Evolution

Evolution refers to the relative change in characteristics of populations

that occurs over time.

The process of evolution suggests that all living things on Earth are here as

a result of descent with modification, from a common ancestor. The

theory implies that species are not fixed, unchanging things but have

evolved through a process of gradual change from pre-existing, different

species. Similarities among different organisms exist because they have

descended from a common, extinct ancestor. Dissimilarities are due to

modification over time. The more distant organisms are thought to be

related, the greater the dissimilarities.

An individual organism cannot evolve, only a population. Individuals are

capable of adapting. An adaptation is a particular structure, physiology or

behaviour that helps an organism survive and reproduce (pass on their

genes) in a particular environment. Not all adaptations are immediately

visible (if caused by mutations in a gene) or applicable (environments are

constantly changing). Thus, a characteristic that may not give an individual

organism a particular advantage now, may become critical for survival later.

This can be demonstrated by the story of the English peppered moth.

The Peppered Moth Story

English peppered moth – Biston betularia

Two colour variations – greyish-white flecked with black dots and

black

In the mid 1800’s, black moths made up less than 2% of the population

near Manchester, England

The moths would hunt during the evening and rest during the day on the

trunks of trees

Greyish-white moths would be camouflaged while black moths were easy

prey for birds

By 1895, the population of black moths skyrocketed to 95%

This drastic change coincided with the

Industrial Revolution

Air pollution from factories, killed the

white coloured lichen on the trees and

the trunks became covered in soot

Flecked moths were now seen and eaten

by birds, while black moths survived long

enough to pass on their genes – increasing

the abundance of the black coloured gene

within the gene pool

A gene pool is the total of all the genes in a population at any one time.

Evolution can also be defined as any shift in a gene pool.

In the peppered moth example, the environment exerted a selective

pressure on the population of moths. When the environment was change,

certain characteristics were selected for (black) and others against (white

flecked).

The black moth can be described as having a high degree of fitness.

Fitness, in an evolutionary sense, refers to how well an organism fits with

its environment

The story of the peppered moth is an example of natural selection.

Natural selection is a process whereby the characteristics of a population

of organisms change because individuals with certain heritable traits

survive specific local environmental conditions and pass on their traits to

their offspring. In order for natural selection to occur there must be

diversity within a species so that there may be selective pressure.

While natural selection is an ongoing, uncontrolled process, people have

been artificially selecting organisms for particular traits for centuries. In

artificial selection, plant or animal breeders select individuals to breed for

the desired characteristics he or she wishes to see in the next generation.

LUCA - Last Universal Common Ancestor

Part 1—The Ancient Sea

3.8 billion years the Earth was very different than it is today. No life

Surrounded by CO2

The temperature was around 950 C

Many atoms and molecules swirled around in the “ancient seas”

High amounts of energy available due to volcanic activity, ultraviolet light and

lightning

When the atoms and molecules bumped into each other at high speeds: bonds were broken

bonds were made

Carbon chains started to form. These carbon chains are the building blocks of organic

molecules. Organic molecules contain both carbon and hydrogen and usually have a

carbon backbone.

Part 2—How was the first Cell Membrane formed?

As atoms rearranged themselves, three important molecules developed: fatty acids chains

glycerol

phosphate

These 3 molecules then “bumped into each other” and bonded through a condensation

reaction. A condensation reaction is a chemical reaction where water is removed. Condensation

reactions build macromolecules.

The macromolecules built for the cell membrane is called a phospholipid. The simplified

model of the phospholipid consists of: o Polar, hydrophilic phosphate head

o Non-polar, hydrophobic fatty acid tail.

When the phospholipids mixed with water, the water “hating”, hydrophobic fatty acid

tails moved away from the water while the water “loving”, hydrophillic phosphate heads

moved toward water. This created the phospholipid bilayer, also known as the cell

membrane.

Overtime substances such as proteins, cholesterol and sugars became attached to or

embedded in the membrane. The selectively permeable bilayer formed naturally to

spherical vesicles due to the properties of the phospholipid. This first closed circuit

cell membrane, trapped matter inside over millions of years. Over time, the fluid on

the inside of the membrane, known as the ICF or intracellular fluid, became very

different from the fluid on the outside of the membrane, known as the ECF or

extracellular fluid.

