IB Topic 5.4: Evolution

Individuals do not evolve, populations do

What is evolution?

Evolution Cumulative change in the heritable

characteristics of a population Change over time Simpson’s couch gag

http://www.jibjab.com/view/131239

What’s the evidence for evolution?

Charles Darwin (1809) Contributed more to our understanding of evolution than

anyone else Traveled on the H.M.S. Beagle

In 1831 set sail (at the age of 22!) from England for a voyage around the world

Made numerous observations & collected evidence that led him to propose a radical and revolutionary hypothesis about how life changes over time

Evolution Evidence: fossil record, artificial selection, homologous

structures

The Fossil Record

Life 500 million years ago is very different from life today

Fish fossils have only been found in rocks ~500 millions years old or younger (less than 15% of the history of life)

Top predators today did not exist at the time of dinosaurs (or before)

Many living organisms today have no identical form in the fossil record

Conclusion: life on Earth is constantly changing

Artificial Selection

Breeding domesticated animals Breeders choose the males and females with the

most desirable genetic characteristics and breed them together Examples?

Conclusion: Evidence that evolution is happening, but the driving force is human choice Not the driving force of evolution in natural ecosystems

Homologous Structures

Similar in form and function, but found in dissimilar species

Example: 5 fingered limb Humans, whales, and bats (see fig. 5.8 page 147) Humerus, radius, ulna, carpals Pentadactyl (penta – 5; dactyl – fingers)

The shape and number of bones may vary, but the general format is the same

Conclusion: evidence that the organisms in question have a common ancestor

Struggle for existence

Organisms produce many more offspring than survive to be mature individuals

Darwin did not coin the term ‘struggle for existence’ but it does sum up the point that over-production of offspring in the wild leads to their competition for resources

Some examples

Organism No. of eggs/seeds/young per

brood or season

Rabbit 8-12

Great tit 10

Cod 2-20 million

Honey bee (queen) 120,000

Poppy 6,000

Many of these offspring die before they reproduce

Darwin’s Observations

1st Observation Populations tend to reproduce rapidly and if every

individual survived, there would be exponential growth Not the case

Seems to be a natural checks and balance system There is a limit to the size of a population that the

environment can support

Deduction There is a struggle for existence

Some live and some die

Darwin’s Observations

2nd Observation Organisms vary

There are differences between individuals of the same specie

These differences affect how well suited an organism is to its environment (fit or fitness)

This is called adaptation Some individuals are better adapted to their environment

than others because they have favorable variations Deduction

In the struggle for existence, the less well-adapted individuals will tend to die

Natural selection

Darwin’s observations

3rd Observation Much of the variation between offspring can be passed

on to offspring It is heritable

Deduction Better adapted individuals reproduce & pass on their

favorable characteristics Over the generations the characteristics of the

population gradually change Evolution by natural selection

Why do individuals within a specie show variation?

Genetic variations arise from gamete formation and fertilization Random assortment Crossing over Random fusion of male and female gametes during sex Mutations

Not as powerful as sexual reproduction Mutations by be advantageous, harmful, or neutral Most mutations are neither harmful or useful

Does this sound familiar?

Evolution in Action

Galapagos Finches Darwin visited the Galapagos Islands in 1835

and collected specimens of small birds (finches) Group of islands off the coast of Ecuador 14 species in all

Darwin observed that the size and shapes of the beaks varied, as did their diet

Further research from Peter and Rosemary Grant suggests a close relation between beak characteristic and diet

Examples of natural selection

Antibiotic resistance in bacteria Pesticide resistance in rats

Antibiotic resistance in bacteria: more evidence for evolution

Antibiotics were one of the great triumphs of medicine in the 20th century

When they were first introduced, it was expected that they would offer a permanent method of controlling bacterial diseases

However, there have been increasing problems of antibiotic resistance in pathogenic bacteria.

The following trends have been established:

After an antibiotic is introduced and used on patients, bacteria showing resistance appear within a few years

Resistance spreads to more and more species of bacteria

Strains of bacteria appear that are resistant to more and more different antibiotics Called multiple resistance

How?

Antibiotic resistance is due to genes in bacteria and so it can be inherited Example of evolution by natural selection

The evolution of multiple resistance has occurred in just a few decades.

It is so important to finish all of your prescribed antibiotics Due to:

Widespread use of antibiotics (humans and animals) Bacteria reproduce quickly (less than 1 hour) Populations of bacteria are HUGE, increasing the chance of a gene

for resistance to form via a mutation If you want more info:

http://www.fda.gov/oc/opacom/hottopics/anti_resist.html

Pesticide resistance in rats

What are pesticides? Typically used by farmers Due to natural variation, some rats are

unaffected (resistant) to the toxin They survive and reproduce, making a new

population in which some or all members possess the genetic resistance

New pesticide must be used The cycle continues …

Take away …

Illustrates how a population can adapt to its environment

Illustrates how humans can be responsible for creating “super-resistant creatures” Why is this not an example of artificial

selection?

Flanders calls Homer an ape and makes a case for evolution revolution in THE SIMPSONS episode "The Monkey Suit"

Exit Slip

3 multiple choice questions 1 minute to answer each question before

screen changes.

Question1

What is natural selection?

A. The mechanism that increases the chance of certain individuals reproducing.

B. The mechanism that leads to increasing variation within a population.

C. The cumulative change in the heritable characteristics of a population.

D. The mechanism that explains why populations produce more offspring than the environment can support.

Question 2

Which factors promote evolution in a species? I. Sexual reproductionII. Environmental changeIII. Overproduction of offspring

A. I and II onlyB. II and III onlyC. I and III onlyD. I, II and III

Question 3

Natural selection is based on which of the following? I. Variation exists within populations.II. There is differential reproductive success within

populations.III. Individuals must adapt to their environment.

A. I onlyB. I and II onlyC. II and III onlyD. I, II and III