Darwin’s Theory of Evolution

Theory vs. Hypothesis

• Theory: unifying idea which is proven through repetitious experimentation

• Hypothesis: conjectures or plausible explanations that may be testable

• Evolution is a theory and not a hypothesis.

Jean Baptiste Lamarck

• Part of why Darwin’s theory of evolution was controversial is that many people still confuse Darwinian evolution with Lamarckian evolution.

• Lamarck’s ideas about evolution are considered to be unsound.

Use and Disuse

• Lamarck believed that if a behavior is useful and beneficial to the organism, then the body parts used to carry out the behavior become prominent and developed.

Example: Giraffe’s have long necks because they needed the longer necks in order to eat leaves from trees.

Use and Disuse

• Example of disuse: Horses and cows eat grass from the ground and therefore do not use their necks in feeding. This is why horses and cows have short necks.

Inheritance of acquired traits

• Lamarck believed that organisms can inherit acquired traits from parents.

• Example: When Arnold Schwarzenegger has kids, his kids will come out looking like him.

Influence of Thomas Malthus

• 1798, economist Thomas Malthus published a work describing the nature of population growth.

• Populations that continue to grow exponentially will eventually run out of resources.

Effect of Competition on Population Growth

• Darwin observed that few species are overpopulated.

• Competition for resources and limitations on birth rate must select out some individuals.

Influence of Artificial Selection

• Darwin also observed that for the time of human existence, humans have selected plant and animal traits which benefit mankind.

• Ex: dog domestication, increased crop yield per plant

The Galapagos The Galapagos Islands lie 500 miles Islands lie 500 miles west of Ecuador in west of Ecuador in the Pacific Ocean, the Pacific Ocean,

directly on the directly on the equator.equator.

The Galapagos The Galapagos Islands lie 500 miles Islands lie 500 miles west of Ecuador in west of Ecuador in the Pacific Ocean, the Pacific Ocean,

directly on the directly on the equator.equator.

Many of Darwin’s Many of Darwin’s conclusions were conclusions were

based on based on observations of observations of

wildlife in the wildlife in the Galapagos Islands.Galapagos Islands.

Many of Darwin’s Many of Darwin’s conclusions were conclusions were

based on based on observations of observations of

wildlife in the wildlife in the Galapagos Islands.Galapagos Islands.

“Galapagos” means turtle.

In particular, In particular, Darwin observed Darwin observed something odd something odd

about the about the finches: they all finches: they all

looked like a looked like a bird he had seen bird he had seen

on the South on the South American American continent.continent.

In particular, In particular, Darwin observed Darwin observed something odd something odd

about the about the finches: they all finches: they all

looked like a looked like a bird he had seen bird he had seen

on the South on the South American American continent.continent.

Darwin wondered if the birds and other Darwin wondered if the birds and other animals had been animals had been createdcreated to match their to match their

environment, why didn’t these birds look like environment, why didn’t these birds look like the birds of the African continent, since the the birds of the African continent, since the

environments of both the Galapagos and environments of both the Galapagos and Africa were similar.Africa were similar.

Darwin wondered if the birds and other Darwin wondered if the birds and other animals had been animals had been createdcreated to match their to match their

environment, why didn’t these birds look like environment, why didn’t these birds look like the birds of the African continent, since the the birds of the African continent, since the

environments of both the Galapagos and environments of both the Galapagos and Africa were similar.Africa were similar.

Darwin guessed that Darwin guessed that some of the birds some of the birds

from South America from South America migrated to the migrated to the

Galapagos.Galapagos.

Darwin guessed that Darwin guessed that some of the birds some of the birds

from South America from South America migrated to the migrated to the

Galapagos.Galapagos.

Once on the islands, Once on the islands, the birds must have the birds must have

changed over the changed over the years.years.

Once on the islands, Once on the islands, the birds must have the birds must have

changed over the changed over the years.years.

This would explain the numerous species of This would explain the numerous species of birds present.birds present.

This would explain the numerous species of This would explain the numerous species of birds present.birds present.

woodpecker finch

cactus finch

large ground finch

Darwin called this…Darwin called this…Darwin called this…Darwin called this…

orororor

(change in species over time)(change in species over time)(change in species over time)(change in species over time)

Darwin concluded:Darwin concluded:

Each species has descended, with changes, Each species has descended, with changes, from other species over time.from other species over time.

Darwin concluded:Darwin concluded:

Each species has descended, with changes, Each species has descended, with changes, from other species over time.from other species over time.

Descent With ModificationsDescent With ModificationsDescent With ModificationsDescent With Modifications

Darwin’s conclusions

• Earth is very old.

• Events observed today must have also happened in the past.

• Organisms struggle for resources and mates.

• Some organisms are better competitors than others.

• Traits can be selected.

Natural Selection

• The forces of nature create a selective pressure so that traits which are better adapted in a particular environment enable the organisms within a population to survive to reproductive age.

• The ability to reproduce is called fitness.• Alfred Wallace coined the term “survival

of the fittest.”

Effect of Natural Selection on Populations

• The organisms selected and survive to reproductive age are more likely to pass on their genes and influence the gene pool then those than die before mating.

Descent with modification

• With sexual reproduction, each generation inherits genes that are varied from the previous generation. These descents are modified from the parental generation.

Observations for Darwin's

theory of natural

selection

1. 1. Organisms produce more offspring than can Organisms produce more offspring than can survivesurvive. Many that survive do not produce . Many that survive do not produce offspring. offspring.

