, B, AB, and O, is a common example of polymorphism. The presence or lack of specific antigens on the surface of red blood cells determines each blood type. Individuals with two instances of the O allele (the OO genotype) will have type O blood, whereas those with one or more A or B alleles will have type A, B, or AB blood. The A and B alleles are dominant, while the O allele is recessive. Human populations contain all four blood types, so this system is regarded as polymorphic.

Genetic variation, environmental factors, or a mix of both can result in polymorphism, which can happen at both the molecular and phenotypic levels. For instance, in some butterfly species, individuals can differ significantly in terms of the color and pattern of their wings within a single community, with some having bright and colorful wings and others having dull or camouflaged wings. Although environmental elements like temperature, light, and nutrition can have an impact, this polymorphism may be caused by genetic variations. Polymorphism can have significant effects on adaptation and development. In addition, various phenotypes may have specific benefits or disadvantages in different environments, affecting an organism's survival and reproduction ability.

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Adaptation and specialization

In evolutionary biology, "adaptation" and "specialization" relate to how organisms change over time to fit their environments better. Adaptation is how an organism develops traits or behaviors that enhance its survival and reproduction in a specific environment. These adaptations can be behavioral or physical, such as the ability to camouflage or unite in social groups for safety. Physical adaptations include the development of sharp claws for hunting. At the molecular level, adaptations can also occur, such as when proteins' structures are altered to improve their performance in particular settings.

On the other hand, specialization describes how an organism develops a strong ecological niche or habitat preference, frequently at the cost of its capacity to survive in different environments. As an illustration, certain bird species have developed specialized beaks that can adapt to various food sources. For example, hummingbirds have long, slender beaks adapted for reaching nectar deep inside flowers, whereas woodpeckers have strong, pointed beaks made for drilling into the wood to locate insects.

While specialization can benefit organisms in their particular environment, it can also make them more susceptible to environmental disruptions or changes. For instance, a species heavily dependent on one type of food source may need help surviving if that food source becomes scarce or unavailable. To live and reproduce in various environments, many organisms display a balance of adaptations and generalist characteristics.

Inbreeding, outbreeding, bottleneck

When related people mate, it is said to be inbreeding. It leads to Inbreeding depression, or reproductive fitness decline, which affects most plant and animal species. This is due to the increased likelihood that when two closely related people mate, their offspring will receive two copies of a deleterious allele, which can result in genetic disorders or decreased fitness. In addition, a population's heterozygosity may decrease due to inbreeding because it can make homozygous alleles more common.

Mating involving people from various populations, subspecies, or species is called outbreeding. A population's genetic diversity may grow due to outbreeding, enhancing its capacity for adaptation and evolution. The expression of advantageous recessive characteristics that were previously hidden by dominant alleles is another benefit of outbreeding. Outbreeding, however, can also raise the chance of hybridization and the introduction of harmful alleles from other groups.

A population can reach a bottleneck when its size suddenly and drastically decreases, frequently due to a natural catastrophe, habitat loss, or human activities like hunting or habitat fragmentation. Because the surviving individuals are likely only to possess a portion of the genetic diversity present in the initial population, bottlenecks can cause a decrease in genetic diversity within a population. Due to this, rare alleles may disappear and dangerous alleles will become more prevalent. As a result, populations that have gone through a bottleneck might have less capacity for adaptation and be more susceptible to environmental stresses.

Evolutionary time: as measured by gradual random changes in the genome

Evolutionary time is the period since two or more species separated from a shared ancestor. The accumulation of slow, random changes in an organism's genome over extended periods can be used to determine the rate of evolution. Random mutations build up in species' DNA sequences, gradually changing the genome. Scientists can determine how long it has been since an organism diverged from another by analyzing the DNA sequences of various species. The history and diversity of species on Earth can be better understood through evolution. Overall, accumulating these small, haphazard changes over a long period can lead to critical evolutionary variations, like the emergence of new species or the evolution of intricate adaptations. Numerous variables, such as climate, genetic drift, gene flow, and natural selection, can affect the speed and direction of these changes.

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