evolution

5. ECOLOGY & EVOLUTION 5.4 Evolution

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Evolution – some vocabulary • Evolution—the process of cumulative change in the heritable

characteristics of a population.• Macroevolution – the change from one species to another. i.e. –

reptiles to birds• Microevolution – the change and variations within a species. i.e. – a

Chihuahua and a Great Dane • Common descent - all living things share a common ancestor if

traced back far enough.• Gradualism - evolutionary change takes places slowly and gradually.

This contrasts with saltation in which changes are sudden and extreme.

• Multiplication of species - the diversity of life is a consequence of speciation. Populations adapting to locations and becoming reproductively isolated from other populations.

• Natural selection: a two stage process in which there is: • production of genetic variation• selection

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The Voyage of the BeagleThe main mission of the voyage was to chart poorly known stretches of the South American coastline for the British navy.

Accompanying the captain was a 22-year-old college graduate, Charles Darwin.

Darwin's main interest was to study the geology, plants, and animals encountered on the voyage. It was a tour that would greatly affect Darwin's thinking and eventually the thinking of many others.

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Darwin

• Charles Darwin proposed a mechanism for evolution – natural selection.

• “On the Origin of Species” published more than 150 years ago, much of it is concerned with evidence for evolution by natural selection.

• Types of evidence:• breeding of domesticated animals and crop plants• fossils• homologous structures• geographical distribution of animals and plants

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Darwin and Evolution• In 1838, Darwin read an essay on human populations

written a few decades earlier by Thomas Malthus. • Malthus contended that much of human suffering, such as

disease, famine, and homelessness, was due to the human population's potential to grow.

• That is, populations can grow much faster than the rate at which supplies of food and other resources can be produced.

• Darwin recognized that Malthus's ideas applied to all species.

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Observations on evolutionOften populations tend to produce more offspring than the

environment can support.Populations tend to grow exponentially, but population

sizes tend to remain constant.

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• Since organism’s traits vary, some organisms are more adapted to survival than others.

• When there is a struggle to survive those with favorable

traits tend to survive long enough to pass them on.

• Those that have less favorable traits die before being able to pass the traits on.

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The Darwin – Wallace Theory• In 1858, another British naturalist, Alfred Wallace, did

come to the same conclusion as Darwin.• Darwin received a letter from Wallace that described the

same basic mechanism for evolutionary change that Darwin had proposed.

• Within a month, some of Wallace's and Darwin's writings were jointly presented in public. Darwin published his book The Origin of Species about a year later.

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Descent with modification• First, Darwin argued from evidence that the species of

organisms living on Earth today descended from ancestral species.

• In other words, life has a history of change. • Darwin proposed that the descendants of the earliest

organisms spread into various habitats over millions of years. In these habitats, they accumulated different modifications, or adaptations, to diverse ways of life

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Natural selection• Natural selection is the process by which individuals with

inherited characteristics well-suited to the environment leave more offspring on average than do other individuals.

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Exam-type question:Explain how sexual reproduction

promotes variation in a species. Meiosis: a) Segregation: When genes are

located on separate chromosomes, they sort independently of each other during meiosis.

b) Crossing over: The exchange of genetic material between homologous chromosomes.

Fertilization: Which alleles are passed on depends on which gametes are fertilized. Diploid organisms have one set of chromosomes from the mother and one set from the father

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Evidence for evolution – Domesticated animals• Humans have deliberately bred and used particular

animal species for thousands of years.• Consider the differences:• The only credible explanation is that change has been

achieved by repeatedly selecting for breeding the individuals suited to human uses (artificial selection).

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Auroch of Western Asia Belgian Blue Cattle

Evidence for evolution - fossils• In the first half of the 19th

century, the sequence of strata of rock gave name to the different geological eras.

• The sequence in which fossils appear matches the sequence in which they would evolve: first bacteria, simple algae, fungi, worms and later land vertebrates.

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• The sequence also fits in with the ecology of the groups: plant fossils before animals, land plants, land animals, plants suitable for pollination and then insect pollinators.

• Many sequences of fossils link together existing organisms with their likely ancestors. Ex.: horses and zebras (Equus) are closely related to tapirs and rhinoceroses.

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Evidence from homologous structures• Some similarities in structures between organisms are

superficial, like those between a whale and a fish = analogous structures.

• Structures that may look superficially different and perform different function, but have a unity of type = homologous structures.

