Evidence for Evolution Student Learning Guide

1. How Fossils are formed

Formation of sedimentary rock layers. Source: Siyavula Education, via Flickr. Click for a larger version.

When an organism dies, most or all of the matter making up its body gets decomposed, leaving no trace. However, if the organism dies in an area where sedimentary rocks are forming, some or all of its body can become fossilized. Sedimentary rocks form as deposited material (sediments) brought by gravity, wind, rivers, or ocean currents accumulate in layers, usually in a body of water, as shown at left.

A fossil is “a remnant, impression, or trace of an organism of past geologic ages that has been preserved in the earth’s crust.” (Merriam Webster). In the example at right, death (1) is followed by burial in sediments (2), keeping the skeleton from being dismembered by scavengers or broken apart by currents. Over time, minerals in the water replace the organic material in the organism’s bones (3), creating a fossil.

Sedimentary strata in Salta, Argentina. Souce: wikimedia commons. Click to enlarge.

Layers of sedimentary rock are called strata. In the field, sedimentary strata (when exposed on land), look like this:

And fossilized fish within these strata might look like this:

200 million year old fossilized fish. Photo by Nkansahrexford via wikimedia commons. Click to enlarge.

Fossils can also be formed as molds or casts of an organism’s body. This can happen as a shell, bone, or other hard part of the organism presses into soft sediments, which then solidify, creating an impression.

Via Wikipedia, by Mark A. Wilson (Department of Geology, The College of Wooster. Click to enlarge.

Sometimes, molds or casts of soft body parts can occur. But in general, hard body parts are fossilized much more easily than soft body parts. Because of that, he fossil record is not representative of all past life, insofar as the record is much more complete for organisms with shells, skeletons, exoskeletons or other hard, durable parts than it is for soft bodied organisms.

A paradox about the fossil record is that it’s simultaneously vast and incomplete. According to an article in the Proceedings of the National Academy of Sciences, “About 192,000 invertebrate fossil species were known in 1970, and at least 3,000 more are named every year.” Based on this reckoning, the number of known fossil species in 2020 would be over 300,000. Since the number of species that have ever lived might be as high as 5 billion, the fossil record represents much less than 1/10,000 of one percent of all life.

2. Knowing the Age of a Fossil

Radioactive Decay. Click to open a larger version.

Knowing how old a fossil is involves two methods. One is radiometric dating. Radiometric dating can only be done with volcanic rock. Since fossils always form in sedimentary rock, scientists date fossils by finding volcanic rocks in nearby strata.

The term “radiometric” combines the words “radiation” with “meter,” and it’s a method of dating specimens by measuring the rate of disintegration of radioactive elements within nearby rocks. Another name for this type of dating is absolute dating. Here’s how it works.

When volcanic rock cools, the atoms within it are locked in place: it’s a closed system. Some of the elements of the atoms within that rock will have radioactive isotopes. These are atoms of an element that have an unstable nucleus. That means that the nucleus will emit subatomic particles. Loss of some of these particles will change that atom’s number of protons, changing that atom into a different isotope of a different element.

The loss of particles from radioactivity is called radioactive decay, and it occurs at a predictable rate. That rate is known as the half-life, and it’s the time it takes for half of a sample of a radioactive isotope of an element to transform itself into some other form.

Here’s a contrived example, with very easy math. Isotope X, with a half life of 100,000 years, decays to isotope Y. If you’re examining a volcanic rock and find that the ratio of X to Y is 50/50, then you know that this rock was formed 100,000 years ago (one half-life ago). If the ratio of X to Y was 25/75, then you’d know that two half-lives had passed, and that the rock had formed 200,000 years ago.

If you want to know the age of a very old rock sample, you need to track radioactive decay in an element with a very long half-life. For example, it takes uranium 238 (U238) 4.5 billion years to decay into lead 206 (Pb206). That means that if the ratio of U238 to Pb206 in a sample of rock is 50/50, then that rock sample would be 4.5 billion years old. An online calculator (or use of a formula) would tell you that if the ratio were 58% U238 to 42% Pb206 , then this rock would be 3.5 billion years old. That takes us back to about the time when life was first emerging on our planet (and you can learn more about evidence for that in the tutorial on the origin of life).

