In the Origin of Species, Darwin marshaled a broad range of evidence to support the concept of descent with modification. Still - as he acknowledged - there were many instances where evidence was lacking.
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| Original Origin of Species |
In the last 150 years, many new discoveries have filled the gaps that Darwin identified. Significantly, the Theory of Evolution has not been weakened but instead strengthened by the genetic revolution. The discovery of DNA has lent itself to more detailed understanding of how descent with modification has occurred.
Let's consider the types of evolutionary evidence:
- Fossil Record (including Transitional Fossils) - Both discussed on other pages (see links)
- Direct observation of evolution - also see "Before Our Very Eyes"
- Homology
- Biogeography
And now, let's examine more closely three of the four below:
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Direct Observations of Evolutionary Change
Biologists have documented observed evolutionary change in thousands of scientific studies. Let's take a look at two examples in particular.
Natural Selection in Response to Introduced Plant Species
Herbivores often have adaptations that help them feed efficiently on their primary food sources. What happens when herbivores begin to feed on a plant species with different characteristics than their usual food source?
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| Corizus hyoscyami |
An opportunity to study this question in nature is provided by soapberry bugs, which use their hollow, needle-like mouth part to feed on seeds located within the fruits of various plants. In southern Florida, the soapberry bug Jadera haematoloma feeds on the seeds of a native plant, the balloon vine (Cardiospermum corindum). But in central Florida, balloon vines have become rare, so instead soapberry bugs in that region now feed on the goldenrain tree (Koelreuteria elegans), a species recently introduced from Asia.
Soapberry bugs feed most effectively when their beak length closely matches the depth at which the seeds are found within the fruit. Goldenrain tree fruit consists of three flat lobes, and its seeds are much closer to the fruit surface than the seeds of the plump, round native balloon vine. Predictably, the beak lengths of soapberry bugs living in central Florida are evolving to be shorter.
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| goldenrain tree |
The Evolution of Drug-Resistant Bacteria
An example of ongoing natural selection that dramatically affects humans is the evolution of drug-resistant pathogens. This is a particular problem with bacteria and viruses because resistant strains of these pathogens can proliferate very quickly.
For example, the evolution of drug resistance by the bacterium Staphylococcus aureus. About one in three people harbor this species in their skin or nasal passages with no negative effects. However, certain genetic varieties (strains) of this species known as methicillin-resistant S. aureus (MRSA) are dangerous pathogens. The past few decades have seen an alarming increase in virulent forms of MRSA such as clone USA300, a strain that can cause "flesh-eating diesase" and potentially fatal infections. How did this strain of MRSA become so dangerous?
In 1943, when penicillin became the first widely antibiotic, S. aureus had already developed a 20% resistance to the drug. The bacteria had mutated and developed an enzyme that it could use to destroy the penicillin. Researchers responded and developed antibiotics that were resistant to the new enzyme, but some S. aureus populations developed resistance to each new drug within a few years.
In 1959, doctors began using the extremely powerful methicillin, but within two years, methicillin-resistant strains appeared. How did these strains emerge?
Methicillin works by deactivating a protein that bacteria use to synthesize their cell walls. However, S. aureus populations exhibited variations in how strongly their members were affected by the drug. In particular, some individuals were able to synthesize their cell walls using a different protein that was not affected by methicillin. These individuals survived the methicillin treatments and reproduced at higher rates. Over time, these resistant strains became increasingly common leading to the spread of MRSA.
Although initially MRSA could be controlled by antibiotics that worked differently than methicillin, it has become increasingly difficult because some MRSA strains are resistant to multiple antibiotics. This is presumably because bacteria can exchange genes with members of their own and other species.Thus, the current multi-drug resistant forms may have emerged over time as MRSA strains that were resistant to different antibiotics exchanged genes.
