Evidence of Evolution: Direct Observation

One of the most compelling pieces of evidence for evolution comes from experimental observations of adaptations in organisms.

Smartnotes

KEY POINTS

  • Many thousands of scientific studies have documented the occurrence of evolution.
  • In Florida, the soapberry bug feeds on the seeds of the balloon vine. As balloon vines become rarer, the bugs move to other plants. Studies show that their beak lengths evolve to become either shorter or longer, depending on the depth of the seeds within the new food source.
  • Since the introduction of penicillin, S. aureus has quickly evolved strains that are resistant to each new antibiotic that has been introduced. A current strain, methicillin- resistant Stapholococcus aureus, is resistant to many antibiotics and has become a serious problem in hospitals.
  • These examples show two things: 1.) Natural selection acts by adapting the patterns of inheritance that already exist in the population. 2.) The type of selection that occurs depends on what is needed by the population in its current environment.

KEY TERMS

  • adaptation

Adjustment to extant conditions: as, adjustment of a sense organ to the intensity or quality of stimulation; modification of some thing or its parts that makes it more fit for existence under the conditions of its current environment

EXAMPLES

  • Figure 2 illustrates the underlying features of drug-resistant bacteria. Infections caused by the bacteria S. aureus are frequently treated using penicillin. However, with penicillin's continued use, the percentage of S. aureus with resistance to the drug, has continued to rise.

FIGURES

1.

fig. 1

Example of Direct Observation Evidence of Evolution: Soapberry Bugs

Studies on beak length in different populations of soapberry bugs demonstrate that their beak length has evolved to adapt to different food sources in their different environments.  For more information, read: S.P. Carroll and C. Boyd (1992). Host race radiation in the soapberry bug: natural history with the history, Evolution 46: 1052-

  1. 1069. (accessible here: http://www.life.illinois.edu/ib/443/Carroll.pdf).

2.

fig. 2

Example of Direct Observation Evidence of Evolution: MRSA

This diagram depicts one recognized way in which bacteria may develop resistance

to antibiotics: if the antibiotic functions by blocking the active site of the enzyme, the bacteria may evolve to produce an enzyme that will not allow the antibiotic to bind to its active site. (This diagram is modeled specifically on the way some strains

of Staphylococcus aureus have evolved to resist the beta-lactam antibiotic  methicillin by expressing the mecA gene.)1. Both enzymes are structurally similar, but they differ in the kind of substances that they will allow in their active sites.

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Examining the evidence for evolution

Evolutionary theory depends on the basic premise that life has existed on Earth for billions of years and, over time, that life has changed. There are four lines of evidence that support this theory: direct observation, homologies between organisms,

evidence in the fossil record, and the distribution of life and land on earth in time and space. The focus here is on direction observations of evolution.

Adaptation

One of the most compelling pieces of evidence for evolution comes from experimental observations of adaptations in organisms. Artificial selection in a laboratory setting or over many generations of selective breeding provides a model for natural selection. Scientists can also observe how organisms interact with their environments and how these interactions change over time or differ between related individuals. They can even conduct experiments that actually demonstrate selection at work. Thousands of scientific studies have documented the occurrence of adaptation of organisms over time.

Soapberry bugs

One striking example of adaptation in an organism, and therefore evidence of evolution, is the soapberry bug, or Serinethinae. (Figure 1) These are brightly colored seed-eaters, comprising three genera and about sixty-five species. These bugs are

specialists on plants in the soapberry family (Sapindaceae), which includes maples, balloon vines, and soapberry trees, among others. Seeds of the plants are the soapberry bug's main resource, used by adults for reproduction and nymphs for growth and development. Jadera haematoloma, the red-shouldered bug, goldenrain- tree bug, or soapberry bug, is a species of soapberry bug that lives throughout the United States and south to northern South America. It feeds on seeds within the soapberry plant family, Sapindaceae, and is known to adapt to feeding on particular hosts. Two populations in southern Florida are particularly notable. The more southern of these two populations has colonized a native host soapberry tree and balloon vine. This vine produces capsules of a fairly uniform size, which the adult bugs feed on by inserting their mouthparts (beak) through the capsule's exterior and into the interior seeds. The soapberry bug is particularly well-adapted to feeding on these plants, as the length of its beak is nearly the same as the depth of seeds within the flowers. In the mid-1950s, a southeast Asian tree, the flat-podded goldenrain

