Darwin’s Theory of Evolution

Who was Charles Darwin?

• Originally a student of theology• Then became a naturalist• In 1831, took ship and sailed around the

world• During his travels, Darwin made

numerous observations and collected evidence that led him to propose a revolutionary hypothesis about the way life changes over time

Darwin’s Voyage

Darwin’s Observations

• Patterns of diversity– HUGE amount of diversity among

living things– Diversity in

• Traits• Offspring production• Habitat

Darwin’s Observations

• Darwin often stopped to collect fossils

•Preserved remains of ancient organisms

•Some looked like modern organisms, some clearly did not

•Where did these organisms fit it? Why had so many species disappeared?

The Galapagos Islands

• Located west of South America• Islands were close but had very

different climates• Very fascinated by both the

tortoises and the finches on these islands– Tremendous variety

Common ancestor? Changed to fit environment?

The ride back home

• Darwin’s return provided a lot of time for reflective thought

• Darwin observed that the characteristics of many animals and plants varied noticeably among the different islands of the Galápagos. – Critical point

The ride back home

• Darwin came up with a possibility– Could the animals on the different

islands have once been the same species?• Was that even possible??

Ideas that shaped Darwin’s Ideas

• Darwin built upon the work of those before him

• Other scientists built upon his work• But…

– Many people were shocked by his ideas and he did not receive a lot of initial support

Evolutionary Roadblocks

• Ideas in Darwin’s time1. Earth was only

a few thousand years old• Later shown to

be billions of years old

Evolutionary Roadblocks

• Ideas in Darwin’s time2. Both the planet

and its organisms were immutable (could not change)• Planet is always

changing…and the life on it

Evolutionary Roadblocks

• Ideas in Darwin’s time3. Any strange geologic features were the

results of infrequent catastrophes that humans rarely witnessed

• Simply not true

People who influenced Darwin

• Scientists had a large influence on Darwin1. James Hutton2. Charles Lyell3. Jean Baptiste Lamarck

• …And an economist4. Thomas Malthus

1. James Hutton

• Layers of rock are built very slowly

• Geological forces shaped Earth’s features (mountains and valleys)

• Took MILLIONS of years

• Earth is not a few thousand years old

2. Charles Lyell

• Wrote Principles of Geology

• Stressed that processes that shaped Earth millions of years ago are the same processes that do so today

3. Jean-Baptiste Lamarck

• Did realize that organisms change over time

• Published his work the year Darwin was born (1809)

Lamarck’s Proposition

• Lamarck proposed that by selective use or disuse of organs, organisms acquired or lost certain traits during their lifetime. These traits could then be passed on to their offspring. Over time, this process led to change in a species. – Had three supporting principles

A. Tendency Towards Perfection

• All organisms have a tendency towards perfection and complexity

• Acquired features that helped them live more successfully in their environments– EX: Giraffe has an urge to be tall, and

tries to do so

B. Use and Disuse

• Organisms could alter the size and shape of their organs by using them in new ways

• If an organ was not used, it would eventually disappear– EX: By trying to be tall, giraffe

stretches its neck out

C. Inheritance of Acquired Traits

• If an organism altered its body structure during its lifetime, then it would pass the new trait to its offspring– EX: If the giraffe stretched its neck

and it got longer, its babies would have longer necks

4. Thomas Malthus

• Looked at population– Notice that babies were being born

faster than people died– Malthus reasoned that if the

human population continued to grow unchecked, sooner or later there would be insufficient living space and food for everyone.

4. Thomas Malthus

• Looked at population– Thought that war, famine, and

disease were the only things that keep the population in check• Darwin realized this was true for every

organism, and not just humans• Most offspring die

– WHY do some die and some survive?

4. Thomas Malthus

• Food supply is linear

• Population growth is exponential

• Problem?

Evidence supporting Darwin’s Ideas

1. Fossil Evidence2. Homologous Structures3. Analogous Structures4. Vestigial Structures5. Comparative Embryology6. Biochemical evidence

1. Fossil Evidence

• Fossil evidence shows that living things have been evolving on earth for millions of years.

• Also showed that the geographic distribution of organisms lines up with the movement of the earth’s crusts & that newer (more modern) forms of fossilized organisms are found in the upper layers of rock

2. Homologous Structures

• Structures that have similar form but different functions– Bones of our forearm, the front flipper

of a whale, the wing of a bat & legs of a turtle.

• Could all four limbed animals with backbones have descended with modification from a common ancestor?

3. Analogous Structures

• Structures that have same function but different structure/structure – Fly and Bird wing

• Was environment similar so it made sense to have this feature?

4. Vestigial Structures

• Structures which no longer have a function to the organism.– Our appendix, hair, coccyx, wisdom

teeth

• These vestiges had a useful function at one time but since they are no longer of an adaptive use to the organisms, they have become reduced with time.

5. Comparative Embryology

• Compares structures of developing organisms (embryos)

• Structures found in embryos that have no function/use or are not present in adults– Early stages of development of many

animals with backbones are very similar

• Common ancestor!

