Theories on Origin and Change © Lisa Michalek. Change Over Time Most scientists agree that organisms have changed over a long period of time – from simple.

Theories on Origin and Change

© Lisa Michalek

Change Over TimeMost scientists agree that organisms have changed over a long period of time – from simple to complex.

Scientists have a good understanding of how life first began & how evolutionary changes occurred.

To explain these events, scientists have proposed various theories.

Some theories have been totally rejected because of lack of supporting evidence.

Other theories have been modified (changed) based on new evidence.

Spontaneous Generation

An Early attempt to explain the origin of life.

The concept that living things come from nonliving things.

People believed, that toads came from mud, flies came from the rotting bodies of animals, and mice came from cheese.

Spontaneous Generation This theory was widely

accepted until the late 1800’s when it was disproved by Louis Pasteur.

Today, we know that life comes from other living things although some scientists believe the first cell must have come from nonliving materials.

Theory of Use and Disuse

In the early 1800’s Jean-Baptiste Lamarck presented this theory.He thought that organisms were able to develop new structures because they needed the structures.He also believed that the size of an organ is determined by how much the organ is used. According to this theory, ballet dancers have big, strong muscles because they uses their muscles a lot. When a dancer stops using a certain muscle, the muscle gets smaller and weaker.

Inheritance of Acquired Characteristics

Lamarck also stated that the useful traits an individual developed during its lifetime can be passed on to its offspring.An acquired characteristic is a trait that is produced during an individual’s lifetime.Lamarck would have said that the children of

dancers would inherit strong muscles.

Lamarck’s theories were widely accepted for a long time.As time went on, scientists began to challenge his theories by showing that there was no data to support his hypothesis.

Disproving Lamarck’s Theory

The idea that acquired traits could be inherited was officially disproved in the late 1800s by the experiments of August Weismann.Weismann cut off the tails from mice.He then mated the tailless mice. He did this for many generations.The offspring of the tailless mice were

always born with normal length tails. This experiment proved that acquired

traits are not inherited by offspring.

Darwin’s Theory

In the 1850’s the theory of natural selection was proposed by Charles Darwin.

In his travels he noticed that a certain species in one geographic area was different from the same species in another area.

His theory of evolution was developed to explain the gradual change in species.

Natural Selection

Nature (environment) acts as the selecting agent of an organism’s traits.Darwin believed organisms better adapted to the environment survive and reproduce more successfully than organisms not as well adapted.Natural Selection suggests that traits that help an organism survive in a changing environment are passed on to the next generation.

Lamarck vs. Darwin

Alfred Wallace

Although Darwin is given credit for the theory of natural selection, another scientist, Alfred Wallace, proposed a theory of evolution that was quite similar to Darwin’s Theory.

Darwin’s Theory of Natural Selection

1. Overproduction: A population generally produces more offspring than can survive in the environment.

• For example: a fish must lay millions of eggs to reproduce a small number of new fish.

2. Competition: Because of overproduction, there is competition or a “struggle for survival,” between organisms for space, food, water, light, minerals, or other limited resources.

3. Variations: Members of a population show variations (differences in traits) that make certain individuals better adapted to survive.

• Differences in structure, size, and color are examples of variations.

Darwin’s Theory of Natural Selection

4. Natural Selection: Since some variations are more helpful than others, there is a natural selection against organisms that cannot adapt. Organisms that cannot adapt, die.

5. Survival of the Fittest: Applies to those individuals that have variations that enable them to live and reproduce.

• In a woodland environment, brown fur color would be a helpful variation and white fur color would not be helpful.

Darwin’s Theory of Natural Selection

6. Inheritance of Variations: Organisms with helpful variations are more likely to survive and to reproduce, passing these variations to their offspring.

7. Evolution of New Species: Over long periods of time, variations accumulate in a population. Eventually, there are so many variations that the population becomes a new species.

Mutation TheoryIn 1901, Hugo DeVries, suggested that inherited mutations caused variations.

He believed that mutations (changes in genetic material) occurred randomly and those mutations that were favorable were inherited by offspring.

DeVries based his theory on his reproduction experiments with the evening primrose plant.

Modern Theory of Natural Selection

The genes of inherited variations that give an organism a better chance for survival tend to be passed on from parents to offspring.

These favorable genes tend to increase in numbers within a population.

Genes for traits with low survival value decrease in numbers from generation to generation.

If the environment changes, genes that previously were neutral or had low survival value may become favorable and increase in numbers.

Geologic TimeThe oldest living thing, a bacteria-like organism,

is estimated to be 3.4 billion years old.Each time period is identified by its dominant (common) animal and plant life.Eras- A time chart that divides the number of years into smaller time units.Cenozoic, Mesozoic, Paleozoic, and Precambrian

Eras are divided into Periods.Periods are divided in Epochs.