tree, was introduced to the Americas as an ornamental plant. It escaped domestication and naturalized. Significantly, the goldenrain tree can be colonized by J. haematoloma, though its capsules are smaller and the seeds less deeply embedded than in the balloon vine. In a seminal paper published in the scientific journal Genetica in 2001, it was shown that evolution that had taken place in this southernmost population of J. haematoloma in a period of only a few decades. The authors showed that the beak length, which in the ancestral type was about 70% the length of the body, was only about 50% the body length in the insects that had colonized the nonnative tree, though the size of the bugs themselves had not changed.

Drug resistant bacteria

Another excellent example of adaptation can be seen in the development of drug- resistant strains of bacteria in response to an increased use of antibiotics in today's medical community. Staphylococcus aureus, literally the "golden cluster seed" or "the seed gold" (also known as "golden staph" and Oro staphira) is frequently part of the skin flora found in the nose and on skin. About 20% of the human population are

long-term carriers of S. aureus. Today, S. aureus has become resistant to many commonly used antibiotics. In the UK, only 2% of all S. aureus isolates are sensitive to penicillin, with a similar ratio in the rest of the world. The β-lactamase-resistant penicillins (methicillin, oxacillin, cloxacillin, and flucloxacillin) were developed to treat penicillin-resistant S. aureus and are still used as first-line treatment. Methicillin was the first antibiotic in this class to be used (it was introduced in 1959), but, only two years later, the first case of MRSA was reported in England, and in the

1990s, there was an explosion in MRSA prevalence in hospitals where it is now endemic. (Figure 2) Antibiotic resistance in S. aureus was uncommon when

penicillin was first introduced in 1943. Indeed, the original petri dish on which Alexander Fleming of Imperial College London observed the antibacterial activity of the Penicillium fungus was growing a culture of S. aureus. By 1950, 40% of hospital S. aureus isolates were penicillin-resistant; and, by 1960, this had risen to 80%.

Bugs, Drugs, and Natural Selection

Both the example of the soapberry bug and that of S. aureus demonstrate that natural selection acts by adapting the frequency of characteristics that already exist in the population and that the type of selection that occurs (positive or negative) depends on what is needed by the population in its current environment.

Evidence of Evolution: Homology

Anatomical similarities, such as homologous and analogous structures, are often used as a reference point for describing evolutionary relationships between species.

Smartnotes

KEY POINTS

  • Homology is the existence of similar traits in related species as derived from a common ancestor, even if the traits may differ in functionality. There are anatomical

resemblances between mammalian forelimbs and homologous structures, although used differently (e.g., walking, swimming).

  • Some homologous structures (e.g., vertebrate tail, throat pouches) are better observed in fetal stages of animals than in adults. The tail disappears in some adults and throat pouches develop into very different structures.
  • Vestigial structures are anatomical features that may serve no purpose in a particular animal today, but may have been of importance to their ancestor. Vestigial pelvic and leg bones in snakes are an illustration of this occurrence.
  • Homology also occurs at the molecular level. Humans share genes with bacteria, some of which have kept the same functions while others have evolved new functions. "Pseudogenes" are the molecular equivalent of vestigial structures -- they have no known function, but are present because the organism's ancestor had them.
  • Many homologous structures derive from distant ancestors, but others have evolved more recently. Tetrapod (four-footed) vertebrates have limbs containing digits, which evolved more recently than the limbs themselves. Therefore, evolution follows a nested pattern, which when considered with ancestral relationships may be represented using an evolutionary tree. Evolutionary trees summarize currently understood relationships between organisms and the pattern of their evolution with respect to common ancestors.
  • Convergent evolution explains why particular organisms may resemble one another despite a lack of common descent. It occurs when similar characteristics arise in unrelated species inhabiting similar environments. The organisms develop similar attributes for survival in their shared environment. Such shared features in unrelated organisms are termed analogous, rather than homologous, structures.

