Evolution of Australian biota -...

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BiologyPreliminary CourseStage 6

Evolution of Australian biota

Part 6: Evolution, survival and extinction

Incorporating October 2002

AMENDMENTS

Part 6: Evolution, survival and extinction 1

Contents

Introduction................................................................................ 2

Mass extinctions ........................................................................ 4

Specialisation and extinction ...............................................................5

Extinction due to human activity..........................................................6

Evolutionary survival and extinction........................................... 9

Summary ................................................................................. 11

Suggested answers ................................................................. 17

Exercises – Part 6 ................................................................... 21

2 Evolution of Australian biota

Introduction

As was discussed earlier in this module, extinction is often part of the

evolutionary process. If conditions in an environment change and no

individuals in a population of a species have a genetic make-up which

permits them to survive and reproduce under the new environmental

conditions, that population of the species will become extinct.

When global environmental changes occurred over time many species on

Earth became extinct. Of all the millions of species, which have evolved

over the 4.7 billion years since life began on Earth, more than 99% of

them are now extinct.

On average, most species of organisms survive for between one and ten

million years, although a few species have survived much longer than

that. Modern humans have been on the Earth for only about 100 000

years so far, but in that time have had considerable effect on the

ecosystems of the planet, including being directly responsible for

processes which have resulted in the early extinction of other species.

The study of the evolution of species through time and an understanding

of the possible changes to past environments may permit some prediction

of the nature of future ecosystems. Although the processes are complex

and the gaps in knowledge must make any predictions quite tentative,

this module discusses such predictions and the evidence which permits

them to be suggested.

In this part you will be given opportunities to learn to:

• explain the importance of the study of past environments in

predicting the impact of human activity in present environments

• identify the ways in which palaeontology assists understanding of

the factors that may determine distribution of flora and fauna in

present and future environments.


In this part you will be given opportunities to:

• gather, process and analyse information from secondary sources and

use available evidence to propose reasons for the evolution, survival

and extinction of species, with reference to specific Australian

examples.

Extract from Biology Stage 6 Syllabus © Board of Studies NSW, originally

issued 1999. Revised November 2002. The most up-to-date version can be

found on the Board's website at

http://www.boardofstudies.nsw.edu.au/syllabus_hsc/syllabus2000_lista.html


Mass extinctions

Throughout geological time, since the appearance of living organisms on

Earth, there have been five mass extinction events. These are events

where more than 75% of species in existence at the time became extinct.

These are shown in the table below.

Period Time Nature of event

end Ordovician 441 mya 85% of all species wiped out

late Devonian 365 mya two waves of extinction, especially marinespecies

end Permian 251.4 mya 96% extinction, including mammal-like reptiles

end Triassic 200 mya 75% of species, especially marine forms extinct

Cretaceous 65 mya 75-80% become extinct, including the dinosaurs

Except for the mass extinction 65 mya at the end of the Cretaceous

Period, all of the other four extinction events appear to have been due to

changes in climate, sea levels, ocean current circulation, volcanic activity

and tectonic plate movements. As was discussed in Part 2 of this

module, all of these things interact with each other to produce change in

climate and therefore changes in ecosystems.

• A rising sea level, associated with global warming at various times

in the geological history of the Earth, not only reduced the amount of

land available for occupation by terrestrial species, it also resulted in

climatic changes in different parts of the planet.

• The movement of the Indo-Australian plate north, after it broke from

Antarctica, brought about temperature and rainfall changes in the

Australian landmass, but also changed ocean currents, which in turn

probably also led to further climatic change.


These changes normally led to complete destruction of habitats to which

the existing species were adapted, leading to their extinction. The few

species which did survive these events became the ancestors for a further

radiation of new species.

It has been estimated that the evolution of new species to fill such

modified ecosystems was normally quite slow. For example, after the

mass extinction at the end of the Permian period it took at least six

million years for the species diversity to return to anywhere near what it

had been before the extinction event.

The mass extinction of species at the same time as the dinosaurs at the

end of the Cretaceous period was probably not due to the factors

discussed above but to the effect of large meteorite impacts, such as that

at the Chicxulub crater on the Yucatan peninsula of Mexico, which is

200 km across! It is suggested that such impacts drove so much dust into

the atmosphere that it reduced the amount of sunlight reaching the Earth

and severely reduced photosynthesis throughout the ecosystems.

It is also possible that the impact also triggered volcanic eruptions by

causing movement of tectonic plates. The ash from these eruptions

further reduced light penetration from the sun.

All of these extinctions however, were not something that just happened

overnight. Even in the Cretaceous extinction, the elimination of species

was thought to be fairly slow, lasting up to at least 100 000 years.

