DARWIN AND EVOLUTION

Chapter 15

Learning Goals

Quiz #9

You will be able to answer questions about Darwin’s historical

voyage on the HMS Beagle

You will be able to explain Darwin’s original theory of

evolution and Natural Selection

Quiz #10

You will be able to explain how Darwin’s theory has evolved

using Hardy-Weinberg equilibrium

You will be able to describe genetic variation, speciation, and

reading a cladogram

Descent with Modifications

In the early 1800’s some extraordinary fossil records were

being uncovered.

In 1815, the first reference of a “dinosaur” being an

ancient species of an animal was published.

Dinosaur bones had been collected since the medieval times,

but were mostly attributed to biblical giants or mythical

dragons

Animals such as the mastodon and apatosaur seemed to

have similarities to modern day animals, such as elephants

and giraffes

Could these animals have been descended from the ancient

dinosaurs, with slight modifications?

The HMS Beagle

Charles Darwin was 22 years old when he joined the

HMS Beagle as the ship’s naturalist.

The role of the naturalist is

Search for valuable plants, spices, minerals, etc.

Collect specimens for research

Serve as a backup to the ship’s doctor, navigator

The Beagle spent five years sailing along the southern

hemisphere, visiting Australia, Africa, South America and

the West Indies

Everywhere they went, Darwin had the chance to

explore

The HMS Beagle

In attempting to build a new collection of specimens,

Darwin made some observations

He found fossils of sea animals in the middle of continents

He found similar animals on totally different continents

Darwin began to subscribe to an “Old-Earth” theory,

which says the earth must be billions of years old

instead of the traditional thousands of years.

He hypothesized that the earth’s oceans and continents must

have had different boundaries in the past

An older earth would have enough time to make changes to

the geography, which would explain the fossil locations.

The HMS Beagle

Were the similarities between animals on different continents

due to random chance? The environments? Common

ancestors? Migrations?

Darwin’s most famous and substantial investigations would

occur when the Beagle made a stop on the Galapagos

Islands.

The Galapagos were an archipelago (a small group of islands.)

Some were flat, some mountainous; some sandy, some rocky; and all

500 miles from South America (which meant migration would be

difficult).

To Darwin, the Galapagos were like a set of miniature continents

and could serve as a model for the earth.

The Galapagos Animals

Tortoises

Each of the islands on the Galapagos had their own species

of tortoises

Darwin noticed on the barren islands, all the tortoises had

longer necks which helped in picking fruit off the cacti

On the lush islands, the tortoises all had short necks. The food

was easier to reach.

Did the islands cause the sizes of the necks in the tortoises to

change? Or did the tortoises live on the islands that were

best suited for their necks?

Harriet the Tortoise: b. 1835? d. 2006

The Galapagos Animals

Finches

Today there are only 13 species of finches on the islands, but

Darwin found many more.

He noticed that although the finch’s appearances differed

greatly, the one similarity was the relationship between food

and beak size.

Finches with large beaks broke seeds or nuts and used a

long tongue to retrieve food inside

Finches with pointed beaks acted like woodpeckers. Once

they created a hole they used tools to force insects out.

Did food choices cause beak sizes? Or did beak sizes

dictate food choices?

Return to England

After Darwin’s return to England in 1836, he waited 20

years before publishing his findings

Partly because his ideas hadn’t formed yet, partly because

he wanted to experiment, and partly because it was such a

radical idea.

Darwin’s basic theory was that if organisms could change

slightly from generation to generation, why couldn’t they

change dramatically over time?

The main problem to tackle was what was the motivation

for change?

Changes obviously were occurring. But why?

Return to England

Darwin finally published his works when, after reading

an article by a fellow scientist with similar ideas, he

became worried that he’d finish in second place.

On November 24, 1859, in the middle of the industrial

revolution of Europe, Darwin published his manuscript

titled On the Origin of Species.

Europe had fallen in love with a mechanized economy.

Darwin’s opinions about a mechanized system of life fit

perfectly with the times.

In the manuscript, Darwin promoted descent with

modifications, a common ancestor, and natural selection

Natural Selection

Natural Selection means that organisms change because

the environment forces them to or they will die.

1) Organisms in a population have inheritable variations

Darwin believed—and we have since proven correct—that

variations are random.

Variations are just as likely to be harmful as helpful

(Keep in mind, Darwin is publishing the same time Mendel is

experimenting. We still don’t know about genes or even what

nucleic acids are.)

2) Eventually, more individuals are produced in a population

than the environment can support

Births are usually much higher than deaths in a population too

Natural Selection

3) Some individuals have characteristics that enable them to

survive AND reproduce better than other individuals

“Fitness” is described as reproductive success relative to other

members of a population. The goal is to be the “fittest”.

If you’re able to survive, you’re more likely to reproduce and, thus,

more likely to pass on the genes that enabled you to survive.

If you’re less likely to reproduce, your unhelpful genes die with you

before you have a chance to pass them on.