KEY TERMS

  • homology

A correspondence of structures in two life forms with a common evolutionary origin, such as flippers and hands.

  • analagous

Having analogy; corresponding to something else; bearing some resemblance or proportion;—often followed by "to"

  • convergent evolution

A trait of evolution in which species acquire similar properties because of their advantageousness.

  • evolutionary tree

A phylogenetic tree or evolutionary tree is a branching diagram or "tree" showing the inferred evolutionary relationships among various biological species or other entities based upon similarities and differences in their physical and/or genetic characteristics.

  • pseudogene

A segment of DNA that is part of the genome of an organism, and which is similar to a gene but does not code for a gene product.

  • vestigial structure

A structure in an organism that has lost all or most of its original function in the course of evolution, such as human appendixes.

  • homologous structure

The traits of organisms that result from sharing a common ancestor; such traits often have similar embryological origins and development. This is contrasted with analogous traits: similarities between organisms that were not present in the last common ancestor of the taxa being considered, but rather evolved separately.

EXAMPLES

  • Figure 1 and Figure 2 show the similarities in mammalian limbs. Since mammals have common ancestry, these are homologous structures, even though these limbs are used for very different functions by a whale versus a bat.
  • Bats and birds have similar limbs that are analogous structures. Their limbs look similar because they use them for the same function (flying), however they evolved these similar limbs independently, rather than from a shared ancestor.

FIGURES

1.

fig. 1

Example of Homology: Forelimbs in Vertebrates

Although they serve different functions, the forelimbs of a human, dog, bird, and whale are structurally similar; they are an example of homologous structures.

  1. 2.    fig. 2

Example of Homology: Embryonic Vertebrates

In the embryonic stages all vertebrates have a tail and pharangeal pouches. These are examples of homologous structures.  Ernst Haeckel's 1892 drawings of various vertebrate embryos are shown here.

fig. 3

Example of Convergent Evolution: Flying Squirrel vs. Sugar Glider

The sugar glider and the flying squirrel both glide through the air, but these traits did not evolve at the same time or in the same lineage. This function is an example of convergent evolution.

fig. 4

Example of Convergent Evolution: Flying Squirrel vs. Sugar Glider

The sugar glider and the flying squirrel both glide through the air, but these traits did not evolve at the same time or in the same lineage. This function is an example of convergent evolution.

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Because evolutionary theory states that organisms are derived from common ancestors, it predicts that related organisms will share similar characteristics. These characteristics that are shared by members of different species due to a common ancestor are called homologies. Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function." Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying structure of bones and muscles. Owen reasoned that there must be a common structural plan for all vertebrates, as well as for each class of vertebrates.

Forelimbs in mammals provide one example of homology. While the forelimb of a frog, a rabbit, a bird's wing, and a whale's ventral fin may seem at first glance to be very disparate, these forelimbs all share the same set of bones, which can also be seen in fossils of Eusthenopteron, an extinct animal. (Figure 1) This provides evidence of their shared ancestry.

Charles Darwin noted especially that many organisms were very similar in the embryonic stage, and he knew that these resemblances were a strong argument in favor of common ancestry. In On the Origin of Species, he stated that "Community of embryonic structure reveals community of descent." Some organisms that do not share many features after birth are remarkably similar in the embryo stage, and this is evidence of shared ancestry. Darwin and his followers found support for evolution in the study of embryology, the science that investigates the development of organisms from fertilized egg to time of birth or hatching. Vertebrates, from fish to lizards to humans, develop in ways that are remarkably similar during early stages, but they become more and more differentiated as the embryos approach maturity. (Figure 2) The similarities persist longer between organisms that are more closely related (e.g., humans and monkeys) than between those less closely related (humans and sharks).