1. 1. Organisms produce more offspring than can Organisms produce more offspring than can survivesurvive. Many that survive do not produce . Many that survive do not produce offspring. offspring.

The female green sea turtle lays a clutch of about The female green sea turtle lays a clutch of about 110 eggs. She may lay several clutches.110 eggs. She may lay several clutches.

The female green sea turtle lays a clutch of about The female green sea turtle lays a clutch of about 110 eggs. She may lay several clutches.110 eggs. She may lay several clutches.

It is likely that It is likely that less than 1%less than 1% of the hatchlings will ever of the hatchlings will ever reach sexual maturity.reach sexual maturity.

It is likely that It is likely that less than 1%less than 1% of the hatchlings will ever of the hatchlings will ever reach sexual maturity.reach sexual maturity.

• #2. Genetic variation exists within members of the same species.

Observations & Inferences on Darwin's

theory of natural selection

Observations & Inferences on Darwin's

theory of natural selection

• #3. Natural resources are limited.• Inference #1: Production of more

individuals than that environment can support leads to struggle for existence among individuals of a population, with only a fraction of offspring surviving each generation.

4. 4. Since more organisms are produced than can Since more organisms are produced than can survive, there is survive, there is competitioncompetition (struggle for (struggle for existence)existence)..

4. 4. Since more organisms are produced than can Since more organisms are produced than can survive, there is survive, there is competitioncompetition (struggle for (struggle for existence)existence)..

Competition exists WITHIN and AMONG species.Competition exists WITHIN and AMONG species.Competition exists WITHIN and AMONG species.Competition exists WITHIN and AMONG species.

foodfoodfoodfood

Within and Among Species for

And Within a Species for

waterwaterwaterwater

matesmatesmatesmates

sheltersheltersheltershelter spacespacespacespace

Observations & Inferences on Darwin's

theory of natural selection

• #5. Organisms that have an adaptation that gives them an advantage have a better chance to survive and reproduce. Much of the variation is heritable

• Inference #2: Survival in the struggle for existence is not random, but depends in part on the hereditary constitution of the surviving individuals. Those individuals whose inherited characteristics fit them best to their environment are likely to leave more offspring than less fit individuals

Descent with modificationDescent with modification: : Living species today Living species today are descended with modifications from common are descended with modifications from common ancestral species that lived in the past.ancestral species that lived in the past.

Descent with modificationDescent with modification: : Living species today Living species today are descended with modifications from common are descended with modifications from common ancestral species that lived in the past.ancestral species that lived in the past.

Over time, genes for less Over time, genes for less favored characteristics will be favored characteristics will be eliminated from the gene pool.eliminated from the gene pool.

Over time, genes for less Over time, genes for less favored characteristics will be favored characteristics will be eliminated from the gene pool.eliminated from the gene pool.

Example: giraffes and their Example: giraffes and their increasingly longer necks.increasingly longer necks.

Example: giraffes and their Example: giraffes and their increasingly longer necks.increasingly longer necks.

Characteristics of fit individuals Characteristics of fit individuals increase in a population over time.increase in a population over time.

Characteristics of fit individuals Characteristics of fit individuals increase in a population over time.increase in a population over time.

Natural Selection: Survival of the FittestNatural Selection: Survival of the FittestNatural Selection: Survival of the FittestNatural Selection: Survival of the Fittest

the variation exists first.the variation exists first.the variation exists first.the variation exists first.

An An adaptationadaptation is any inherited characteristic (a is any inherited characteristic (a genetic variation) that can increase an genetic variation) that can increase an organism’s chance of survival.organism’s chance of survival.

An An adaptationadaptation is any inherited characteristic (a is any inherited characteristic (a genetic variation) that can increase an genetic variation) that can increase an organism’s chance of survival.organism’s chance of survival.

the environment changes.the environment changes.the environment changes.the environment changes.

a variation may give an advantage a variation may give an advantage to survive environmental change.to survive environmental change.

a variation may give an advantage a variation may give an advantage to survive environmental change.to survive environmental change.

An organism does not change An organism does not change because of need or desire to because of need or desire to survive. The organism either survive. The organism either already has the variation that already has the variation that enables it to survive or it dies.enables it to survive or it dies.

An organism does not change An organism does not change because of need or desire to because of need or desire to survive. The organism either survive. The organism either already has the variation that already has the variation that enables it to survive or it dies.enables it to survive or it dies.

Evidence of Evolution

• The main evidence that supports evolution is the fossil record.

• The fossil record shows how organisms have changed over time, and how new groups have formed – examples of intermediates between fish and amphibians, reptiles and birds, and reptiles and mammals.

Evidence for evolution

• Fossils showing extinct and transitional species

Cooksonia, earliest evidence of

branching plant

Evidence for evolution

• Gene and protein comparisons

Evidence for evolution• Homologous structures –different functions

similar structure – common ancestor• Analogous structures – different structure

same function – indicates similar pressure

Evidence of Evolution

• Vestigial Structures – structures that are reduced in function and importance. Examples include appendix in humans, toes in snakes, and hip bones in whales.

Evidence for evolution

• Embryology

Speciation

• If one population becomes isolated and inbreeds, their gene pool becomes smaller.

• This population may not be able to breed with another population of the same species, thus leading to speciation (evolution of new species).

Debate over Evolution

• Many people confuse Darwin’s ideas with Lamarck’s ideas.

• People proclaim that to believe in evolution is to disregard God.

• Darwin does not propose how man came to be but how all organisms have the capacity to change.

• We know today that mutations and genetic recombination can lead to differences in DNA from one generation to the next, making some organisms better adapted to the environment than others.