Result of the formation of the cell membrane: Molecules can now be made faster

inside the cell membrane because the limited space increases the collisions (i.e.

Molecules bang into each other more often.) The membrane also controls what can come

in and out.

Part 3: How were nucleic acids formed?

Similar to the formation of the cell membrane atoms continued to rearrange forming

both organic and inorganic molecules. Three important molecules for formation of

nucleic acids were:

Phosphate

Ribose sugar

Nitrogen containing bases (A, C, G, U or T)

These molecules combined by condensation reactions (releasing water) to form

nucleotides. Nucleotides consist of a nitrogen base bonded to a sugar bonded to a

phosphate. These RNA nucleotides then bonded to each other through condensation

reactions to form long chains.

Over time, deoxyribose sugar developed (has one less oxygen molecule than ribose).

The same condensation reactions occurred to form DNA nucleotides (nitrogen base

A,T,C,G bonded to sugar, bonded to phosphate). The DNA nucleotides then bonded

together to form long chains AND the nitrogen bases weak attraction to each other (A

to T and C to G) formed “hydrogen bonds” which resulted in a double chain.

Part 4: How was protein formed?

Inside the membrane (ICF) molecules are still moving quickly and banging into one

another. The number of collisions increased due to the decreased space.

An amino acid is formed when an amino, an R group and carboxyl bond. Over time this

produced 20 different amino acids (the R group is what makes each amino acid unique).

When amino and carboxyl groups bump into each other they join through a condensation

reaction, where water is released and this time a peptide bond is formed. As amino

acids bump into each other they also join through condensation reactions and ultimately

build proteins.

Simplifying the model: Amino acid (represented by a circle)

Dipeptide (two amino acids bonded together)

Polypeptide (several amino acids bonded together)

Protein (over 30 amino acids bonded together and folded into specific shapes)

Long chains of amino acids begin to fold due to the weak attraction of the molecules

(“hydrogen bonds”)

Proteins have many roles in the cell some of which include: o Proteins channels (transport molecules in and out of the cell)

o Cytoskeleton (hold organelles in place)

o Worker proteins (to move molecules)

o Enzymes (to make chemical reactions in the cell occur faster)

Part 5: What did the first cell(s) look like?

EACH MACROMOLECULE (phospholipid, nucleic acid, and protein) WAS BONDED

THROUGH A CONDENSATION REACTION TO ESTABLISH THE EARLIEST FORM

OF LIFE, 3.8 BILLION YEARS AGO. This earliest form of life is referred to LUCA (last universal common ancestor).

This form of life is called a prokaryotic cell. Prokaryotic cells are… small & simple

no organelles

mainly anaerobic (no O2)

no membrane bond nucleus….most DNA is clustered together is the nucleoid

o May also have a plasmid (a ring of DNA outside the nucleoid)

Part 6: How did the first eukaryotic cell form?

Eukaryotic cells have been around for about 2 billion years. The main theory is that the

eukaryotic cell represents the merger of 2 or more simpler cells. This is known as

endosymbiosis. While prokaryotic cells lack many internal structures, two organelles in

the eukaryotic cell present the strongest evidence of endosymbiosis in early eukaryotes.

One is the chloroplast, an organelle found in photosynthetic eukaryotes that converts

solar energy into sugar. The other is the mitochondrion, an organelle that does the

opposite-it converts sugar into energy so the cell can do work.

There is much evidence supporting the endosymbiotic theory:

Both Chloroplasts and Mitochondria: Are about the same size as a simple bacterium

and are surrounded by two membranes that are

similar to living prokaryotes and suggests that they

were engulfed by endocytosis

Have their own ribosomes, which is similar to bacterial ribosomes and significantly

different from eukaryotic ribosomes

Contain their own simple loop of DNA (a circular chromosome) and many gene

sequences matching living prokaryotes (i.e. chloroplast genes match cyanobacteria)

Reproduce by simple binary fission (splitting in two) (text pg. 68) independently

of the host cell cycle and mitosis

It is thought that over time colonies of cells started to specialize based on instructions

from genes. The first multicellular organism existed 1.2 to 1.5 billion years ago (which

means half as long as unicellular organisms). The oldest such fossils are of red algae

found in rocks in artic Canada. These were still simple organisms by modern standards.

