Evolution: Changes Over Time

Chapter 14

Evolution: History Of IdeasOver time many ideas have been proposed in

order to account for the biodiversity that resides on Earth.

Some of these ideas were strange:For example in the 1700’s Benoit De Millet

described that the appearance of birds was due to flying fish being chased out of the water.

As time went on, our ideas of evolution have changed and evolved.

Nineteenth-century ViewsIn the 19th Century, there were three main

individuals which have influenced our ideas of evolution significantly:Erasmus Darwin Jean Baptiste LamarckRobert Chambers

Erasmus Darwin (1731-1802)

Erasmus Darwin was the grandfather of Charles Darwin.

He was a physician, a well known poet, philosopher, botanist, and naturalist.

He was the one who initially came up with the ideas on evolution that Charles later elaborated on.

Erasmus argued that all organisms on Earth originated from a common ancestor.

Jean Baptiste Lamarck (1744 – 1829)

• Lamarck thought that if an animal acquired a characteristic during its lifetime, it could pass it onto its offspring.

• Hence giraffes got their long necks through generations of straining to reach high branches.

Robert Chambers(1802 – 1871)

Was a supporter of the idea of evolution.

In 1844, he anonymously published a book outlining his ideas on evolution, Vestiges of the Natural History of Creation.

Chambers’ main idea on evolution was that over a period of time, species had the capability of changing in order to better suit their environment

Species: Unchanging or Not?Through most of the 1800s scientists believed

that species cannot change and they were fixed, as they had been since their creation.

Creationism was viewed as the only means by which different organisms on Earth existed.

Charles Darwin (1809-1882) and Alfred Russel Wallace (1823-1913)

• During a five year voyage on the HMS Beagle, Darwin was introduced to biological diversity all over the world.

• He gathered specimens that he eventually used as support for his theory of natural selection.

• He proposed that random genetic variations exist in populations that allow some individuals to produce more viable offspring, changing the population.

• Darwin worked closely with Wallace who was another naturalist at the time that had come to the same conclusions as Darwin, but without the extensive evidence that Darwin had.

The Darwin-Wallace viewAs Darwin and Wallace had come to the same

conclusions regarding evolution, they decided to publish their ideas together in a joint publication.

Wallace and Darwin published their works on natural selection in 1859 together. This publication was 20 years after Darwin first drafted Origin of the Species and sent it to other colleagues to review.

Evolution, or change over time, is the process by which modern organisms have descended from ancient organisms.

Neo-DarwinismNeo-Darwinism is a term used to describe the

'modern synthesis' of Darwinian evolution through natural selection with Mendelian genetics.

Although Darwin and Mendel had hypothesised about their respective ideas about Evolution and genetics around the same time, neither knew of the other’s work.

Today, it is hard to imagine studying Evolution without having studied Mendelian genetics.

Social and Political Influences on Theories of Evolution

The reason why it took Darwin such a long time to publish his theories was because it may not have been received well by the scientists at the time.

Many scientists held the teachings of the church as sacred, and Darwin did not want to be against the church

Darwin also did not know what the general reaction to his ideas would be.

Time Scales in EvolutionThe study of time scales is integral to the study

of evolution.

By studying time scales it makes it possible to study the age of fossils and rocks and also to estimate the age of the Earth itself.

Estimating the Age of the Earth

Up until the mid-1800s, it was thought that the Earth was only about 6,000 years old. This date was derived by Archbishop James Ussher in the 1600s from the Bible.

It is thought that the actual age of the Earth is 4.54 billion years old.

This makes much more sense to scientists as it provides the time frame over which evolution has had a chance to occur.

The age of the Earth is measured by using radiometric techniques on the oldest known meteorites on Earth.

Geologic Time

• This time scale was developed in the nineteenth century when geologists observed that particular rocks where characterised by distinctive groups of fossils.

• Names were based on areas where they were first recognised or on the distinctive nature of the rocks.

• For convenience the geological time scale divided the Earth’s history into hierarchical intervals. The most widely used time intervals are Periods.

• The time interval predating the appearance of the first abundant fossils is called the Precambrian.

How old is it?The difference between absolute and relative

age:Absolute age is the actual age of something. For

example 7 years old.Relative age is a comparison of ages. For example

object A is 12 years older than object B.

In studying evolution, we use both absolute and relative dating in order to figure out the age of fossils and rocks.

Relative AgeWhen we look at layers of different types of

rocks in the Earth’s crust, we are looking at relative age.

For example if we find a fossil above a certain layer of rock, we can conclude that the fossil is younger than that layer of rock.

Absolute AgeIn order to find the absolute age of rocks, we use

radiometric dating.

This is based on the rate of decay of radioactive elements which are present in rocks.

We use the half lives of these elements in order to backtrack and find out when the rock was originally formed.

There are many different elements that we can use in order to determine absolute age.

Potassium-Argon (K-Ar) Dating

The Potassium-Argon (K-Ar) dating method is the measurement of the accumulation of argon in a mineral.

This method works by using the radioactive from of Potassium (K40), whose half life is around 1,250 million years.

The decay of K40 yields Ar40 and Ca40.

So this method works by counting the amount of Ar40.

Argon39/Argon40 DatingThis technique is similar to the K-Ar dating

technique.

In this type of dating, the object to be tested is irradiated in order to convert the K39 to Ar39.

The amounts of Argon are then measured and hence an age can be estimated.

