Natural Selection & Evolution

• Biology is the scientific study of life, examining the structure, function, growth, origin, evolution, and distribution of living organisms
• It encompasses a wide range of fields, including botany, zoology, microbiology, genetics, and ecology
• The principles of biology are essential for understanding the natural world and addressing challenges related to health, agriculture, and the environment
• Selection is a fundamental mechanism of evolution, referring to the process by which certain traits become more or less common in a population over time due to differential survival and reproduction
• Evolution is the change in the heritable characteristics of biological populations over successive generations
• These evolutionary changes give rise to diversity at every level of biological organization, including species, individual organisms, and molecules

Natural Selection

• Natural selection is a key mechanism of evolution
• It is the differential survival and reproduction of individuals due to differences in phenotype
• Phenotype is the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment
• Natural selection leads to adaptive evolution, where populations become better suited to their environment over time
• The principles of natural selection were famously described by Charles Darwin in his book "On the Origin of Species"
• Darwin outlined that more individuals are produced each generation than can survive
• He also stated that phenotypic variation exists among individuals and the variation is heritable
• Darwin also proposed that individuals with traits better suited to their environment are more likely to survive and reproduce
• Over generations, this process leads to a greater proportion of individuals with advantageous traits in the population

Artificial Selection

• Artificial selection, also known as selective breeding, is the process by which humans intentionally breed animals and plants for desirable traits
• Unlike natural selection, where the environment drives the selection process, artificial selection is driven by human preferences
• Artificial selection has been used for millennia to improve crop yields, enhance livestock productivity, and create new breeds of pets
• Artificial selection provides strong evidence for the power of selection to drive evolutionary change
• The rapid and dramatic changes observed in domesticated species demonstrate the potential for selection pressures to reshape populations over time
• An example of artificial selection is seen in the various breeds of domestic dogs, all of which are descended from wolves and have been selectively bred for different traits such as size, coat type, and temperament

Sexual Selection

• Sexual selection is a specific type of natural selection in which individuals with certain heritable traits are more likely to obtain mates
• It can lead to the evolution of traits that enhance mating success, even if those traits are detrimental to survival
• Sexual selection often results in sexual dimorphism, where males and females of a species exhibit different physical characteristics
• Examples of sexual selection include the bright plumage of male birds, the antlers of male deer, and the elaborate courtship displays of many animals
• These traits can increase a male's attractiveness to females or allow them to compete more effectively with other males for access to mates
• Sexual selection can occur through two main mechanisms namely intersexual selection and intrasexual selection
• Intersexual selection (mate choice) occurs when individuals of one sex (usually females) choose mates based on certain traits
• Intrasexual selection (male-male competition) involves competition between individuals of the same sex (usually males) for access to mates

Genetic Drift

• Genetic drift is a random process that can cause changes in allele frequencies in a population over time
• Unlike natural selection, genetic drift does not lead to adaptation
• It is most pronounced in small populations, where chance events can have a significant impact on the genetic makeup of the population
• Genetic drift can lead to the loss of genetic variation within a population, as some alleles become more common by chance while others disappear
• The founder effect and bottleneck effect are two special cases of genetic drift
• The founder effect occurs when a small group of individuals establishes a new population, the allele frequencies in the new population may differ from those in the source population due to chance
• The bottleneck effect occurs when a population undergoes a drastic reduction in size, the surviving individuals may not represent the genetic diversity of the original population

Gene Flow

• Gene flow, also known as gene migration, is the transfer of genetic material from one population to another
• Gene flow can introduce new alleles into a population or alter the frequencies of existing alleles
• Gene flow can reduce genetic differences between populations and prevent them from diverging
• The extent of gene flow between populations depends on the dispersal ability of individuals and the presence of barriers to movement
• For example, if pollen is able to move between different plant populations, this is an example of gene flow
• Additionally, if animals move between different populations, this is an example of gene flow in animals

Mutation

• Mutation is the ultimate source of all new genetic variation
• Mutations are random changes in the DNA sequence that can occur during DNA replication, repair, or cell division
• Most mutations are neutral or harmful, but some can be beneficial
• Beneficial mutations can increase an individual's fitness and be favored by natural selection
• Mutations can be small-scale (e.g., point mutations) or large-scale (e.g., chromosomal rearrangements)
• The rate of mutation varies among different genes and organisms
• Without mutation, there would be no new variation for natural selection to act upon, and evolution would grind to a halt
• Mutations, in combination with other evolutionary mechanisms, are responsible for the diversity of life on Earth

Adaptation

• Adaptation is the process by which populations evolve traits that enhance their survival and reproduction in a particular environment
• Adaptations can be structural, physiological, or behavioral
• Examples of adaptations include the camouflage of insects, the drought tolerance of desert plants, and the migration patterns of birds
• Adaptations arise through natural selection, as individuals with advantageous traits are more likely to survive and reproduce
• Adaptations are not always perfect such as trade-offs and constraints can limit the extent to which populations can adapt to their environment
• For example, a trait that is beneficial in one context may be detrimental in another
• Also, physical and genetic constraints can prevent populations from evolving certain traits, even if they would be advantageous

Speciation

• Speciation is the process by which new species arise
• It is the evolutionary process by which populations evolve to become distinct species
• Speciation typically occurs when populations become reproductively isolated, preventing gene flow between them
• Reproductive isolation can arise through various mechanisms, including geographic isolation (allopatric speciation) and reproductive barriers (sympatric speciation)
• Allopatric speciation occurs when populations are geographically separated, such as by a mountain range or a body of water
• Sympatric speciation occurs when populations diverge in the same geographic area, often due to reproductive barriers such as prezygotic or postzygotic isolation
• Prezygotic barriers prevent mating or fertilization from occurring
• Postzygotic barriers occur after the formation of a hybrid zygote and can lead to reduced hybrid viability or fertility
• Speciation is a key process driving the diversification of life on Earth, it has resulted in the vast array of species we see today

Evidence for Evolution

• The evidence for evolution comes from a variety of sources, including:
• Fossil Record which provides a historical record of life on Earth, documenting the evolution of different species over time
• Comparative Anatomy which reveals similarities and differences in the anatomy of different species, reflecting their evolutionary relationships
• Molecular Biology shows similarities and differences in the DNA and protein sequences of different species, providing further evidence for their evolutionary relationships
• Biogeography examines the geographic distribution of species, which can provide insights into their evolutionary history
• Direct Observation involves observing evolution in real-time, such as the evolution of antibiotic resistance in bacteria
• This wealth of evidence supports the conclusion that evolution is a well-supported scientific theory

Explore natural selection, a key mechanism driving evolution. Understand how differential survival and reproduction lead to changes in heritable traits over generations. Learn how these evolutionary shifts contribute to biological diversity across various organizational levels.