Charles Darwin and Charles Darwin and Natural SelectionNatural Selection

Darwin journeyed on the HMS Beagle as a naturalist

• 5 year journey• studied and collected many biological specimens• on Galapagos Islands, off coast of Ecuador, observed

animals such as finches, tortoises, and iguanas • Thirteen different but similar species of finches, each

with a distinctive bill that is specialized for a particular food source.

• Suggested that these birds migrated from Ecuador

and changed after they arrived.

Darwin’s ideas were influenced by:

• Jean Baptiste Lamarck, who hypothesized that acquired traits were passed onto offspring

•Charles Lyell, a geologist, who suggested that the Earth was much older than 6000 yrs•Thomas Malthus, who wrote that human populations grow much faster than their food supply

•Alfred Wallace, who suggested natural selection after studying wildlife in the Malay Archipelago.

Darwin observed differences among island species.

Marine iguana

Land iguana

Thirteen different but similar species of finches, each with a

distinctive bill that is specialized for a particular food.

Suggested that these birds migrated from South America and changed after they arrived

Key insights led to Darwin’s idea for natural

selection. • Darwin noticed a lot of variation in

domesticated plants and animals.

• Artificial selection is the process by which humans select traits through breeding.

• Natural selection is a mechanism by which individuals that have inherited beneficial adaptations produce more offspring on average than do other individuals.

• Heritability is the ability of a trait to be passed down.

• There is a struggle for survival due to overpopulation and limited resources.

• Darwin proposed that adaptations arose over many generations.

Principles of Natural Selection1. Variation. What can cause variation in a

population?

• Genetic differences and mutation

2. Overproduction. What are pros and cons of overproduction?

• Having many offspring increases the chance for survival, but also results in competition for resources.

3. Adaptation. What determines whether an adaptation is beneficial or not?

• A certain variation that allows an individual to survive better than other individuals it competes against.

4. Descent with Modification. How does natural selection change a population over time?

• Over time, more members of the species will have adaptations that are well suited for survival and reproduction in an environment.

Elephants in Queen Elizabeth National Park, Uganda, Africa

Normally, nearly all African elephants, male and female, have tusks. In 1930, only one percent of the elephant population in Queen Elizabeth Park was tuskless because of a rare genetic mutation. Food was plentiful, and by 1963 there were 3,500 elephants in the park.

In the 1970’s, a civil war began in Uganda. Much of the wildlife was killed for food, and poachers killed elephants for their ivory tusks. By 1992, the elephant population had dropped to about 200. But by 1998, the population had increased to 1,200. A survey revealed that as many as 30 percent of the adult elephants did not have tusks. Ugandan wildlife officials also noted a decline in poaching.

Natural selection acts on distributions of traits. • A normal distribution graphs as a bell-shaped curve.

• Populations have a normal distribution when they are not undergoing natural selection

• Microevolution is evolution within a population.– observable change in the allele frequencies – can result from natural selection

Directional selection – favors one of the extreme variations

• Woodpeckers with long beaks capture the most insects, as they can reach the insects deep in the tree trunk.

• Stabilizing selection – favors the average

• Small spiders have a hard time capturing prey

• Large spiders easily spotted by birds

• Medium sized spiders are best suited to survive in their environment, reproduce more often, leave more offspring.

Disruptive selection - favors both extremes

• On light colored rocks, the light limpets are camouflaged and survive the best

• On dark rocks, the dark limpets are most successful

• Tan (intermediate) limpets are visible on both the light rocks and dark rocks, and their numbers decline due to predation

Evidence of Evolution

A. Fossils

Fossil links found between

• fish and amphibians

• reptiles and birds

• reptiles and mammals

Whales from land mammals

Fossil linking fish and amphibians

• 365 million years old

• arm bone with fish fin characteristics

• found in Pennsylvania

• thought to be from a lobed-finned fish

Archaeopteryx – links reptiles and birds

                                                                                                                                         

A fossil of Archaeopteryx was discovered at about the same time Darwin published On the Origin of Species. This pigeon-size creature had a dinosaur like shape, complete with a long bony tail, heavy jaws with serrated teeth, and three long fingers. It also had feathers like those of modern birds.

Hind leg bones in whales

An amphibious reptile found in Texas, 2005

Diarthognathus, an animal with reptile and mammal characteristics

Early mammals may have looked like this

Evolution of the horse

B. Geography– island species most closely resemble nearest mainland

species– populations can show variation from one island to

another

Larva

Adult barnacleAdult crab

– Crabs and barnacles with similar larvae, different adult body forms

C. Embryology

• Early in development, vertebrate embryos have similar characteristics such as a tail, buds that become limbs, and pharyngeal pouches that hold the gills of fish and amphibians.