To date a fossil, radioactive dating has to be combined with another geological phenomenon called superposition. Because of the way sediments are laid down, sedimentary rock layers that are closer to the surface are assumed to be younger, while those lower down are assumed to be older. For example, if you were digging through sedimentary layers and found a sequence of fossils like the one shown at left, you could assume that A is the youngest and C is the oldest. Using superposition enables you to determine the relative age of a fossil; in other words, that it’s older than fossils in strata that are higher up, and younger than fossils in strata that are lower down. This is called relative dating. 

By combining relative dating with radiometric dating, you can determine a fossil’s age. Assume that between layer A and B at left, there was a layer of volcanic rock. If that rock were 10,000,000 years old, then you’d know that the fossils in layer B were older than 10,000,000 years old, and that the fossils in layer A were younger.

3. The Fossil Record as Evidence for Evolution

We started this module on evidence for evolution by looking at the two claims made by the theory of evolution.

  1. The pattern of life should show descent with modification.
  2. As evolution occurs, there will be underlying change in a population’s genetic makeup.

In almost all cases, the organic material in a fossilized specimen is lost. That means that genetic changes within a lineage can’t be traced. However, fossils provide several pieces of evidence supporting the idea of descent with modification.

A trilobite. These extinct arthropods were once as common as cockroaches. Source: wikipedia. Click to enlarge.

First, fossils show that life has changed over time. If you examine fossils from a million years ago, you won’t find any modern humans. We didn’t exist (at least not in our modern form). Go back 100 million years, and the only mammals are the pouched marsupials. Our type of mammal (placental mammals) didn’t exist. The dominant land animals at that time were dinosaurs. Go back 450 million years ago and there’s no life on land. Go back to 500 million years ago, and the seas would be crawling with trilobites, an arthropod so common (and so well preserved because of its hard exoskeleton) that you can easily buy an actual specimen. Go back a billion years, and all life is unicellular. Go back two billion years, and all life is prokaryotic. Go back four billion years, and there’s no life at all.

Second, not only has there been change, but the type of change is the kind that evolutionary theory predicts. The evolutionary biologist J.B.S. Haldane famously joked that one fossil find could disprove the idea of evolution. What would that be? Finding a fossilized rabbit in pre-Cambrian rocks (rocks older than 600 million years old). That’s because evolutionary theory requires that simple life forms precede more complex life forms. Fish have to come before tetrapods (four limbed vertebrates) because tetrapods are elaborations on a fish theme. Amphibians have to come before mammals (because the more complex mammalian structures are an elaboration on tetrapod themes first shown by amphibians). In the plant world, mosses precede ferns, and ferns precede plants with seeds (like pine trees), and plants with seeds precede plants with flowers (like magnolias). On the grandest time scale, the 4 billion years of life’s history prokaryotic life precedes eukaryotic life. Unicellularity precedes multicellularity.

Note that this trend toward complexity is only seen in life’s grandest scale. Within any particular lineage, evolution can go in any direction, including toward simplification.

Third, through transitional fossils, the fossil record shows descent with modification in a variety of lineages. Transitional fossils have features that are common both to an ancestral group and its descendants. While it’s impossible to know whether the transitional fossil is a direct ancestor of more modern groups, species with transitional features can be seen as models of how the transition occurred. (Wikipedia). Here are a few examples.

3a. Whales

Here’s a reconstruction of a series of mammalian fossils in the whale lineage. The lineage shows transitions that include a reduction of hind legs, movement of nostrils to the top of the head, and modifications of the skull that made sonar and echolocation possible.

Much of the unearthing of proto-whale skeletons has been done by Philip D. Gingerich at the University of Michigan, and you can read his summary of some of the findings here. To learn more, you can read related articles at Smithsonian Magazine, at Wikipedia , or at the NY Times

RUSHELLE KUCALA – © Copyright 2018. Used by permission. www.rushelle.com

3b. Birds

Birds are a branch of the dinosaurs. As difficult as this might be to fathom, the robin in your front yard is a dinosaur whose ancestors survived the Cretaceous extinction that killed off the rest of the dinosaurs. That’s true of that hawk soaring overhead, and the pigeons downtown.