An anatomical feature that appears to serve no purpose to a particular animal but may have been of importance to its ancestor is referred to as vestigial. Vestigiality describes homologous characters of organisms that have seemingly lost all or most of

their original function in a species through evolution. These may take various forms such as anatomical structures, behaviors, and biochemical pathways. Some of these disappear early in embryonic development, but others are retained in adulthood. The vestigial versions of the structure can be compared to the original version of the structure in other species in order to determine the homology of a vestigial structure. Homologous structures indicate common ancestry with those organisms that have a functional version of the structure.

Vestigial structures are often called vestigial organs, although many of them are not actually organs. These are typically in a degenerate, atrophied, or rudimentary condition, and tend to be much more variable than similar parts. Although structures usually called "vestigial" are largely or entirely functionless, a vestigial structure may retain lesser functions or develop minor new ones. However, care must be taken not to apply the label of vestigiality to exaptation, in which a structure originally used for one purpose is modified for a new one. For example, the wings of penguins would not be vestigial, as they have been modified for a substantial new purpose (underwater locomotion), while Darwin pointed out that those of an emu would be, as they have

no major function now.

Vestigial characters range on a continuum from detrimental through neutral to marginally useful. Some may be of some limited utility to an organism but still degenerate over time; the important point is not that they are without utility, but that they do not confer a significant enough advantage in terms of fitness to avoid the random force of disorder that is mutation. It is difficult, however, to say that a vestigial character is detrimental to the organism in the long term — the future is unpredictable, and that which is of no use in the present may develop into something useful in the future. Vestigiality is one of numerous lines of evidence for biological evolution.

Examples of vestigial organs can be found throughout the animal kingdom:

In whales and other cetaceans, one can find small vestigial leg bones deeply buried within the back of the body. These are remnants of their land-living ancestors' legs. Many whales also have undeveloped, unused pelvis bones in the anterior part of their torsos.

The wings of ostriches, emus, and other flightless birds are vestigial; they are remnants of their flying ancestors' wings.

The eyes of certain cavefish and salamanders are vestigial, as they no longer allow the organism to see, and are remnants of their ancestors' functional eyes.

Boas and pythons have vestigial pelvis remnants, which are externally visible as two small pelvic spurs on each side of the cloaca. These spurs are sometimes used in copulation, but are not essential, as no colubroid snake (the vast majority of species) possesses these remnants.

Homology between seemingly highly divergent organisms can often be seen at the molecular level. Genes homologous to those in humans can be found in almost any organism, including bacteria. In many cases, these homologous genes have the same function in these different species, while others have evolved new functions. The equivalent of "vestigial" genes can be found in the pseudogene--a gene that has no known function in an organism, but are retained in the DNA sequence because of their usefulness to the organism's ancestor.

Because pseudogenes are generally thought of as the last stop for genomic material that is to be removed from the genome, they are often labeled as junk DNA. Nonetheless, pseudogenes contain fascinating biological and evolutionary histories within their sequences. This is due to a pseudogene's shared ancestry with a functional gene: in the same way that Darwin thought of two species as possibly having a shared common ancestry followed by millions of years of evolutionary divergence, a pseudogene and its associated functional gene also share a common ancestor and have diverged as separate genetic entities over millions of years.

Homology between a pseudogene and its functional counterpart is determined by comparing the DNA sequences of the two genes. After aligning the two sequences, the percentage of identical base pairs is computed. A high sequence identity (usually between 40% and close to 100%) means that it is highly likely that these two

sequences diverged from a common ancestral sequence (are homologous), and highly unlikely that these two sequences were independently created.

