Darwin’s theory of evolution

Section 16.1 – his voyage of discovery

Charles Darwin

• One of the most famous British scientists of all time

• Born in 1809• Exciting time for science – humanity

understanding of the Earth was changing• Evolution refers to the idea of changes

over time• Darwin developed a scientific theory that

explains how modern organisms change over long periods of time through descent from common ancestors

Darwin’s journey• Darwin sailed on a 5 years voyage of the

Beagle in 1835• Plan of journey was to map the coastline of

South America• Darwin was a naturalist – planned to collect

specimens of plants and animals• He had no idea that he would have a scientific

breakthrough that would change our understanding of how the world works

• His research has implications for the modern day

• Living world constantly changing• How drug resistant bacteria come about• How diseases such as Avian flu evolve

Observations aboard the beagle

•Darwin was a geologist

•He was also a lover of all things natural• He collected 68 species of

beetles in a single day trip to Brazilian rainforest

•He made detailed observation in his notebook, but didn’t stop there• He wanted to explain why there

was so much biological diversity in a scientific way

Darwin’s key observations (#1)

• Species vary globally• Darwin noticed that different, yet

ecologically similar animal species inhabited separately, not ecologically similar habitats around the globe• Example – Rhea (South America), Ostrich

(Southern Africa) and Emu (Australia)

• Rabbits found in grassland in Europe, but not South America or Australia despite having ideal habitats for them

• Why are kangaroos only found in Australia?

Darwin’s key observations (#2) • Species vary locally• Darwin noted that different yet related

animal species often occupied different habitats within a local area•Many examples from the Galapagos

islands• Islands are very close, but they have

different ecological conditions• Different islands have animals that have

specific characteristics• Tortoises are famous, but also finches and

other birds display variation

Darwin’s key observation (#3) • Darwin also collected fossils, not just living

specimens• Scientists already knew that fossils

preserved remains or traces of ancient organisms• Some looked very strange, others look like

modern organisms• Example – Glyptodont (ancient giant

armored mammal)• Found in same region that armadillos

presently found in•Was there a connection?

Darwin’s explanations • Darwin discovered that each of his

finches he collected from the Galapagos were different species of finch, found nowhere else in the world

• Same applied to tortoises – each island had a unique species of tortoise

• Although all species on the Galapagos looked like, and were related to a species found on the South American mainland

• This lead to a lot of research, culminating in his theory of evolution, and the idea that species are not fixed and that they can change by some natural processes

Ideas that shaped Darwin’s thinking

Section 16.2

Geological breakthroughs • During Darwin’s time, humanity's

understanding of the Earth was changing rapidly • James Hutton and Charles Lyell

famously suggested that the Earth of extremely old, and that processes that occurred in the past are the same processes that operate in the present• Uniformitarianism • Hutton presented his work in 1785• Lyell published his first great work –

principles of geology in 1830

James Hutton – Age of the Earth

• Hutton made a number of observations of the Earth

• He linked together a number of geological process• Cooling magma to make igneous rocks

• Uplift to make mountains

• Erosion to reduce mountains and make sedimentary rocks

• All of these processes take a very long time

• Introduce the concept of deep time, and that the Earth must be more than a few thousand years old

Charles Lyell – Uniformitarianism • Lyell argued that laws of nature are constant, and

that scientists must be able to explain past events in terms of processes that occur in the present• Concept of uniformitarianism - the present is the

key to the past• Volcanoes release lava and gas in the past the same

way they do today• Rivers erode canyons the same way they do today

• The Earth needs to be old enough to be able to form the numerous structures and phenomena that we can see across it’s surface• Darwin read Lyell’s book, and it influenced his

understanding of the world• Darwin asked if the Earth can change over long periods

of time, can animals?

Did anyone come up with the idea of evolution before Darwin? • Yes!• Throughout the 19th Century, the fossil

record gave the idea that life had somehow evolved • A Frenchman – John Baptiste Lamarck

proposed that organisms could change during their lifetimes, by selectively using or not using parts of their bodies• Acquired traits could be passed on to

offspring, enabling species to change over time• Published in 1809 - the year Darwin

was born

What does this mean? • Lamarck suggested that all organisms have an

inbuilt drive to become more complex and perfect

• Organisms will change and acquire features that make them live more successfully in their environment

• Example – water bird could develop long legs as it began to wade in deeper water looking for bird

• The body could adapt to it’s new surroundings• Traits altered by an organism were called

acquired characteristics • He proposed that if a bird acquired a new trait,

such as longer legs it could pass it on to it’s offspring

Why were Lamarck's ideas incorrect?

