The Record of Life
*Everything changes over time. The Earth has changed dramatically during the estimated 4.6 billion years it has existed.
– All of the biodiversity present on Earth today is a result of the evolution of unicellular organisms that lived billions of years ago.
– Evolution: the change in populations over time
– the idea that modern organisms have descended from ancient organisms
– a natural change in the relative frequencies of alleles in a population’s gene pool
– theory: a well-supported and testable explanation
Fossils- Clues to the Past
*The Earth’s rocks are like a journal; they provide information about the diversity of life that has existed on the planet, and the change it has undergone.
– Paleontologist: scientists who study ancient life
– Fossil: evidence of an organism that lived long ago
– Most fossils are formed when the remains of plants or animals become buried under layers of sediment, these layers compress
over time, and their bodies either leave an impression on the hardened sediment, or become preserved within the rock itself. (sedimentary rock)
– By studying the location, condition, and position of fossils, scientists can make deductions about:
o past geography. (where the continents and oceans were located)
o past climates. (if it was hot, cold, humid, arid, etc.)
o past environments. (if it was a lake, desert, forest, grassland, etc.)
– Few organisms become fossilized because bacteria and fungi (decomposers) usually decompose the body before it is preserved, therefore, the fossil record is incomplete.
The Age of a Fossil
*The fossils in different layers of sedimentary rock vary in age. Scientists use 2 main methods to determine the age of fossils.
- Relative Dating: determining the age of fossils by observing where the fossil was found within the sedimentary rocks in which it lay (p.380)
o Follows the Law of Superpostion wh/states that the layers of rock near the Earth’s surface are younger than those in deeper layers.
o Scientists can use this to determine the order of appearance and extinction of species over time:
– episodic speciation: when in Earth’s timeline certain species originated
– mass extinctions: when in Earth’s timeline entire groups of organisms disappeared from the fossil record almost at once!
o It can NOT determine the actual age of a fossil→ only the order that it appeared in the fossil record.
o To find the specific ages of fossils, scientists use…
- Radiometric Dating: determining the age of fossils by comparing the amount of radioactive isotope found in the rocks to the new element into which it decays
o Radioactive isotopes: atoms that break down, or decay, over time, giving off radiation.
§ Each isotope forms a new element after it decays. (Ex.- Carbon-14 → Carbon-12)
§ Every isotope has a characteristic decay rate (half-life), wh/can be used as a type of clock!
-Ex.- C-14 decays to half its original amount in 5730 years (50% C-14 + 50% C-12).
§ Ex.- Let’s say a rock contains a radioactive isotope that decays to half of its original amount in 1 million years. When this rock formed, it was made up of 100% of the radioactive isotope, and 0% of the new element that it decays into. If you find this rock and analyze it, and find that it contains only 50% of the radioactive isotope and 50% of the new element, how old must this rock be?
- 1 million years old – 1 half-life – half of the radioactive isotope is gone and has decayed into the new element
*By examining layers of sedimentary rock and dating the fossils that are found within them, scientists have been able to put together a calendar of Earth’s history known as The Geologic Time Scale.
– If the Geologic Time Scale (4.6 billion years of Earth’s existence) were scaled down to one calendar year…
o Earth formed = January 1st
o Unicellular life 1st appears = March 20th
o Multicellular organisms fill the oceans = Oct.15th
o 1st mammals appear on land = Dec. 10th
o 1st birds appear in the air = Dec. 15th
o Modern man 1st appears = the evening of Dec. 31st
*We’ve only just arrived…
The Theory of Evolution
*Recall that evolution is the change in populations over time. But why do organisms change? Where do these changes come from? What causes genetic variety?
1. mutations: a random change in the DNA sequence; can be passed on to offspring
2. sexual reproduction: the shuffling of genes during meiosis and the random fertilization of gametes; crossing over also increases variation
*The differences that appear that help organisms survive and reproduce become more common, while differences that aren’t as beneficial become less common.
– The greater the variation amongst a species, the greater the likelihood that at least some of the members of a species will survive under changed environmental conditions. (Ex.- p.407- Peppered Moths)
Charles Darwin and Natural Selection
– Charles Darwin (1809-1882): and English scientist; The 1st person to propose
the theory of evolution by natural selection
– In 1831, at age 21, joined the crew of the HMS Beagle as the ship’s
– The ship sailed on a 5-year route from Europe to S. America to the S.
Pacific to the Indian Ocean, back to Europe.
– During his travels, he studied and collected evidence at every port along
the route that led him to propose a revolutionary hypothesis about the way life changes over time!!
