Theories Of Evolution

Theories of Evolution

1766 Georges Buffon (French)-said that some fossil forms may be

ancient versions of living species

Theories of Evolution

Jean Baptiste Lamarck (1809-French) Organisms constantly strive to improve

themselves and become more advanced This effort to improve causes the most used

body structures to develop, while the unused structures waste away (principle of use and disuse) Once the structure is modified, the modification is inherited by the organism’s offspring (inheritance of acquired

characteristics)

Lamarck’s Theory of Evolution

More examples Traits Acquired During Ones Lifetime Would

Be Passed To OffspringClipped ears and tails of dogs could be passed to

offspring!

Lamarck’s Mistakes

Was he correct?? NO!

Traits are passed down from one generation to the next by genes, not by an individual’s life experiences or activities

Lamarck did NOT know how traits were inherited (Traits are passed through genes)

Genes Are NOT Changed By Activities In Life

Change Through Mutation Occurs Before An Organism Is Born

Theories of Evolution continued…

Charles Darwin (Born Feb. 12,1809-British) Voyage on H.M.S. Beagle (1831-

1836) voyage around the world; author of On the Origin of Species

Purpose of voyage

was to map the South

American coastline

Darwin was the naturalist on the

ship.

H.M.S. Beagle voyage

Voyage of the Beagle

During His Travels, Darwin Made Numerous Observations

Collected Evidence That Led Him To Propose a

Revolutionary Hypothesis about The Way Life Changes Over

Time

Darwin’s Belief’s

Survival of the fittest OR

Natural selection

Food and resources are limited So organisms have to fight to get

them (lions fight for food, etc)

• Too many organisms, they will fight to survive• not all offspring will survive

What happened to the Giraffe’s?

Survival of the fittest or

Natural selection

Natural selection said the giraffes with short necks had less food to eat

Why? the food resources changed to leaves only on

the upper branches What happened?

short necks could not reach upper branches and did not survive

Long neck giraffes survived because they were able to reach the food

Evolution is the slow, gradual change ina population of organisms over time…

a looooooooong time

Darwin’s Theory

1. Overproduction – Many more offspring are born than can be supported by the environments’ food, space, and resources. These offspring then compete for survival.2. Individual variation – Individuals within a species have different traits

Sources of variation:a. Mutations (rare)b. Sexual recombination -independent assortment and crossing over during meiosis, and fertilization

Darwin’s Theory

3. Natural Selection – Those individuals with traits best suited to the environment are the ones that survive long enough to reproduce and pass their traits on to their offspring. Those organisms less fit tend to die before they are able to reproduce. Their genes are not passed on.

This leads to an accumulation of favored traits in the population over generations (evolution).

The The unequal unequal ability of individuals to ability of individuals to survive and reproducesurvive and reproduce leads to a gradual leads to a gradual change in a change in a populationpopulation,, with with favorable characteristicsfavorable characteristics accumulating over generationsaccumulating over generations

““natural selectionnatural selection””New species evolveNew species evolve

Where do Where do we see this we see this

change?change?

Let’s look at a classic example of

natural selection

Peppered moths!

Theories of Evolution continued…

Alfred Wallace – He also developed a theory of evolution based on natural selection. He sent a memoir concerning his ideas to Darwin in 1858. This prompted Darwin to publish his book.

1859 – Darwin publishes his book, On the Origin of Species.

• Living species of today arose from a succession of ancestors through evolution

• Natural selection is the mechanism for how life evolves.

Populations evolve, individuals do not!

Population = a group of individuals of the same species living in the same place at the same time

Common MISCONCEPTION – Individual organisms evolve during their lifetime

Populations evolve over generations!