These ancestral relationships between organisms can be represented by an evolutionary, or phylogenetic, tree. Phylogenetic trees illustrate the nexted pattern of evolution and summarize the currently understood relationships between organisms and the pattern of their evolution with respect to their common ancestors. A phylogenetic tree is a specific type of cladogram, or branched diagram, where the branch lengths are proportional to the predicted or hypothetical evolutionary time between organisms or sequences. The taxa joined together in the tree are implied to have descended from a common ancestor. In a rooted phylogenetic tree, each node with descendants represents the inferred most recent common ancestor of the descendants, and the edge lengths in some trees may be interpreted as time estimates. Each node is called a taxonomic unit. Internal nodes are generally called hypothetical taxonomic units (HTUs) as they cannot be directly observed.

Evolution is most commonly thought of as "divergent" evolution--that is, the process by which a single species diverges into two or more descendant species. However, evolution can also be classified as convergent. Convergent evolution describes the acquisition of the same biological trait in unrelated lineages. (Figure 3) The wing is a classic example of convergent evolution. Although their last common ancestor did not have wings, birds and bats both possess wings which evolved independently of the other, and both are capable of powered flight. The wings are similar in construction, due to the physical constraints imposed upon wing shape.

Traits arising through convergent evolution are termed analogous structures, in contrast to homologous structures, which have a common origin. Bat and bird wings are an example of analogous structures, while the bat wing is homologous to human and other mammal forearms, sharing an ancestral state despite serving different functions. Therefore, analogous traits are similarities between organisms that were not present in the last common ancestor of the taxa being considered but rather evolved separately. Analogous traits may or may not share a common embryology; in the example of the wings of bats and birds, both evolved from the vertebrate forelimb and therefore have similar early embryology.

Evidence of Evolution: Fossil Record

Understanding evolution is done by comparing living species to one another, and by comparing them to extinct species (i.e.fossils).

Smartnotes

KEY POINTS

  • The fossil record allows us to see the evolution that has occurred in the past. Through fossils, we can study extinct organisms and contrast them to their present-day counterparts, allowing us to hypothesize evolutionary relationships.
  • The size of the stickleback pelvic bone shows a consistent change within a group.
  • The fossil record was used to back up molecular data in the case of cetaceans, thereby demonstrating the formation of new species and their transition from land back to water.

KEY TERMS

  • clade

A group of animals or other organisms derived from a common ancestor species.

  • fossil record

The collective record of biological development that is reflected in the fossilized remains of organisms through geological history

EXAMPLES

  • An isolated population of three-spined stickleback, a species of fish, has much shorter spines than individuals in non-isolated populations. The fossil record shows that this reduced spine size evolved rapidly. Ecological theories for this evolutionary pattern have been supplemented by molecular analyses that have revealed the genetics behind spine size.

FIGURES

1.

fig. 1

Fossil of Pakicetus, an extinct cetacean

Pakicetus is a genus of extinct terrestrial carnivorous mammal of the family Pakicetidae which was endemic to Pakistan from the Eocene (48–49 Ma). Many paleontologists regard it as a close relative to the direct ancestors of modern day whales.

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Fossils as evidence for evolution

Understanding the evolution of complex life and the roles that changing terrestrial and extraterrestrial environments played in life's history depends upon synthetic knowledge of the fossil record. Paleontologists have been describing fossils for more than two centuries. The fossil record acts as a series of snapshots of evolutionary history that together tell a story of the process of evolutionary change over the past four billion years. Because fossilization is a rare occurrence, it is impossible to see every stage of an organism's development, but fossils discovered across the earth clearly show that life has evolved over time.