Specialisation and extinction

Species which evolve into a specialised way of life are thought to be

more prone to extinction than those that have more general adaptations.

The megafauna may have been dependant on specific attributes of their

environments and were unable to cope with changes to these

environments brought about by climate change.

The platypus is highly specialised to obtain its food from the bottom of

water bodies using its bill. The modern species must obtain all of its

food in the water and so is very susceptible to any environmental changes

which might reduce the amount of aquatic habitat. The other fossil

platypus species may have become extinct as a result of arid conditions

increasing over their distribution.


Extinction due to human activity

Since modern humans evolved, a sixth mass extinction of species has

been in progress due to a number of human activities, including:

• hunting for food, animal products (eg. furs) and for sport

• pollution – especially water pollution, which has eliminated many

susceptible species

• introduction of predators, diseases and competitors

• climate change – mainly due to the greenhouse effect and global

warming

• habitat destruction for agricultural land, plantation forests and urban

development.

You are almost certainly aware that carbon dioxide and other gases in the

lower atmosphere permits the Sun’s heat to reach the Earth but prevents

some of it from being re-radiated back into space. Without this

greenhouse effect the average temperature on the Earth would be several

degrees lower than it is now and some species would be unable to

survive. However, the burning of fossil fuels, and a number of other

human activities, are increasing the amount of these gases in the

atmosphere, enhancing the greenhouse effect. As a result of this global

warming is occurring.

There are a few scientists who still are unconvinced by the evidence that

global warming is occurring, but the idea is generally accepted by the

majority of scientists. The world body of scientists called the

Intergovernmental Panel on Climate Change (IPCC) concluded that the

rate of warming at the present was unprecedented. They also found that

the rate matched the model for the enhanced greenhouse effect.

The prediction is that sea levels will rise by 7 metres over the next

thousand years.

While most scientists would agree with the findings of the IPCC report

there is much less agreement about what climate and sea level changes

will result from global warming. Increased global warming is happening

as a result of human activity but what sea level and climatic change it

will lead to in the future, and how far off these changes will be

experienced, is much less definite.

Scientific knowledge of changes in climate, habitats and the diversity of

species occurring in an area have been deduced in various ways by

scientists, including palaeontologists, climatologists and geologists.

But, predictions from these data need to be made with extreme caution

due to the incomplete nature of the data. Remember that humans have

only been around for the last 100 000 years and for a good deal of that


time they did not have the technology either to measure the data or to

record it.

Wild species of organisms, which are not yet extinct, but occur in small

numbers or in restricted habitats are often given conservation

descriptions like threatened, endangered or vulnerable. These are species

which have not had suitable genetic variations in their populations which

have permitted some individuals to survive and reproduce in the face of

the rapid changes brought about in their environments by the activities of

humans. Any climatic changes, brought about by global warming, have

the capacity to affect the status of such species and so predictions of what

will happen to the climate in various parts of the world are important

from the point of view of their conservation.

Species of organisms used for food by humans will also be affected by

climate change. Various complex climate models suggest that some

areas of the world, which are now well supplied with rainfall, will

become arid and some arid areas will be well supplied by rainfall.

This is predicted to have a major impact on some of the current food

producing areas of the world. At present we know that even weather

forecasting from day to day is quite imprecise and so longer range

predictions using these climate models are at best quite speculative.

It is a sobering thought that, even if the extinction brought about by

human activities were to cease now, the descendants of the modern

humans responsible for this extinction would probably not witness a

recovery. It has been estimated that recovery from previous mass

extinctions has taken millions of years. As the average length of time

any species has lasted from its evolution to its ultimate extinction is only

between one and ten million years, it seems likely then that Homo

sapiens will itself have become extinct before the recovery of diversity

has occurred!

In Parts 2 and 3 of this module you considered that climate, geology, habitat

and changes in flora and fauna over geological time need to be deduced or

inferred from indirect evidence. You will need to go back through your

notes to find this information. This will be good revision and will also help

you with the self test at the end of this part of the module.

1 Choose one period of geological time since Australia split away

from Antarctica and describe the climate, sea level, geology and

flora and fauna of the period.

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2 Describe two pieces of evidence scientists may have used to work

out what the conditions would have been like at that time.

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3 Greenhouse conditions resulting in global warming have occurred

previously in the geological history of the Earth. One reason for this

is thought to have been slight changes in the Earth’s axis and another

the greenhouse gases being released from increased volcanic activity

at various times. However, the latest global warming is thought to

be due to human activity.

Outline one method which scientists may have used to work this out.

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Check your answers.