– This is the Theory of Evolution– a scientific explanation for the diversity of life on this planet
– What exactly did he observe on his journey?
o He collected fossils and noticed some looked like today’s creatures, and some looked totally different. . . what happened??
o On the Galapagos Islands, he noticed that the tortoises (shell shape/size), birds (beak shape/ size), and plants all varied from isle to isle.. why??
o He noticed that organisms seemed perfectly suited to live in their particular environments.
o This all led Darwin to think… Had these organisms once been members of the same species that had changed or evolved over time after becoming isolated from each other??
Darwin Continues His Studies
*Several other scientists’ work helped to shape Darwin’s thinking. . .
– James Hutton’s Theory of Geological Change: Explained that natural forces formed and shaped the Earth (wind, rain, plate movement, etc.) and that these processes take millions of years!!
o The Earth must be MUCH older than people thought→ millions instead of thousands of yrs.old
– Charles Lyell’s Principles of Geology:
Explained that the processes that formed and shaped the Earth in the past are continuing to change the Earth in the present!!
o These 2 scientists got Darwin thinking: ‘If the Earth can change, what about life? If the processes that change Earth take millions of years, could life, if given millions of years, also change and be continually changing?’
– Thomas Malthus’ Theory on Human Population Growth: Explained that if the human population continued to grow unchecked, sooner or later there would be insufficient living space and food for everyone!!
o Darwin realized this applied more to plants other animals, because they reproduce quicker and have more offspring.
o So why hadn’t the Earth been overrun by these organisms? Why did so many die?? Which individuals survive???
Darwin Explains Natural Selection
*In 1859, Darwin published his book- ‘On the Origin of Species by Natural Selection’.
– He pointed out that organisms display natural variation: diff.s of a species that can be inherited
– He explained how humans ourselves had been breeding organisms in order to obtain variations that were desirable to us. (selective breeding)
o He called this artificial selection.
– He explained that natural selection was similar to artificial selection, only in natural selection: organisms w/certain variations are better able to survive, reproduce, and pass these variations on to the next generation, and thus, these variations accumulate within a population, ultimately resulting in the change/evolution of the population.
o Organisms w/o these variations are less likely to survive and reproduce, thus eliminating their traits from the population.
– This book disproved that species were fixed and unchanging, and explained that natural selection had been taking place for millions of years and continues today!
– Natural selection works b/c there are many struggles for survival: organisms are constantly competing for food, space, mates, shelter, escape from predators, etc.
– Natural selection is also known as Survival of the Fittest: those better suited (fit) for their environment survive; those that are not suited (not fit) die and do not reproduce
o Fitness is the result of the accumulation of adaptations: any inherited characteristic that increases an organisms chance for survival in its environment
– Ex.- The birds beaks on the Galapagos Islands were adapted for their particular food source! – Long beaks = get insects out of trees; the island they live on has lots of trees w/insects!
– Short beaks = good at cracking open nuts; the island they live on has lots of nuts and seeds.
Structural Adaptations Arise Over Time
*Advantageous adaptations are how organisms survive the struggle for existence. Examples??
– smarter, faster, more aggressive, more attractive, better protection, better senses, better shaped limbs, mimicry, camouflage, etc.
– Mimicry: when a harmless species phenotype resembles that of a harmful species (p.406)
o Predators that know to avoid the harmful species will also avoid the harmless species b/c they look alike, thus enabling it to survive and reproduce!
– Ex.- yellow jacket wasp and a syrphid fly
– Ex.- Monarch butterfly and Viceroy butterfly
– Camouflage: when a species phenotype blends in with that of its surroundings (p.406)
o Predators can not easily find the organism, thus enabling it to survive and reproduce!
– Ex.- chameleon (changes color to blend in w/its surroundings)
– Ex.- a walking stick (looks like the stick of a tree, in which it lives)
*Besides developing structural adaptations, organisms can also develop physiological
adaptions: changes in an organism’s metabolic (internal) processes
– Ex.- Bacteria have evolved to prevent being killed by certain types of medications, like penicillin!
– Ex.- Many species of plants/insects are now resistant to pesticides/insecticides which used to kill them!
o These examples are direct evidence for evolution.
Other Evidence for Evolution
*Darwin argued that living things have been evolving on Earth for millions of years. Evidence for this has been found in the fossil record (Chp.14), as well as in the anatomy, embryology, and biochemistry of organisms.
- Anatomy: the basic arrangement of the bones in each limb is similar (p.409)
– Homologous Structures: structural features with a common evolutionary origin
– Ex.- a bat wing is homologous to a human arm
– Analogous Structures: structural features that are similar in function, but do NOT have a common evolutionary origin
– Ex.- a bat wing is analogous to a butterfly wing
– If they are not related, then why are they similar? Because these animals have adapted to similar ways of life!
