What is a species?It can seem obvious when two individuals are of different species, e.g. the lion and the tiger. What about the Bengal tiger and the Siberian tiger – are these different species?

Similarly, the capacity to interbreed cannot be tested in animals that are extinct, such as the dinosaurs. To overcome this problem, other definitions of a species are needed.

If two populations are geographically isolated, it can be difficult to tell if they are capable of interbreeding to produce fertile offspring. This is one aspect of the species problem.

Defining a speciesThe biological species concept is the most common definition of a species. It defines a species as a set of individuals who can reproduce to produce fertile offspring. As well as the problem of geographical separation, another disadvantage of this definition is that it only applies to organisms that reproduce sexually.

These happy face spiders look different, but since they can interbreed, they are considered the same species: Theridion grallator.

Speciation

A lineage-splitting event that produces two or more separate species.

Imagine that you are looking at a tip of the tree of life that constitutes a species of fruit fly.

• Move down the phylogeny to where your fruit fly twig is connected to the rest of the tree.

That branching point, and every other branching point on the tree, is a speciation event. At that point genetic changes resulted in two separate fruit fly lineages, where previously there had just been one lineage.

How do new species arise?

Here is one scenario that exemplifies how speciation can

happen:The scene: a population of wild fruit flies minding its own business on several bunches of rotting bananas, cheerfully laying their eggs in the mushy fruit...

Disaster strikes: A hurricane washes the bananas and the immature fruit flies they contain out to sea.

The banana bunch eventually washes up on an island off the coast of the mainland.

The fruit flies mature and emerge from their slimy nursery onto the lonely island.

At this point, speciation has not occurred — any fruit flies that got back to the mainland could mate and produce healthy offspring with the mainland flies.

The populations diverge: Ecological conditions are slightly different on the island, and the island population evolves under different selective pressures and experiences different random events than the mainland population does. Morphology, food preferences, and courtship displays change over the course of many generations of natural selection.

So we meet again: When another storm reintroduces the island flies to the mainland, they will not readily mate with the mainland flies since they've evolved different courtship behaviors. The few that do mate with the mainland flies, produce inviable eggs because of other genetic differences between the two populations. The lineage has split now that genes cannot flow between the populations.

Differing selection pressures on the two islands can complete the differentiation of the new species.

EVIDENCE FOR EVOLUTIONEVIDENCE FOR EVOLUTION

1. Fossils Evidence2. Anatomy3. Embryology4. Biochemistry

EVIDENCE FOR EVOLUTIONEVIDENCE FOR EVOLUTION

1. Fossils

Fossils tell a story…

The rocks at the top of the Grand Canyon, Arizona, are 250 million years old. Those at the bottom are 2 billion years old.

Why did the rocks form in layers?

Why do so many people want a closer look at them?

Looking back in time

The Evidence for Evolution• Fossils

– Found in sedimentary rocks

– Fossils can be rocks with imprints of dead matter.

– Can be amber; ice; tar.

The rocks are formed from sediments and many contain fossils.

What are fossils?

Fossils are the preserved remains, impressions or traces of animals, plants and other organisms that lived millions of years ago.

Some fossils contain the mineralized remains of the organism. In other cases, the remains have been completely dissolved and what is left is an impression, which may later fill with minerals to produce a cast of the organism.

fish skeleton fossilisedolder sedimentbecomes rock

more recent sediment collects

Layers of sediment

land raised abovewater level

recent rock

older rock

earth movements fracture rock

fossilised skeleton exposed

Fractured rock

When rock strata become exposed, it can be assumed that,in most cases, the lowest layers are the oldest.

This means that the fossils of organisms preserved in the lowestlayers represent animals and plants that lived many millionsof years ago

rock strata of increasing age

Rock strata

This is a fossil of a fish which lived 40 million years ago

Fossil fish

Fossilization is a rare event. Different types of fossil form under different conditions and environments.

Fossilized remains only form in the absence of microbes, which need food, oxygen, water and warmth.

A rare event

ambermud

ash/mud

ice and frozen soil

What type of fossil forms whether there are microbes present or not?

The history of life on Earth as shown by fossils is called the fossil record.

What does the fossil record show?

The fossil record can also show how different species evolved from common ancestors.

Although there are gaps in the fossil record, it helps to tell the evolutionary story of past and present-day organisms. It can showhow the changes in an organism were linked to changes in its habitat.

living fish sediment from river

fish skeleton partly buried by sediment

The Evidence for Evolution

Why is the fossil record incomplete?

– Only hard parts become fossilized.

– Not all organisms die in the right conditions for fossilization

– Not all fossils have been discovered

Whale Evolution

The Fossil Record

• The fossil record may show slow gradual changes and transitional forms

• It may also show jumps in evolution, either due to an explosion in life forms or an incomplete fossil record

Land Mammal

?

??

?

Where are the

intermediate

fossils?Where are the

intermediate

fossils?

Ocean Mammal

Complete seriesof transitional

fossils

We found the fossil — no joke!

Evolution from sea to land2006 fossil discovery of early tetrapod

4 limbs

Missing link from sea to land animals

Fossil of Archaeopteryx• lived about 150 mya• links reptiles & birds

Today’s organisms descended from ancestral species

Today’s organisms descended from ancestral species

Evolution of birds

feathers

claws

thinribs

teeth

long tail

wing-likeforelimbs

Reptile-like features

Bird-like features

Replica of Archaeopteryx fossil; half bird half reptile

Evolution all around usDarwin said, “It is not the strongest of the species that survives, nor the most intelligent, but rather the one most responsive to change.”

The environment today is changing rapidly as a result of habitat destruction, pollution and the artificial control of populations.

How is environmental change affecting the evolution of animals, plants and microbes?

What happens when habitats change?

Adaptive evolution ensures that individuals within a species have traits allowing them to survive and reproduce in their habitat.If the habitat changes, however, successful traits can become a disadvantage.

Individuals that fail to reproduce, compete effectively for food or survive against new predators will eventually die out. If the last individual of a species dies, the species is extinct.

For example, if global warming caused arctic snow to melt, brown rabbits living in arctic regions might be better camouflaged and so more likely to survive than white rabbits.

Why did the dodo become extinct?The dodo was a large flightless bird that lived on the island of Mauritius.It nested on the ground in forests, producing one egg at a time.

When human settlers arrived on the island in the mid-1600s, they brought animals such as rats and dogs to the island, which ate the dodos’ eggs.

The dodo became extinct sometime in the late 1600s.What traits might have helped the dodo to survive longer?

The settlers chopped down the forests in which the dodos lived, and may have even hunted the dodo for food.

Why does extinction happen?

destruction of natural habitats

over-hunting

climate change

pollution.

Extinction has always taken place, even before humans evolved. For example, the dinosaurs are thought to have become extinct due to a comet striking the Earth.

How does human activity cause extinction?

Do you know of any extinct animals?

Lack of sustainability threatens many organisms with extinction.

Countdown to extinction

Over 1.5 million different species of organisms have been identified, but scientists think there are many millions still to be discovered.

However, extinction rates are the highest since the death of the dinosaurs. On average, one species of plant or animal becomes extinct every 20 minutes.