Evolution

What is Evolution?

Evolution involves inheritable changes in a population of organisms through time

Fundamental to biology and paleontology

Paleontology is the study of life history as revealed by fossils

Explains the development of life as seen in the fossil record

Misconceptions regarding Evolution

Evolution proceeds strictly by chance

Nothing less than fully developed structures, such as eyes, are of any use

There are no transitional fossils (the so-called missing links connecting ancestors and descendants)

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.

ancient Greeks, philosophers, theologians, and all others who tried to explain fossils during the Middle Ages

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

Lamark (1744-1829)

Theory of Inheritance of Acquired Characteristics.

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

Example

The fossil record shows that giraffe necks got longer over time. This also coincides with the development of taller trees

Darwin (1809-1882)

On the Origin of Species, published in 1889

details his ideas on evolution formulated 20 years earlier

proposes a mechanism for evolution called natural selection

What Darwin notices

By selecting desirable traits and then breeding plants and animals with those traits we produce more useful 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 SelectionOrganisms in all populations posses heritable variations.

size, speed, agility, visual acuity, digestive enzymes, color, and so forth

Some variations are more favorable than others and give an organism 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 be numerous variation in all inherited traits

For 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 limited

the most easily concealed

those that adapt most readily to a new food source

those having the ability to detoxify some substance

and so on...

Apparent limits to darwin’s theory

Darwin’s theory of Natural selection works on existing variation in a population.

It could not account for the origin of new variations within a population

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, 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

Labs, pugs, boxers, shepherds are examples of true breeding strains of dog

Mendel’s workMedel first cross pollinated to different strains of garden peas; one strain that produced red flowers and one strain that produced white flowers

The resulting seeds produced plants that only produced red flowers

Mendel’s experimentMendel then allows the second generation to self fertilize and produced a third generation. The third generation seed produces the following result.

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 this is important

His work showed that genes controlling traits do not blend during inheritance

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

Therefore, some variation in populations results from alternate expressions of genes (alleles) based on inheritance

Modern GeneticsComplex, 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 Genetics

Modern evolution incorporates chromosome theory of inheritance into Darwin’s theory of natural selection

along with alternate expression of variation described by Mendel and the environmental forces described by Darwin genes may also mutate.

mutation occurs constantly in all animals

Most mutations have no visible effect

only mutations that occur in sex cells (sperm and egg) can be passed on.

Politics and ScienceLamarck’s idea of inheritance of acquired characteristics no longeraccepted as a valid scientific theory

Russian 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

The SpeciesSpecies 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

Genetic DriftThe randomness with which alleles are transferred through a population as a result of sexual reproduction

Recipe for a speciesSpeciation 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

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

Process of changePhyletic gradualism

the gradual accumulation of minor changes eventually bring about new species

process of change

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

Icthyosaur-reptile

Parallel evolution

Death the driving force of evolution

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 to 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

As species go extinct new species usually quickly exploit the opportunities the other species’ extinction creates

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