EVOLUTION

Descent with Modification/

Change over Time

CHANGE OVER TIME Evolution – theory that concerns how species

change over time. the process by which modern organisms have

descended from ancient organisms. change is very slow and only in minor ways Can individual organisms evolve? NO!!! Populations of organisms do evolve as a result of a

change in gene pool. Theory – a well supported, testable explanation of

phenomena that have occurred in the natural world.

Charles Darwin –joined the crew of the HMS Beagle and set sail for a voyage around the world. During his travels, Darwin collected evidence that led him to propose the theory of evolution.

DARWIN’S THEORY OF EVOLUTION Galapagos Islands – Darwin observed that the

characteristics of many animals and plants varied noticeably among the different islands of the Galapagos.

LAMARCK’S EVOLUTION HYPOTHESIS

Jean-Baptiste Lamarck – proposed that by selective use or disuse of organs, organisms acquired or lost certain traits during their lifetime.

Use and disuse of organs – organisms develop new organs or change the structure and function of old organs. Continual use of an organ would result in the acquired trait for a

new organ. Disuse would result in the disappearance of an organ because it

was no longer needed. Inheritance of acquired traits – believed that offspring

would inherit useful structures developed by parents. Lamarck’s hypothesis is now known to be incorrect. While

organisms do adapt to their environment, an individual’s genetic makeup cannot change.

Only genetic variations are passed on from generation to generation.

In other words, the discovery of genes proved Lamarck’s hypothesis invalid.

DARWIN PRESENTS HIS CASE

On the Origin of Species – Darwin’s book, published in 1859.

Here he poses a mechanism for evolution that he called natural selection.

EVOLUTION BY NATURAL SELECTION

Struggle for Existence – members of each species compete to obtain food, living space, and other necessities for life.

Fitness – the ability of an individual to survive and reproduce in its specific environment.

Adaptation – any inherited characteristic that increases an organism’s chance for survival; gives an organism an advantage in its particular environment. Successful adaptations enable organisms to

become better suited to their environment and thus better able to survive and reproduce.

EVOLUTION BY NATURAL SELECTION

Survival of the Fittest – individuals that are better suited to their environment and have adaptations that enable fitness, survive and reproduce most successfully. Thus, passing the genes that allow them to be better suited on to their offspring. Individuals with characteristics that are not well suited to their environment have low levels of fitness, and either die or leave few offspring. Darwin referred to this as natural selection. Over time, natural selection results in changes in

the inherited characteristics of a population. These changes increase a species’ fitness in its environment.

DESCENT WITH MODIFICATION

Descent with Modification – over time, natural selection produces organisms that have different structures and establish different niches. As a result, species today look different from their ancestors.

Common Descent – all species – living and extinct – are derived from common ancestors, therefore a single “tree of life” links all living things.

SUMMARY OF DARWIN’S THEORY1. Variation exists within a species.

2. Organisms produce more offspring than can survive.

3. All organisms compete for limited natural resources (food, water, shelter).

4. Each unique species has different advantages and disadvantages in the struggle for existence.

5. The environment selects organisms with beneficial traits.

Natural selection – individuals with adaptive traits have a greater chance than others to survive and pass on the genes for these beneficial traits to their offspring.

EVIDENCE OF EVOLUTION

1. The fossil record

2. Geographic distribution of living species

3. Similarities in embryology/early development

4. Similarities in anatomical (body) structures

5. Biochemical analysis

6. Ice core drillings

THE FOSSIL RECORD• This stratigraphic

column shows the order in which organisms appeared. Each layer represents a particular time frame and shows an organism which was found during that time.

• The oldest fossils appear in lower layers, and the most recent fossils at the top. This allows for placement of fossils to be used as an aid in dating the organism found.

GEOGRAPHIC DISTRIBUTION

Armadillos and anteaters descended from a common ancestor, yet on different continents?

GEOLOGIC PROCESSES

• Tectonic plates have drifted atop Earth’s mantle

• Locations of continents and ocean basins influence Earth’s climate

• Species move, adapt to new environments

EMBRYOLOGY

ANATOMICAL EVIDENCE The more closely related a species, the more

similar their structures will be. Homologous structures – body parts with structures

that are very similar, even though they have entirely different functions.Ex. Bones in a whale’s flipper, a human’s arm or bat’s wing

Vestigial structures – body parts that have degenerated or reduced in size and seem to serve no function.Ex. Tail-bone, wisdom teeth, appendix Boas and whales – small hip and leg bones (pelvic girdle)

Analogous structures – body parts that have the same function but are NOT similar in structure.

