The Theory of Evolution [BIOLOGY]

• On the Origin of Species: Natural Selection

  Darwin’s experience with the voyage allowed him to form his notion of evolution: “descent with modification.” Darwin isn’t the first to propose evolution but his work provided a mechanism to explain the process
• On the Origin of Species: Natural Selection

  In the process called natural selection, individuals with certain traits will survive and reproduce more than individuals who lack traits that fit into their living environment. Over generations, these traits accumulate and modify the organisms to better suit their specific ways of life in their environment. These accumulated traits and modifications are called adaptations.
• Evidence for Evolution

  He was particularly intrigued by the organisms in Galapagos Islands, located 900 km towards the Pacific from South America, as most animal inhabitants are not found elsewhere in the world but resemble South American species.
• Evidence for Evolution

  In addition, he witnessed how an earthquake was able to lift Chile’s coastline by a meter and this helped him refine ideas that natural forces affect the Earth’s surface and, in turn, how life developed. This also helped explain how he was able to find fossils of marine snails high up on the Andes Mountain.
• Evidence for Evolution

  Paleontologists, scientists who study fossils, gain access to very old fossils when erosion carves through the upper, younger stratum into the deeper, buried layers. When fossils are found in the same layer/stratum, it can be assumed that they are found living in the same point in time and this can be confirmed through dating methods. Because these layers sort of chronicle the evolution over millions of years in the order in which fossils appear in the rock strata, this has been referred to as the fossil record and is one of the pieces of evidence for evolution.
• Evidence for Evolution

  Another evidence for evolution is the presence of anatomical structures that may have different functions but are structurally similar because they come from a common ancestor. Similarity coming from this common ancestry is called homology and these features are called homologous structures. An example is the forelimbs of mammals. Comparing the limb of a human to that of a whale (that has become a flipper) or that of a bat (that has been modified for flight), the functions differ but when observed anatomically will have similarities.
• Evidence for Evolution

  This differs from analogous structures which are similar features that evolved independently for distantly related organisms. For example, the marsupial sugar glider from Australia is able to glide like the flying squirrel, a eutherian, because they modified their flap of skin tissues to allow gliding. Analogous structures come to be because of convergent evolution, the independent evolution of similar features in different lineages
• Evidence for Evolution

  Some homologous structures turned into remnant structures that have become marginal or perhaps of no importance to the organism. Called vestigial structures or rudimentary structures, these features served important functions for the ancestors of the lineage. An example in the human body is the appendix. In ancestral species, diet is composed primarily of plant material hence the appendix then functioned as sites where symbiotic microorganisms would help digest cellulose from plant material.
• Evidence for Evolution

  Having proposed a mechanism for evolution, Darwin conceived the notion that artificial selection – selective breeding of domesticated plants and animals to get desirable traits in their offspring – was key to understanding change brought by evolution. By talking to farmers about livestock breeding, he learned two essential components of artificial selection: variation and heritability.
• Evidence for Evolution

  Variation, differences among individuals in the same group, allows breeders to select animals or plants with the most desirable combination of characters as breeding stock. Heritability, meanwhile, refers to the transmission of a trait from parent to offspring. Despite lacking underlying knowledge of genetics, breeders are already aware of the importance of heritability in artificial selection.
• Mechanisms of Microevolution

  This condition is known as the Hardy-Weinberg equilibrium and for a population to be in this equilibrium

  • The population must be large. Smaller populations are more prone to have their allele frequencies fluctuate due to chance events or encounters, depending on the magnitude of the event.
  • No gene flow between populations. When individuals move into or out of populations, they add or remove alleles which alter the gene pool. Think of animal hybrids as clear examples.
• Mechanisms of Microevolution

  This condition is known as the Hardy-Weinberg equilibrium and for a population to be in this equilibrium

  • No mutations.
  • Random mating. As in the case of artificial selection if individuals mate preferentially, specific traits are favored and this modifies the gene pool.
  • No natural selection.
• Darwin was the first to examine sexual selection, natural selection that favors individuals with certain traits fashioned for obtaining mates. Secondary sexual characteristics, noticeable differences not directly related to reproduction, manifest as sexual dimorphisms. These may come in the form of size difference; but can also include forms of adornment like horns, manes, and plumage with males usually being the showier sex, at least among vertebrates.
• Sexual selection can be intrasexual or intersexual. When individuals of the same sex compete directly for mates, this refers to the former and is usually found in species where the winning individual acquires a harem of mates. The latter is the more common type of sexual selection wherein individuals of one sex (usually females) are choosy in selecting mates.