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7.2 How do species evolve?Dogs have been bred for centuries in all human cultures. They were originally bred for qualities such as hunting, herding, companionship and protection but since the 1850s have also been bred for their looks. since that time, pure-bred dogs have seen a rise in popularity, and standards for producing breeds have been established. These standards ensure a uniform appearance. in other words, the genotype and phenotype of a pure-bred dog will not really change over time. in contrast, populations of wild dogs are constantly evolving due to variations in genotype.

How does natural selection work?To date, the theory that best explains the diversity of life forms and the evidence of change over time is Darwin’s theory of evolution by natural selection. Darwin argued that it was entirely possible for one species to evolve gradually into a separate species, with its own unique traits, over many generations.

As Charles Darwin and Alfred Russel Wallace pointed out, an individual doesn’t evolve but populations do. A population is considered to be a group of interacting individuals of a species living in a particular area.

Natural selection is the principal mechanism for evolution. Darwin proposed four means by which natural selection works:

1 Not all individuals that are produced in a population survive to produce offspring.

2 Variation exists between individuals in a population.

3 Those individuals in a population that are ‘fitter’ contribute more to the next generation than those that are less fit. This is known as survival of the fittest.

4 The differences between individuals in a population are inherited.

What causes variations in populations?Natural selection acts on the variation of traits within a population, but where does this variation of traits come from? All members of a species share a set of common traits that help define them as a species. For example, Homo sapiens (i.e. humans) are identified, for purposes of classification, as:

an animal (kingdom Animalia)•

with a backbone (phylum Chordata)•

with a segmented spinal cord •(subphylum Vertebrata)

that suckles its young (class •Mammalia)

that gestates its young with the aid •of a placenta (subclass Eutheria)

that is equipped with five-digit •extremities, a collarbone and a single pair of mammary glands on the chest (order Primates)

and that has eyes at the front of the •head, stereoscopic vision and a proportionately large brain (suborder Anthropoidea).

Fig 7.11 In the Victorian common froglet, �� Crinia signifera, like most frog species, the female is much larger than the male. These differences between sexes of the same species are known as sexual dimorphism. Common froglets are also polymorphic (from the Greek polymorphos, meaning ‘having many forms’). This means that no two frogs have the same colouration or pattern regardless of their sex.

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Our species belongs to the family Hominidae and the genus Homo (larger brained hominids that appeared about two million years ago), and is characterised by a higher and more vertical forehead, a round and gracile cranium, small face and teeth, a prominent chin, and a more slender and elongated post-cranial skeleton.

Yet despite these unifying traits no two people (except for identical twins) look the same. Most traits differ from one individual to another, especially in sexually reproducing species.

Much of the variation between individuals has an inheritable basis and is due to genetic differences. Individuals of the same population generally have the same number and kind of genes, but different alleles. All the genes in the entire population can be thought of as a gene pool—a pool of genetic resources. The gene pool includes all the alleles for all the genes in the population. Individuals inherit different combinations of these alleles, which leads to variations in phenotypes (see Chapter 6).

A human gamete can have one of 10600 possible combinations of alleles. Not even 1010 humans are alive today. So unless you have an identical twin, it is extremely unlikely that another person with your exact genetic make-up has ever lived or ever will. Which alleles end up in a given gamete and, later, in a new individual? The outcome depends on five different events but, of these five, only mutation creates new alleles.

mutations result in variationAs you learnt in Chapter 6, mutations can occur at the gene level or the chromosome level. Some mutations may be lethal, having a drastic effect on an individual’s phenotype that results in its death. Others may be neutral, neither helping nor harming an individual. Natural selection does not increase or decrease the frequency of neutral mutations in a population because these

do not influence an individual’s chance of surviving or reproducing.

A mutation may also give an individual an advantage. Even if the advantage is small, chance events or natural selection may preserve the mutated gene and assure its representation in the next generation. Beneficial mutations, and neutral ones, have been accumulating in different lineages for billions of years. Through all that time, they have been the raw material for evolutionary change—the basis for the staggering range of biological diversity, past and present.

Allele frequencies and directional selectionIn the real world, populations are always evolving. The frequency of different alleles changes as a result of the ability of individuals to survive long enough to reproduce and pass these alleles on to the next generation.

