5.5. Evidence of Evolution

Evidence of common descent of living things has been discovered by scientists working in a variety of fields over many years. This evidence has demonstrated and verified the occurrence of evolution and provided a wealth of information on the natural processes by which the variety and diversity of life on Earth developed. This evidence supports the modern evolutionary synthesis, the current scientific theory that explains how and why life changes over time. Evolutionary biologists document the fact of common descent: making testable predictions, testing hypotheses, and developing theories that illustrate and describe its causes.

Genetics

One of the strongest evidences for common descent comes from the study of gene sequences. Comparative sequence analysis examines the relationship between the DNA sequences of different species, producing several lines of evidence that confirm Darwin’s original hypothesis of common descent. If the hypothesis of common descent is true, then species that share a common ancestor inherited that ancestor’s DNA sequence, as well as mutations unique to that ancestor. More closely related species have a greater fraction of identical sequence and shared substitutions compared to more distantly related species.
Universal phylogenetic tree of life based on comparisonof small subunit rRNA (ribosomal RNA) sequences; source
Universal phylogenetic tree of life based on comparison
of small subunit rRNA (ribosomal RNA) sequences; source

The simplest and most powerful evidence is provided by phylogenetic reconstruction. Such reconstructions, especially when done using slowly evolving protein sequences, are often quite robust and can be used to reconstruct a great deal of the evolutionary history of modern organisms (and even in some instances such as the recovered gene sequences ofmammoths, Neanderthals or T. rex, the evolutionary history of extinct organisms). These reconstructed phylogenies recapitulate the relationships established through morphological and biochemical studies. The most detailed reconstructions have been performed on the basis of the mitochondrial genomes shared by all eukaryotic organisms, which are short and easy to sequence; the broadest reconstructions have been performed either using the sequences of a few very ancient proteins or by using ribosomal RNA sequence.

Phylogenetic relationships also extend to a wide variety of nonfunctional sequence elements, including repeats, transposons, pseudogenes, and mutations in protein-coding sequences that do not result in changes in amino-acid sequence. While a minority of these elements might later be found to harbor function, in aggregate they demonstrate that identity must be the product of common descent rather than common function.

Other techniques used are:

  • Universal biochemical organisation and molecular variance patterns. All known extant (surviving) organisms are based on the same biochemical processes: genetic information encoded as nucleic acid (DNA, or RNA for viruses), transcribed into RNA, then translated into proteins (that is, polymers of amino acids) by highly conserved ribosomes.
  • DNA sequencing. Comparison of the DNA sequences allows organisms to be grouped by sequence similarity, and the resulting phylogenetic trees are typically congruent with traditional taxonomy, and are often used to strengthen or correct taxonomic classifications. Sequence comparison is considered a measure robust enough to correct erroneous assumptions in the phylogenetic tree in instances where other evidence is scarce.
  • Endogenous retroviruses. (ERVs) are remnant sequences in the genome left from ancient viral infections in an organism. The retroviruses (or virogenes) are always passed on to the next generation of that organism that received the infection.

    sanger
    DNA sequence in photograph paper
  • Proteins. The proteomic evidence also supports the universal ancestry of life. Vital proteins, such as the ribosome, DNA polymerase, and RNA polymerase, are found in everything from the most primitive bacteria to the most complex mammals.
  • Pseudogenes. Pseudogenes, also known as noncoding DNA, are extra DNA in a genome that do not get transcribed into RNA to synthesize proteins. Some of this noncoding DNA has known functions, but much of it has no known function and is called “Junk DNA”. This is an example of a vestige since replicating these genes uses energy, making it a waste in many cases. Pseudogenes make up 99% of the human genome (1% working DNA).

Evidence from comparative anatomy

  • Atavism is the tendency to revert to ancestral type. In biology, an atavism is an evolutionary throwback, such as traits reappearing which had disappeared generations before.
  • Embryological evidence, comes from the development of organisms at the embryological level with the comparison of different organisms embryos similarity. Remains of ancestral traits often appear and disappear in different stages of the embryological development process

    “Ontogeny recapitulates phylogeny”

Ernst Haeckel (1834–1919), a physician, was so influenced by Charles Darwin’s The Origin of Species that he gave up medicine and devoted himself to comparative anatomy. He disagreed with Darwin’s theory of natural selection, and suggested that the environment acted directly on organisms, producing new species. In 1868, he proposed the biogenetic law, which sought to explain evolution as a series of stages in which the new characteristics of the next animal to evolve are simply added on to the lower animal. Briefly put, his biogenetic law stated that ontogeny recapitulates phylogeny (the embryological development of a particular species repeats the evolutionary history of that species).