These two examples highlight two key points about natural selection. First natural selection is not a process of editing, not a creative mechanism. A drug does not create resistant pathogens; it selects for resistant individuals that are already present in the population. Second, natural selection depends on time and place. It favors those characteristics in a genetically variable population that provide advantage in the current local environment. What is beneficial in one situation may be useless or even harmful in another. Beak lengths arise that match the size of the typical fruit eaten by a particular soapberry bug population. However, a beak length suitable and beneficial for fruit of one size can be a disadvantageous when the bug is feeding on fruit of another size.
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| MRSA |
In 1959, doctors began using the extremely powerful methicillin, but within two years, methicillin-resistant strains appeared. How did these strains emerge?
Methicillin works by deactivating a protein that bacteria use to synthesize their cell walls. However, S. aureus populations exhibited variations in how strongly their members were affected by the drug. In particular, some individuals were able to synthesize their cell walls using a different protein that was not affected by methicillin. These individuals survived the methicillin treatments and reproduced at higher rates. Over time, these resistant strains became increasingly common leading to the spread of MRSA.
Although initially MRSA could be controlled by antibiotics that worked differently than methicillin, it has become increasingly difficult because some MRSA strains are resistant to multiple antibiotics. This is presumably because bacteria can exchange genes with members of their own and other species.Thus, the current multi-drug resistant forms may have emerged over time as MRSA strains that were resistant to different antibiotics exchanged genes.
Understanding the Implications
These two examples highlight two key points about natural selection. First natural selection is not a process of editing, not a creative mechanism. A drug does not create resistant pathogens; it selects for resistant individuals that are already present in the population. Second, natural selection depends on time and place. It favors those characteristics in a genetically variable population that provide advantage in the current local environment. What is beneficial in one situation may be useless or even harmful in another. Beak lengths arise that match the size of the typical fruit eaten by a particular soapberry bug population. However, a beak length suitable and beneficial for fruit of one size can be a disadvantageous when the bug is feeding on fruit of another size.
Homology
Another type of evidence for evolution comes from analyzing similarities among different organisms. As discussed, evolution is a process of descent with modification: Characteristics present in an ancestral organism are altered (by natural selection) in its descendants over time as they face different environmental conditions. As a result, related species can have characteristics that have an underlying similarity yet function differently. Similarity resulting from common ancestry is known as homology.
Anatomical and Molecular Homologies
The view of evolution as a remodeling process leads to the prediction that closely related species should share similar features - and they do. Of course, closely related species share the features use to determine their relationship, but they also share many other features. Some of these shared features make little sense except in the context of evolution.
For example, the forelimbs of all mammals, including humans, cats, whales, bats, and many others, show the same arrangement of bones from the shoulder to the tips of the digits, even though these appendages have very different functions: lifting, walking, running, swimming, or flying.
Such striking anatomical resemblances would be highly unlikely if these structures had arise 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.
Comparing early stages of development in different animal species reveals additional anatomical homologies not visible in adult organisms.
For example, at some point in their development, all vertebrate embryos have a tail located posterior to the anus as well as structures called pharyngeal pouches (gills).
These homologous throat pouches ultimately develop into structures with very different functions, such as gills in fishes and parts of the ears in humans and other mammals.
Perhaps the most interesting homologies concern the "left over" structures of marginal, if any importance to the organism. These vestigial structures are remnants of features that served a function in the organism's ancestors.
For example, the skeletons of some snakes retain vestiges of the pelvis and leg bones of walking ancestors. Another example is provided by eye remnants that are buried under the scales of blind species of cave fishes. We would not expect to see these vestigial structures if snakes and blind cave fishes had origins separate from other vertebrate animals, or if they had been created.
Here are a few more examples of vestigial organs:
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| vestigial human appendix |
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| Vestigial wisdom teeth |
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| Blind mole rat has flaps of skin that seal its eyes |
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| Vestigial pelvis in whales |
Similarities among organisms can also be observed at the molecular level. All forms of life use the same genetic language of DNA and RNA, and the genetic code is essentially universal. thus it is likely that all species descended from a common ancestors that used this code. But molecular homologies go beyond a shared code.
