Evolution Satirical cartoon by Thomas Nast, from Harper’s Weekly, August 19, 1871

Evolution Evolution

Satirical cartoon by Thomas Nast, from Harper’s Weekly, August 19, 1871.

ObjectivesObjectivesDefine evolution.Outline the evidence for evolution.State that populations tend to produce more

offspring than the environment can support.Explain that a consequence of over-reproduction

of offspring is a struggle for survival.State that members of a species show variation.Explain how sexual reproduction promotes

variation in a species.Explain how natural selection leads to evolution.Explain two examples of evolution in response to

environmental change.

Charles DarwinCharles DarwinBritish; lived from 1809 to 1882.Began to develop the theory of

evolution on a trip around the world aboard the HMS

Beagle.

Charles DarwinCharles DarwinCame to realize that the earth had

changed over a long period of time, which caused

some creatures to go extinct, opening up space for new creatures to appear.

The history of life on earth wassimilar to a branching tree, tracing back to some common ancestor. Nature selected which creatures survived and passed on their characteristics. Species change!

Evolution definedEvolution definedEvolution: the change over

time in the frequency of inherited variations in a

population over generations.

Natural selection amplifies or diminishes inherited (not acquired)

variations.

Evolution definedEvolution definedEvolution: the change over time in the

frequency of in-herited variations in a population (not individuals).

Evolution definedEvolution definedLamarckian vs. Darwinian evolution: change over time

Lamarck was a Frenchman who lived before Darwin and first proposed creatures change over time (evolve).Example: giraffes have a long neck because one stretched its neck to reach high branches and passed the trait on.


Darwin would say that giraffe’s necks come in various sizes. Those with a longer neck can reach higher branches and survive better during hard times and pass the trait on to their offspring.


Common errors“Potato beetles evolved resistance to DDT in order to survive.”

There is no intention to evolve.The beetles either had or didn’t have the genes needed to

survive the chemical.Offspring of the survivors shifted the population toward a greater likeli- hood of resistance overall.Peppered moths come in various colors, white to black. Population colors shift as colors of tree trunks change over time, and predators choose different colored moths..

Natural selectionNatural selectionNatural selection: the differential success in the

reproduction of different phenotypes resulting from the interaction of organisms with their environment.

Charles Darwin’s mechanism of evolution.Developed from 5 observations:

1) Species are so fertile that populations would rise exponen- tially if all individuals reproduced successfully.

2) Populations tend to remain stable in size, except for seasonal fluctuations.

3) Environmental resources are limited.4) Individuals of a population vary; no two

individuals are exactly alike.5) Much of this variation is heritable.

Natural selectionNatural selectionHow natural selection works:

Populations tend to produce more offspring than the environment can support.

As a consequence, overproduction of offspring leads to a struggle for existence, with only a fraction of the offspring surviving each generation.

A generation of mice lasts 45 days, then there are 4 – 7 babies. After 45 days there are perhaps 25 mice, then

after another 45 days there could be 125, then

625.


Individuals in a species vary.


Sexual reproduction promotes variation in a species.

Genes are redistributed and mixed among individuals.Asexual reproduction would produce no differences.

Natural selectionNatural selectionNatural selection leads to evolution

Survival is not random, but depends in part on the hereditary make-up of the individuals.

Individuals more fit in their environment are likely to leave more offspring than those less fit.

Preferential survival & reproduction leads to a gradual change in a population, with favorable characteristics accumulating over the generations.

Examples of natural selectionExamples of natural selectionEvolution of resistance to in-

secticides in insect species.Effectiveness of insecticide

decreases with time.Each generation there are

more resistant insects.Natural selection edits

existing variation.Natural selection favors

characteristics that fit the current, local environment.

Examples of natural selectionExamples of natural selectionDrug-resistant strains of HIV evolve rapidly in

the viral population infecting any particular patient.

Like pesticide resistance: the drug 3TC interferes with HIV replication in human cells. Resistant strains become 100% of the population in just a few weeks.Bacterial resistance

to antibiotic drugs works in the same

way.

Evolution Evolution

Satirical cartoon by Thomas Nast, from Harper’s Weekly, August 19, 1871.

Evidence for evolutionEvidence for evolutionEvidence of evolution is everywhere in biology.

Fossil recordHomologous structuresBiogeographyEmbryonic developmentMolecular biology Selective breedingNatural selection

within human lifespan

Evidence for evolutionEvidence for evolutionFossil record

Fossils are “any traces of dead organisms”: bones, tracks (foot-prints), leaf impressions, excrement, actual organisms frozen in ice, in amber, or in tarpits.


Most fossils are found in sedimentary rock where deeper rock is older, formed from sand or clay deposits.

Stratigraphy – dating fossils by charting the rock layers.