Ex. Wing of a bird and wing of a butterfly

HOMOLOGOUS STRUCTURES

VESTIGIAL STRUCTURES

Vestigial organs associated with eye structures

Vestigial remains of a pelvic girdle in a whale

ANALOGOUS STRUCTURES

• Analogous structures are a contrast to homologous structures.

• These structures are of no use in classifying organisms or in working out their evolutionary relationships with each other.

BIOCHEMICAL ANALYSIS

Biochemical analysis – study of chemicals (DNA, cells, proteins, blood, enzymes) that are within organisms.

Nutall precipitation technique – used to test similarities of blood proteins, particularly antibodies. The stronger the agglutination, the more closely

related the organisms.

Species - a group of similar organisms that share a common ancestor. species do not reproduce outside the group and

produce fertile offspring.

GENES AND VARIATION

Changes in genes produce heritable variation on which natural selection can operate.

Gene pool – consists of all genes, including all the different alleles that are present in a population.

Relative Frequency – the number of times an allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur. Typically expressed as a percentage Has nothing to do with dominant/recessive

In genetic terms, evolution is any change in the relative frequency of alleles in a population.

GENES AND VARIATION Inherited variation – changes in genes

provided by nature. Artificial selection – “controlled breeding”

Process controlled most directly by humans Humans select the variations that they find

useful.

SOURCES OF GENETIC VARIATION Mutation – a change in DNA

Mutations are a source of new genetic material. They add genetic material to a population’s gene

pool thereby increasing variation within the population.

Mutations can be harmful, beneficial or neutral. Causes of mutations:

Ultraviolet light X-rays Radioactivity Certain chemicals (mutagens) Random errors in DNA coding

SOURCES OF GENETIC VARIATION

Gene shuffling – genetic recombination (in sexual reproduction only)

The 23 pairs of chromosomes found in humans can form 8.4 million different combinations of genes – before crossing over.

Gene shuffling produces many different combinations of genes, but does not alter the relative frequencies of each type of allele in the population.

SINGLE GENE AND POLYGENIC TRAITS The number of phenotypes produced for a given trait

depends on how many genes control the trait. Single gene trait – a trait controlled by a single gene

with two alleles. (ex. Widow’s peak) Polygenic trait – trait controlled by two or more

genes. Each of these genes often has two or more alleles. As a result, one polygenic trait can have many possible genotypes and phenotypes. (ex. Height in humans).

NATURAL SELECTION ON SINGLE GENE TRAITS

Populations evolve, not individual organisms. Natural selection on single-gene traits can

lead to changes in allele frequencies and thus to evolution.

NATURAL SELECTION ON POLYGENIC TRAITS

Directional selection – when individuals at one end of the curve have higher fitness than individuals at another end of the curve, directional selection takes place.

NATURAL SELECTION ON POLYGENIC TRAITS

Stabilizing selection – when individuals near the center of the curve have higher fitness than individuals at either end of the curve. This type of selection leads to a reduction of

variation in a species.

NATURAL SELECTION ON POLYGENIC TRAITS

Disruptive selection – when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle.

GENETIC DRIFT Genetic drift – random change in allele

frequencies that occurs in small populations. Founder effect – allele frequencies change

as a result of the migration if a small subgroup of a population.

EVOLUTION VS. GENETIC EQUILIBRIUM

Hardy-Weinberg Principle – allele frequencies in a population will remain constant unless on or more factors cause those frequencies to change.

Genetic equilibrium – the situation in which allele frequencies remain constant.

For evolution to occur some external factor must upset the genetic equilibrium of a population.

EVOLUTION VS. GENETIC EQUILIBRIUM

Five conditions are required to maintain genetic equilibrium:1. There must be random mating.2. The population must be very large.3. There can be no movement into or out of the

population.4. There can be no mutations.5. There can be no natural selection.

If these conditions are not met, then genetic equilibrium will be disrupted and the population will evolve.