4 Major Eras

Cenozoic EraMost recent era, began approx. 60 million

years agoAge of Mammals (mammals became the

dominant form of animal life.)Flowering plants were the dominant form

of plant life.Man appeared late in this era.

4 Major Eras

Mesozoic EraBegan over 200 million years ago.Known as the Age of Reptiles.Reptiles, such as dinosaurs were the

dominant animal life.The first birds were thought to appear during this

era.Fern forests were the dominant plants.

4 Major Eras

Paleozoic EraLongest era, lasting over 400 million years.Divided into:

Age of AmphibiansAge of FishesAge of Invertebrates

The first land plants developed during this time and fern trees were widespread along with swampy forests.

Modern insects also appeared, amphibians, giant insects, trilobites, crustaceans and fishes were common.

4 Major Eras

Precambrian EraOldest time period, over 4.6 billion years ago.Simple multicellular organisms, such as

sponges and worms, were numerous.The first protists (one-celled organisms)

appeared during this time and probably bacteria and algae were dominant.

Fossil Evidence

Fossils – the remains or traces of organisms that once lived.

Paleontologists – Scientists who study fossils.

The study of fossils in the earth provides evidence to support the idea that life changed over time from simple to complex.

Fossil distribution shows that life began in the sea and then moved to land.

Becoming a Fossil

Fossil Evidence

It also provides evidence for the time of origin (beginning) of various forms of life.

Fossils help scientists understand how climates and land surfaces have changed.

By the process of radioactive dating, scientists determine the age of the earth’s rocks and its fossils.

Fossil EvidenceIn Sedimentary Rock

Many Fossils are found in sedimentary rock.Where the crust of earth is undisturbed, the oldest rock layers lie under the younger layers.Sedimentary rock is formed from layers of slowly deposited sediments. Rock particles Silt Mud

After long periods of time and great amounts of heat and pressure, sediments harden into rock forming visible layers.

Fossilization

Amber - A yellowish-brown sap secreted by pine trees.

Tar

Ice

Mold - An indentation in rock shaped like an organism.

Fossilization

Cast - The decayed organism forms a mold, and the mold becomes filled with a different substance.

Petrification - The tissues of the organism are slowly replaced by minerals.

Imprints – Occur when a print is made in a soft sediment, that later turns to rock.

Skeletal EvidenceComparative AnatomyThe science that studies the structures (anatomy)

of plants and animals.When scientists compare skeletal structures of

different vertebrates, they see a similar basic structures.

This observation shows that organisms with similar bone structures may have evolved from a common ancestor population.

Example - Evolution of Eye

Skeletal Evidence

Organs or structural parts that seem to have a common evolutionary origin are referred to as homologous structures.

Although homologous structures are similar in structure, they do not always have the same function.

Vestigial Structures

Parts of an animal’s body that are not usedThese structures look like structures that are fully developed and used by other animals.Example: Human AppendixScientists think that perhaps some ancestor of humans used their appendix and, as evolution continued, humans stopped using this organ.Other Examples: human ear muscles, and the leg bones of the python and porpoise

Comparing Cell Structure

Cells and cell organelles are basically alike from one group of organisms to another.

For example, all cells have a nucleus, cell membrane, cytoplasm, ribosomes, mitochondria, chromosomes, and other organelles.

This is evidence that different kinds of living things may share a common origin.

Comparing Embryos

Comparisons of early stages of embryonic development show the possibility of common ancestry and evolutionary relationships.

The science that studies the structural similarities among vertebrate embryos is called comparative embryology.

Example - Embryos

Comparing EmbryosAt early stages, vertebrate embryos show gill slits, tails and two-chambered hearts.Do you see many differences among the embryos during their early embryo stages?

Similarities in BiochemistrySimilarities in the body chemicals (biochemistry) of living things, such as DNA, hormones, and enzymes, show a close relationship between various forms of life.

Organisms that are closely related, like the cat and the lion, have a greater similarity in their protein structure.

Greater differences in cell biochemistry are thought to show a lesser evolutionary relationship.

Examplesof

Evolution

The ancient ancestor of the horse, was about the size of a fox. It had four toes on its front feet and three toes on its hind feet.

The horse gradually got bigger and the length of its feet increased.

As time passed, some of the toes disappeared, until today the modern horse is one-toes.

The middle toe is the one that remains, but the horse retains tiny splints of two other toes.

The skull grew longer and the teeth became flat-topped.

Examples of Evolution The ancestor of the present

elephant was the size of a pig and had no tusks.

Over time, the size of the elephant’s body and head increased tremendously.