Sticklebacks

Evidence for evolution in the fossil record is especially evident in a species of fish called the three-spined stickleback. Sticklebacks are most commonly found in the ocean, but they can be found in some freshwater lakes. These fish have three dorsal spines, two pelvic spines, and one anal spine at the posterior of the anal fin. The dorsal and pelvic spines are protected from collapse by a pelvic girdle that connects them in a strong, crush-proof structure surrounding the body. The spines can be locked in an erected position for defense against predators. Some units of this small fish were isolated from other groupings during the Ice Age, approximately 10,000 years ago. Recent discovery of stickleback fossils in a quarry in Nevada shows evolution of the stickleback occurring in this relatively short timespan. Through a progression of fossils, the fish population can be seen to dramatically reduce the size of their pelvic spines.

Fossils help in understanding modern sticklebacks

It is hypothesized that these freshwater lakes did not contain larger predators so the fish had less need to have these spines for protection. Recent genetic have been able to identify the genetic cause for this reduction in pelvic spines. Stanford scientists used genome mapping to identify a single gene that is the major contributor to pelvic spine development. This gene, Pitx1, is expressed in marine sticklebacks that possess pelvic spines, but is turned off in sticklebacks that show complete loss of the pelvic skeleton (Chan et al. 2010).

Cataceans

A particularly interesting example of large-scale evolution is that of the cetaceans. The cetaceans (whales, dolphins, and porpoises) are marine mammal descendants of land mammals. Their terrestrial origins are indicated by their need to breathe air from the surface of the water, the bones of their fins, which are homologous to those of land mammals, and the vertical movement of their spines, characteristic more of a running mammal than of the horizontal movement of fish. The question of how land

animals evolved into ocean-going leviathans was a mystery until recent discoveries in

Pakistan revealed several stages in the transition of cetaceans from land to sea.

Determining the relationship between whales and hippos

The traditional theory of cetacean evolution was that whales were related to the mesonychids, an extinct order of carnivorous ungulates (hoofed animals). These animals possessed unusual triangular teeth that are similar to those of whales. For this reason, scientists long believed that whales evolved from a form of mesonychid; however, more recent molecular phylogeny data suggest that whales are more closely related to the artiodactyls, specifically the hippopotamus. However, hippos' anthracothere ancestors do not appear in the fossil record until millions of years after Pakicetus, the first known whale ancestor, leaving a large gap in our understanding

of how whales may have evolved. Hippo fossils are not observed until the Miocene, but whale ancestors have been dated to the Eocene. The whale/hippo hypothesis contains a gap of nearly 30 million years when no hippo ancestors are present. The skeletons of Pakicetus (Figure 1) demonstrate that whales did not derive directly from mesonychids. Instead, they are a form of artiodactyl (even-toed ungulate) that began to take to the water after the artiodactyl family split from the mesonychids. This hypothesized ancestral group likely split into two branches around 54 million years ago. One branch would evolve into cetaceans, possibly beginning with the proto-whale Pakicetus from 52 million years ago with other early whale ancestors collectively known as Archaeoceti, which eventually underwent aquatic adaptation into the completely aquatic cetaceans. In other words, the proto-whales were early artiodactyls that retained aspects of their mesonychid ancestry (such as the triangular teeth) which modern artiodactyls have since lost. An interesting implication is that the earliest ancestors of all hoofed mammals were probably at least partly carnivorous or scavengers; today's artiodactyls and perissodactyls switched to a plant diet later in their evolution. Later molecular analyses included a wider sampling of artiodactyls and produced a more complete tale. Hippos were determined to be the closest relative of whales, ruminants were related to a whale/hippo clade, and pigs were more distant.

Evidence of Evolution: Biogeography

Biogeography, the study of the distribution of organisms in space and time, can yield important information about evolutionary processes.

Smartnotes

KEY POINTS

  • The patterns of species distribution across geographical areas can usually be explained through a combination of historical factors such as speciation, extinction, continental drift.
  • Islands are of particular importance in biogeography due to the fact that islands are typically geographically isolated from mainlands.  This isolation can result in speciation, one possible manifestation of evolution.
  • Continental drift can play an important role in biogeography because connections between continents dictate what species are geographically isolated or united.
  • Island biogeography demonstrates that species on a given island may more closely resemble species on a nearby mainland, rather than species on a distantly located island with similar environmental conditions.