Archaeopteryx is a small dinosaur that lived about 150 million years ago. Its fossil was discovered in Germany in 1861. It has many transitional features: unlike modern birds, it has jaws with sharp teeth. Its wings end with three clawed fingers. Unlike tail-less modern birds, it had a long bony tail. However, like modern birds, archaeopteryx had wings and feathers.

Here’s the fossilized skeleton.

Archaeopteryx lithographica, specimen displayed at the Museum für Naturkunde in Berlin. (This image shows the original fossil – not a cast.). Click to enlarge.

And here’s a reconstruction. Note the claws on the wings.

Copyright 2018, Rushelle Kucala. Used by permission. www.rushelle.com

3c. Tiktaalik and the transition from fish to tetrapods

Tiktaalik is an animal that’s transitional between lobe-finned fish and the first tetrapods. Lobe-finned fish have fins with stout bones, unlike the more slender rays found in the fins of fish like salmon, tuna, guppies, or goldfish. Tetrapods are the four limbed vertebrates that walked on land. The group includes the amphibians, reptiles, birds, and mammals.

Tiktaalik fossils. source: Wikipedia

The images above show fossilized remains of Tiktaalik’s snout and upper body (left) and its forelimb. Below is a reconstruction showing what Tiktaalik might have looked like.

Reconstruction of Tiktaalik’s head and upper torso. Souce: https://paleozoo.com.au/

Tiktaalik, whose fossils were unearthed in Northern Canada in 2004, lived during the late Devonian period, about 375 million years ago. Tiktaalik’s features link the lobe-finned fish to their tetrapod descendants. Tiktaalik had the scales and gills that all fish have. But its fin bones, attached by large muscles to well-developed shoulder bones, suggest that this animal could have supported its weight. It also had holes above its eyes (spiracles) that indicate that it had lungs in addition to gills. Its long snout and teeth suggest that it could raise its head just above the water line (like a crocodile) as it scanned for prey in tidal waters.

 

Model of Tiktaalik, from https://paleozoo.com.au/. Click to enlarge.

To learn more about Tiktaalik, you can read this brief article in National Geographic, or  this much more detailed article in Nature from its three discoverers, Neil Shubin, Ed Daeschler, and Farish Jenkens.

4. Fossils as Evidence for Evolution: Checking Understanding

[qwiz qrecord_id=”sciencemusicvideosMeister1961-Evidence for evo: fossils”]

[h]Fossils as Evidence for Evolution

[i]Biohaiku

The Fossil Record

Evolution, extinction

Written in rock

 

[q multiple_choice=”true”] Fossils form in which of the following rock types?

[c] Igneous

[f] No. Igneous rocks form from lava or magma. This molten material is lifeless, and if any living material were trapped within lava or magma, it would be incinerated. Find some rocks that form in gentler ways.

[c] Metamorphic

[f] No. Metamorphic rocks are brought deep within the earth’s crust, where intense pressure and heat transform the original sedimentary or igneous rock into another form. Even if the parent rock was sedimentary, any fossils within it would be destroyed by this metamorphic process.

[c*] sedimentary

[f] Yes. Fossils form within sedimentary rocks.

[q] The type of rock in which this fossil is found is a   [hangman]  rock.

[c] sedimentary

[f] Great!

[q] A remnant or impression of an organism that has been preserved in the earth’s crust is known as a [hangman] .

[c] fossil

[f] Correct!

[q multiple_choice=”true”] Radioactive dating can only be done with which of the following rock types

[c*] Igneous

[f] Yes. When igneous rocks are formed, radioactive atoms within the rock are trapped inside. Subsequently, they’ll decay at a predictable rate, and this decay can be used to date the age of the rock.

[c] Metamorphic

[f] No. Metamorphic rocks can’t be used for radioactive dating. Look for a rock type which, when formed, traps radioactive atoms inside.