The two upper incisor teeth increased in size and length and gradually developed into tusks.

The early trunk was much shorter than the trunk of today’s elephant.

Evolution In Our Time

Staphylococcus bacteriaWhen the antibiotic was

first used, this bacteria had genes that made them resistant to antibiotics.

When the use of antibiotics became widespread, these genes increased in numbers, producing a population of bacteria that was not killed by antibiotics.

Evolution In Our Time

Today, scientists continuously develop new antibiotics because the bacteria population mutates and produces new antibiotic-resistant strains.

Mutations are not caused by environmental change. Mutations occur randomly.

Mutations with positive survival value allow organisms to be better adapted to their environment.

The environment selects those variations, or adaptations, that may have survival value.

Evolution in Our TimeEnglish Peppered Moth Peppered moths have two basic colors: light color with

dark markings or dark color with light markings. Before the industrial revolution, most peppered moths

were light colored. This enabled them to blend with their light-colored environment, such as the trunks of trees and the side of buildings.

By blending with the environment, the light-colored moths were almost invisible.

Insect eating birds could not see them.

Evolution in Our Time The soot and other air pollutants from the industrial

revolution gradually changed the environment from light to dark.

The light-colored moths became visible and were eaten by birds.

The dark-colored moths could not be seen against the dark background. These moths reproduced more dark moths and the population shifted from light to dark-colored.

Recently, as a result of environmental pollution laws, the moth population is slowly changing back to light-colored moths.

Evolution in Our Time

Artificial SelectionSometimes plant and animal breeders purposely

modify organisms by mating plants and animals that have certain desirable traits.

By selective breeding, man may cause evolutionExample: racehorses and greyhounds have been

produced that are faster than their predecessors.

Geographic Isolation

Occurs when a population is physically separated into smaller populations by geographic barriers. Mountain ranges, deserts, oceans, rivers, other bodies of

water, big expressways, or shopping malls.

Changes may occur in these separated populations that, over a long period of time may result in the production of different species. The production of a new species is known as speciation.

Reproductive isolation occurs when members of the isolated population and the main populations can no longer interbreed, even if the barriers are removed.

Adaptive Radiation

The process by which many new species of organisms evolve from a common ancestor.

The new species evolve and fill different environmental niches where there is less competition.

Adaptive RadiationOrganisms move into new niches in the environment through chance mutations that have positive adaptive value.

A positive adaptation would be one that allows an organism to live successfully in a new niche. If there is little competition in the niche, the

organism has a better chance to survive and reproduce.

Rate of Change

Gradualism

A theory that evolutionary change is slow, gradual, and continuous.

New species would arise by the very gradual collection of minor changes in a population.

Rate of Change

Punctuated EquilibriumA theory that species are relatively stable for long periods of time (several million years). This stability is interrupted by brief periods during which major changes occur. These changes result in the evolution of new species.

The minor changes that occur in a population over time might produce new varieties of an existing species, but not a new species.

Heterotroph HypothesisThis theory, proposed by A.I. Oparin in the late 1930’s, stated that groups of organic molecules were formed from the chemical elements in the Earth’s primitive ocean.These organic molecules combined, using energy from heat, lightning, solar radiation, and radioactive materials in the rocks.The first living things were thought to be heterotrophs. Since no free oxygen gas existed in the atmosphere,

these forms of life carried on anaerobic respiration.

Heterotroph Hypothesis

They used the free organic molecules in the sea for food.Over time, genetic changes occurred in the first organisms.As a result of the genetic changes, photosynthetic forms of life evolved.These organisms, autotrophs, released oxygen into the atmosphere.Aerobic forms of life evolved from the anaerobic forms.

Human Evolution

Darwin only suggested that humans, along with other mammals, could have shared a common ancestor.

Some human-like fossil forms have been discovered.

Their exact place in human ancestry has yet to be determined.

Human Fossil Sites

Out of Africa Theory

Human Evolution

Modern scientists assume that human evolution, like evolution in other animals, is continuing.

Because of their superior reasoning ability, humans are able to control their environment.

Because of this, the evolutionary effect of natural selection is not as great as in other organisms.

Factors That May Affect Human Evolution

Medical KnowledgePermits the survival of individuals with genetic

traits such as diabetes, hemophilia, and PKU. Without medical knowledge these people would

die and the genes for these diseases would decrease in number in the population.

With modern medicine the number of genes are maintained or increased.

Modern Transportation Humans are less affected by the evolutionary

force of geographic isolation.

Factors That May Affect Human Evolution

Advanced Technology Gives humans better nutrition and greater control

over their reproductive process. It has also increased the number and kinds of

mutagenic agents in the environment.

Genetic Engineering May possibly lead to the appearance of new traits

and the elimination of others.