Evolution

Ch 7Pt 1

Wooly mammoth skeleton: Mammoths lived 2 million to about 9,000 years ago. They were about 9 to 15 feet tall. Where would scientists look for possible reasons for extinction?

5 mass extinctions1 underway

• Evidence of five major mass dying of life forms on Earth

• Each is followed by a succession of distinctly different organisms which can survive in the absence of predecessors– Example: dinosaurs died

» Gave way to rise of mammals» Eventually humans appear

Fossil succession shows :

Life, as we can best define it:

• The definition must be “built” on descriptions that fit all living things.– Living things are both complex and organized– Living things grow and reproduce– Living things respond to stimuli– Living things acquire and use materials and

energy– Living things have (use) DNA to store

information

Misconceptions about

evolution:

?? Evolution proceeds strictly by chance

?? Evolved species must be more complex than the predecessor

?? Where are the missing links?

?? humans evolved from monkeys so monkeys should no longer exist

Theory of Evolution

• A theory explains a series of observations and often unifies

related facts through supportive evidence.

• Evolution is based on the observed accumulated generation to generation changes within a defined group.

• Evolution accounts for

– similarities among life forms– differences among life forms

Jean-Baptiste de Lamarck

– (1744-1829) is best remembered for his theory of Inheritance of Acquired Characteristics:

-- new traits arise in organisms because of their needs and repetitive behaviors to meet those

needs– Acquired traits are somehow passed on to their

descendants

• Lamarck’s theory seemed logical at the time and was widely accepted

Lamark’s Theory

Rethinking…why would giraffes “develop” or actually

“express” the trait of longer necks over time?Was the “acquired” trait passed on?

Darwin• In 1859, Charles Robert

Darwin (1809-1882) published On the Origin of Species– In it he detailed his ideas

on evolution formulated 20 years earlier

– he proposed a mechanism for evolution

– From 1831 to 1836, he had traveled along the southern continents and Europe.

What most don’t know about Charles Darwin

• Darwin born in England on Feb. 12, 1809 – same day as Abraham Lincoln.

• Father a doctor. Mother affluent (Wedgwood pottery family) .

• Both families were free thinkers and philosophers.

• His private school education was useless for his interests; his hobbies hunting, observing natural world, collecting things, and chemistry.

About the same time…..

– James Hutton was promoting his old earth and plutonic (igneous) rocks

– and Abraham Werner promoted Neptunism (all rocks precipitate from oceans).

– Darwin was exposed to Zoomania which promoted Lamarckian ideas. Learned taxidermy from a former black slave who inspired Darwin with his stories of the tropics.

And a little more….• Off to Cambridge, he studied to be a clergyman.

Baptized into the Church of England, where it was compatible to be a naturalist.

– He hunted, caroused, and collected beetles.

– he learned to observe variations within a species from botanist and clergyman Professor Henslow.

– Studied geology with Adam Sedgwick, a catastrophism proponent who later named the Cambrian system of rocks in England.

– Learned to do field work and record observations meticulously.

Fig. 7-1, p. 115

Darwin spent only 5 weeks in the GalapagosIslands. Here he made observations aboutdistinct differences among similaranimals that were directly related to foodsupply. He published his ideas in 1859, 24 yrs. later.

Observation of finchesin the Galapagos Islands

Berry eater

Insect eaters

Insect eaters

Cactus eaters

Seed eaters

What Darwin noticed

• Plant and animal breeders practice artificial selection – breeding plants and animals with desirable traits

• A process of selection among variant types in nature could also bring about change

• Thomas Malthus’s timely essay on population suggested that human competition for resources and high infant mortality limited a population size

Natural Selection (Key Points)The mechanism

• Organisms in all populations posses heritable variations.– size, speed, agility, visual acuity, digestive enzymes,

color, and so forth

• Some variations are more favorable than others– some have a competitive edge in acquiring resources

and/or avoiding predators

• Not all young survive to reproductive maturity

Those with favorable variations are more likely to survive and pass on their favorable variations

Survival of the Fittest?

• Natural selection is sometimes expressed as “survival of the fittest”

“Survival of the Fittest” What does it mean??

• Misconception:• among animals only the biggest, strongest, and fastest

are likely to survive

– Maybe these traits are an advantage-- but natural selection may favor species other than the obviously bigger, stronger, or faster. Examples?

Natural Selection involves differential rates of survival and reproductionSurvival of the species depends on producing offspring.

the smallest if resources are limited

the most easily concealed

those that adapt most readily to a new food source

those having the ability to detoxify some substance

Others?

It helps to know what organisms had survived stressful conditions in the past ….

Again, rethinking………

The peppered mothPeppered moths had light colouration, which effectively camouflaged them against the light-coloured trees and lichens which they rested upon During the Industrial Revolution

in England, many of the lichens died out, and the trees that peppered moths rested on became blackened by soot,

So, is “natural selection” the mechanism for evolution?

• Natural selection works on existing variations in a population– It could not account for the origin of variations

• Critics reasoned that should a variant trait arise, it would blend with other traits and would be lost

• The answer to these criticisms existed even then in the work of Gregor Mendel, but remained obscure until 1900

Gregor Mendel

• controlled genetic experiments with true-breeding strains of garden peas– strains that when self-fertilized always display the

same trait, such as flower color

• Traits are controlled by a pair of factors now called genes

• Genes occur in alternate forms, called alleles– One allele may be dominant over another– Offspring receive one allele of each pair from each parent

Mendels Work

• The parental generation consisted of true-breeding strains

RR = red flowers rr = white flowers

• Cross-fertilization yielded a second generation

all with the Rr combination of alleles, in which the R (red) is dominant over r (white)

Mendel’s Work• The second generation, when self-fertilized

produced a third generation with a ratio of three red-flowered plants to one white-flowered plant

Why is this important?