Evolution, survival and extinction

The last thylacine or Tasmanian tiger, Thylacinus cynocephalus, was

captured in 1933 and died in captivity in 1936. This large carnivorous

marsupial once occurred on the mainland of Australia, as well as in

Tasmania and Papua New Guinea, but became extinct in all but

Tasmania around 2000 years ago.

Thylacine.

The Tasmanian tiger was not a fast runner and hunted at night in pairs,

tiring its prey by relentless pursuit, before killing it with a powerful bite.

Since kangaroos and wallabies were its main source of food it inhabited

open forest and woodlands.

Being at the top of the food chain the species probably did not occur

in large numbers. Numbers were reduced by human hunting (it

unfortunately developed a taste for sheep as food!) and possibly by an

outbreak of disease.

The species became extinct in other parts of Australia, probably as a

result of competition with the dingo, introduced into Australia around

3500-4000 years ago by Asian sailors. There are no dingos in Tasmania,

which became separated from the Australian mainland between

8 000 - 10 000 years ago.


If you think about the evolution, survival and extinction of this species of

marsupial then you may draw the following conclusions.

• The thylacine must have evolved from some marsupial ancestor after

Australia broke away from Antarctica.

• During the changes in climate, as Australia drifted north, the new

species evolved to be adapted to the role of ‘top’ carnivore in the

woodland ecosystems, which arose as a result of the drying of the

continent.

• However, no individuals in thylacine populations had the genetic

make-up necessary for the species to adapt to:

– the competition from the introduced dingo on the mainland,

– the hunting pressure from humans in Tasmania

– some deadly disease organism.

Like more than 99% of species which have evolved on Earth, the

thylacine became extinct. But, it was helped along the way by human

intervention by the introduction of a new competitor, hunting and

possibly the introduction of a disease not normally found in Australia and

one to which some animals had no natural resistance.

Do Exercise 6.1.


Summary

Now attempt the self-test, which will test a range of learning material and

skills which you have covered in this module.

Before starting on this exercise, read back through your notes and

assignments to familiarise yourself again with material which perhaps

you have not looked at for some time.

Multiple choice

1 The concept of the environment favouring the survival and

reproduction of the most adapted features or the elimination of the

least adapted was proposed by:

A Charles Darwin and Thomas Huxley.

B Thomas Huxley and Bishop Wilberforce.

C Alfred Wallace and Charles Darwin.

D Alfred Wallace and Thomas Huxley.

2 On his visit to Australia on the HMS Beagle Charles Darwin is often

said to have been uninterested by the flora and fauna of this country.

Which statement is the best interpretation of Darwin’s experience

with the flora and fauna of Australia?

A Darwin was unable to identify most of the species and so took

little interest in them.

B Darwin carried out considerable collecting and although

naturalists before him had already identified many species, he

also found new ones.

C Darwin was very interested by the geology of Australia but paid

little attention to its flora and fauna.

D Darwin watched platypuses swimming in the Coxs River near

what is now Lithgow, but apart from that, he saw few of

Australia’s native species.


3 More than half of the land area of Australia is said to be arid.

Which statement is the best definition for arid conditions?

A An environment where rainfall is very low and evaporation is

very high.

B An area with hot but humid conditions

C A hot, windy and dusty environment.

D An environment which is always has low rainfall and high

temperatures.

4 There are two types of cell division, one involved with growth,

repair and asexual reproduction and the other strictly with sexual

reproduction. Meiotic cell division results in the formation of:;

A two daughter cells which are genetically different from each

other

B four daughter cells with half the genetic complement of the

original cells

C four daughter cells which have fairly similar genetic make-up

D two daughter cells which are genetically identical to each other.

5 The giant southern landmass of Gondwana consisted of the

following tectonic plates:

A African, South American, Indo-Australian and Pacific

B Indo-Australian, Antarctic, African, South American

C Antarctic, Pangaean, Indo-Australian and South American

D African, South American, Pangaean and Pacific.


Short Response Questions

6 The following have been used as evidence that Australia was once

part of a single large southern continent.

• continental margins

• spreading zones between continental plates

• fossils evidence

• similarities between modern and fossil species

Choose two of these and briefly explain how they show a

relationship between the past and present landmasses.

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7 Scientific study has led to a better understanding of the biology of

many species of organisms in Australia. Use an example you have

studied to answer the following questions.

a) Describe an Australian species which has become better

understood as a result of the use of modern technology.

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b) Briefly describe the technology used to determine the nature of

this adaptation.

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8 After Australia broke from Antarctica it drifted north and

experienced considerable climatic and geographical change. During

this time new species evolved as a result of the following factors.