[c] sedimentary

[f] No. Radioactive dating can’t be done with sedimentary rock. Look for a rock type which, when formed, traps radioactive atoms inside. 

[q] When [hangman] rocks are formed, [hangman] atoms within the rock are trapped inside. Subsequently, they’ll [hangman]at a predictable rate, and this decay can be used to date the age of the rock.

[c] igneous

[f] Good!

[c] radioactive

[f] Great!

[c] decay

[f] Great!

[q] The type of rock shown in this cartoon is a  [hangman] rock, and the layers within are known as [hangman].

[c] sedimentary

[f] Correct!

[c] stata

[f] Correct!

[q] The idea that deeper sedimentary strata formed before the strata closer to the surface is called [hangman].

[c] superposition.

[f] Excellent!

[q] In radiometric dating, the time it takes for half of a sample of a radioactive isotope to transform itself into some other form is called the [hangman]-[hangman]

[c] half

[f] Great!

[c] life

[f] Good!

[q] In the following question, both answers begin with “r.” Dating a fossil based on its position within sedimentary strata is known as [hangman] dating. Dating a fossil based on the decay of radioactive isotopes in nearby volcanic rocks is known as [hangman] dating.

[c] relative

[f] Good!

[c] radiometric

[f] Excellent!

[q] The fossil record shows that [hangman] has changed over [hangman].

[c] life

[f] Correct!

[c] time

[f] Correct!

[q] [hangman]fossils have features that are common to both an ancestral species, and its [hangman].

[c] transitional

[c] descendants

[f] Correct!

[q] Transitional fossils such as Tiktaalik validate the evolutionary idea of [hangman] with [hangman].

Tiktaalik

[c] descent

[f] Excellent!

[c] modification

[f] Great!

[q] Archaeopteryx shows features found in both dinosaurs and [hangman]. That makes it a [hangman] fossil.

[c] birds

[f] Great!

[c] transitional

[f] Excellent!

[q]Isotope M has a half life of 1000 years, during which it decays into isotope N. You’re studying a sample of material that has a relative proportion of 25% M and 75% N. How old is this material?

[c] 500 years old.

[f] No. 500 years is, in this case, half of one half life. In this case, two half lives have passed.

[c] 1000 years.

[f] No. 1000 years is one half life. Here’s how to think about this. In one half life of this element, you’d expect half of M to decay into N, for a 50/50 ratio. That’s not the ratio here. Based on the question, find the number of half lives, and multiply that by the duration of a half life.

[c*] 2000 years old

[f] Excellent. If only 25% of the original material is left, then two half lives have passed, for a total of 2000 years.

[/qwiz]

5. Biogeography: Convergent Evolution and (more) Adaptive Radiation

Charles Darwin’s Origin of Species (1859) opens with these words:

When on board H.M.S. Beagle as naturalist, I was much struck with certain facts in the distribution of the inhabitants of South America, and in the geological relations of the present to the past inhabitants of that continent. These facts seemed to me to throw some light on the origin of species…

Today, we refer to the study of the geographic distribution of species as biogeography. Both Darwin and Alfred Russel Wallace (the co-discoverer of the idea of natural selection) made important early contributions to this field. Not surprisingly, biogeography also provides evidence for evolution.

In what follows, I’m going to closely follow an argument laid out by Jerry Coyne in his book Why Evolution is Truewhich I strongly recommend to anyone interested in learning more about the evidence for evolution.

5a. Biogeography and Convergent Evolution

Take a look at the following plants.

[qwiz summary=”false” repeat_incorrect=”false” qrecord_id=”sciencemusicvideosMeister1961-Evidence for Evo: cacti and euphorbs”]

[h] Mystery Succulent

[q] Plant 1

[q] Plant 2

[q] Plant 3

[q] Most people would categorize these plants into which plant family? [hangman]

[c] Cactus

[f] Right…That’s indeed what most people would do, but they’re all wrong. These plants are all euphorbs. They’re superficially like cacti, but not closely related to them. Unlike cacti, which (with one exception), are limited to North and South America, euphorbs live in the deserts of Africa, Asia, and the driest parts of Europe.