4) As generations progress, a higher percentage of

individuals will have these traits than previous generations

Adaptations are the genes that help organisms to survive in their

environments

5) The result are populations built for their local environment

Natural Selection

Tortoises

In the past all Galapagos islands probably had tortoises with

both long and short necks

On drier islands, longer necks helped tortoises to eat. They

were beneficial.

On lush islands, longer necks would have entangled the

tortoises more easily. Shorter necks were safer.

The tortoises with the best traits were the most likely to

reproduce and pass on longer-necked genes

Eventually, only these tortoises inhabited these islands

Natural Selection

Finches

Similarly, the finch’s beaks were more or less beneficial

based on the food on their island.

If you had a big beak on a nut-filled island or a

narrow beak on a vegetation-filled island you had an

easier time eating.

Since you didn’t waste time with food, you had more

time to focus on mating.

Since you mated more often, you passed your traits on

more often.

Extra Credit for Quiz #9

The evolutionary organization of life is broken into

six groups: Animals, Plants, Fungi, Protists,

Bacteria and Archaea. Which common group of

organisms do not fit into any accepted kingdom or

domain in this group?

Evidence

The fossil record

The fossil record is the history of life told through remains

from the past (plant, animal, bacterial, geologic)

The fossil record shows plants and animals from the past that

are currently extinct but similar to today’s plants and animals

Example: The ancient mesohippus and the modern-day horse

Bones of animals millions of years old—and 5-6 times the

size of modern day animals—are remarkably similar

Some even show hybrids, like the archaeopteryx, a reptile-

like creature with avian-like feathers

Reptile Characteristics

-Teeth

-Skull Size

-Solid Bones

-Tail

Bird Characteristics

-Leg Shape

-Wings

-Feathers

-Wishbone (collarbone)

Evidence

Biogeography

Biogeography is the study of the distribution of plants and

animals throughout the world

Why does South America have no rabbits? No cacti in

northern Africa? The Galapagos tortoises and finches only in

the Galapagos?

These species would thrive if they were found there.

Darwin concluded that these species could not have

inhabited these lands because they had no ancestors who

lived in these lands.

Organisms do not randomly emerge. Someone has to

migrate there

Evidence

Common Anatomy

An enormous number of species all show common

homologous structures (anatomically similar in different

species)

Forelimbs of humans, horse, whale, cat, bird

This should not be confused with analogous structures, or

structures which serve a similar purpose but are not

ancestrally related

Wings of a bird vs an insect

Evidence

Common Anatomy cont…

Vestigial structures are anatomical features that are

functional in some species but not in others

Wings of an ostrich

Eyes of cave-dwelling salamanders

Spending time building structures that serve no purpose

seems like a waste.

Most likely they either USED to serve a purpose, or

eventually WILL serve a purpose.

Evidence

Biochemical evidence

The final piece of evidence is biochemical evidence and was

formed years after Darwin

The system of chemistry that codes for organisms is universal

All life is built on DNA, which contains the same 4 nucleotides

All life is built on proteins containing the same 20 amino

acids

The same codons code for the same amino acids

The same basic proteins exist in single and multicellular

organisms

The Modern Theory of Evolution

Darwin provided compelling evidence that species and

populations change.

What he didn’t know (and neither did anyone else at the

time) was the how

What are the mechanisms for passing on traits?

How are they passed on?

How does this turn into a trait?

Now we will talk about how the parts of the theory of

evolution changed, was altered, thrown out, or confirmed

after the discovery of genes

Microevolution

In a population, all of the alleles for a specific gene

within the population are called a gene pool

Gene pools are described using allele frequencies

If we count a population of 1000 people for the earlobe

trait, each person has two alleles for a total of 2000 alleles

in the population

If 500 of those alleles are for unattached earlobes, then the

allele frequency is 500/2000=25%.

Today, the study of evolution is the study of tracking the

allele frequencies in a population

Microevolution

Some common misconceptions about gene frequencies (in

other words, these are incorrect):

The majority of the gene pool is always the dominant allele

Gene frequencies over generations will tend to favor the

dominant allele

The dominant allele is the best allele.

Which allele is dominant in a gene pool has nothing to

do with whether or not an organism wants the allele

Polydactylism, Huntington’s Disease

Allele Frequency

Evolutions cannot occur unless the allele frequency of a

population changes.

In order for the frequency to not change, five

characteristics about a population must be true for each

gene.

1. No mutations.

Either mutations don’t occur or two mutations cancel each other out

2. No gene flow, either immigration or emigration

Immigration: into your population

Emigration: out of your population

Allele Frequency

3. Random mating.

Individuals pair by chance, not according to genotypes or

phenotype

4. Large population size

The population is large enough that chance alone does not affect

frequencies

5. No natural selection.

No selective agent in the environment favors one genotype over

another.

