Evolution

Biology 20

Classification of Organisms

• Taxonomy – classification of organisms

• Approximately, 10 million species on the planet, 2 million classified.

• Carl Linnaeus – 1707-1778 Developed a naming system called Binomial nomenclature (Genus-Species) – Ex. Homo Sapiens

Order of Classification

Six-Kingdom Systems of Classification

Monera

Monera

Phylogeny

Evidence of Evolution

• Evolution: all changes that have occurred in living things since the beginning of life.

• If history of Earth is condensed to one 24-hour day:– Earth is formed at the beginning of midnight– Prokaryotes appear about 5:00 in the

morning– Eukaryotes present at 4:00pm in the

afternoon– Invasion of land starts about 10:00pm at

night– Humans appear 30 seconds before midnight

at the end of the day

Fossil Evidence

• Fossils are remains, traces or other direct evidence of past life forms

• Most fossils form from the burial of plants and animals in the sediment

• Most fossils are embedded in sedimentary rock

• Paleontologists study the fossil record based on the boundaries between the strata, this helps determine the relative age of fossils

• Fossil links combined with modern comparative anatomy allows us to deduce vertebrate descent:– Fish amphibians reptiles birds and

mammals

Evolution of the Modern Horse

One of the few animals for which we have a fairly complete evolutionary record is the horse, as all the main stages of horse evolution have been preserved in fossil form. Over 60 million years the horse evolved from a dog-sized rainforest-dwelling creature, into an animal adapted to plains-dwelling and standing up to 2 metres high.

In the process it traded-in its multi-toed feet, adapted for walking across the forest floor, for single-toed hooves, suited for running over open country.

Geological Time Scale

• Geological history of Earth is divided into eras and periods

• Fossil record provided relative dating of rock layers; top layer of rocks are younger than lower layers of rock

• Absolute dating methods uses radioactive isotopes– Isotopes each have particular

half-life or time it takes for half of the isotopes to decay and become non-radioactive

– Carbon 14 is used to date organic matter; half decays to nitrogen 14 each 5730 year; limited to about last 50,000 years.

Mass Extinctions• Extinction is death of all members of a species in wild;

mass extinctions are extinctions of many species in a short time.

• Five mass extinctions in the fossil record• Following extinctions, remaining groups expand to fill

habitats vacated by extinct species• Proposed that most mass extinctions are caused by

asteroid impact– Impact of the asteroid would create climate similar to a

nuclear winter with worldwide cooling (ice age)– Cretaceous-Tertiary(64-1.8 million years ago) border has a

higher level of iridium, rare in the Earth’s crust but common in meteorites

– Huge meteorite crater of correct age ground in Caribbean Ocean and Yucatan peninsula; suspected sight of impact of meteor that results in dinosaur extinction

• Marine animals fossil record indicates mass extinction occurs every 26 millions year; corresponds to movements of solar system within the Milky Way galaxy.

• Craters

Biogeographical Evidence• Biogeography is the study of distribution of plants and

animals throughout the world• Current distribution of organisms reflects the

evolutionary history, organisms evolve in one location and spread out into other regions; for example, no rabbits are found in South America – they originated elsewhere and did not reach South America

• Physical factors, including locations of continents, limit population ranges

• Continental drift states that continents have slowly moved over time– Explains close puzzle-piece fit of east coast of South

America with west coast of Africa, and other continent edges

– Explains distribution of seed ferns throughout southern continents

– Explains distribution of early reptiles across many continents from time when land was joined

– Explains divided distribution of mammals that evolved after continents parted.

Movement of the Continents

Anatomical Evidence

• Many organisms share a unity of plan, for example, the vertebrate forelimbs contain same sets of bones used for different functions in bat wings, whale fins, etc

• Simplest explanation in having a common ancestor whose basic forelimb plan was modified in succeeding groups as each continued along it own evolutionary pathway.