[q]Study the pictures below of a cactus (left) and a euphorb) right

Barrel Cactus, Arizona, USA Euphorbia grandialata, Canary Islands, Spain

Read below to see how this relates to evolution.

[q]Cacti and euphorbs are both ___________ to living in deserts. To avoid __________ loss, their ___________ have become reduced to spines. Their ________ are thick,coated with wax, and adapted for water_________.

[l]leaves

[fx] No. Please try again.

[f*] Excellent!

[l]adapted

[fx] No. Please try again.

[f*] Good!

[l]stems

[fx] No, that’s not correct. Please try again.

[f*] Good!

[l]storage

[fx] No, that’s not correct. Please try again.

[f*] Great!

[l]water

[fx] No, that’s not correct. Please try again.

[f*] Good!

[q]But cacti and euphorbs evolved these adaptations separately. In other words, as plant species go, they’re not closely [hangman] to one another. They did not inherit their adaptations from a common [hangman].

[c]related

[c]ancestor

[x]Convergent evolution is the process by which similar selective pressures act upon distantly related lineages, resulting in superficially similar adaptations in the descendants. To converge, means to come together, as in “lines converging.”  But in terms of convergent evolution, the coming together is superficial.

Read on to see how convergent evolution supports evolutionary theory.

[/qwiz]

 

Why do distant areas with similar climates support unrelated species that, like cacti and euphorbs, have similar adaptations?  Because of descent with modification. Over the course of evolution, one lineage of plants became adapted to the deserts of North and South America. This lineage gave rise to the cacti. Another lineage became adapted to the deserts of Eurasia, Africa, and Australia. This lineage gave rise to the euphorbs. If you look superficially, they’re similar. But a deeper look (one that includes, for example, looking at their DNA), reveals that these species aren’t closely related.

An example of convergent evolution occuring in an entire group of animals involves the marsupial mammals of Australia and the placental mammals of North and South America. Study the pairs of animals in the diagram below.

The marsupials are the pouched mammals (think of kangaroos). Their young are born in an extremely immature state. After being born, the young drag themselves to their mother’s pouch, latch onto her nipple, and remain there until they they can survive outside of the pouch. The pouch is called a marsupium: hence the name, marsupials. Placental mammals —like primates (our group), rodents, carnivores, elephants, bats, and so on — are born in a much more mature state.

To understand Marsupial evolution, we need to see how the globe has changed over the past hundreds of millions of years. These changes are referred to as continental drift, and explained by the geological theory of plate tectonics (which is outside the scope of this course. If you’re interested, read about it on Wikipedia).

Take a moment to study the series of maps below. Pay attention to the times and the position of the continents.

The Marsupials first emerged in the Americas during the time of the dinosaurs, about 125 million years ago. At that time, the continents were arranged in a way that’s intermediate between the Jurassic and Cretaceous maps above. At that time South America had recently disconnected from Africa, and land connected South America, Antarctica, and Australia. Antarctica, at that time, was not as cold as it is today. Marsupials spread from the Americas, through Antarctica, and arrived in Australia about 60 million years ago.

Placentals emerged after the marsupials: about 65 million years ago, close to the time when dinosaurs became extinct. The extinction of the dinosaurs led to a widespread diversification of placental mammals. Though it’s not clear why, it looks like placentals outcompeted the marsupials in many parts of the world, where many marsupials became extinct. That’s why, for example, almost all of the mammals of North America are placentals: the opossum is a notable exception.

But before placentals could make it to Australia, that continent had separated from the rest of the continents. Except for bats, Australia remained free of placental mammals: a marsupial safe-haven.

So we have two distantly-related groups of mammals. In each region, similar ecological niches were available. Into each of these niches, evolution unfolded in a parallel way. There was a niche for a burrowing mammal that could prey on worms and other organisms in the soil. Into that niche evolved the placental Eastern mole in the Americas, and the marsupial mole in Australia. Into the ant-eating niche evolved the placental South American anteater, and the Australian marsupial banded anteater, also called the numbat. Into the tree-living, small gliding niche evolved the placental flying squirrel in the Americas, and the marsupial sugar glider in Australia. And into a top carnivore niche evolved the wolf in the Americas, and the Tasmanian wolf (extinct since the 1930s) in Australia.