For example, in the 1950s, scientists in England documented changes in the colour of a moth species, Biston betularia. These moths range in colour from light grey to nearly black. During the day they rest motionless on tree trunks. In unpolluted areas, tree trunks are covered with light grey lichens against which light grey moths are well

camouflaged. In areas with severe air pollution, lichens cannot survive, so tree trunks are lichen-free and dark, exposing lighter moths to predation from birds.

It seemed to researchers that, as areas became more polluted, dark moths increased in frequency—natural selection seemed to be changing allele frequencies in the population.

In 1952, strict pollution controls went into effect, so that the lichens returned

Fig 7.12 Polydactyly is the presence of an extra digit. Polydactyly can occur by itself or, more commonly, as one ��feature of a syndrome of congenital anomalies. When it occurs by itself, it is associated with autosomal dominant mutations in single genes. The condition occurs in one in every 500 live births.

Fig 7.13 Dark-coloured moths of the species ��Biston betularia increased in numbers owing to air pollution killing lichen on trees.

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and tree trunks became free of soot, for the most part. As might be predicted, selection started to operate in the reverse direction. Where levels of pollution declined, the frequency of dark moths also declined. The process of natural selection favouring a particular phenotype is known as directional selection.

Other examples of directional selection include the evolution of pesticide-resistant insects and antibiotic-resistant bacteria. In these cases, our use of chemicals (e.g. pesticides or antibiotics) has selected for variants that are resistant to the chemicals.

Evolution by natural selection occurs as a result of competition between individuals in a population with different traits. This competition may be for prey, for nutrients from the soil or for mates, for example. Selection for traits that provide an advantage with regard to mating is called sexual selection.

Sexual selectionThe differences between males and females of the same species is known as sexual dimorphism. Sexual selection within a species may take a variety of forms. In some species, females mate with whatever males are around, so males battle with one another for control of areas where females gather. In these species, sexual selection favours traits that allow a male to out-compete other males, thus favouring larger, stronger males. This is known as intrasexual selection. Both stag beetles and red stag deer have developed horns that are used in contests with other males over females. In other species, the females make the choice. When females choose their mates, males often compete for female attention by means of their colouring, ornamentation or elaborate courtship displays. In these species, intersexual selection favours traits that make males more appealing to females. Females may choose males based on their ability to provide resources, such as food, or on

practivity 7.1

Understanding natural selectionwhat you need: Different tools (e.g. spoons, forks, chopsticks or straws), lollies

This is a whole class activity.

1 each different group of students has a different type of eating utensil, such as spoons, forks, straws and chopsticks.

2 each group has the same mix of lollies.

make sure you check for food allergies and avoid using lollies that present a risk of anaphylaxis.

3 using only the tool provided, try and collect as many of the lollies as possible into a bowl.

Questions to consider ...

which of the eating utensils was the •most successful at gathering lollies.

over several generations, what might happen to the frequency of the alleles in the population that gave rise to this phenotypic trait?

which of the eating utensils was the •least successful at gathering lollies. over several generations, what might happen to the frequency of the alleles in the population that gave rise to this phenotypic trait?

A mutation, such as a vacuum-type •mouth part, occurred that gave rise to a phenotypic trait that enabled lollies to be gathered much more easily than with other phenotypes. what might the population look like after 10 generations?

Fig 7.14 Stag beetles use their horns to contest with other males over females.��

Fig 7.15 The male bird of paradise performs an elaborate courtship display to attract a female.��

Geographic barrier

New species

New speciesPolymorphism

New species

New niche

Originalspecies

Originalspecies

Originalspecies

Original population

Original population

Original population

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their genetic qualities. Female choice can result in females choosing males with more elaborate adornments, which will result in sons also with such elaborate adornments and daughters who prefer to choose such adornments.

The concept of fitnessIn evolution, fitness means the ability of a particular genotype to contribute to subsequent generations. An individual’s fitness is measured through its phenotype. As phenotype is affected by both genes and environment, so the fitness of different individuals with the same genotype are not necessarily equal but depend on the environment in which the individuals live. However, since the fitness of the genotype is an averaged quantity, it will reflect the reproductive outcomes of all individuals with that genotype.

The process of natural selection chooses those individuals within a population that have the best characteristics for survival. This process is known as survival of the fittest.

What do you know about how natural selection works?

1 variation in individuals can occur in different ways but only one way creates new alleles. what is this process called?

