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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