There’s a side note to this story that serves as additional evidence for evolution. As stated above, based on the fossil record, it was known that marsupials evolved in the Americas and migrated through Antarctica to Australia. So, one could predict that fossils of marsupial mammals should be found in Antarctica. In 1982, a team of American scientists found exactly that: fossils of rat-sized marsupials (identified by their distinctive teeth and bones) that dated to about 40 million years ago. These weren’t the ancestors of Australia’s marsupial mammals, because Australia had already broken off from Antarctica at that point. But they confirm that Antarctic had once been home to marsupials, which used the continent while en route to their present Australian home.

Finally, it’s worth noting that while we can explain biogeographical phenomena by referring to continental drift, fossil evidence was an important clue that set the stage for the discovery of continental drift and the underlying theory of plate tectonics. By the early 1900s, geologists had mapped the location of a variety of fossils of what seemed, based on morphology, to be the same species. But the distances separating these fossil finds were vast, often crossing oceans. Look, for example at the presence of the extinct fern Glossopteris in South America, Africa, India, Australia, and Antarctica. Or the presence of the extinct reptile Lystrosaurus in Antarctica, India and Africa.

This distribution makes sense if the continents, rather than being locked in their current positions, actually moved. This idea was proposed by Albert Wegener in 1912. His solution to the puzzle is below.

Source: wikipedia

Because Wegener lacked a mechanism to explain the movement of the continents, his idea of continental drift was widely rejected during his lifetime. However, it was revived and accepted when the theory of plate tectonics arose in the 1950s and 1960s.

5b. Convergent Evolution happens when similar selective pressures elicit similar adaptations in distantly related groups

Convergent evolution can be rooted in biogeography, as with the marsupial and placental mammals discussed above. But that’s just one instance of a more general phenomenon of convergent evolution. Look at the three organisms below.

An ichthyosaur is an extinct marine reptile that perished at the same time as the dinosaurs became extinct, 65 million years ago. But from its fossils, we know that it had a hydrodynamic (streamlined) shape, similar to that found in sharks and dolphins. The similar phenotype shared by all three animals is not from common ancestry. It’s an example of convergent evolution. In each group of animals, similar selective pressures resulted in similar adaptations.

The adjective used to describe the similar phenotypes that result from convergent evolution is analogous. It’s important to keep the terms analogous and homologous (and the related nouns analogy and homology) distinct. Practice making this distinction in the questions below.

[qwiz qrecord_id=”sciencemusicvideosMeister1961-Evidence for evo: homology or analogy”]

[h] homology or analogy

[q multiple_choice=”true”] The wing of a butterfly and the wing of a bat.

[c] homology

[f] No. These wings are both adaptations for flying. But they didn’t result from inheritance from a common ancestor.

[c*] Analogy

[f] Excellent. These wings are the result of convergent evolution, not common ancestry. They’re analogous (not homologous).

[q multiple_choice=”true”] The beak of a platypus (a mammal) and the beak of a duck (a bird).

[c] homology

[f] No. These beaks are both adaptations for eating. But they didn’t result from inheritance from a common ancestor.

[c*] Analogy

[f] Nice job. These beaks are the result of convergent evolution, not common ancestry. They’re analogous (not homologous).

[q multiple_choice=”true”] The forearm of a human being and the flipper of a dolphin.

[c*] homology

[f] Fabulous. These forearms are both variations on a basic vertebrate forearm theme. The similarity in the underlying bones shows evidence of common ancestry. Each forearm has been modified as an adaptation for a specific environment. That makes them homologous.

[c] Analogy

[f] No. Analogous structures result from convergent evolution. They’re analogous in terms of their function. In this case, the function of the forelimb is quite different.

[q multiple_choice=”true”] WARNING: Think carefully! The forelimb of a bird and a bat.

[c*] homology

[f] Fabulous. If you are thinking of these as forelimbs (as opposed to as wings) then they’re both variations on a basic vertebrate forearm theme. The similarity in the underlying bones show evidence of common ancestry.

