© Boardworks Ltd 20091 of 7. © Boardworks Ltd 20092 of 7 What is a SCIENTIFIC THEORY? A scientific...


What is a SCIENTIFIC THEORY?

• A scientific theory = a well-supported testable explanation of phenomena

• *Best explanation for something we have at this time*


Darwin’s Observations

• During his trip he made several observations noting the variation among plants and animals, and how remarkably well suited to the environment these animals were


How is a polar bear adapted to its extremely cold climate?

How is a polar bear adapted?

white greasy fur repels water and acts as camouflage

thick fur and body fat insulate from the cold

large, wide feet spread the body’s weight and act as good paddles and snow shoes


Other adaptations that polar bears have evolved to cope with conditions in the harsh polar environment include:

More polar bears adaptations

small ears and small body surface area to volume ratio reduces heat loss

eyes have brown irises to reduce the glare from the Sun’s reflection

black skin is a good absorber of heat


• BIG QUESTION –

• HOW WERE ORGANISMS THIS WELL ADAPTED TO THEIR ENVIRONMENT??


The short-necked ancestors of modern giraffes needed to reach the leaves on tall trees when food was scarce.

Lamarck’s theory of evolution

How would this theory explain a giraffe’s long neck?

Over their lifetimes these giraffes stretched their necks; a trait which was then passed on to their offspring.

Jean-Baptiste Lamarck (1744-1829) was a French botanist who believed that species evolved because they inherited traits acquired through the over or under-use of body parts.


Darwin’s theory of evolution

The British naturalist Charles Darwin (1809–1882) later suggested a more persuasive argument for evolution.

Darwin proposed that evolution took place through natural and sexual selection.

Darwin developed his theory of evolution after noticing close similarities between certain fossils and the adaptations of modern day animals he saw during his round-the-world voyage on the HMS Beagle.


Darwin knew that in religious Victorian society his findings would be controversial and blasphemous. He was unwilling to publish and risk his reputation.

However, when Darwin learned that another British naturalist, Alfred Russel Wallace, had proposed an almost identical theory of natural selection, he published his findings.

Controversial research

Luckily, a handful of influential scientists were convinced by Darwin's work and spoke out in public to promote his ideas.


Natural selection


Due to natural variation, the ancestors of modern giraffes would have had necks of different length.

How does Darwin’s theory explain a giraffe’s long neck?

How the giraffe got its neck

As a result, the long-necked giraffes were more likely to be healthy and live to produce more high-quality offspring.

This, in turn, would increase the chances of their long-necked characteristic (an adaptive trait) being passed on to future generations.

Giraffes with longer necks would have been able to reach more food than those with shorter necks.


• Darwin proposed that over long periods, natural selection produces organisms that have different structures, establish different niches or occupy different habitats.

• He called this idea DESCENT WITH MODIFICATION


Galápagos finches

Darwin thought all the finches could have evolved from one type of finch that came from the mainland.

Some finches had strong and claw-like beaks, suitable for crushing seeds.

Other finches had thin and delicate beaks, suitable for picking insects from holes in the ground.

In the Galápagos, Darwin noticed that different islands had different types of finches, with different types of beak.

Natural variation meant that some finches had slightly different beaks. These finches would have been able to eat different types of food and avoid competition. They wouldtherefore have survived and passed on their genes.


Natural selection


Darwin made extensive use ofspecimens and fossil evidence toexplain his theory of evolution,but because DNA andgenes had not yet beendiscovered, he was unableto explain why traits varied within individuals or how they were inherited.

Could Darwin explain everything?

Victorian scientists found it difficult to test Darwin’stheory. For his theory to work, the Earth needed to be millions of years old, but its age was not known at that time.

In addition, little was known about the process of fossilization or how to explain gaps in the fossil record.


ENTER…

MENDEL!(and his pea plants)


Uniquely you…

The population of the Earth is more than 6 billion people, and no two individuals (apart from identical twins) are genetically the same. Why?

People are different because they inherit different characteristics (or traits) from their parents.

Like all babies, this child carries a unique set of genes; half from his mother and half from his father.

A person’s unique characteristics are caused by:

the set of genes they inherited from their parents (nature)

the environment in which they developed (nurture).


Inherited and acquired characteristics

Differences in some characteristics are due to a combination of both inherited and environmental factors. In some cases, it can be difficult to say how much influence each factor has.

Other types of characteristics, such as scars and hair length, are not inherited but depend on environmental factors. These are called acquired characteristics.

Some characteristics, such as eye color and earlobe shape, are only determined by genes. These are called inherited characteristics.


Phenotype and genotype

The overall appearance of an organism depends on two things:

The full set of genes of an organism is called its genotype.

All the observable characteristics of an organism are called its phenotype.

1. its genes (inherited characteristics)

2. the effects of the environment in which it lives.

An organism’s phenotype therefore depends on its genotype plus environmental effects.

phenotype = genotype + environmental effects


Environmental causes of variation

The effects of the environment in which an organism lives can cause significant variation between individuals.

Plants are affected by water, sunlight, temperature and the availability of nutrients.

Animals are similarly affected by water and nutrients.

When these factors areplentiful the plants thrive.

When these factors arescarce the plants wither.


The conditions for evolution

Means of selection – There must be a mechanism or pressure that selects some variables for the next generation at the expense of others.

There are three factors needed for a population of organisms to be able to evolve:

Variation – There must be differences between the individuals in a population.

Heredity – The differences between organisms must be heritable.


The origin of genetic variation

Genetic variation is heritable. It is this variation that natural selection acts upon. The causes of genetic variation are:

Mutation Sexual recombination

deletion, addition or substitution of a nucleotide

independent assortment of chromosomes in meiosis

deletion or translocation of part of a chromosome

crossing-over during meiosis

aneuploidy – loss or gain of a single chromosome

random fertilization.

polyploidy – the addition of whole chromosome sets.


Environmental causes of variation

Organisms can be affected by their environment. Variation caused by the environment is not heritable, so it is not subject to natural selection.

However, the ability of organisms to develop differently in different environments can be genetic. This means organisms can evolve to be flexible.

Plants are a good example of this. The number of leaves, growth pattern and size of any individual plant is dependent on the environment, e.g. availability of light and nutrients.


98% of a human and a chimpanzee’s genes are the same. What does this tell you about their evolution?

DNA from different organisms can be compared. The fewer differences, the less time since they shared a common ancestor.

It is a relatively short time since they both evolved from earlier mammals.

New lines of evidence

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