Based on fossil evidence, scientists think that large, complex eukaryotes first

developed about 550 million years ago. (see text pg. 333)

Fossil Evidence

Palaeontology Palaeontology is the study of fossils.

Fossil A fossil is any sort of remains of an organism that lived in a past geological

age.

In order for fossils to form, there must be a way to preserve the dead

remains of animals and plants for a time so that they do not decay

completely. The most common way that this occurs is on the bottom of

bodies of water.

When an animal or plant dies and falls into the water, sediments sometimes

cover up the remains quickly. The layers of sediment form a protective

covering to slow the process of decay. Over thousands of years, the

sediments around the remains harden into rock. The dead animal or plant

remains eventually decay leaving an empty space inside the sedimentary

rock. Minerals filter down into this space and harden into rock, forming a

shape just like the animal or plant. This process is called fossilization. The

mineral remains are called fossils.

Sometimes there are no minerals that filter down into the empty space in

the rock. The space that is left is called an imprint. Some common fossil

imprints are dinosaur tracks, which are formed when the large animals left

their tracks on the bottom of shallow seas or rivers.

There are other ways that fossils can be preserved. Many animals have

been found preserved in ice in Siberia. Freezing preserves a fossil of the

highest quality. It preserves the organism with little alteration to the

chemical composition. Other fossils, especially insects, are found

imbedded in amber (a sticky sap from trees that covers the insects and

then hardens).

There are four main types of fossils:

1. Whole Animal/Plant

Preservation of soft and hard body parts

Very rare

Insects entombed in amber (preserved so well, to be studied as though they had

just died)

Frozen Mammoth carcass and human remains (even after 40 000 years)

2. Petrification/Replaced Remains

Remains of the organism are turned to stone

Organic substances (soft parts) decay, but water containing

Minerals soak into the cavities and pores of hard structures (bones, shells, eggs

etc…)

Water slowly dissolves original hard parts

3. Imprints

Outlines of leaves, feathers, footprints etc…

Carbon print – transfer of atoms to rock (painting a picture)

Significance of footprints: depth, size and distance between provide information

about weight, length and bone structure

4. Mould/Casts

Living organism is buried in mud/clay, which eventually hardens

Body dissolves away, leaving a cavity within the hard material

Cavity is filled with rock in the shape of the original creature

The fossil record reveals a history of life on Earth and shows the kind of organism that

were alive in the past. The millions of species on Earth today are only a small fraction

of the species that have ever lived. In fact, it is estimated that 99% of all species that

have ever lived are now extinct.

One of the largest collections of fossils unearthed is in the Burgess Shale – a rich fossil

bed in the Rocky Mountains of Yoho National Park, British Columbia, shows a time (about

500 million years ago during the Cambrian period) in which a stunning burst of

biodiversity occurred. A large number of the fossils are unlike anything in our modern

oceans.

Fossils from more recent geological periods are much more similar to species alive

today. A fact that supports the idea that life has evolved over time. Species that were

alive long ago have had a longer time to change.

Age of Fossils

Relative Dating

The fossil record supports the idea of evolution in that fossils appear in chronological order.

The Law of Superposition states that in an undisturbed horizontal sequence of rocks, the oldest

rock layers will be on the bottom, with successively younger rocks on top of these. By

correlating fossils from various parts of the world, scientists are able to give relative ages to

particular strata (layer of rock). This is called relative dating.

The fact that organisms do not all appear in the fossil record simultaneously supports the idea

that organisms have slowly evolved from ancestral forms.

When studying the fossil record there is evidence of a number of transitional fossils;

intermediary links between groups of organisms and share characteristics common to two

separate groups.

Archaeopteryx, which lived approximately 150 million years ago, appears to be the link between

reptiles and birds. The creature had feathers, claws on its wings and a bony tail. This is one of

many fossils that suggest birds evolved from dinosaurs.