  • Homologous structures and divergent (adaptive) evolution. If widely separated groups of organisms are originated from a common ancestry, they are expected to have certain basic features in common. The degree of resemblance between two organisms should indicate how closely related they are in evolution:

    • Groups with little in common are assumed to have diverged from a common ancestor much earlier in geological history than groups with a lot in common;
    • In deciding how closely related two animals are, a comparative anatomist looks for structures that are fundamentally similar, even though they may serve different functions in the adult. Such structures are described as homologous and suggest a common origin.
    • In cases where the similar structures serve different functions in adults, it may be necessary to trace their origin and embryonic development. A similar developmental origin suggests they are the same structure, and thus likely derived from a common ancestor.

    When a group of organisms share a homologous structure that is specialized to perform a variety of functions in order to adapt different environmental conditions and modes of life are called adaptive radiation. The gradual spreading of organisms with adaptive radiation is known as divergent evolution.

  • Skeleton of a Baleen whale with the hind limb and pelvic bone structure circled in red. This bone structure stays internal during the entire life of the species.
    Adaptation of insect mouthparts: a, antennae; c, compound eye; lb, labrium; lr, labrum; md, mandibles; mx, maxillae.
    (A) Primitive state — biting and chewing: e.g. grasshopper. Strong mandibles and maxillae for manipulating food.
    (B) Ticking and biting: e.g. honey bee. Labium long to lap up nectar; mandibles chew pollen and mould wax.
    (C) Sucking: e.g. butterfly. Labrum reduced; mandibles lost; maxillae long forming sucking tube.
    (D) Piercing and sucking, e.g.. female mosquito. Labrum and maxillae form tube; mandibles form piercing stylets; labrum grooved to hold other parts.

    A strong and direct evidence for common descent comes from vestigial structures. Rudimentary body parts, those that are smaller and simpler in structure than corresponding parts in the ancestral species, are called vestigial organs. They are usually degenerated or underdeveloped. The existence of vestigial organs can be explained in terms of changes in the environment or modes of life of the species. Those organs are typically functional in the ancestral species but are now either nonfunctional or re-purposed. Examples are the pelvic girdles of whales, haltere (hind wings) of flies and mosquitos, wings of flightless birds such as ostriches, and the leaves of some xerophytes (e.g. cactus) and parasitic plants (e.g. dodder). However, vestigial structures may have their original function replaced with another. For example, the halteres in dipterists help balance the insect while in flight and the wings of ostriches are used in mating rituals.

Paleontology

One piece of evidence offered by Darwin is found in the science of paleontology. Paleontology deals with locating, cataloging, and interpreting the life forms that existed in past millennia. It is the study of fossils—the bones, shells, teeth, and other remains of organisms, or evidence of ancient organisms, that have survived over eons of time.

Paleontology supports the theory of evolution because it shows a descent of modern organisms from common ancestors. Paleontology indicates that fewer kinds of organisms existed in past eras, and the organisms were probably less complex. As paleontologists descend deeper and deeper into layers of rock, the variety and complexity of fossils decreases. The fossils from the uppermost rock layers are most like current forms. Fossils from the deeper layers are the ancestors of modern forms.

Current distribution of Glossopteris placed on a Permian map showing the connection of the continents. (1, South America; 2, Africa; 3, Madagascar; 4, India; 5, Antarctica; and 6, Australia)

Evidence from geographical distribution

Data about the presence or absence of species on various continents and islands (biogeography) can provide evidence of common descent and shed light on patterns of speciation.

Continental distribution

All organisms are adapted to their environment to a greater or lesser extent. If the abiotic and biotic factors within a habitat are capable of supporting a particular species in one geographic area, then one might assume that the same species would be found in a similar habitat in a similar geographic area, e.g. in Africa and South America. This is not the case. Plant and animal species are discontinuously distributed throughout the world:

  • Africa has Old World monkeys, apes, elephants, leopards, giraffes, and hornbills.

    Present day distribution of marsupials. (Distribution shown in blue. Introduced areas shown in green.)
  • South America has New World monkeys, cougars, jaguars, sloths, llamas, and toucans.
  • Deserts in North and South America have native cacti, but deserts in Africa, Asia, and Australia have succulent native euphorbs that resemble cacti but are very different, even though in some cases cacti have done very well (for example in Australian deserts) when introduced by humans
A dymaxion map of the world showing the distribution of present species of camelid. The solid black lines indicate migration routes and the blue represents current camel locations.
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