Evolution

What is evolution?

A basic definition of evolution…“…evolution can be precisely

defined as any change in the frequency of alleles within a gene pool from one generation to the next." - Helena Curtis and N. Sue Barnes, Biology, 5th ed. 1989 Worth Publishers, p.974

So what does the definition mean?

• Evolution is a change in the number of times specific genes that code for specific characteristics occur within an interbreeding population

• Individuals don’t evolve, populations do

• There is no implied “improvement” in evolution

• Evolution: changes through time1. Species accumulate difference

2. Descendants differ from their ancestors

3. New species arise from existing ones

Genetic Variation and Evolution

A brief history of evolution

• Jean-Baptist Lamack

• First person to come up with a theory of how evolution occurred.

• Inheritance of acquired characteristics: changes that an organism aquired during it’s life were passed on. (no genetic link).

• WRONG

A brief history of evolution

Charles Darwin was born on February 12, 1809 in Shrewsbury, England.

From 1831 to 1836 Darwin served as naturalist aboard the H.M.S. Beagle on a British science expedition around the world.

He observed much variation in related or similar species of plants and animals that were geographically isolated from each other.

These observations were the basis for his ideas.

Evidence of Evolution

• The Fossil Record

• Geographic Distribution of Living Things

• Homologous Body Structures

• Similarities in Early Development

Evidence for Evolution

• The Fossil Record-Layer show change

• Geographic Distribution of Living Things

• Homologous Body Structures

• Similarities in Early Development

Evidence of Evolution• The Fossil Record• Geographic

Distribution of Living Things-similar environments have similar types of organisms

• Homologous Body Structures

• Similarities in Early Development

Homologous Structures

• Homologous Structures-structures that have different mature forms in different organisms, but develop from the same embryonic tissue

Evidence for Evolution

• Vestigial organs-organs that serve no useful function in an organism

• i.e.) appendix, miniature legs, arms

Similarities in Early Development

Conclusion

• After making all these observations Darwin came up with a theory as to how populations evolve.

• NATURAL SELECTION!

Natural selection: proposed by Darwin as the mechanism of evolution

• individuals have specific inherited characteristics • they produce more surviving offspring• the population includes more individuals with

these specific characteristics• the population evolves and is better adapted to its

present environment

Natural selection: mechanism of evolutionary change

Natural Selection

Darwin knew nothing of genes, but what he did have were two observations and a little inference that provided the motive force for evolution.

•Darwin: Evolution is descent with modification

Natural Selection

Observation 1: Organisms generally have more offspring than can survive to adulthood.

Observation 2: Offspring are not identical. There is variation in their appearance, size, and other characteristics.

Natural Selection

Inference: Those organisms that are better adapted to their environment have a greater likelihood of surviving to adulthood and passing these characteristics on to their offspring.

Survival of the “fittest.”

Darwin’s theory for how long necks evolved in giraffes

Survival of the “fittest.”

Evolution of species

• Based on 3 mechanisms– 1. Sources of variation

– 2. Method of selection for those characteristics that would be

passed on

– 3. A mechanism for retaining changes

1. Sources of variation

• Genetic diversity thru mutations that are not lethal– Physical or behavioral traits

• Sexual reproduction between genetically different individuals

1. Sources of variation

Gene Flow• Many species are made up of local populations whose members tend

to breed within the group.

• Each local population can develop a gene pool distinct from that of other local populations.

• Members of one population may breed with occasional immigrants from an adjacent population of the same species.

-introduce new genes or alter existing gene frequencies in the residents.

This is called hybridization.

2. Method of selection• Natural selection: environmental

conditions determine which individuals in a population produce the most offspring

• Reproductive fitness is the method of selection.– Competition, escaping from and eluding

predators– Finding a good mate

Fitness and Its Measurement

• Fitness: A phenotype better fitted to it’s environment usually increases in frequency.

• Fitness is a combination of:– Survival: how long does an organism live

– Mating success: how often it mates

– Number of offspring per mating that survive

3. Maintenance of Variation

• Fittest organisms reproduce.

• The beneficial alleles are passed down to the offspring.

• This maintains the ability for change.

Selection in 3 Directions

• There are three ways a population can change.

• Stabilizing Selection

• Disruptive Selection

• Directional Selection

Stabilizing Selection• Organisms with the average traits have the

highest fitness.

Disruptive Selection

• Individuals that have either extremes have the highest fitness.

Directional Selection

• Individuals with extreme characteristics in one direction have the highest fitness.

Selection• Artificial selection: a breeder selects for

desired characteristics

Hardy–Weinberg principle states that the genotype frequencies in a population remain constant or are in equilibrium from generation to generation unless specific disturbing influences are introduced. Those disturbing influences include

Genetic equilibrium is a basic principle of population genetics.

Genetic Drift• If a population is small, Hardy-Weinberg principal may be violated.

• Chance alone may eliminate certain members out of the population. In such cases, the frequency of an allele may begin to drift toward a certain trait or set of traits.

• Ultimately, the allele may represent 100% of the gene pool or, just as likely, disappear from it.