Carbon14 DatingThis dating technique also works in the same as

the previous techniques we have discussed (using the half-life of the isotope).

As the half-life of carbon14 is a lot shorter than any of the other dating techniques (5,740 years), it cannot date things which are too old.

This dating method is used to date things which are up to about 60,000 years old.

Electron Spin ResonanceThis technique is mostly used to date minerals.

It works by using radiation to cause electrons to separate from the atoms.

These electrons then become trapped in the crystal lattice of minerals.

This changes the magnetic field of the material at a rate that is predictable, allowing it to be used to date an item.

It can be used to date when mineralisation, sedimentation, or the last heating of minerals took place.

Evidence of EvolutionThe evidence of evolution lies in many aspects

that have been studied in biology.

This includes:The Fossil RecordComparative anatomyHomology and Analogy Embryology

The Fossil Record

dinosaurs humansbacteriaorigins© NASA

complex cells

The fossil record shows a sequence from simple bacteria to more complicated organisms through time and provides the most compelling evidence for evolution.

Transitional fossils

Archaeopteryx

• Many fossils show a clear transition from one species or group to another.

• Archaeopteryx was found in Germany in 1861. It shares many characteristics with both dinosaurs and birds.

• It provides good evidence that birds arose from dinosaur ancestors

Comparative Anatomy

Human and Gorilla

• Similar comparisons can be made based on anatomical evidence.

• The skeleton of humans and gorillas are very similar suggesting they shared a recent common ancestor, but very different from the more distantly related woodlouse…

Woodlouse

Homology: Similar Structures

Analogy: Similar Function

Analogous structures do not derive from the same ancestral structures, but serve a similar or same purpose.

Vestigial Structures and Organs

The coccyx is a vestigial tail

• As evolution progresses, some structures are no longer useful to an organism.

• These are known as vestigial structures.

• The coccyx is a reduced version of an ancestral tail, which was adapted to aid balance and climbing.

• Another vestigial structure in humans is the appendix.

Comparing Embryos

Comparative biochemical and genetic studies

If organisms are close relatives of one another, then it is true that they would share a high degree of genetic and chemical similarities.

Comparing proteinsAll proteins are built from the 20 different amino

acids.

Proteins from different species can be compared, and the similarities and differences in the amino acid sequence can be studied.

Comparing DNA by hybridisation

Another comparing technique is DNA hybridisation to compare how similar two strands of DNA are.

In this technique, DNA is heated until the two strands separate. This is done for both species to be compared.

These separated strands are then mixed and let to cool.

Once the DNA strands cool, down, they have a tendency to pair up again.

At this stage, a DNA strand from one species would be pairing with the DNA from another.

Hybridised sequences with a high degree of similarity will bind more firmly, and require more energy to separate them: i.e. they separate when heated at a higher temperature than sequences which share less similarities,

Comparing DNA: Gene Sequences

Comparing gene sequences is an automated, computer-based examination of DNA in order to reveal genetic or evolutionary relationships between organisms.

Comparing Chromosomes

Involves comparing chromosomes and their banding patterns. (H = human, C = chimpanzee

Bio-geographic Distributions

Another way we can compare organisms is by their bio-geographic distributions.

For example, when Charles Darwin visited Australia, he noticed that there were no rabbits even though that the conditions seemed perfect for them.

The ‘molecular clock’ concept

The molecular is a technique that uses rates of molecular change to find the time in geologic history when two species diverged.

It is used to estimate the time of occurrence of events called speciation.

Calibrating and testing the ‘clock’

In order to calibrate the clock, the absolute time of a speciation event must be known.

Molecular ClockObservations about amino acid changes that occurred during the divergence between species show that molecular evolution takes place at an approximately constant rate. Further evidence for this came from the relative rate test. This suggests that molecular evolution is constant enough to provide a molecular clock of evolution. This means that the amount of molecular change between two species measures how long ago they shared a common ancestor. Molecular differences between species are therefore used to infer phylogenetic relations.

Molecular evolution in living fossils provides an example of a constant rate of molecular evolution independent from morphologic evolution.

Figure: the constant rate of evolution of the alpha-globin. Each point on the graph is for a pair of species, or groups of species.

Figure: the constant rate of evolution of the alpha-globin. Each point on the graph is for a pair of species, or groups of species.

Patterns of evolutionThere are many different patterns of evolution.

These include:Divergent evolutionConvergent evolutionParallel evolutionCo-evolution

Divergent EvolutionDivergent evolution is the accumulation of

differences between groups which can lead to the formation of new species.

Usually this happens as a result of the same species adapting to different environments, leading to natural selection defining the success of specific mutations.

Convergent EvolutionConvergent evolution describes the

acquisition of the same biological trait in unrelated lineages.

Parallel EvolutionParallel evolution is the development of a

similar trait in related, but distinct, species descending from the same ancestor, but from different clades.

Co-EvolutionIn a broad sense, biological co-evolution is "the

change of a biological object triggered by the change of a related object". Co-evolution can occur at multiple levels of biology: it can be as microscopic as correlated mutations between amino acids in a protein, or as macroscopic as traits between different species in an environment.

SpeciationSpeciation is the evolutionary process by which

new biological species arise.

Evolution: Gradual or Intermittent?

There has been continual debate in the scientific community about whether evolution is gradual or intermittent (also known as punctuated).

Gradual evolution refers to the gradual change of organisms over a period of time.

Intermittent evolution refers to the sudden change in organisms

Punctuated Equilibrium

An interesting example: flounder