• genetic variation

• habitat changes

• isolation and

• natural selection.

Use these factors in a description of proposed steps which take place

in the formation of a new species.

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9 In your studies you investigated variation within two species.

a) Name one of these species and state what variable characteristic

you decided to measure.

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b) Describe how you went about measuring this characteristic,

including how many individuals you measured.

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c) Present your results either as a table of values or as a graph

showing the distribution of different measurements.

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Check your answers.



Suggested answers

Mass extinctions1 The Miocene period in Australia was thought to have been wet and

to have varied in temperature between being hot and cold, with

rainforests dominated the region. However, by late in the period

species in the Eucalyptus and Casuarina genera were beginning to

be more common. Mammals and birds dominated the vertebrate

fauna, probably including aquatic forms such as several

platypus species.

2 Fossils. For example, a sequence of fossilised leaves or pollen from

rainforest and then Eucalyptus plants would indicate wet and then

drier conditions. Fossils of parts of such species as platypus, turtles

and water birds indicates a past aquatic environment.

Higher growth rates seen in tree rings at the start of the period

indicate wet conditions and drier later.

3 Air trapped in ice cores indicate higher levels of carbon dioxide

since the start of the Industrial Revolution than previous years.

Growth rings in trees can also be used as plant growth increases with

increased carbon dioxide in the atmosphere.

Summary1 C Darwin and Wallace came up with the idea of natural selection

independently of each other at about the same time.

2 B Darwin expressed the thoughts in a letter that there would be

few new species which had not been identified by naturalist who

had visited Australia before him but also carried out his own

collecting and described new species.

3 A Although all of Australia’s arid areas are also hot in summer,

conditions are often cold in winter, and in other areas of the

world arid conditions can be cold and dry.


4 B The genetic material in the daughter cells (gametes) is halved by

the second division during meiosis.

5 B Gondwana included India and Australia (Indo-Australian),

South America, Africa as well as Antarctica.

6 The margins of continents, especially the continental shelves and

rock strata show good fit between parts of the continents which are

now separated, suggesting that they were once joined.

The existence of spreading zones between plates provides evidence

that the plates are in fact moving in relation to each other.

Fossils of similar groups, like the marsupials and early plants, such

as species of Glossopteris have been found on other southern

continents, suggesting that they got there when the continents were

initially joined to each other.

Living species, such as the flightless ratite birds (emus and

cassowaries in Australia, rheas in South America, the extinct

elephant birds of Africa and the kiwi and extinct moas of

New Zealand), suggest that they evolved before the break-up of

the supercontinent.

7 a) The platypus was thought to be like a reptile and was not able to

regulate its body temperature. However, measurements of body

temperature in captive and free-ranging platypuses enabled

scientists to discover that it did regulate its body temperature,

enabling it to survive in the cold conditions found in much of its

range in winter.

b) Radio telemetry was used in these studies where a transmitter

measuring body temperature was attached to animals and the

signal picked up by a receiver on the river bank.

8 As conditions change, many individuals do not have genetic

variations which adapt them to the new conditions. As a result most

become extinct but those have a suitable genetic make-up survive

and reproduce. Their offspring are adapted to the new conditions

and so these characteristics are passed on to the next generation.

If the group of organisms is isolated from other groups of the same

species this process of natural selection may quite quickly lead to the

formation of a new species.

9 a) Number of striped periwinkles with stripes of varying widths.

b) 20 individuals in each of 3 stripe classes were measured using

vernier callipers.


c) Stripe category Frequency (numbers ofindividuals in the sample)

wide stripes

(> 2 mm wide)3

medium stripes

(1-2 mm wide)25

narrow stripes

(< 1mm wide)2



Exercises - Part 6

Exercises 6.1 Name: _________________________________

Exercise 6.1: Extinction

Choose one or more species of Australian animal or plant which has

either become extinct throughout Australia or has reduced its previous

distribution in Australia. Find out the following:

• Which group the organisms belongs to and therefore what might

have been its ancestral group.

• Its adaptations to the habitat in which the organisms was found.

• The changes in the organism’s ecosystem to which the population

has been unable to adapt and which have led to its extinction.

To do this you will need to consult a range or sources, including the

Internet (if you have access to it) CD ROMs, library books and

magazines, such at Australian Geographic and Nature Australia

(published by the Australian Museum). You may also choose to use the

information in this part.

a) Outline reasons for the extinction of at least one Australian species.

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b) Describe the adaptations that assisted the survival of at least one

Australian species.

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c) Name one Australian species that has become extinct.

Propose reasons for the extinction of this organism.

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