•Organisms don’t automatically want to become perfect• Evolution does not mean that organisms want to become better, nor does it

happen in a predetermined direction

• Traits acquired by individuals can not be passed on to offspring

•But, he did recognize that species are not fixed• He tried to explain evolution using natural processes• He recognized a link between environment and body structures• His work paved the way for people like Darwin

Thomas Malthus• In 1798, Thomas Malthus recognized that humans

were being born faster than they were dying• This resulted in overcrowding

• He proposed that if population growth continued unchecked there would not be enough space for everyone• War, famine and disease were three factors that control

population growth

• Darwin took Malthus’s work and applied it to nature• In nature, most offspring die before they reach

maturity, and only a few manage to reproduce

• Darwin used this theory to investigate why certain individuals in a population survive, and look to see if there was any pattern

Artificial selection• Darwin investigated changes produced by

plant and animal breeders• Breeders know that individual organisms

vary, and it is possible to pass on these traits to offspring• Used to improve crops and livestock• The process of farmers breeding specific

animals or plants for certain characteristics is called artificial selection• Nature provides the variation, and humans

use what is useful• Darwin tested the idea of artificial selection

by breeding fancy pigeons

Why is this important?

•At first Darwin didn’t recognize the importance of variations within a species• Thought to be minor defects

•Darwin recognized that these minor defects provided the possibility for evolution• This is the basis for his theory of evolution

Assignment

•Discuss with you neighbor the key people who influenced Darwin and his theory of evolution

•Create a timeline in your notes displaying this key bits of information• Use the textbook to help you

•Add as much detail as possible• Current understanding in the world at each time period• What each scientist did• How their contributions helped to change the general understanding of the

world

•Basic example to help you on page 459 – but go into more detail!

Darwin presents his caseSection 16.3

Why did it take Darwin so long to publish his work? • It took Darwin 20 years to publish his Ideas on Evolution

• He was scared of being ridiculed• Radical idea• Revolutionary groundbreaking research• He knew he would face a lot of opposition

•He received a letter from Alfred Russel Wallace• English naturalist in Malaysia• Wallace’s thought were very similar to Darwin’s

•Darwin didn’t want to get scooped and published his work

Evolution by natural selection• He described a process in nature that operated like artificial selection• He used ideas proposed by Malthus and Lamarck in on the origin of species.

• Malthus• If more individuals are produced than survive, there will be competition for food, space and other

necessities for life• Struggle for existence• Which species come out on top?

• Variation and Adaptation• Darwin knew that individuals have natural variations• He proposed that some of these variants are better suited to life in their environment than others• Faster predators, or those with longer claws, or sharper teeth might catch more prey• Prey who are better camouflaged are less likely to be caught • Any heritable characteristic that increases and organisms ability to survive and reproduce is called

an adaptation

Examples of adaptations

Survival of the fittest

• Darwin recognized a connection between the way an organism ‘makes a living’ and the environment that it lives in

• Differences in adaptations affect an organisms fitness• Fitness refers to how well an organism survives and can reproduce in it’s

environment

• Individuals which flourish – and do well in their environment have a high fitness

• Individuals which have characteristics not suited to their environment have a low fitness,

• Die without reproducing or have a low fitness

• Survival in evolutionary terms refers to reproducing and passing adaptations on to the next generation

Natural selection

•The name for Darwin’s mechanism for evolution

•Organisms with variation most suited to their local environment survive and leave more offspring

•Difference to artificial selection is that nature, rather than a farmer or breeder makes the choice!

• It will occur whenever:• more individuals are born than can survive • There is natural heritable variation• There is variable ‘fitness’ amongst individuals

Natural selection explained

Natural selection….

•Natural selection does not make an organism “better”

• It doesn’t make the organism perfect

• It just means that the organism is able to pass on their genes to the environment

• It does not move in a fixed direction – it will depend on the local environment

• If the local environment conditions change, some traits that were once adaptive may no longer be useful, and different traits may become adaptive

Peppered moths

Common descent• Natural selection relies on the ability to leave descendants

• There are different scales of natural selection• Well adapted individuals in a species survive and reproduce• Well adapted species will survive over time

• Darwin proposed that over time, and many generations, adaptation could cause successful species to evolve into a new species

• He also proposed that living species are descended from common ancestors

• Descent with modification

• This requires a large amount of time – only possible with deep time, and Hutton and Lyell’s view of the Earth