Vertebrate embryos

D. Anatomy•Homologous structures – similar in structure, with different functions

• Vestigial Structures

• Structures that are reduced in size and either have no use or a less important use than they do in other, related organisms.

• Examples: wings on flightless birds, Human ear muscles, human wisdom teeth human appendix , hind leg bones in whales

The cassowary, a flightless bird with wings

Wisdom teeth in human

Human appendix

Human hand

Bat wing

Mole foot

Fly wing

– Analogous structures are not evidence of a common ancestor.

– Analogous structures have a similar function.

Evolutionary Biology Today• DNA sequence analysis: two closely-related organisms

will have similar DNA

• Homeobox genes indicate a very distant common ancestor.– control the development of specific structures– found in many organisms

Homeobox genes

1. Natural Selection: certain traits might be an advantage for survival

2. Mutation: creates new genetic variation

3. Sexual selection: certain traits may improve mating success; alleles for these traits increase in frequency

Mechanisms of Evolution

Sexual selection occurs when certain traits increase mating success.

• Sexual selection occurs due to higher cost of reproduction for females.– males produce many

sperm continuously– females are more

limited in potential offspring each cycle

Sexual dimorphism

Mating systems

Monogamy One male mates exclusively with one female

Polygamy Individuals mate with more than one partner

Polygyny Some males mate with more than one female

Polyandry Some females mate with more than one male

Monogamy Polygyny Promiscuity

Promiscuity Males mate with more than 1 female and vice versa

Harem polygyny

Elephant seals breeding females cluster together on beaches

- allows males to defend a harem of many females at once

Male elephant seals engagein violent, bloody fights overfemales – large size provides an advantage in male combat

Male elephant seals weigh up to 3x more than females!

Male reproductive success is highly variable:

8 individual males inseminated 348 females in one study!

Alternative reproductive tactics

Remember…there may not be a “best” way to be a male (or female)

The Red Queen

Evolutionary arms-races

The Red Queen: “It takes all the running you can do, to keep in the same place.”--Lewis Carroll, “Through the Looking Glass”

The Red Queen Hypothesis: “For an evolutionary system, continuing development is needed just in order to maintain its fitness relative to the systems it is co-evolving with.” --Leigh Van Valen (1973)

4. Gene flow: movement of individuals to or from a population (also known as migration). Immigrants add alleles, emigrants take alleles away.

Example: troops of baboons in eastern Africa. Females remain with the troop, but younger or less dominant males leave their birth troop, eventually joining another troop. This ensures gene flow.

5. Genetic drift is a change in allele frequencies due to chance.

• Genetic drift causes a loss of genetic diversity.• It is most common in small populations.

A) A population bottleneck can lead to genetic drift. – It occurs when an event

drastically reducespopulation size.

– The bottleneck effect isgenetic drift that occursafter a bottleneck event.

Example: In the 1800’s, northern elephant seals were overhunted. The population was reduced to about 20 individuals. Hunting has ended, and there are now about 100,000 seals. However, the population has little genetic variation.

B) The founder effect is genetic drift that occurs after start of new population

–It occurs when a few individuals start a new population.

What is speciation?

The rise of two or more species from one existing species

What causes speciation?

Types of reproductive isolation

• Geographic isolation

2. Temporal isolation

• timing of reproductive periods prevents mating

3. Behavioral isolation

• includes differences in courtship or mating behaviors

Example: female fireflies only respond to light patterns of males of their own species

Satin bowerbird

and McGregor’s

bowerbird

• What process keeps the number of total species on Earth from growing exponentially through speciation?

Draw a visual for divergent evolution and one for convergent evolution.

Give an example of each. (convergent and divergent)

• Which type leads to analogous structures?

Convergent or divergent?

Convergent or divergent?

Polar bears and grizzly bears?

• Convergent or divergent?

(Adaptive radiation)

What is coevolution?

Evolutionary Arms Race

True or false:

When sexual selection is acting in a species, all males tend to look alike.

• There are two groups of pine trees that appear to be very similar phenotypically and genotypically. However, one releases pollen in January, when the female structures of that group are receptive, and one in March. What kind of reproductive barrier is this?

Microevolution• a change in gene frequency in a population —

such as all the individuals of one beetle species living on a particular mountaintop.

Industrial Melanism

• Example is the peppered moth.• Peppered Moth Simulation

Macroevolution

• generally refers to evolution above the species level

• Macroevolution