If just one of these five is not met, an evolution can occur

Genetic Mutations

Without mutations, there could be no new variations

among members of a population

Many mutations are not immediately detected because

they do not affect phenotypes

Once a mutation has occurred, the right combinations of

genes and/or environment factors may make a mutated

genotype more advantageous than the wild genotype

In this case, the favorable genotype will be passed on

over generations more than the unfavorable genotype

Gene Flow (or Gene Migration)

Gene flow is the movement of alleles between

populations by migration of the individuals

Large migrations of individuals, into or out of a

population, will carry their alleles with them

This inevitably affects the frequencies of the population

It is possible that one allele’s arrival or departure may

be so dramatic that the result is only one allele existing

in the population in future generations

Nonrandom Mating

While we’d like to think everyone’s equal, almost all

species on the planet are picky about their mates

If an organism has features that are undesirable or

unhealthy, their alleles have a smaller chance of being

passed along

Therefore, their allele frequencies will decrease in future

generations

Assortative mating: individuals mate with those with

specific phenotypes

Sexual selection: males compete over the right to mate,

but the winner gets as many mates as he chooses

Genetic Drift due to Small populations

Genetic drift is changes to allele frequencies due to

chance issues in the population

Larger populations are less likely to have a natural

disaster, disease, or other issue to dramatically affect

the frequencies

Drift is random, so a change in one population may not

have any effect whatsoever on a neighboring population

A drift that occurs in elm trees in northern California will

probably not be seen in a similar population in northern

Idaho

Genetic Drift due to Small Populations

Disasters can sometimes cause a near-extinction called a

bottleneck effect, where most members of a species die.

Only a few will make it through the “bottleneck”

The few remaining survivors’ alleles then make up the

new allele frequencies

Cheetahs are an example of this. All cheetahs, no matter

their locations, have a dramatically similar genome

It is believed sometime between 4,000 and 10,000

years ago they suffered a near-extinction, the result

being low variations in genotypes and alleles

Genetic Drift due to Small Populations

The founder effect is when a rare allele or combination

of alleles is unusually high in a population of a species

that is isolated from the other populations

The founding members of this population happen to

carry these rare alleles.

Thus, all their offspring over the generations contain the

same rare combination

Since the population doesn’t interbreed with other

populations, the frequency is much higher than usual.

Example: the dwarfism fingers in the Amish population of

Pennsylvania

Natural Selection

Natural Selection is when the environment favors one allele

and/or genotype instead of another.

Darwin hypothesized a form of natural selection. The

modern version of natural selection is as follows:

At least two phenotypes exist, & they are controlled by genes.

Genes are able to be passed from parents to offspring

One allele is better suited for an environment than the other

Organisms with the better allele will have offspring; organisms

with the less favored allele will not.

Offspring of parents with the better allele will also have the

better allele. The less favored allele has died off.

Natural Selection

In short, if an individual has an allele that allows them to

survive better in an environment than a different allele,

they will have a better chance of reproducing

If they have a better chance of reproducing, the next

generation will have a higher percentage of offspring

with the alleles for the favorable trait

Eventually, the alleles for the favorable trait will be the

only alleles in the population

Generation 1

Generation 2

Generation 3

Generation 4

Generation 5

?Generation 10

Generation 1

Generation 10

Speciation

Speciation is the splitting of one species into two or more

species, or the transformation of one species into a new

species over time.

A species is a group of organisms all capable of

interbreeding and producing fertile offspring

It’s important to identify and separate species from each

other because genes only flow between populations of

similar species, not different ones.

It’s also important to note when species separate

themselves to the point where a new species emerges

Speciation

For two species to separate, they must be reproductively

isolated. There are many ways this occurs

1) Habitat isolation

When two species occupy different habitats, even within the

same geographic range, they are less likely to meet and

attempt reproduction.

Red Maples of the East Coast vs Bigleaf Maples of the West Coast

2) Temporal Isolation

Two species live in the same area but reproduce at different

times of the year

The leopard frog, which mates in April and the bull frog, which mates

in July

Speciation

3) Behavioral Isolation

Two species have different courtship patterns that allow

males and females to recognize one another.

Fireflies of different species have different flashing patterns to

attract mates

Gypsy moths give off pheromones only detectable by members of

their own species

4) Mechanical Isolation

When two species genitalia or structures are incompatible,

reproduction cannot occur

Male dragonflies’ anatomy are so uniquely shaped they only fit

with other members of their own species

Speciation

5) Gamete Isolation

Two species may be able to mate, but their gametes may be

incompatible with each other

The Gray Fox has 66 chromosomes; the Red Fox has 34

6) Zygote Mortality

Fertilization occurs, but the zygote does not survive

Many zygotes of coral hybrids do not survive development

7) Hybrid Sterility

The zygote of a hybrid is able to develop and be born, but

the offspring are sterile

When a male donkey and a female horse reproduce, the result is a

mule. Mules live healthy lives, but they are sterile

Evolution Extra Credit

QUIZ 10: In a given area of a lodge pole pine

forest, why are all of the trees literally the same

age?