2 natural selection cannot increase or decrease the frequency of neutral mutations in a population. why is this?

3 in many species, females are larger than the males because they carry the reproductive burden. however, there are exceptions to this. what might be the cause of these exceptions?

4 in your own words, describe the mechanism by which natural selection can influence the frequency of alleles in a population.

How does one species become two?Natural variation in populations allows natural selection and sexual selection to operate on the gene pool of a population causing changes to the allele frequencies. As we have seen, this gives fitter individuals within a population a greater survival advantage. So how, though, are new species formed?

Allopatric speciationAllopatric speciation has been regarded as the main process by which new species arise. Imagine a friend from primary school whom you haven’t seen for a long time. They moved away and have had a lot of adventures that changed them so much that years later when you catch up you no longer recognise the way they look or behave. Allopatric speciation is an evolutionary process in which one species divides into two because the original homogenous population has become separated and both groups diverge from each other.

Fig 7.16 The processes of speciation: (a) allopatric; (b) sympatric; and (c) parapatric.��

a

b

c

In their separate niches, or environmental regions in which they exist, the two groups go their own evolutionary ways, accumulating different gene mutations, being subjected to different selective pressures and experiencing different historical events.

Under normal conditions, genes in a given population are exchanged through breeding, so that even if some variation occurs, it is limited by this gene flow. But the gene flow is interrupted if the population becomes divided into two groups. If there is no gene flow between the two groups, for example, if animals cannot move from one population to the other, then they may begin to look and behave differently from each other. Given enough time for evolution to work, the two populations may become so different that they become incapable of interbreeding should they ever come together again.

Allopatric speciation occurs in three ways.

1 The populations become physically separated, often by a long, slow geological process like an uplift of

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land, the movement of a glacier or formation of a body of water.

2 The separated populations diverge, through changes in mating tactics or use of their habitat.

3 The populations become reproductively separated so that they cannot interbreed and exchange genes.

Allopatric speciation is thought to have occurred among rock wallabies in Australia. Rock wallabies live in rocky outcrops, which are often separated by long distances, resulting in their geographical isolation from each other. There are about 20 different genetically and chromosomally different forms of rock wallaby in Australia, of which 16 are classified as different species.

Fig 7.17 Allopatric speciation is thought to have ��occurred in rock wallabies.

Fig 7.18 Species of coffee plant with 22, 44, 66 and ��88 chromosomes are known. This suggests that the ancestral condition was a plant with a haploid (n) number of 11 and a diploid (2n) number of 22, from which evolved the different polyploid descendants.

Sympatric speciationSpeciation does not always require physical barriers between populations. In sympatric speciation, daughter species arise within the home range of an existing species. In flowering plants, sympatric speciation can occur instantly. During meiosis (Chapter 6), the failure of chromosome pairs to separate properly can cause polyploidy in which the daughter plant can have three sets of chromosomes. (Remember from Chapter 6 that a normal cell is diploid, containing just two sets of chromosomes.) Or two plant species can cross, creating a polyploid hybrid.

In either case, the polyploid offspring is unable to breed with its parents’ population. However, self-fertilisation or asexual reproduction can produce others like it.

How do species become isolated?Common to all forms of speciation is the concept of population isolation. What causes one population of a species to be cut off from another? In populations that have diverged because of sympatric speciation, it is usually some form of genetic isolation, but other forms of isolating mechanisms also exist.

Temporal isolationTemporal isolation occurs when individuals of divergent populations reproduce at different times. For example, periodical cicadas mature underground. One species emerges and

What do you know about how one species becomes two?

1 how does gene flow influence the process of speciation?

2 what are the three main processes of speciation called?

3 why is sympatric speciation more common in plants than in animals?

Sympatric speciation also may occur in a few animal groups, although polyploidy is generally lethal in most animals and certainly in humans.

Parapatric speciationThe third mechanism of speciation, parapatric speciation, is less obvious and less well understood. In this model, neighbouring populations become distinct species while maintaining contact along a common border. Interbreeding individuals produce hybrid offspring in this region, which is called a hybrid zone.

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reproduces every 17 years. A ‘sibling’ species, which is almost the same morphologically, emerges every 13 years to reproduce. At this rate, the two species would release gametes at the same time only once every 221 years!