• Homologous structures do not prove that organisms have evolved, nor that they had common ancestry.

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DNA mutations

Amit Mishra - NES International School Mumbai 17

• The sequences of bases in DNA molecules are passed from parents to offspring.

• These information-rich molecules are the records of an organism’s ancestry

• If two species have genes and proteins with sequences that match closely, it can be concluded that the sequences must have been inherited from a relatively recent common ancestor.

• In contrast, the more differences in DNA sequences, the less likely they share an ancestor.

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Evidence from molecular biology

• Figure 14-16


DNA mutations lead to identifying relationships• Organism 1 ABCDEFG• Organism 2 AbcdefG• Organism 3 ABCDefG• Organism 4 ABCDEfG• Organism 5 ABCdefG

• Each letter represents a key gene locus that each organism possesses.

• For each locus, upper- and lower-case letters indicate different genes.

• Draw a flowchart of how these organisms relate to each other

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ICT Database exerciseMultiple alignment


The mechanism of natural selection


Embryological evidence


The Cambrian explosion


Part 2


Group discussion exercise

• Discuss the theory that species evolve by natural selection

• Discuss how natural selection leads to evolution

• Put together a few arguments and conclusions to share with the other groups.

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Summarizing the Theroy of Evolution

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Group discussion exercise• Greater survival and reproductive success of individuals with favorable heritable variations can lead to change in the characteristics of a population.

• Variations: refer to the differences among members of the same species.

• There is evidence that the traits of populations change over time in relation to changes in their environment.

• However, these recently observed changes are relatively small.

• These observations do not prove that the different species evolved from other species.

• Evolution is a theory.

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Examples of evolution

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Example 1: Antibiotic resistance bacteria.• The bacteria becoming resistant to antibiotics, such as

penicillin. Is an example of natural selection. • Penicillin, an antibiotic works by stopping bacteria from

forming cell walls.• When a infected person is treated with penicillin, the

bacteria are unable to grow new cell walls, and they burst open.

• However the population of bacteria in the person’s body may be several millions.

• The chances are that any of the bacteria will get mutated and is not affected by penicillin.

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• The mutant bacteria will go on reproducing• Soon it may form a huge population of penicillin resistant bacteria.

• If some bacteria become resistant to one antibiotic, they may be treated with another.

• This happens quite frequently and hence there are so many different antibiotics available.

• This rapid evolution is due to the following:• Very widespread use of antibiotics, including in animal feeds• Bacteria can reproduce very rapidly, with a generation time of less than

an hour.• Populations of bacteria are often huge.• Bacteria can pass genes on to other bacteria in several ways.

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Example 2: Staphylococcus aureus


• This bacteria is associated with a variety of conditions including skin and lung infections.

• The bacteria exists in two forms (that’s why it’s an example of evolution);

• 1) Methicillin-resistant Staphylococcus aureus (MRSA) also known as the epidemic MRSA against which Methicillin antibiotic has no effect.

• 2) Methicillin Susceptible Staphylococcus aureus (MSSA). This form is still contained by the use of the antibiotic Methicillin.

How MRSA evolved• The antibiotic is the selection pressure on the population of

Staphylococcus aureus. • Random mutation within the species produces a resistance

gene at low frequency in the population • The gene can be transferred via plasmids to other bacteria • Frequent use of the antibiotic puts MRSA at a selective

advantage to survive and MSSA at a selective disadvantage • MRSA therefore survives the antibiotic and is able to

reproduce. • The descendants of MRSA will also carry the resistance gene • The resistance gene increases in frequency or there is a

process of cumulative change in the heritable characteristics (resistance gene) in the population

• The species has evolved into two new forms


• Genetic transfer 4-video series


• The existence of homologous structures is evidence that living organisms share ancestors. What is the deduction from the cartoon?

• This deduction was very shocking to people living in the second half of the 19th century. Do you think the cartoon is making fun of the people who were shocked or of Darwin for implying something so absurd?

• Darwin was well aware that there would be hostile reactions to this theory and probably delayed publishing for as long as he could. Which do you think Darwin feared more, hostility from other scientists or from religious groups?


Cartoon published in 1881 by Punch, an English satirical magazine.

evolution

Documents

darwin charles darwin

evolution fossils

species of organisms

darwin wallace theory

land animals

ancestral species

land plants

history of change