PATTERNS OF EVOLUTION

Macroevolution – the large scale evolutionary patterns and processes that occur over long periods of time.

Extinction – More than 99% of all species that have ever lived

are now extinct. Several times in earth’s history, mass extinctions

wiped out entire ecosystems. Each mass extinction provides opportunities for

organisms that survive and often results in a burst of evolution that produces many new species.

PATTERNS OF EVOLUTION

1. Divergent evolution – the process by which related species become less alike; evolve into a variety of species

Leads to speciation –the formation of a new species.

Ex. Polar bears diverged from brown bears Camels and llamas

Also results in adaptive radiation – the process by which members of a species adapt to a variety of habitats.

Ex. Darwin’s (Galapagos) finches – 13 different variations

Biodiversity is believed to be the result of speciation and extinction.

PATTERNS OF EVOLUTION Adaptive radiation – when a single species or small group

of species evolves into several different forms that live in different ways.

SPECIATION As new species evolve, populations become

reproductively isolated from each other.

Reproductive isolation – when members of two populations cannot interbreed and produce fertile offspring.

TYPES OF REPRODUCTIVE ISOLATION

Behavioral isolation – occurs when two populations are capable of interbreeding but have differences in courtship rituals or other reproductive strategies that involve behavior. (ex. Birds with different mating songs).

Temporal isolation – two or more species reproduce at different times. (ex. Flowers releasing pollen at different times of the year).

Geographic isolation – two populations are separated by geographic barriers. (ex. Colorado river separates two types of squirrels).

GEOGRAPHIC ISOLATION The movement of tectonic plates influences

evolution by changing the locations of continents, causing some species to be geographically isolated from others.

Geographic isolation can result from Mountain ranges Volcanic eruptions – lava flows Rivers Earthquakes Deforestation Continents Islands

TESTING NATURAL SELECTION IN NATURE

SPECIATION IN DARWIN’S FINCHES Founders arrive – finches from South American mainland

(species A) populated the Galapagos. Geographic isolation – birds from species A crossed to

another Galapagos island. The two populations became isolated from one another and had different gene pools.

Changes in the gene pool – populations on the new island adapted to their environment, forming population B.

Reproductive isolation – because of their adaptations and different beaks, species A and B do not mate together if they come in contact.

Ecological competition – during dry season, individuals compete for food, and over time birds evolve better beaks (species C).

Continued evolution – this process continues over several generations and over time produces the 13 species seen today.

PATTERNS OF EVOLUTIONConvergent evolution – the process by which unrelated organisms develop similar characteristics; analogous structures.• occurs when different species share the same environmental

surroundings– Ex. Whales and dolphins (mammals) now resemble fish

CONVERGENT EVOLUTION

• Can often lead to cases of mimicry – the evolution of one organism so it comes to resemble or look like another.– Ex. Queen Anne’s butterfly closely resembles the toxic

Monarch butterfly.

• Over the course of time, the change of the gene pool of one species may lead to the change of the gene pool of another species – coevolution – when two species evolve in response to changes in each other.– Ex. Bats and moths– Flowers and insects

MIMICRY

Queen Anne’s Butterfly Monarch Butterfly

COEVOLUTION

PATTERNS OF EVOLUTION

Punctuated equilibrium – when long stable periods are interrupted by brief periods of more rapid change. Gradualism – the idea that biological change is

slow and steady. (supported by Darwin) Scientists suggest that most new species are

produced by periods of rapid change, because organisms evolve rapidly to fill available niches.

Example of punctuated equilibrium –Dinosaur species dominated Earth for over

100 million years. During this time, most mammals were small mouse-sized nocturnal organisms. Following the mass extinction of the dinosaurs, the small mammals rapidly diversified to fill available habitats and niches.

EVOLUTION – A SUMMARY

Biological evolution By natural selection – explains how life changes

over time Adaptation or adaptive traits enables an

organism to survive through natural selection to reproduce under prevailing environmental conditions.

Biological evolution is based on changes in a population’s genetic makeup over time.

Populations evolve due to variations within, but individuals can’t develop new structures.