Behavioural isolationBehavioural isolation prevents gene flow between related species that live in the same territory through differences in behaviour. For example, in many bird species an elaborate species-specific courtship ritual precedes copulation. Different species of bowerbird construct elaborate bowers and decorate them with different colours in order to attract females. The satin bowerbird builds a channel between upright sticks and decorates the bower with bright blue objects. The New Guinean Macgregor’s bowerbird builds a tall tower of sticks

and decorates this with bits of charcoal. Evolutionary changes in mating rituals, such as bower construction, can contribute to speciation.

mechanical isolationMechanical isolation occurs when there is a physical incompatibility between body parts of potential mates or pollinators. For example, the flowers of some plants are sized and shaped so that they allow some pollinators to land but exclude others.

Physical isolationPhysical isolation can result from a geographic barrier. For example, two populations can be separated by rivers, mountains or even the formation of deserts. The plants and animals of south-eastern Australia and south-

western Australia are separated by a desert barrier.

Extreme events can also result in physical isolation. In the summer of 1995, Hurricane Luis, followed by Hurricane Marilyn, ripped through the northern Lesser Antilles in the Caribbean. Fifteen green iguanas survived on a raft of uprooted trees for nearly a month, floating over 300 km before reaching the northernmost island of Anguilla. These few individuals were the first recorded of their species, Iguana iguana, to reach the island. The iguanas have since established themselves as a breeding colony on the island, although they are now completely cut off from the larger population. In time, natural selection may operate on this isolated group to the point that they become a new species.

Fig 7.19 The different bowers of (a) the satin ��bowerbird and (b) the Macgregor’s bowerbird attract only females of their own species.

Fig 7.20 The green iguana. Island hopping has been a significant form of allopatric speciation and one that gave ��rise to the amazing diversity of forms that puzzled Darwin on the Galapagos Islands.

a

b

What do you know about how species become isolated?

1 what are the four different ways in which species may become isolated?

2 explain in your own words what an isolating mechanism is.

3 Give an example of how physical isolation could create a new species.

Africa

South America

Antarctica

India

AustraliaFossils of the fern Glossopteris, found in all of the southern continents, show that they were once joined.

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>>FRESH IDEAS<<What causes continents to drift?If you look at the coastlines of South America and Africa you may notice something quite remarkable—they seem to fit together like pieces of a large jigsaw puzzle. Such order in what would appear to be a random event is one of the keys that led Darwin to formulate a hypothesis about species and many other scientists to develop scientific theories.

A German scientist, Alfred Wegener, developed one such theory in 1914. Like others before him, Wegener had been struck by the remarkable fit of the coastlines of South America and Africa. What also puzzled Wegener and others was the distribution of some living plants and animals and numerous types of fossils. Based on this and on the distribution of these

animals and plants, Wegener came up with a hypothesis to explain such anomalies and ultimately the theory of continental drift. This theory suggests the surface of the Earth is in constant motion and therefore the continents change their position over time. Continental drift helps to explain the distribution of fossils and even the jigsaw-like fit of many continents.

Fig 7.21 Given that the fossil fern �� Glossopteris cannot walk, swim or fly, how can its isolated occurrence in so many different parts of the world be explained?

Fig 7.22 An underwater volcano erupted, with accompanying earthquakes, in Tonga in March 2009. The island ��nation of Tonga is part of the Pacific ‘rim of fire’, which is an arc of earthquakes and volcanic zones that stretches from Chile in South America, through Alaska and down through Vanuatu to Tonga.

Plate tectonicsIt is often hard for us to conceive that the Earth is a dynamic planet and that the ground we stand on is in motion and has been for millions of years—unless you happen to live along some very narrowly defined lines of volcanic activity and earthquakes. And then a clear pattern starts to form. The Earth, it seems, is divided up into a number of plates, which move relative to one another. You learnt about plates in Chapter 7 of Big Ideas Science Book 1.

To understand the process by which this occurs we need to understand the theory of plate tectonics. According to this theory, molten rock from the mantle spreads out beneath the crust and, in places, forces its way through. At these points the crust is pushed further apart. Where plates meet, one is forced under the other and, in doing so, sites of volcanic activity and mountain building are created.

The movement of continents has had a profound influence on how land animals and plants have evolved.

If two continents split, the •populations of animals and plants on both evolve in different directions than they would have had they stayed as a single population.

