The Theory of Evolution by Natural Selection

Evolution of Evolutionary thinking (Pre-Darwinian)

Jean-Baptiste Lamarck

(1744-1829) – French naturalist, proposed a theory that organisms were driven by some inner force toward greater complexity. But thought that org. could pass on traits to their offspring that they acquired during their lives. (“Lamarckism”, proposed in 1809)

Lamarckism

Lamarck based his theory on two observations thought to be true in his day: 1.

“Use it or lose it” - Individuals lose characteristics they do not require and develop those which are useful. 2.

Inheritance of acquired traits - Individuals inherit the acquired traits of their ancestors.

Lamarckism

 Examples include: the stretching by giraffes to reach leaves leads to offspring with longer necks;  Strengthening of muscles in a blacksmith's arm leads to sons with like muscular development. This theory was later disproved!

Darwin’s Voyage

 Charles Darwin   Set sail on the HMS Beagle in 1831 Became the ship’s naturalist  Arrived in the Galapagos Islands in 1835  Observed that the animals on the islands were similar to those on the mainland

Darwin’s Voyage

 Galapagos Animals  The Galapagos animals, while similar, were also different from island to island as well as to the mainland   Most obvious difference were the sizes and shapes of the finches’ (small birds) beaks Sizes and shapes of the beaks were adapted to what the birds ate

Galápagos Finches

Insect eaters Berry eater Seed eaters Cactus eaters

 Beak shape varies depending on diet

Darwin’s Voyage

 On the Origin of Species (Darwin’s book)  For the 20 years that followed his return to England Darwin studied plants, animals and adaptations  Darwin wrote about how species can change gradually over many, many generations and become better adapted to new environmental conditions.

Evolution

 The gradual change in a species over time.

Natural Selection

Organisms that are better

adapted

to an environment are more likely to survive and reproduce than organisms that are less well adapted.

Adaptations

Katydids have camouflage to look like leaves. Non-poisonous king snakes mimic poisonous coral snakes.

Factors that affect Natural Selection:

Overproduction

most species produce far more offspring than will/can survive

Overproducers

mature rapidly short-lived: most die before they reproduce

Producers

mature slowly live long lives: low juvenile mortality rate have many offspring - tend to overproduce have few offspring at a time invest little in individual youngsters population not regulated by density: boom and bust population figures care for their young population stabilizes near carrying capacity

Factors that affect Natural Selection:

Competition:

since food and resources are limited, the offspring have to compete to survive Darwin called it: “Struggle for existence”

Factors that affect Natural Selection:

Variation:

Members within a species exhibit individual differences – these differences must be inheritable Natural selection won’t work in a population of clones! Remember that a key to variation is sexual reproduction.

Factors that affect Natural Selection:

Survival to reproduce:

Only those individuals that are better suited to the environment will survive and reproduce (“Survival of the fittest”).

Fit individuals pass on to a portion of their offspring the advantageous characteristics.

How do new species form?

 Natural Selection  Continental Drift  Changes in environment  Mutations  Man

Continental Drift

 Fossil records show that when the continents were connected animals walked across. When the continents separated, the animals were separated.

Changes in the Environment

Example, the pepper moth.

Originally, the pepper moth was white, which was good because it could blend in.

Then, trains were invented and the soot they produced covered the trees. Making the trees black. The moths that were black could now survive better.

Mutations

Some species are more susceptible to mutations. Some mutations allow the animal to survive; other mutations do not allow the animal to survive. The mutations that are not decrease the chance of survival remain.

Man: Artificial Selection

 Selective breeding as practiced by humans on domesticated plants and animals….

 For example: Dogs

Evidence of Evolution

 Fossil Record  Similarities in Body Structure  Similarities in Early Development  Vestigial Structures  Similarities in DNA

Fossil Record

 How fossils form  An organism dies and becomes buried in sediment  Minerals gradually replace the bones and more sediments cover the fossil Pterodactyl Trilobite

Similarities in Body Structure

If the two organisms have body structures that are similar, they must have had a common ancestor .

Similarities in Early Development

 Scientists look at embryos of different organisms and find that many embryos resemble one another.

Vestigial Structures

 Vestigial Structures A structure found in an organism that is no longer in use but may have been useful at some point in the organism's life. Tail Present in human and all vertebrate embryos. In humans, the tail is reduced; most adults only have three to five tiny tail bones and, occasionally, a trace of a tail-extending muscle.

Whales possess a femur and pelvis, but these bones are no longer useful to the mammals.

Vestigial Structures

Why do dogs have tiny, functionless toes on their feet (dewclaws)?

Ancestral dogs had five toes on each foot

Similarities in DNA

 The more similar the sequences of DNA are, the more closely related the organisms are.

 Humans and chimpanzees DNA is more similar than human DNA is to dog DNA.

Patterns of Evolution

 Divergence  Adaptive radiation  Convergence

Divergent Evolution

 New species develop traits that differentiate them from their ancestors  Divergence accounts for descendants that differ from their ancestors and from one another

Convergent Evolution

 Unrelated animals develop similar body forms to fill same niche

Adaptive radiation

   Mammals filled ecological niches vacated by dinosaurs Greatest mass extinction occurred millions of years before us- the dinosaurs More than 90% of all species died out – the animals that remained filled the gap

How we know what happened when  Radiometric Dating  Stratigraphy  Molecular clocks

Radiometric Dating

 Radioactive Dating (Absolute Dating)  Rocks that fossils are found near contain radioactive elements.

 The half-life of a radioactive element is the time it takes for half the atoms in an element to change into a stable element (carbon 14 into carbon 12)

Stratigraphy

Fossils from organisms that died longer ago are buried deeper in the sediment/rock than fossils from organisms that died more recently.

Molecular clocks

 When a stretch of DNA does indeed behave like a molecular clock, it becomes a powerful tool for estimating the dates of lineage splitting events.

The End (?)