Natural Selection at Work

1. Directional Selection- characteristics at one end of the phenotypic range become more common than midrange characteristics Examples:

Widespread use of chemical pesticides in agriculture

Antibiotic Resistance 2. Stabilizing Selection- midrange

characteristics are favored (extremes are not favored) Example: babies that weigh far more or far

less than average at birth have a lower chance of survival

Natural Selection at Work (continued)

3. Disruptive Selection – characteristics at both ends of the range are favored (midrange characteristics are selected against)

Example: black-bellied seedcracker bird found in rainforests. These birds have either a small beak or a large beak. Birds with small bills can eat the soft seeds available to them, large-billed birds are better at cracking larger, harder seeds. Birds of this type with medium sized bills do not exist.

Types of Natural Selection

Mechanisms of Microevolution

Microevolution = the generation to generation change in the frequency of alleles in a population Population Genetics – the study of the genetic make-up of populations over time1. Genetic Drift – occurs when populations shrink

A. Bottleneck effect – disasters such as earthquakes, floods, and fires may kill large numbers of individuals. The small surviving population has a smaller sample of alleles left to pass on to the future generations. Reduces the overall genetic variability in a population because some alleles are likely to be lost from the gene pool

Gene pool = all the alleles (genes) in a population

Endangered SpeciesCheetahs were once widespread in Africa and Asia. Their numbers have significantly decreased. Only a few populations exists in the wild. They have very low genetic variability (as low as highly inbred lab mice!)

The cheetahs remaining are being crowded into nature preserves thus increasing the chance for the spread of disease. With little genetic variability within the species, they may have a reduced ability to adapt and survive any environmental changes.

Mechanisms of Microevolution

B. Founder effect = A few individuals colonize a habitat. The new populations gene pool would be different from the parent population it came from.

Ex. During a hurricane a few birds from a population of birds are blown off course and settle on an island. These birds survive and reproduce eventually resulting in a different population of birds from the original.

Mechanisms of Microevolution2. Gene Flow – a population may gain or lose alleles when individuals move into or out of populations.

It tends to reduce differences between populations

Ex. The armies of Alexander the Great brought the genes for green eyes from Greece all the way to India!

Mechanisms of Microevolution

3. Mutations – changes in the DNA Mutations are very rare, however, the

cumulative effect over many generations can be significant. Mutations are the original source of genetic variation.

Microevolution does not necessarily lead to the evolution of new species. Natural selection is needed for new species to evolve.

Macroevolution: The Origin of New Species

Macroevolution – the major changes in the history of life Origin of new species Origin of new anatomy (wings, feathers,

bigger brains) Explosive diversification (Diversification =

evolution of a variety of species) Mass extinctions – clears the way for new

adaptations

Macroevolution

Speciation = when one or more species branch from a parent species, which may continue to exist.

Creates biological diversity by

increasing the number of species.

Species = a population whose members have the potential to interbreed with one another to produce fertile offspring

How does speciation occur?

In order for a new species to evolve, some of the species must be kept separate from the other members of the species.

Now the separated population can follow its

own evolutionary course.

Divergent evolution

when isolated populations evolve independently

Reproductive Barriers

Examples between species:

Temporal isolation – time based (Ex. Western spotted skunks breed in the fall, eastern spotted skunks breed in late winter)

Habitat isolation – they may live in the same region but not the same habitat (Ex. N. American garter snake: one species lives on land, a closely related species lives in the water)

Behavioral isolation – traits that enable individuals to recognize potential mates (odor, color, etc…)

Geographic barriers – mountains may merge, land bridges form, large lake becomes several smaller lakes; can also occur if members of a species colonize a new and remote area.

How does speciation occur?

Adaptive radiation – Overtime, many new species may come from one ancestral species. Multiple branching of a family tree occurs. A new environment may have many new niches. If organisms becomeisolated from one another, new species can evolve To occupy the niches.

How does speciation occur?

Convergent evolution – species with very different ancestors may evolve to have similar characteristics because they evolved to occupy similar niches (they do not have a relatively recent Common ancestry, though). Ex. Penguins and porpoise – have similar body structure for fast

swimming