BIO 112 Notes: Evidence of Evolution

Fossils and Evolution

Fossils are found in sedimentary rocks formed in the sand and mud that settle to the bottom of seas, lakes, and swamps. New sediment layers cover old ones and compress them into layers of rock called strata.

  • The fossils found in the strata provide a glimpse to the organisms that populated the earth at the time of the layer forming. This contributes to the theory of evolution, because originally it was thought the earth was only a few thousand years old and floods were the cause of valleys and canyons, but with the theory of evolution, it was proposed they were formed by ==slow, continuous actions==, and the earth must be much older than people thought.
  • This supports the theory of evolution because it takes organisms millions of years to evolve into what they are today, and because the earth is so old as well, it is probable evolution is likely.

Evolution: the process by which different kinds of living organisms are thought to have developed and diversified from earlier forms during the history of the earth.

  • It occurs due to a changing environment, forcing organisms to adapt over time and show altered genes, novel traits, and new species.

Artificial Selection

Artificial selection: when humans modify species over many generations by selecting and breeding individuals with desired traits. People have been artificially selecting animals and plants for years.

Scientific evidence that supports the theory of evolution

  • Direct observations: Biologists have documented evolutionary change in thousands of scientific studies
  • Homology: Similarity resulting from common ancestry
  • The fossil record: documents the pattern of evolution, showing that past organisms differed from present-day organisms and that many species have become extinct.
  • Biogeography: the scientific study of the geographic distributions of species.

Closely related species share the features used to determine their relationship, but they also share many other features that make little sense except in the context of evolution.

  • For example, the forelimbs of all mammals, including humans, cats, whales, and bats, show the same arrangement of bones from the shoulder to the tips of the digits, even though the appendages have very different functions: lifting, walking, swimming, and flying. Such anatomical resemblances would be highly unlikely if these structures had arisen anew in each species. Rather, the underlying skeletons of the arms, forelegs, flippers, and wings of different mammals are homologous structures that represent variations on a structural theme that was present in their common ancestor.

Microevolution

Microevolution: a change in allele frequencies in a population over generations. The three main mechanisms that cause it are natural selection, genetic drift, and gene flow.

Natural Selection

Natural selection: a differential success in reproduction which results in certain alleles being passed to the next generation in greater proportions.

  • Natural selection causes adaptive evolution, a process in which traits that enhance survival or reproduction increase in frequency over time.

Genetic Drift

Genetic drift: a process where allele frequencies fluctuate unpredictably from one generation to the next.

  • An example of genetic drift is the ==bottleneck effect==, which can eliminate alleles from the entire gene pool of the species due to a drastic reduction in population size from a sudden environmental change.
  • Another example of genetic drift is the ==founder effect==, which is when a few individuals in a population colonize a new location that's separate from the old population. This also greatly reduces the population size, as well as reduces the genetic variability of the population.

Natural Selection

The first main mode of natural selection is directional selection.

  • Directional selection occurs when conditions favor individuals exhibiting one extreme of a phenotypic range, causing a population to become more frequent in that phenotype.

The second main mode of natural selection is disruptive selection.

  • Disruptive selection occurs when conditions favor individuals of two different extreme phenotypes over individuals with intermediate phenotypes. Therefore, a population begins to become more frequent in extreme phenotypes.

The third main mode of natural selection is stabilizing selection, which favors intermediate phenotypes over extreme phenotypes.

  • The result is a population becoming more frequent in intermediate phenotypes rather than extreme.

Directional (pointing one way, extreme), Disruptive (rupturing the middle), Stabilizing (middle ground stable)

Pace of Evolution

Gradualism states that evolutionary change happens continuously and gradually.

  • Many genetic changes that would be insignificant by themselves build up until there is a significant phenotypic difference and an organism distinctly different from the original arises, and it can be considered a new species.

Contrarily, punctuated equilibrium suggests that the pace of speciation is much more random and dynamic.

  • It explains that species maintain a relatively unchanged phenotype and genotype for many generations, followed by an extremely rapid change in traits, resulting in a new species over a short amount of time.

Reproductive Isolation

Reproductive isolation is the existence of biological factors (barriers) that impede members of two species from interbreeding and producing viable, fertile offspring.

  • Prezygotic barriers: a reproductive barrier that impedes mating between species or hinders fertilization if interspecific mating is attempted.
    • Example: Such barriers typically act in one of three ways: by impeding members of different species from attempting to mate, by preventing an attempted mating from being completed successfully, or by hindering fertilization if mating is completed successfully.
    • Prezygotic mechanisms prevent the formation of a zygote, whereas post-zygotic mechanisms block the development of a viable and fertile individual after fertilization has taken place.
    • Some prezygotic isolating mechanisms examples: habitat isolation (geographic barrier), temporal isolation (species reproduce at different times of the day or year), behavioral isolation (very little interspecies mating happens because of difference of behavior like a bird’s song), mechanical isolation (when morphological features like size prevent 2 species from mating), gametic isolation (2 species try to breed but gametes fail to unite) Here are some postzygotic isolating mechanisms she listed as examples: hybrid inviability (fertilized egg cannot develop past early embryonic stages), hybrid sterility (when an interspecies hybrid may be viable but sterile, like a mule)
  • Postzygotic barriers: a reproductive barrier that prevents hybrid zygotes produced by two different species from developing into viable, fertile adults.
    • Example: If a sperm cell from one species overcomes prezygotic barriers and fertilizes an ovum from another species, a variety of postzygotic barriers (“after the zygote”) may contribute to reproductive isolation after the hybrid zygote is formed. For example, developmental errors may reduce survival among hybrid embryos. Or problems after birth may cause hybrids to be infertile or decrease their chance of surviving long enough to reproduce.