Evolutionary trees• Darwin based his explanation for the diversity of

life on the idea that species change over time• Evolutionary trees suggest that all organisms are

related• Looking back in time you can find common

ancestors shared by tigers, panthers and cheetahs

• If you go further back there will be common ancestors with dogs, horses ,bats

• If you keep heading back there will be a common ancestor for all living things

• A tree of life exists for all living things

Evidence of EvolutionSection 16.4

Support for Darwin’s work

• Darwin’s work over 150 years ago occurred before our undersatnding of certain fields of science was capable of testing the theory• Related fields are:

• Geology

• Physics

• Paleontology

• Chemistry

• Embryology

• Molecular biology

• Genetics

• Since his work was published, every scientific test has supported evolution

Biogeography• Study of where organisms live now, and where their ancestors lived in

the past

•Patterns in both living and fossil species provide a wealth of information

• Show how modern species descended from ancestors

•Two patterns relate to Darwin’s theory• Closely related species differentiate in slightly different climates

• Galapagos species suggested descent from common ancestors• Natural selection on each island lead to different but related species

• Very distantly related species develop similarities in similar environments• Emu, Rhea and Ostrich• Body structures show they are only distantly related • Similar selection pressures results in similar adaptations

Fossils

•The age of the earth and the fossil record caused problems for Darwin• The Earth is now a lot older

than previously thought – 4.6 billion years!

• Gaps in the fossil record in 1850 have been infilled • Allow the trace of modern

species from extinct ancestors

Anatomy and Embryology•By Darwin’s time, it had been shown

that all vertebrate limbs share the same basic bone structure

•But they have different uses

• Some used for crawling

• Some used for climbing

• Some used for running

• Some used for flying

•Why do they all have the same bone structure?

Homologous structures• Darwin proposed that animals with similar structures

evolved from a common ancestor • This ancestor will have had a basic version of this structure

• Evolutionary theory explains how homologous structures can be adapted to different purposes through descent with modification form a common ancestor

• Anatomical details are studied, alongside stages of embryo development and when they appeared in evolutionary history

• Common appearances are used to link certain animals

• Reptiles and birds share a common front limb suggesting they share a common ancestor

• This common ancestor is more recent than the common ancestor of reptiles, birds and mammals

• Common ancestor for all four legged animals was an ancient lobe-finned fish that lived over 380 million years ago

Analogous and vestigial structures

•Analogous structures • When we talk about common descent, we talk about common structure not

common function

• Body parts that share common function but not structure are analogous • Example – wing of a bee and the wing of a bat

•Vestigial Structures• Not all homologous structures have important functions• Vestigial structures are inherited from a common ancestor but have lost most

or all of their original function

Embryology

• Researchers have known for a long time that the early developmental stages of many animals with backbones look very similar

• Groups of embryonic cells grow in the same order,

• Similar patterns of embryonic development provide further evidence that organisms have descended from a common ancestor

Genetics and Molecular Biology

•Darwin had no idea how heredity worked

•Work on genetics has gone to support Darwin’s theory

•The universal genetic code and homologous molecules provide evidence of common descent

•We understand how mutation and reshuffling of genes during sexual reproduction produces variation that leads to natural selection

Common Genetic code

•All living things use DNA and RNA to carry information an direct protein synthesis

•This is nearly identical in all organisms including bacteria, yeast, plants, fungi and animals

•Powerful evidence that all living things share a common ancestor

Homologous molecules

•Homology is not limited to physical structures

•Homologous proteins have been found in a number of places

•Cytochrome c which functions in cellular respiration is found in almost all living cells

•Hox genes are found in almost all vertebrates• Determine the order of body parts

Testing natural selection• As most kinds of evolution have occurred over millions of years, it makes it hard to test….• Some however have been observed and studied repeatedly in labs, and controlled in

outdoor environments• Best evidence comes from observations on the Galapagos islands and Darwin’s finches

Testing Darwin’s hypothesis

• Peter and Rosemary grant tested Darwin’s hypothesis

• They have studied Galapagos finches for more than 35 years

• Recognised that Darwin’s hypothesis depended on two assumptions• For beak size and shape to evolve, there must be enough natural heritable variation • Differences in beak size and shape must produce differences in fitness

• Tested on the small island of Daphne Major

• All finches were caught, tagged and identified

• Periodically re-caught and anatomical features like wing length were measured,

• Their data showed a lot of variation in heritable traits in Galapagos finches

Testing natural selection

•The Grant’s showed that individual finches with different sized beaks have better or worse chances of surviving both seasonal drought and longer dry spells

•Birds with larger beaks are more likely to survive during dry spells

•Natural selection can take place in finch populations both frequently and rapidly

•They also showed that competition and environmental change drive natural selection