AN ANCIENT, CHANGING EARTH

• Earth is constantly changing and throughout history has had changes in1. The atmosphere2. Climate3. Positions of continents4. Geography5. Types and number of organisms

GEOLOGY AND A CHANGING EARTH

James Hutton – proposed that layers of rock form slowly and that rocks are shaped by natural forces that operate over millions of years.

Charles Lyell – wrote Principles of Geology, which explained that processes that changed Earth in the past are still operating in the present.

ATMOSPHERE AND CLIMATE CHANGES

• Atmosphere – Changes in the atmosphere were the result

of collisions between earth and large asteroids (catastrophic events)

• Climate change– Alternate periods of heating and cooling– Advance and retreats of ice sheets over

northern hemisphere

INTERPRETING FOSSIL EVIDENCE

Paleontologists – scientists who study fossils in order to determine animal behavior and climate conditions of the past.

Paleontologists determine the age of fossils using two techniques; 1. Relative dating – the age of a fossil is determined by

comparing its placement with that of fossils in other horizontal layers of rock.

Problems – erosion, earthquake and volcanic activity can twist, fold and bend rock layers.

2. Radioactive dating – scientists calculate the age of a sample based on the amount of remaining radioactive isotopes it contains.

More exact method because the decay rate of each element is known.

Fossil record – provides evidence about the history of life on Earth. It also shows how different groups of organisms, including species, have changed over time.

HOW FOSSILS FORM

For fossil to form, either the remains of the organism or some trace of its presence must be preserved.

FOSSIL EVIDENCE

Most fossils are formed in sedimentary rock (mud, silt, clay) Upper layers = most recent Lower layers = older fossils

Types of fossils include:1. Casts2. Amber – preserved or frozen3. Trace fossils4. Petrified fossils5. Imprints6. Molds

GEOLOGIC TIME SCALE

Geologic time scale – divisions used to represent evolutionary time. Geologists divide

time into four major eras: precambrian, the paleozoic, the mesozoic, and the cenozoic eras.

Each era is subdivided into periods.

GEOLOGIC TIME SCALE

Precambrian 88% of Earth’s history Few multicellular fossils

Paleozoic Many vertebrates and invertebrates lived during

this time. Mesozoic

“The Age of the Dinosaurs” Mammals began to evolve during this time.

Cenozoic “The Age of the Mammals” The last 65 million years

FORMATION OF THE EARTH

Geologic evidence shows that Earth is about 4.6 billion years old and was created over many years.

Earth was formed from cosmic debris which collided with on one another over the course of millions of years.

Early Earth was extremely hot, and unsuitable for life because of violent earthquakes and asteroid impacts.

HISTORY OF THE EARTH

• The evolution of life is linked to the physical and chemical evolution of the earth.

• Life on earth evolved in two phases over the past 4.7 -4.8 billion years

1) Chemical evolution (1 billion years) of the organic molecules and systems of chemical reactions needed to form the first proto-cells.

2) Biological evolution (3.7 billion years) of single-celled organisms and then multicellular organisms

FIRST ORGANIC MOLECULES

1950s – Stanley Miller and Harold Urey tried to simulate conditions of early Earth in the laboratory. Filled flask with fluids and gases similar to those

in Earth’s early atmosphere. Passed electric sparks through the mixture to simulate lightning. Their experiments produced amino acids, the building blocks of ____________?

This exhibited the process of chemical evolution.

THE PUZZLE OF LIFE’S ORIGINS

FREE OXYGEN

Microfossils of unicellular organisms resembling bacteria have been found in rocks more than 3.5 billion years ago, which means that they must have evolved in the absence of oxygen, because there was little/no oxygen in Earth’s early atmosphere. (Cyanobacteria)

Photosynthetic organisms evolved later, which created a rise in oxygen levels in the atmosphere. This change likely caused some organisms to become extinct, while others evolved more efficient respiration mechanisms which utilized oxygen.

THE ENDOSYMBIONT THEORY

One large prokaryotic cell containing DNA, engulfed several other smaller prokaryotic cells with specialized functions. Together these cells lived symbiotically and were interdependent on one another. Collectively, they formed the first eukaryotic cell, which went on to divide and give rise to other eukaryotic cells.

Main Point – one condition necessary for the evolution of first life on Earth was the presence of DNA.

THE ENDOSYMBIONT THEORY