For example, organisms as dissimilar as humans and bacteria share genes inherited from a very, very distant common ancestor. Some of these homologous genes have acquired new functions, while others, such as those coding for the ribosomal subunits used in protein synthesis, have retained their original functions. It is also common for organisms to have genes that have lost their function even though the homologous genes in the related species may be fully functional.
Like vestigial structures, is appears that such inactive vestigial genes may be present simply because a common ancestor had them.
Homologies and "Tree Thinking"
Some homologous characteristics, such as the genetic code, are shared by all species because they date to the deep ancestral past. In contrast, homologous characteristics that evolved more recently are shared only within smaller groups of organisms.
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| Map of Biological Domains and Kingdoms |
Consider the tetrapods, the vertebrate group that consists of amphibians, mammals, and reptiles. All tetrapods have limbs with digits, whereas other vertebrates do not. Thus, homologous characteristics form a nested pattern: All life shares the deepest layer, and each successive smaller group adds its own homologies to those it shares with larger groups.
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| Example of Nesting |
This nested pattern is exactly what we would expect to result from descent with modification from a common ancestor.
Convergent Evolution
Although organisms that are closely related share characteristics because of common descent, distantly related organisms can resemble one another for a different reasons: convergent evolution, the independent evolution of similar features in different lineages.
For example, marsupial mammals are distinct from another group of mammals the eutherians - few of which live in Australia. Eutherians complete their embryonic development in the uterus, whereas marsupials are born as embryos and complete their development in an external pouch.
Some Australian marsupials have eutherian look-alikes with superficially similar adaptations. For instance, a forest-dwellling Australian marsupial called the sugar glider is superficially very similar to flying squirrels, the gliding eutherians that live in the North American forests. But the sugar glider has many other characteristics that make it a marsupial, much more closely related to kangaroos and other marsupials than to flying squirrels or other eutherians.
Once again, an understanding of evolution can explain these observations. although they evelved independently from different ancestors, these two mammals have adapted to similar environments in similar ways.
Biogeography
Another type of evidence for evolution comes from biogeography, the geographic distribution of species. The distribution of organisms is influences by many factors, including continental drift, the slow movement of Earth's continents over time.
About 250 million years ago, these movements united all of Earth's landmasses into a single large continent called Pangaea. Roughly 200 million years ago, Pangaea began to break apart; by 20 million years ago, the continents we know today were within a few hundred kilometers of their present locations.
We can use evolution and continental drift to predict where fossils of different groups of organisms might be found.
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| Horse Evolution |
For example, scientists have constructed evolutionary trees for horses based on anatomical data. these trees and the ages of fossils of horse ancestors suggest that present-day horse species originated 5 million years ago in North America. At that time, North and South America were close to their present locations, but they were not yet connected, making it difficult for horses to travel between them. Thus, we would predict that the oldest horse fossils should be found only on the continent on which horses originated - North America. this prediction and others like it for various other groups have proven true, continuing to supply more evidence for evolution.
An understanding of evolution also helps understand biogeographic data. For example, islands generally have many species of plants and animals that are endemic (found nowhere else in the world) Yet as Darwin described in The Origin of Species, most island species are closely related to species from the nearest mainland or a neighboring island. He explained this observation by suggesting that islands are colonized by those same species. these colonists eventually give ries to new species as they adapt to their new environment. An example of this is currently occurring in the 35 year old Pan Mrcrau experiment.
Such a process also explains why two islands with similar environments in distant parts of the world tend to be populated not by species that are closely related to each other, but rather by species related to those of the nearest mainland, where the environment is often quite different.
Concluding Thoughts
Clearly, the pattern of evolution - the observation that life has evolved over time - has been documented directly and is supported by a great deal of evidence. In addition, Darwin's explanation of the process of evolution - that natural selection is the primary cause of the observed pattern of evolutionary change - helps to make sense of massive amounts of data. The effects of natural selection can be observed and tested in nature.
The next blog post will contain information regarding micro evolution of populations and a discussion of the Hardy-Winberg equation.
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