Evolution

What is Evolution?

The theory that organisms gradually and slowly change over time into new species. Results from natural selection

acting on genetic variation present among individuals of a species

Sources of variation: mutations and genetic recombination (mutations can be good, bad, or indifferent)

Example of Evolution: Pesticide resistance

In the 1960s DDT was used to kill mosquitoes (to stop the spread of malaria). At first, the results were good, but soon DDT use had to stop. Why?

The pesticide killed most insects, but a few survived. These survivors are mosquitoes with genes that somehow enable them to resist the chemical.

These survivors then reproduce and may pass on their pesticide resistant genes to their offspring.

In each generation, the number of pesticide resistant insects increase, making the pesticide spraying less effective.

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)

Theories of Evolution continued…

Charles Darwin (1859-British) Voyage on H.M.S. Beagle

(1831-1836) to the Galapagos Islands; 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

Theories of Evolution continued…

Darwin collected several type of finches on the islands. He noted that the birds beaks were adapted to the type of food they ate. Darwin hypothesized that the finches strayed from the mainland and then diversified on the different islands.

While on the voyage he read Principles of Geology (Charles Lyell) which discusses the gradual changes the earth goes through over time (ex. mountain building)

Through his research Darwin composed his theory of evolution (1840s). He delayed publication, however, fearing the public outcry that would occur against his ideas.

Darwin’s Theory of Natural Selection

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 of Natural Selection

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).

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 time

1. 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.

2. 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

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 of reproductive barriers 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 bodystructure for fast swimming

How Fast are Species Created?

Two models: Gradualism – Species diverge from a common ancestor

gradually and slowly Punctuated equilibrium – A species undergoes most of its

modifications early in its existence (punctuated). Little change occurs after that (equilibrium). Paleontologist have observed in the fossil record that

most species appear in new forms relatively suddenly (in geologic terms), then change little during their time on earth, they then disappear from the fossil record.

Ex. A certain species survives 5 million years on earth. Most of its modifications occur during the first 50,000 years or so and then little change occurs during the rest of its existence. (Gradual changes during the first 50,000 years, but in geologic time 50,000 years is short/abrupt)

Evidence for Evolution

Fossils-reveal changes in the characteristics of organisms Reveals the appearance of organisms in a historical sequence

Oldest know fossils are prokaryotes (3.5 bya)

Paleontologist = scientist who studies fossils

Evidence For Evolution

Comparative anatomy-shows that organisms have common ancestors Homologous

structures-characteristics that are similar because they were inherited from a common ancestor

Evidence for Evolution Comparative Anatomy

Analogous structures-characteristics that are similar in purpose, because the organisms share a common role in the environment, but are not inherited from a common ancestor(example: wings of bat and wings of insects)

Homology vs Analogous

Evidence for Evolution

Vestigial structures-structures that are inherited but unused. These structures were fully developed and functional in some ancestor (example: snakes-leg

and pelvic bones; horse-reduced toe; humans-appendix)

Evidence for Evolution continued…

Comparative embryology-characteristics among different species are displayed only in embryonic development and are lost in adults example: All vertebrates

have gill pouches on the sides of the throat during embryonic development (fishes, frogs, snakes, birds, apes, humans, etc…)

Evidence for Evolution continued…

Comparative biochemistry-universal DNA!! The four bases that make up DNA are the same in all

species. The more similar the bases, the more related the organisms are. (The DNA of humans is 98% similar to that of Bonobos.) The greater number of sequences that match, the more recent the common ancestor.

The codon chart applies to all living things… the triplet bases code for the same amino acids in all living things

(the process of protein synthesis is almost identical in all eukaryotes)

More Evidence for Evolution

All forms of life use RNA All living things depend on water All life is based on five elements –

carbon, hydrogen, nitrogen, oxygen, and phosphorus. These elements, along with small amounts of others, make up the organic compounds common to all cells: lipids, carbohydrates, proteins, and nucleic acids