[c] Analogy

[f] No. This is very tricky. If the question was about WINGS, you’d be right, because the two groups (birds and mammals) converged on the same solution. But the question is about forelimbs. As forelimbs go, these structures are homologous: same bones; same embryonic tissue; inherited from a common ancestor.

[q multiple_choice=”true”] WARNING: Think carefully! The wing of a bird and the wing of a bat.

[c] homology

[f] No. If you are thinking of these as WINGS, these are not shared features that were inherited from a common ancestor. The ancestors of bats were rodent-like mammals who evolved into a flying niche. The ancestors of birds were small dinosaurs who similarly evolved into a flying niche. But they did this independently. The wings are analogous, not homologous

[c*] Analogy

[f] Wow! This is very tricky, but you still got it right! Two groups (birds and mammals) converged on the same solution. That makes them analogous structures, resulting from convergent evolution.

[q]This diagram is saying that the streamlined form of sharks, dolphins and ichthyosaurs is [hangman]. That’s because this phenotype evolved from [hangman] evolution, and not from a common [hangman]/

[c]analogous

[c]convergent

[c]ancestor

[/qwiz]

5c. More Biogeography: Life on Oceanic Islands

The Galapagos and Hawaiian islands are both oceanic islands. They’re volcanic in origin. Geologically speaking, they’re relatively young, which means that they’re made of rocks that are millions of years old. Continents, by contrast, have rocks that are billions of years old.

Oceanic islands are the small, usually volcanic islands that are located at a considerable distance away from the nearest continent. Think of islands like the Galapagos (1000 kilometers away from Ecuador), or the Hawaiian islands (2500 kilometers away from California).

Biologically, these islands have a few things in common.

  • They are missing many types of animals and plants found on continents or on large continental islands. Examples of continental islands are the British isles, Taiwan, Long Island, etc.
  • On all oceanic islands, the same types of organisms are missing: amphibians, freshwater fish, land mammals and, with a few exceptions, reptiles.
  • They are rich in native insects, birds, and plants. Many of these species are endemic: found only in that particular group of islands, and nowhere else.
  • Often, these endemic species are members of groups of similar species. We’ve already discussed the finches on the Galapagos islands. Hawaii has a similar abundance of biodiversity within a group of birds called honeycreepers, six of which are shown below.
    Modified from an image by the bioninja.

    Here’s a group of 18 Hawaiian moth species, all in the genus Hyposmocoma. Like the birds above, they’ve evolved into a wide variety of ecological niches, including aquatic species and predatory species.

    (c) David Rubinoff. Used by permission of the author.

[qwiz qrecord_id=”sciencemusicvideosMeister1961-Evidence for evo: islands and biogeography”]

[h]Explaining the biology of remote oceanic islands.

[i]How do we explain the high diversity of endemic species on remote islands? How do we account for the lack of entire categories of animals?

[q labels = “top”]The reason why there are no terrestrial mammals, reptiles, amphibians, or freshwater fish on remote islands is because these types of animals have no way of ____________ these islands. The _________ surrounding these __________ makes ____________ by these kinds of animals impossible.

[l]colonization

[fx] No, that’s not correct. Please try again.

[f*] Correct!

[l]islands

[fx] No, that’s not correct. Please try again.

[f*] Correct!

[l]colonizing

[fx] No. Please try again.

[f*] Excellent!

[l]oceans

[fx] No. Please try again.

[f*] Excellent!

[q]Why are there so many _______, plants, and insects? Because while these volcanic islands might be geologically ________ compared to continents, they’re still __________ of years old. While colonization is __________, it does happen as small populations of birds, insects, or plant seeds get blown to these islands in ________, or simply fly off course.

[l]difficult

[fx] No, that’s not correct. Please try again.

[f*] Excellent!

[l]birds

[fx] No, that’s not correct. Please try again.

[f*] Good!

[l]millions

[fx] No. Please try again.

[f*] Excellent!

[l]storms

[fx] No, that’s not correct. Please try again.

[f*] Great!