Acanthostega, which lived approximately 360 million years ago, show the link between fish and

amphibians. The creature had gills and lungs, stumpy legs, limbs and toes, a long tail, a jaw and

teeth.

The oldest fossil discovered thus far are of stromatolites (rings formed by cyanobacteria) that

lived over 3.8 billion years ago!

Radioactive Dating

Some atoms on Earth have nuclei that are unstable and emit forms of radioactive decay over

time.

The 3 main types of radioactive decay are:

Alpha particle emission ()

Beta particle emission ()

Emission of gamma radiation ()

Alpha Decay

An alpha particle is a helium nucleus, 42He, composed of two protons and two neutrons.

Since alpha particles have no electrons, they carry a charge of +2.

Radium-226 is an example of an atom that emits alpha particles:

Beta Decay

Beta decay occurs when an isotope emits an electron, 0-1e.

The emission of a beta particle is accompanied by the conversion, inside the nucleus, of a

neutron into a proton, therefore this emission involves a change in the number of protons in the

nucleus; therefore the atom becomes a different element.

Carbon-14 is an example of an atom that emits beta particles:

Eventually all radioactive atoms will emit enough particles that they reach a stable nuclear

configuration.

Examples: Uranium 238 – lead 206

Potassium 40 - Argon

Carbon 14 – Nitrogen 14

Half Life

Atoms that display radioactive decay of their nuclei can be used as a tool to determine the

exact age of a fossil. This can be accomplished because each radioactive element has a unique,

predictable half life; the time it takes for a given sample of a radioactive element to decay to

one-half of its initial amount.

Examples: Uranium - 4.5 billion years for ½ to decay to lead

Potassium - 1.3 billion years

Carbon 14 - 5760 years

The radioactive dating process measures the ratio of the amount of the original element (i.e.

Uranium 238) found in the substance, compared to the amount of the end product (i.e. Lead

206)

Using the half life of this substance and this ratio, the age of the specimen can be determined.

The half life of Uranium is sufficiently long enough to date the oldest rocks and fossils on

Earth.

Carbon dating is used to measure the age of the organic material of much younger specimens.

(Up to 30 000 years old, but is most reliable for objects no more than 7000 years old)

Equipment called a mass spectrometer is used to determine the amounts of C-14 (or other

elements present)

Evidence of Evolution by Natural Selection

Evolution by natural selection is a scientific theory that explains how Earth’s vast biodiversity developed

in the past, continues to develop in the present and will continue to develop in the future. Natural

selection is a process by which populations change over many generations as organisms with advantageous

heritable traits survive and reproduce, passing their traits to their offspring.

There is an abundance of evidence supporting the idea of natural selection:

Fossils Fossils in young layers are more similar to organisms living today than fossils found in deeper stratum

Fossils appear in chronological order

Not all organisms appear at the same time

Transitional fossils show links between groups of organism

Comparative Anatomy

Similarities in the basic pattern of anatomy is evidence of inheritance from a common ancestor

(homologous vs analogous)

Vestigial Structures/Features Vestigial features are those that are underdeveloped and/or non-

functioning structures that are homologous to a fully functioning

structure in closely related species. Every organism that has been

studied in detail has vestigial organs including humans.

Examples: tailbone, appendix and goosebumps in humans; pelvic bone in

whales and some snakes; dewclaws on dogs and some other animals.

Embryology

Related species have similar embryological development

Examples: tails and gill slits in human embryos, arm buds on

snakes, hair and teeth in baleen whales

Geographical Distribution

Related species are more likely to be found in areas that are geographically close to each other rather

than are locations that are geographically separate but environmentally similar.

Example: Cacti species - only found in the Americas, even though suitable habitats exist on other

continents. This would indicate that they have descended from a single common species and have not

been able to cross the ocean.

Island species

Animals found on islands often closely resemble animals found on the closest continent.