6. Biochemical evidence

• Not used by Darwin but in use today!– Using modern biotechnology, examine

the amino acids (proteins) and genes (DNA) of organisms.

– Similar organisms will have more in common. The more closely related they are, the more recently they descended from a common ancestor

On the Origin of Species

• Darwin’s book• Hesitant to publish

– Alfred Russel Wallace—incentive

• Never talks about origin of life but how new species come to be

Darwin’s Theory of Evolution by Means of natural selection

1. Variation– In a population, there are variations in

different traits2. Overpopulation

– In nature, more organisms are born than can survive

3. Competition and Struggle for existence– There are not enough resources for all

organisms!– Certain species have natural

advantages

Darwin’s Theory of Evolution by Means of natural selection

4. Survival of the fittest / Natural Selection– You’ve got the correct combination of

advantageous traits (adaptations)?• You survive! Try to have babies and pass

on those stellar genes/traits!

– You do not have the correct variations and adaptations?

• You die!...that’s it.

5. Species change over time– Due to natural selection– S L O W process– New forms / species arise; others disappear

Natural Selection

Comparing: Lamarck and Darwin

Comparing: Lamarck and DarwinL

AMARCK

DARWIN

What was Darwin missing?

• Darwin published his work in 1836– Did not have all pieces to the

puzzle1. What was the source of variation?2. How are variations passed down

from generation to generation?

What was Darwin missing?

• Mendel did not publish his work until 1900

• Watson and Crick did not publish their work until 1953

• We can now answer the questions Darwin couldn’t using genetics, molecular biology, and evolutionary theory experiments

Sources of Genetic Variation

1. Mutation – may increase, decrease or have no effect on the organism’s fitness

2. Gene shuffling – during meiosis & fertilization, genes get shuffled

3. Crossing over – new combinations result during prophase I of meiosis

Why does it all change?

• The environment changes!– Adaptive radiation—the idea that as organisms

spread out to a new environment, they will acquire changes that allow them to better survive in the environment

• Radiate—spread out• Adapt—change

– Best seen with Darwin’s finches on the Galapagos islands

Adaptive Radiation

• Darwin’s Finches – small brown birds– Ancestral finches evolved to adapt to

open niches on the various Galapagos Islands.

– Some evolved a shorter, fat beak for cracking large seeds, some have a smaller, pointier beak for opening small seed, while others have a long pointed beak for getting into creases in bark for insects

Adaptive RadiationDarwin’s Finches

Evolutionary paths

• Based on the environment…1. …species can become

more different– DIVERGENT evolution– Results in speciation– Leads to homologous

structures– Idea of adaptive radiation

Evolutionary paths

• Based on the environment…2. …species can become

more similar– CONVERGENT evolution– Leads to analogous

structures

Genetic Drift and the Founder Effect

• Genetic Drift– In a small population (isolated

one), a particular allele may occur more frequently even though it doesn’t lead to the fitness of the population, it is just there

• Founder Effect– Is the change in gene frequency

as a result of the movement of a small group of a population.

The founder effect

CAUSES genetic

drift.

An Australian Aboriginal cave painting

Sicilian family

Isolating Mechanisms

• These are barriers lead to reproductive isolation

• Prevents reproduction…further isolates genesA. Behavioral isolationB. Temporal IsolationC. Geographic isolation

A. Behavioral Isolation

• Different courtship patterns, hibernation or estivation patterns, living habits, etc.

• May totally be different where two populations are no longer able to reproduce

B. Temporal Isolation

• Different mating times for species• Different pollination times for

plantsWood frogs mate in late March, Leopard frogs mate in mid AprilSince both are of the genus Rana, they can interbreed but do not Wood & Leopard Frogs

C. Geographical Isolation

• Physical barriers separated populations.

• Over time, they develop two totally different gene pools

The story of the fruit flies

1. A bunch of flies were minding their own business eating a banana2. A hurricane washed the banana & flies onto an island3. Since conditions & food are different on the island, the flies evolve

separate from their mainland relatives4. When some of the flies mix with the mainland relatives, they can no

longer produce viable offspring when they mate. Speciation has occurred!!

Extinction

• 99% of all species that ever inhabited this earth have gone extinct!!

• Mass extinctions– Lead open habitats/niches to be

filled by surviving populations.– Leads to evolution of new species– Actually considered GOOD for

evolution

Coevolution

• Two totally unrelated species evolve in response to changes in each other over time.

• Bees don’t see red, but do see yellow, blue, and UV. Thus, bee-pollinated flowers are mostly yellow or blue with UV nectar guides (landing patterns) to guide the bee

• Birds, like hummingbirds have good eyes which can see red

Evolutionary Rates

• Gradualism – evolution is a constant process and occurs at a steady rate

• Punctuated equilibrium- Periods of equilibrium followed by rapid periods of change.

– Change occurs with environmental pressures

– Species arise abruptly then have long periods of little change