[l]young

[fx] No, that’s not correct. Please try again.

[f*] Good!

[q]When they do arrive, they’re often the only examples of their kind on the island. This sets the stage for  _________ ___________. Few ______________ species might serve as the original __________. But over the subsequent years, each ancestor _______________ into many descendant species. That’s why remote oceanic islands are the home to so many ____________ species.

[l]ancestral

[fx] No. Please try again.

[f*] Excellent!

[l]adaptive

[fx] No. Please try again.

[f*] Good!

[l]colonists

[fx] No, that’s not correct. Please try again.

[f*] Good!

[l]diversifies

[fx] No. Please try again.

[f*] Great!

[l]endemic

[fx] No. Please try again.

[f*] Good!

[l]radiation

[fx] No. Please try again.

[f*] Great!

[q]While terrestrial mammals can’t reach oceanic islands, one type of mammal that can is the the only flying mammal: the [hangman]. Also, not surprisingly, mammals that can [hangman], like seals, are also capable of colonizing oceanic islands.

[c]bat

[c]swim

[q]In terms of reptiles, most oceanic islands are completely free of native snakes and lizards. The Galapagos islands have three species of iguanas. Their ancestors probably arrived through rafting on logs or mats of vegetation. Once on the Galapagos, one lineage into the marine iguana (shown below), the world’s only aquatic lizard.

[/qwiz]

6. Biogeography and Convergent Evolution: Checking Understanding

[qwiz qrecord_id=”sciencemusicvideosMeister1961-Evidence for Evo: biogeography and convergent evolution”]

[h]Biogeography and Convergent evolution (with a few questions about fossils)

[i]Biohaiku

An island’s fauna

So many birds and insects!

But no land mammals

[q]The study of the geographic distribution of species is called [hangman].

[c]biogeography

[q]Species such as the euphorbs of Africa, Eurasia, and Australia and the cacti of the Americas are similar because of [hangman] evolution. Similar [hangman] pressures resulted in the evolution of similar [hangman] in each group.

[c]convergent

[c]selective

[c]adaptations

[q]The marsupials of Australia and the placentals of the Americas separately evolved into similar ecological [hangman]. While the marsupial mole of Australia and the Eastern mole of north america have similar [hangman] for a life spent burrowing through the soil, their phenotypic similarity is only superficial, a result of [hangman] evolution

[c]niches

[c]adaptations

[c]convergent

[q]The euphorbs of Africa, Eurasia, and Australia and the cacti of the Americas aren’t particularly closely [hangman]. Their superficially similar adaptations weren’t inherited from a common [hangman]. The best way to describe them is as [hangman] traits.

[c]related

[c]ancestor

[c]analogous

[q] [hangman] traits results from shared ancestry. Their is a similar underlying structure, but the [hangman] might be different.

[c] homologous

[c]function

[q][hangman] traits result from convergent evolution. The function of the part might be similar, but the underlying [hangman] is usually quite different. On the level of the entire organism, we can say that organisms are in a similar ecological [hangman], and therefore require a similar suite of adaptations.

[c]Analogous

[c]structure

[c]niche

[q]Many mysteries related to the distribution of fossil species (such as the one shown below) can be solved by the idea of [hangman] [hangman]

[c]continental

[c]drift

[q]Oceanic islands, like the Galapagos Islands or the Hawaiian islands, are rich in native species found nowhere else. The word for that is [hangman].

[c]endemic

[q]The diversity of Hawaiian Honeycreepers can be explained through the concept of [hangman] [hangman]. All of these Honeycreepers, in other words share a common [hangman]. Over time, each species evolved to fill a specific ecological [hangman].

[c]adaptive

[c]radiation

[c]ancestor

[c]niche

[q]One feature of island biology is that islands lack specific types of animals: amphibians, freshwater fish, land mammals, and reptiles. That’s because these types of animals are unable to cross the [hangman] that isolate these islands, and thus are never able to [hangman] the islands.

[c]oceans

[c]colonize

[/qwiz]

Links

  1. Developmental and Molecular Homologies
  2. Evidence for Evolution Main Menu