Islands get colonized by organisms that dispersed from the nearest mainland

Once on the island, the colonizing species is isolated and may be faced with different selective

pressures

The colonizing species will then evolve

differently from its relatives on the mainland

Example: no native amphibians or large land

mammals have ever been discovered on

remote ocean islands

Galapagos Islands

13 varieties of land birds found nowhere else

on Earth and ALL are finches

Each variety is a result of descent with

modification from a common finch ancestor

which somehow reached the islands from

South America

Comparative Biochemistry Living organisms are highly similar at the

molecular level, suggesting that they have descended from a common ancestor, which carried the

information to make these molecules.

All cells are made of the same basic types of organic compounds: nucleotides, proteins, lipids and

carbohydrates

In all organisms, organic reactions are controlled by enzymes

All proteins are made of the same 20 amino acids

The major carbohydrate molecule of cells are six carbon sugars such as glucose and its polymers, like

starch and cellulose

All cells carry out anaerobic respiration - glycolysis

All cells contain DNA, which determines protein structure

The GENETIC CODE is universal (human DNA has been incorporated into the DNA of bacteria and

animals)

All species, studied in detail, also have pseudogenes - genes that have undergone mutations and no

longer code for a functioning protein

Example: Mammals have about 1000 functioning olfactory receptor (OR) genes that code for the

receptors that detect airborne chemicals; dolphins, like all mammals, breathe though their noses, but

have no need for a sense of smell in the air. Dolphins do have these same 1000 genes, but only about

200 of them are functional. Biologists suspect that after dolphins evolved an aquatic lifestyle, their

OR genes were of no value, so any mutations that made the genes dysfunctional would have been

neutral and not selected for or against, so over millions of years most of the dolphins’ OR genes have

become pseudogenes.

Mechanisms of Evolution

Genetic variation of individuals within a population makes evolution possible. Sexual

reproduction creates a large amount of this diversity as individuals inherit a combination of

alleles (traits). In addition, mutations (permanent changes in DNA) happen randomly providing

for the potential of new traits to emerge on their own. Although variation in a population occurs

randomly, natural selection acts upon that variation in a non-random way.

Evolution can be divided into two main categories:

Macro-evolution is evolution on a grand scale, such as the evolution of a new species

from a common ancestor

Micro-evolution is the change in the gene frequencies (a shift in the gene pool) within a

population over time - it is evolution within a species

Factors that can lead to the shift in gene frequencies include:

Mutations - Random changes in DNA that can affect the gene pool

Gene Flow – The movement of alleles from one population

to another due to migration.

The addition of new alleles to a population will

increase diversity and may help the population

survive.

Non-random Mating - Individuals in a population select mates often based on their physical

features.

Examples: caribou antlers

and peacock feathers

Genetic Drift - Change in gene frequencies

because of chance breeding

events. Drift has a greater

effect on smaller populations.

Founder Effect - a change in a gene pool that

occurs when a few individuals start a new isolated population. Example: island

colonization.

Bottleneck effect - a change in the gene pool caused by a rapid decrease in population.

Example: natural disaster

Natural Selection - alleles that help an individual survive and reproduce will accumulate in the

population, leading to a shift in the gene frequencies.

Types of Selection

Selective Pressures can result in different patterns of natural selection. Characteristics can be

selected for (positive selection) or against (negative selection).

Directional Selection – occurs when selection

favours individuals with a more extreme variation of

a trait. The result is a shift away from the average

condition. Example: strawberries are selected for

larger and sweeter fruits, thoroughbred horses for

running speed.

Stabilizing Selection – occurs when the average

characteristic within a population is favoured by the environment. Example: human birth weights

are subject to stabilizing selection.

Disruptive Selection – favours individuals at either end of the extremes of a characteristic.

Example: male coho salmon – small 500 g or large 4500 g.

Sexual Selection – the favouring of any trait that specifically enhances the mating success of

an individual. Sexual selection often leads to the males and females of a species evolving

appearances and behaviours that are quite different from each other (sexual dimorphism).

Example: males often evolved larger body size and other attributes (antlers) that are used in

direct competition for mates. Some of the features selected for by sexual selection, can

become a disadvantage for the longevity of the individual. Example: large or brilliant coloured

plumage make for easy prey.

Patterns of Evolution

Adaptive Radiation – occurs when a single species evolves into a number of distinct but closely

related species (Galapagos finches), usually occurs when a variety of new resources not being

used by other species becomes available.