If two continents collide or form a •land bridge between them, two different populations of animals or plants begin to mix. Sometimes, this results in extinctions of species due to competition with, or predation by, members of other species.

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When a continent moves north or •south across the Earth, the animals and plants are then exposed to a wider range of environments than if the continent were to move east or west. This results in a greater change in the rate of evolution.

The theory of continental drift supports the theory of evolution by natural selection. If we apply these theories, we might expect that those continents that were once joined would have more closely related animals and plants than those that were not. And this is exactly what we get. The continents of India, Africa, South America, Antarctica and Australia were once joined as a supercontinent called Gondwana. Their connection to each other is still obvious in the plants and animals alive today.

The southern beech tree belongs to an ancient group of plants that evolved before the break-up of Gondwana. Its fossil and current distribution in Gondwana fragments, such as Australia, New Guinea, New Caledonia, New Zealand, Antarctica and South America, confirms this connection.

Similarly, the frogs of Australia show their closest evolutionary relationships to frogs in Africa and South America. Marsupial mammals are also an excellent illustration of the links

between continental drift and the theory of evolution. Marsupials are found in the Americas as well as Australia and New Guinea and there appear to be no routes of migration between the two populations. How did marsupials get

from their place of origin to locations half a world away? The answer lies in the theory that, 160 million years ago, these southern places were joined together as Gondwana.

1 using your knowledge of continental drift, explain how the current distribution of marsupials may have occurred?

2 many great scientific breakthroughs and theories have developed out of finding order in what could have been a random event. what observation of order initially led to the theory of continental drift?

3 Draw up a cause and effect chart to show how continental drift is related to the evolution of species.

Fig 7.24 The distribution of fossil and living examples of the southern beech tree, �� Nothofagus.

Fig 7.25 The distribution of marsupials today.��

Fig 7.23 Cross-section through a plate boundary.��

Lithosphere

Asthenosphere

ContinentIsland arc Mid-oceanic ridge Trench Volcano

Faulting

Hot spot

Subduction plateMesosphere

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>>CONNECTING IDEAS<<

Remember1 of the five ways in which alleles can be mixed, only

one process results in the formation of new alleles. which is it?

2 what is the difference between sexual selection and directional selection?

3 what are the three processes of speciation?

Understand4 explain the difference between incorrectly suggesting

an organism has evolved as opposed to correctly suggesting that a population of organisms has evolved.

5 Define the term ‘gene pool’ in your own words.

6 Distinguish between the concepts of continental drift and plate tectonics.

Apply7 callistemons (bottlebrushes) are unusual as their

stems (branches) do not terminate in flowers. instead, the stem keeps growing out past the old flower. As a consequence, a mature plant may contain the ripe seed of numerous years in its branches. how has this adaptive feature enabled callistemons to exploit the current Australian environment?

8 connect the term ‘allopatric speciation’ to ‘gene flow’.

Analyse9 The tortoises of the Galapagos islands either have

a domed shell with short neck—in islands with significant rainfall—or a shell with the front flared up and a long neck—in islands that are more arid.

The tortoises feed on prickly pear cactus. on islands with no tortoises, the prickly pear plant is low and spreading, but on islands with long-necked tortoises, the prickly pear plant is tall and has harder spines protecting it.

a why might the tortoises have two very different phenotypes?

b would the tortoises that originally reached the islands be likely to resemble any of the current tortoises?

c using the terms ‘variation’ and ‘survival of the fittest’, explain why the prickly pear plant is so different on the islands with long-necked tortoises from those plants growing elsewhere.

d what type of speciation is occurring on these islands?

Evaluate10 only two species of native non-marine mammals

(both bats) existed on new Zealand before the Polynesians introduced the rat and dog some 1500 years ago. This unusually small number of mammal species, along with new Zealand’s separation from Gondwana 60–80 million years ago, has led many to speculate on which land mass mammals originally evolved. The earliest known mammal-like fossil remains are over 160 million years old. considering this information, explain whether a Gondwanan origin for mammals is likely.

Create11 create a range of scenarios in which a population

of a species gets reproductively isolated from other populations.

How do species evolve?

12 create a mind map linking all of the following key terms: natural selection, survival of the fittest, variation, mutation, alleles, sexual selection, directional selection, allopatric speciation, gene flow, physical isolation, continental drift.

7.2

Big ideas