• The factors (genes) controlling traits do not usually blend during inheritance

• Traits not expressed in each generation may not be lost even if not seen!

• Some variation in populations results from alternate expressions of genes (alleles)– Variation can be maintained!– **Why is variation important to survival of

a species?

Modern Genetics

Complex, double-stranded helical molecules of deoxyribonucleic acid (DNA) called chromosomes are found in cells of all organisms

Specific segments of DNA are the basic units of heredity (genes)

The number of chromosomes varies from one species to another

fruit flies 8; humans 46; horses 64

Modern Thinking

• During the 1930s and 1940s, – paleontologists, population biologists, geneticists, and

others developed ideas that merged to form a modern synthesis or neo-Darwinian view of evolution

• They incorporated chromosome theory of inheritance into evolutionary thinking– They saw changes in genes (mutations) as

only one source of variation

Most Importantly

• They completely rejected Lamarck’s idea of inheritance of acquired characteristics

• They reaffirmed the importance of natural selection

• But since then, some scientists have challenged the emphasis in modern synthesis that evolution is gradual

The Species

• Species – a population of similar individuals that in

nature interbreed and produce fertile offspring

– Species are reproductively isolated from one another

• Goats and sheep do not interbreed in nature, so they are separate species

• When artifically bred in captivity, offspring are most often sterile.

Remember…

• Evolution by natural selection works on variation in populations– most of which is accounted for by the reshuffling of

alleles from generation to generation during sexual reproduction

• The potential for variation is enormous with thousands of genes each with several alleles (varieties), and with offspring receiving 1/2 of their genes from each parent

• New variations arise by mutations– change in the chromosomes or genes

Mutations

• Mutations result in a change in hereditary information

• ONLY mutations that take place in sex cells are inheritable,– Can be chromosomal mutations (affecting a large

segment of a chromosome)– or point mutations (individual changes in particular

genes)

• Mutations are random with respect to “fitness”– they may be beneficial, neutral, or harmful to survival!

The Species

• Species – a population of similar individuals that in

nature interbreed and produce fertile offspring

– Species are reproductively isolated from one another

• Goats and sheep do not interbreed in nature, so they are separate species

• When artifically bred in captivity, offspring are most often sterile.

Recipe for a species

• Speciation is the process by which a new species arises from an ancestral species

• It involves change in the genetic makeup of a population, – which also may bring about changes – in form and structure

• During allopatric speciation, – species arise when a small part of a population

becomes isolated from its parent population

Berry eater

Insect eaters

Insect eaters

Cactus eaters

Seed eaters

Variations among “Darwin’s finches” were naturally selected from among the

existing variations within the gene pool and mutations that may have occurred.What would cause the selection of the

observed variations?

Allopatric Speciation

• Reduction of the area occupied by a species may leave a small isolated population– Two peripheral isolates evolved into new

species (i.e. Darwin’s finches)

How long does it take for changes to appear?

• Although widespread agreement exists on allopatric speciation scientists disagree on how rapidly a new species might evolve

• Phyletic gradualism- the gradual accumulation of minor changes which eventually bring about new species

Punctuated Equilibrium

holds that little or no change takes place in a species during most of its existence then evolution occurs rapidly

Styles of Evolution

• Divergent evolution occurs when an ancestral species giving rise to diverse descendants adapts to various aspects of the environment– Divergent evolution leads to descendants that differ

markedly from their ancestors

• Convergent evolution involves the development of similar characteristics in distantly related organisms

• Parallel evolution involves the development of similar characteristics in closely related organisms

Divergent Evolution

Convergent Evolution

Parallel Evolution

Evolutionary Novelties

• All land-dwelling vertebrate animals posses bone and paired limbs so these characteristics are primitive and of little use in establishing relationships among land vertebrates

• However, hair and mammary glands are derived characteristics. – Only one subclade, the mammals, has them

It wouldn’t be Geology without Death and Destruction…..

• Perhaps as many as 99% of all species that ever existed are now extinct

• Organisms do not always evolve toward some kind of higher order of perfection or greater complexity

• Vertebrates are more complex but not necessarily superior in some survival sense than bacteria – after all, bacteria have persisted for at least 3.5 billion

years

• Natural selection yields organisms adapted to a specific set of circumstances at a particular time

Extinction• The continual extinction of species is referred to as

background extinction

• It is clearly different from mass extinction during which accelerated extinction rates sharply reduce Earth’s biotic diversity

• Extinction is a continual occurrence– …so is the evolution of new species that usually quickly

exploits the opportunities another species’ extinction creates

– Mammals began a remarkable diversification when they began occupying niches the extinction of dinosaurs and their relatives left vacant

Extinction

• The mass extinction of dinosaurs and other animals at the end of Mesozoic Era is well known…but not the greatest loss of biologic diversity!

• The greatest mass extinction occurred at the end of the Paleozoic Era – end of Permian– More than 90% of all species died out

– We will discuss these extinctions and their possible causes throughout the rest of the term

Some predictions from the Theory of Evolution

Oldest fossil-bearing rocks should have different organisms than more recent rocks

There should be fossils connecting orders and classes of descendant organisms.

Closely related species should be similar in anatomy, biochemistry, genetics

Classification of organisms should show a nested pattern of similarities

Isolated populations should closely resemble nearer populations rather than distant ones.

Organisms should show a predicted succession in the fossil record: fish, reptiles, mammals

Animals that diverged from a common ancestor should evolve to be more different over time

Homologous or Analagous?

• Homologous structures:– Similar structure– Different purpose

• Analagous structures:– Distinctly different structures

• Similar purpose Homologous or analagous?

See Page 148 Text.

Forelimb of humans, whales, bats

Wings of birds, bats, flies