Evolution

Wooly mammotn skeleton: Mammoths lived 2 million to about 9,000 years ago. They were about 9 to 15 feet tall. What was the reason for the extinction?

What is Evolution?

• Evolution involves inheritable changes in organisms through time – Fundamental to biology and paleontology

• Paleontology is the study of life history as revealed by fossils

• Explains the development of life

First, Let’s Define “Life”

• 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 of evolution

• Evolution proceeds strictly by chance• Nothing less than fully developed structures,

such as eyes, are of any use• There are no transitional fossils

• so-called missing links connecting ancestors and descendants

• Evolved species must be more complex than the predecessor

• humans evolved from monkeys so monkeys should no longer exist

Historical Perspective

• Evolution is usually attributed solely to Charles Darwin, but actually considered long before he was born.– By some ancient Greeks and by philosophers and

theologians during the Middle Ages• Nevertheless, the prevailing belief in the 1700s

was that the Bible’s book of Genesis explained the origin of life.

Contrary views were heresy!Contrary views were heresy!

Historical Perspective• During the 18th century, naturalists were discovering

evidence that could not be reconciled with literal interpretation of Scripture

Scientists gradually accepted a number of ideas: • The principle of uniformitarianism,• Earth’s great age• Many types of plants and animals had become extinct• change from one species to another occurred• Species existed on Earth that were no longer living (extinct)

• What was lacking, though, was a theoretical framework to explain evolution

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 both– Life’s unity: similarities among life forms– Life’s diversity: differences among life forms

Lamark• Jean-Baptiste de Lamarck

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

• According to this theory – new traits arise in organisms because of their needs – Once acquired new traits are somehow passed on to

their descendants

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

Lamark’s Theory

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 and proposed a mechanism for evolution

Fig. 7-1, p. 115

Darwin is most associated with his time at theGalapagos Islands (only 5 weeks!). Here he Observed distinct differences among similaranimals that were directly related to foodsupply. He published his ideas 24 yrs. later.

Berry eaterInsect eaters

Insect eaters

Cactus eaters

Seed eaters

What he noticed• Plant and animal breeders practice artificial selection

– by selecting desirable traits and then breeding plants and animals with those traits

• Observing artificial selection gave Darwin the idea that a process of selection among variant types in nature could also bring about change

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

Natural Selection (Key Points)• 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– However, Those with favorable variations are more likely

to survive and pass on their favorable variations

Back to the Giraffes…why would giraffes “develop” or actually

“express” the trait longer necks over time?

Survival of the Fittest

• In colloquial usage, natural selection is sometimes expressed as “survival of the fittest”

• This is misleading because natural selection is not simply a matter of survival but involves differential rates of survival and reproduction

What does “Survival of the Fittest” actually mean over time??

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

are likely to survive

– These characteristics might provide an advantage but natural selection may favor species other than the obviously bigger, stronger, or faster. Examples?

• 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?

The Cretaceous – Tertiary Boundary also known today as the Cretaceous – Paleogene

BoundaryWhat life forms were lost, and which survived?

• Photosynthesizing organisms, including phytoplankton and land plants, formed the foundation of the food chain in the late Cretaceous as they do today. Evidence suggests that herbivorous animals died out when the plants they depended on for food became scarce; consequently, top of the food chain predators such as Tyrannosaurus rex also perished.

• Coccolithophorids and molluscs, including ammonites, rudists, freshwater snails and mussels, and those organisms whose food chain included these shell builders, became extinct or suffered heavy losses. For example, it is thought that ammonites were the principal food of mosasaurs, a group of giant marine reptiles that became extinct at the boundary.

• Omnivores, insectivores and carrion-eaters survived the extinction event, perhaps because of the increased availability of their food sources. At the end of the Cretaceous there seem to have been no purely herbivorous or carnivorous mammals. Mammals and birds that survived the extinction fed on insects, worms, and snails, which fed on dead plant and animal matter. Scientists hypothesize that these organisms survived the collapse of plant-based food chains because they fed on detritus

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

Observers knew that life on earth has changed with time.

Some explanations were plausible, such as the extinction or inability for an organism to survive.

Other observations were not as easy to explain, and more scientific investigation and application was needed to support the hypotheses.

Limits on Natural Selection• Natural selection works on existing variation 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• During the 1860s, Gregor Mendel, performed a series of

controlled 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 blend during

inheritance

• Traits not expressed in each generation may not be lost

• Therefore, some variation in populations results from alternate expressions of genes (alleles)– Variation can be maintained!– Why is variation important to survival of a species?To be continued! Read Chapter 7

Modern Genetics

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

except bacteria which have ribonucleic acid (RNA)

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

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, 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!

Mutations

• If a species is well adapted to its environment, most mutations would not be particularly useful and perhaps could be harmful

• But what was once a harmful mutation can become a useful one if the environment changes

The Species

• Species is a biological term for 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)

Hmmm…but 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

Misconceptions

• One antievolution argument is “If humans evolved from monkeys, “why are there still monkeys?”

• This involves two misconceptions– No scientist has ever claimed that humans

evolved from monkeys– Even if they had, that would not preclude

the possibility of monkeys still existing

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