Evolution

What is Evolution?

Evolution involves inheritable changes in organisms through time

Fundamental to biology and paleontologyPaleontology is the study of life history as revealed by fossils

Explains the development of life

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

humans evolved from monkeys so monkeys should no longer exist

Historical PerspectiveEvolution is usually attributed solely to Charles Darwin, but actually considered long before he was born.

ancient Greeks and by philosophers and theologians during the Middle Ages

Nevertheless, the prevailing belief in the 1700s was that Genesis explained the origin of life.

Contrary views were heresy!Contrary views were heresy!

Historical PerspectiveDuring the 18th century, naturalists were discovering evidence that could not be reconciled with literal reading of Scripture

Scientists gradually accepted a number of ideas: The principle of uniformitarianism,Earth’s great ageMany types of plants and animals had become extinctchange from one species to another occurred

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

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 In 1859, Charles Robert

Darwin Darwin (1809-1882)(1809-1882) published published On the Origin of SpeciesOn the Origin of Species details his ideas on details his ideas on

evolution formulated 20 evolution formulated 20 years earlier years earlier

proposes a mechanism for proposes a mechanism for evolutionevolution

What he noticedPlant and animal breeders practice artificial selection

by selecting desirable traits and then breeding plants and animals with those traits to produce more usegul species

dogs, cats, vegetables, flowers

What if natural processes could do the same thing?

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

What was different about the animals that survived?

Natural Selection (Key Points)

Darwin proposes the idea of Natural SelectionOrganisms in all populations posses heritable variations.

size, speed, agility, visual acuity, digestive enzymes, color, and so forthSome variations are more favorable than others

some have a competitive edge in acquiring resources and/or avoiding predators (i.e. baby birds and rabbits)

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 GiraffesIn any population there is bound to benumerous variation in all inherited traitsFor example giraffe with all different neck lengths (some long some a bit shorter)

As environments changed and trees grew taller and taller those giraffe with longer necks had distinct advantage over those with shorter necks. These giraffe were more likely to survive and therefore pass on there characteristics

Survival of the FittestIn colloquial usage, natural selection is sometimes expressed as “survival of the fittest”

This is misleading because natural selection is not simply a matter of being the strongest; it involves differential rates of survival and reproduction

One characteristic might provide an advantage to the individual in a specific circumstance but nature may favor the something else

the smallest if resources are limitedthe most easily concealedthose that adapt most readily to a new food sourcethose having the ability to detoxify some substanceand so on...

The End….right?

Limits on Natural Selection

Darwins theory of Natural selection works on existing variation in a population.

It could not account for the origin of new variations

Some critics also reasoned that traits would blend with other traits and be lost

Red hair+blonde hair = strawberry blondeLong neck+ Short neck= medium neck

The answer to these criticisms existed but remained hidden 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 • or for example dog breeds—Boxer+boxer

=baby boxer – Boxers are a true breeding strain

Mendel’s Work

• The parental generation consisted of true-breeding strains : One strain that produced red flowers and one strain that produced white flowers

• Mendel Cross-fertilized the two strains to yield a second generation –all of which had red flower

Mendel’s Work

• Mendel then allowed the second generation to self fertilize and produced a third generation

• From his experiments Gregor determined that 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

Why is this important?

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

• Although traits may not be expressed in each generation they are not lost

• Therefore, some variation in populations results from alternate expressions of genes (alleles) based on inheritance– Variation can be maintained!

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

• Modern evolution incorporates chromosome theory of inheritance into Darwin’s theory of natural selection– changes in genes (mutations) only one

source of variation

Most Importantly• Lamarck’s idea of inheritance of

acquired characteristics no longeraccepted as a valid scientific theory– Problems with politics

• LisenkoismRussian agronomist Lisenko believed Lamarcks ideas fit much more closely with communist ideology (no gene could be better than another).

• Eventually put in charge of Russian Science and purges all evolutionary scientist

• Responsible for massive wheat famine

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:– Random with respect to fitness– May be beneficial, neutral, or harmful

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

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

Allopatric Speciation

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

population becomes isolated from its parent population

– The peripheral isolates evolve as a result of genetic constriction and new environmental factors

Yeah…but how long?

• 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 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

Current thought is that evolution is most likely a mixture of these two ideas

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

Icthyosaur-reptile Dolphin- mammal

Parallel Evolution

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. Humans and monkeys evolved from a common ancestor.

– Even if they had, that would not preclude the possibility of monkeys still existing—dogs

have been bread from wolves and wolves still exist

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 exploit 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 tiny

• The greatest mass extinction occurred at the end of the Paleozoic Era– More than 90% of all species died out– We will discuss these extinctions and their

possible causes throughout the rest of the term