KEY TERMS

  • continental drift

The slow movement of continents explained by plate tectonics.

  • endemic

(Especially of plants and animals.) Peculiar to a particular area or region; not found in other places.

  • speciation

The process by which new distinct species evolve.

EXAMPLES

  • Lemurs are found only on Madagascar, an island which has been separated from the mainland for a long time. Since they are found nowhere else, they are an endemic species.

FIGURES

1.

fig. 1

The Galapagos Islands

Darwin's trip to the Galapagos was an important experience in terms of helping him piece together his ideas on evolution.

  1. 2.    fig. 2

Koala

The geographical isolation of Australia has resulted in a wide variety of animals that are found no where else on earth.

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What is biogeography?

Biogeography is the study of the distribution of species (biology) spatially

(geography) and temporally (throughout history). Biogeography aims to reveal where organisms live, at what abundance, and why they are or are not found in a certain geographical area. The patterns of species distribution across geographical areas can usually be explained through a combination of historical factors such as speciation, extinction, continental drift (see below), and associated variations in sea level, river routes, habitat, and river capture. These natural forces work in combination with the geographic constraints of an area, such as landmass and isolation, and the available ecosystem energy supplies to generate the biodiversity in a given area.

Biogeography and islands

Biogeography is most keenly observed on the world's islands. These habitats are often much more manageable areas of study because they are more condensed than larger ecosystems on the mainland. Islands are very diverse in their biomes, ranging from tropical to arctic climates. This diversity in habitat allows for a wider range of species studied in different parts of the world. Charles Darwin recognized the importance of these geographic locations, and he remarked in his journal "The Zoology of Archipelagoes will be well worth examination." (Figure 1) Two chapters in On the Origin of Species were devoted to geographical distribution. Islands are also

ideal locations because scientists can look at habitats that new species have only recently colonized, and they can observe how those species disperse throughout the island and the success they achieve in these places. They can then apply this information to similar mainland habitats.

Continental drift and evolution

Continental drift is the movement of the Earth's continents relative to each

other. Similar plant and animal fossils are found around different continent shores, suggesting that they were once joined. For example, the fossils of Mesosaurus, a freshwater reptile resembling a small crocodile, have been found both in Brazil and South Africa; fossils of the land reptile Lystrosaurus have been discovered in rocks of the same age in locations in South America, Africa, and Antarctica. There is also living evidence—the same animals being found on two different continents. For instance, some earthworm families (e.g.: Ocnerodrilidae, Acanthodrilidae, Octochaetidae) are found in both South America and Africa. The process of continental drift and the resulting isolation of endemic species to particular land masses can play an important role in evolution. The establishment and evolution of the present-day fauna in Australia was apparently shaped by the unique climate and the geology of the continent. As Australia drifted, it was, to some extent, isolated from the effects of global climate change. The unique fauna that originated in Gondwana (which was part of Pangea) such as marsupials, survived and adapted in Australia. An understanding of continental drift is therefore essential in understanding evolutionary relationships among organisms. (Figure 2)

Geographical isolation and speciation

Data about the presence or absence of species on various continents and islands can provide evidence of common descent and shed light on patterns of speciation, as previous connections between the continents may dictate which species have been geographically isolated or united. An endemic species is one that is unique to a defined geographic location, such as an island, nation, or other defined zone or habitat type, and found only there; organisms that are indigenous to a place are not endemic to it if they are also found elsewhere. For example, all species of lemur are endemic to the island of Madagascar; none are found anywhere else in the world. Physical, climatic, and biological factors can contribute to endemism. Endemic types or species are especially likely to develop on biologically isolated areas such as islands because of their geographical isolation. This includes remote island groups, such as Hawaii, the Galápagos Islands, and Socotra, and biologically isolated but not

island areas, such as the highlands of Ethiopia, or large bodies of water, such as Lake

Baikal.