• Homologous structures are similar in structure derived through descent from a common ancestor

• Analogous structures have similar functions but differ in anatomy and did not derive from the same ancestral structure; for instance, an insect wing and a bird wing

Homologous and Analogous Structures

Embryological Development

• Theory of Recapitulation– “ontogeny recapitulates phylogeny”– “life history repeats evolutionary history”

• All vertebrates have a tail, notochord and pharyngeal pouches during development

• In humans– The notochord develops into the vertebral

column– The tail becomes the sacral vertebrae (tailbone)– The pharyngeal pouches become the Eustachian

tubes, middle ear and thyroid gland• Simplest explanation is that the notochord

and pouches are primitive fish features and fish are ancestral to other vertebrates

Comparative Embryology

Biochemical Evidence

• Almost all living organisms use the same basic biochemical: DNA, ATP, many identical enzymes, DNA triplet codes and 20 amino acids

• Similarity of biochemistry is explained by descent from a common ancestor

• DNA base sequences differences is DNA between a number of organisms shows less difference the more closely related they are; for example, 2.5% difference between humans and chimpanzees but 42% difference between humans and lemurs.

Contributors to the Theory of Evolution

• Linneaus– Developed the classification system (binomial

nomenclature)– Linneaus did not believe in evolution, but his

classification system helped to organize many organisms to show similar ancestry

• Buffon– Anticipated many of Darwin’s ideas– Extreme conflict with the church

• Larmark– Recorded the first theory of evolution– Two major disputable points:

• Nature has an innate tendency to evolve in the direction of increasing complexity, determined by the interaction with environment

• Acquired characteristics will be passed onto offspring (not genetically acquired)

Other Contributors to the Theory of Evolution

• Cuvier– Founder of paleontology– Also did not believe in evolution, but did lay the

ground work for Darwin’s theory

• Hutton– Geologist– Proposed the Earth is always changing and is very

old

• Charles Darwin– Studied medicine and theology– At 22 became a naturalist on the HMS Beagle

which sailed through the Galapagos Islands– Proposed the Theory of Natural Selection

• Alfred Wallace– Proposed the same idea as Darwim

Theory of Natural Selection

• Natural selection is a theory that proposes that individuals whose genetic variation best adapt them to their environments will be most likely to survive and pass on those traits.

Darwin’s Discovery of the Theory of Natural Selection

(Fact 1) There is a potentially exponential increases in population Overpopulation

+(Fact 2) Population sizes are relatively

stable (observation)+

(Fact 3) Resources are limited=

(Inference 1) There is a struggle for existence

+

Darwin’s Discovery of the Theory of Natural Selection

(Fact 4) Individuals are unique (observation) Variation

+(Fact 5) Many characteristics are inherited

=(Inference 2) There is a differential

survival, that is natural selection Survival of the Fittest

+(Fact 6) The Earth is very old

=(Inference 3) Through many generations

species evolve Origin of species by inheritance of successful variation

Key Points of Natural Selection• Individual differences are essential for

evolution• The struggle in nature that is of greatest

consequence to evolution is not that between different species; rather, it is the competition within a population of a single species to obtain food, attract a mate, escape predators, etc.

• Only the difference that are inherited are relevant to evolution

• Evolution generally takes a great deal of time and is ongoing.

Sources of Variation

• Mutation create the variation necessary for Natural Selection– Mutations are random changes in the

DNA sequence in a chromosome– Caused by environmental factors

(chemical, radiation or viruses) or in errors when the cell makes copies of the DNA

– Only the mutation to the chromosomes in the sex cells are passed to the next generation.

– Mutations can be harmful, neutral or beneficial. (most are harmful or neutral)

Sexual Reproduction and Variability

• Asexual Reproduction = no variation

• Sexual Reproduction = variation– In sexual reproduction there are two

parents and the offspring inherit a copy of each gene from each parent

– The chromosomes are assorted randomly. So each sex cell has a different combination of chromosomes.

– Sexually reproducing species choose different mates.

Speciation

• Species – a group of organisms that look alike and can interbreed under natural conditions to produce fertile offspring.

• Speciation – the creation of a new species

• Allopatric Speciation – speciation by geographic then reproductive isolation.

Geographical Isolation

– Species become geographically isolated by some physical barrier. A mutation occurs in one group that does not occur in the other.

– Natural selection takes place and the populations evolve independently.

– In time, the changes that occur between the species are so pronounced that if the were reunited they can no longer mate.