Divergent Evolution - one or more new species are produced by changes to the initial organism

("parent"), each branch gives rise to a new species and "parent" can still exist.

Example: Northern Ontario rodents – beaver, flying squirrel, porcupine

Convergent Evolution – occurs when two different species

evolve to occupy similar ecological niches. Example:

streamlined body shape of the dolphin and shark

Coevolution – a process in one species evolves in response to

the evolution of another species. Example: the Madagascar

long-spurred orchid and the hawk moth

Speciation: How Species Form

Modes of Speciation

A species is a population of organisms that can interbreed and produce a viable offspring. Speciation is

the formation of a new species (macroevolution). In order for a new species to form, individuals from the

original species must evolve to become reproductively isolated from the remainder of the population. The

creation of a new species is continuous (microevolution) and it is often difficult to pinpoint exactly when a

new species forms.

Reproductive Isolation -

biological barriers that keep

species from reproducing

Pre-zygotic isolating

mechanisms either impede

mating between species or

prevent fertilization of

the eggs. These include:

Behavioral isolation –

different species use

different courtship

and other mating clues

to find and attract a mate. Examples: songs of birds and species specific pheromones

Ecological/Habitat isolation - two species may live in the same region but in different habitats

and may never encounter each other. Example: elevation within a mountain range

Temporal isolation - many species are kept separate by timing barriers (mate at different times

of the day, different seasons, or even different years)

Mechanical isolation - may attempt to mate but are anatomically incompatible (lock and key

genitals of some insects)

Gametic isolation - if gametes of different species do meet, male gametes may not be able to

recognize and fertilize an egg of a different species. Example: chemical markers on the surface of

fish eggs.

Post-zygotic barriers are when sometimes a sperm from one species is able to fertilize the egg of

another species and a zygote is produced, but post-zygotic barriers prevent these hybrid zygotes from

developing into normal, fertile individuals. These include:

Zygotic Mortality – mating and fertilization is possible, but genetic differences result in a zygote

that is unable to develop past an early stage. Example: goat and a sheep

Hybrid Inviability – the hybrid individual develops and either dies before birth or if born alive

cannot survive to maturity. Example: tiger and leopard

Hybrid Infertility - offspring produced is sterile. Example: donkey and horse = mule

Types of Speciation

Sympatric Speciation – populations within the same geographical areas diverge and become

reproductively isolated. This type of speciation is far more common in plants than in animals.

Examples: 1. A mutation called polyploidy, where an organism has three or more sets of

chromosomes; common in plants that self-pollinate – potatoes

2. Two different species breed to produce a sterile offspring that then reproduces

asexually - wheat.

Allopatric Speciation – formation of a new species as a

result of evolutionary changes following a period of

geographic isolation. i.e. mountains, rivers or roadways.

Eventually the split populations become so distinct that

they are unable to breed even if brought back together.

Example: Darwin's Finches

Parapatric Speciation – the formation of a new species in

an area where an organisms range does not significantly

overlap. Individuals at the periphery of a population tend

to have a slightly different gene pool than that of the

parent population and are subjected to the founder effect

and genetic drift.

The Speed of Evolution Current models of evolution suggest that main types of change are at work:

Gradualism – a model of evolution that views evolutionary change as slow and steady and constant. Big

changes occur by the accumulation of many small changes.

Punctuated Equilibrium – a model of evolution that views history as long periods of stasis (equilibrium)

interrupted by periods of divergence. It seems that a lot of species undergo

changes when they first diverge from the parent species. After that, the

change is relative slow.

Human Impact on Speciation and Mass Extinctions

Human activities can affect the genetic diversity of a population in various ways:

Converting wilderness into croplands or areas for recreation or tourism

Building roads

Building urban subdivisions

Flooding areas to build dams

Hunting

In addition to the affect humans have on species, there have been five notable mass extinction events

identified throughout history where rapid change in the environment caused by a major shift in the

species on the planet. The most recent of these was the extinction of the dinosaurs thought to have

been caused by the ice age triggered by the impact of a large asteroid. This event was the catalyst for

the adaptive radiation of mammals.