Since the late 1940s, fossils are dated by the decay of

radioactive isotopes. This is called radiometric

dating.


The fossil record in rocks provides relative ages.Radiometric dating can

determine absolute ages. Organisms accumulate radio-

active isotopes when alive. Isotopes decline after death

- they decay (transform) into another element.

Most carbon is 12C, but there is a small yet constant amount of 14C in the air, and therefore in our living bodies – 1 part per trillion.This amount declines after death.

Most carbon is 12C, but there is a small yet constant amount of 14C in the air, and therefore in our living bodies – 1 part per trillion.This amount declines after death.

Evidence for evolutionEvidence for evolutionHalf-lifeHalf-life: time for ½ of the isotope atoms to

decay.Use 40K to date old rocks: half-life = 1.3 billion yr.Use 235U for early vertebrates: half-life = 700 mil. yr.Use 14C for recent fossils: half-life = 5,730 years.

Evidence for evolutionEvidence for evolutionUsing 14C dating to determine the age of organic

materials. Half-life of Half-life of 1414C is 5,730 years.C is 5,730 years.

Work backward from the amount present today toa time when there was maximum 14C,1 part per trillion.


Fossils give evidence about the major branches of descent in the tree of life.

Order establishedEx: fossil fishes predate other

vertebrates; amphibians are

next, followed by reptiles, then mammals and

birds.


Transitional forms link old & new species.

Evolution of horse’s hooves from 5 toes.

Evolution of whale from horse-like animal: whale

retains a pelvis where hind legs

once attached; forelegs now flippers

Evidence for evolutionEvidence for evolutionHomologous structures

Features of new species are altered versions of ancestral features. Similarity in characteristics resulting from common ancestry is known as homology.

All cats have a common ancestor.All cats have a common ancestor.

Evidence for evolutionEvidence for evolutionHomologous structures

For example, the forelimbs of human, cats, whales, and bats share the same skeletal elements, but different functions because they diverged from the ancestral tetrapod forelimb.They are homologous structures.

Evidence for evolutionEvidence for evolutionVestigial organs are homologous structures that

have marginal, if any, importance to a current organism, but which had important functions in ancestors.

Skeletons of some snakes & fossil whales retain vestiges of pelvis and leg bones of walking ancestors.In humans - wisdom teeth, tailbone, appendix.

Evidence for evolutionEvidence for evolutionBiogeography - the study of the distribution of

life forms over geographical areas. If evolution is true, then we should expect to find

related species living near each other Except in cases of great mobility (like sea animals, birds, and animals distributed by humans) or over long periods of time (due to plate tectonics).

If, however, we find that species are distributed in a random geographic manner, with closely related species no more likely to be found close to each other than unrelated species, then this would be strong evidence against evolution and common descent.

Evidence for evolutionEvidence for evolutionBiogeography

Plate tectonics – the continents are on plates that glide over the surface of the earth carrying life with them.


Identical fossils in parts of the world now widely separated indicate that the continents were once joined.

The southern part of Pangaea


Australian example: marsupials vary widely but are more closely relat-

ed to each other than to similarly-

appearing animals on other continents.

All have a pouch!Placental predators

out-competed them on

other continents, and they disappeared. Australia (with no placentals) was isolated.


Marsupials ori-ginated in SA

70 million yrs ago then spread

to Australia.


Species tend to be more closely related to other species from the same area than to other species with the same way of life, but living in different areas.

The sugar glider from Australiais more closely related to othermarsupial mammals in Australiathan to the flying squirrel, a pla-cental mammal of North America.This is an example of convergentevolution.

Evidence for evolutionEvidence for evolutionEmbryonic development

Genes for embryonic development are conserved in many different species making the embryos similar.All vertebrate embryos have structures called pharyngeal pouches in their throat at some stage in their development. These develop into different, but still homologous, adult structures: gills of fish or Eustachian tubes in mammals.

Evidence for evolutionEvidence for evolutionMolecular biology corroborates

evolutionary trees.Evolutionary relationships among species are documented

in their DNA and proteins.Ex: the Cytochrome c protein

is more similar when crea-tures are closely related:

Human & chimp have the same 104 amino acids,

dog has 13 differences, rattle- snake has 20 changes.

Evidence for evolutionEvidence for evolutionSelective breeding

Humans have domesticated many animals,

giving them new characteristics over

time (they evolved).

Dogs domesticated from wolves about

15,000 years ago.Sheep, cattle, horses

goats, pigs, chickens

Evidence for evolutionEvidence for evolutionSelective breeding

Plants: corn, wheat, potato, bean, cabbage, etc.

Documents

Evolution Satirical cartoon by Thomas Nast, from Harper’s Weekly, August 19, 1871