– Vestigial Structures: a body structure that has no function in a present-day organism but was probably useful to an ancestor
– Does NOT affect fitness, so is not eliminated
– Ex.- appendix; goose bumps; tail; etc.
- Embryology: embryos look similar in the early development of many different organisms (p.410)
– suggests evolution from a common ancestor
- Biochemistry: the DNA and RNA sequences of many different species has been compared and found to be very similar
– Used to determine the evolutionary relationships of species. (The more similar the DNA = the more closely related the two species)
*All of this evidence can also be calibrated against each other to help estimate how long ago various groups of organisms diverged evolutionarily from one another!
Mechanisms of Evolution
*Individuals do not evolve- populations do! Evolution occurs as a population’s genes and their frequencies change over time.
– population: all of the members of the same species that live in a given area (share a common gene pool)
– gene pool: all of the different allele’s in a populations genes; the combined genetic info. of all of the members of a population
– allelic frequency (relative frequency): the % of any given allele in the gene pool; the # of times an allele (T) occurs in a gene pool compared w/the # of times other alleles (t) occur
Natural Selection Acts on Variations
*Natural selection does not act directly on genes. In stead, it acts on phenotypes (physical, behavioral, biochemical characteristics).
– Even though natural selection does not operate directly on genes, it can change the relative frequencies of alleles in a population over time.
– Evolution can be defined in genetic terms as any change in the relative frequencies of alleles in a population’s gene pool.
There are 3 different types of natural selection that act on variation: stabilizing, directional, and disruptive.
- stabilizing selection: natural selection that favors average individuals in a population
– individuals near the center of the curve have higher fitness than individuals at either end
– the overall bell curve narrows
- directional selection: natural selection that favors one of the extreme variations of a trait
– individuals at one end of the
curve have higher fitness than
individuals in the middle or at the
– increases the # of individuals w/the trait at one end
- disruptive selection: natural selection that favors both of the extreme variations of a trait
– individuals at both ends of the
curve have higher fitness than
those near the middle
– increases the # of individuals w/the trait on both ends; acts against the
average individuals/intermediate trait
*In small populations, an allele can become more or less common simply by chance (leave more offspring; killed off by accident; migration of individuals; etc.)
– Genetic Drift: the random change in allelic frequencies of a population
o changes the diversity of organisms in a population
The Evolution of Species
*So we’ve learned how mutations, sexual reproduction, natural selection, and genetic drift can change a population’s gene pool over time. But how do these changes lead to the evolution of a new species?
– speciation: the evolution of new species
o occurs when members of similar populations no longer interbreed to produce fertile offspring→ this is known as reproductive isolation. It can develop in 3 ways:
- geographic isolation: occurs whenever a physical barrier divides a population
– Ex.- mountains, rivers, oceans, lava flow, etc.
– Nat. sel. now works separately on each group
– Genetic changes that appear in one group are not passed on to the other group
- behavioral isolation: occurs when organisms are capable of interbreeding, but have differences in courtship rituals or other behaviors (diff. songs to attract mates, etc.)
- temporal isolation: occurs when organisms reproduce at different times
– Ex.- flowers release pollen at diff. times of day
– Ex.- certain frogs mate in the fall, other frogs mate in the summer
Patterns of Evolution- Speciation in Darwin’s Finches:
*Darwin discovered 13 different species of finches on the Galapagos Islands. Speciation of these finches occurred in six steps: (p.420)
- Founders arrive– A few finches from South America came to the islands.
- Separation of Populations– Finches moved to different islands.. essentially isolated from each other and no longer shared a common gene pool.
- Changes in the Gene Pool– Over time, populations on each island became adapted to their local environments. (Natural selection would have formed different populations.)
- Reproductive Isolation– Finches choose their mates carefully.. differences in beaks and mating behavior lead to reproductive isolation.
- Ecological Competition– Species evolve in ways that increases the differences in each bird population. (Ex.- Shape of beaks adapted to eat different foods in order to avoid competition.)
- Continued Evolution– After many generations, 13 species of finches have evolved.
– Darwin’s finches display a type of evolution known as adaptive radiation: when an ancestral species evolves into an array of species to fit a number of diverse habitats
– Adaptive radiation is a type of divergent evolution: the pattern of evolution in which species that once were similar to an ancestral species diverge or become increasingly distinct
– Divergent evolution is the opposite of convergent evolution: the pattern of evolution in which unrelated species evolve similar traits due to similar environmental pressures (Ex.- insects, bats, and birds)
Evolution vs. Genetic Equilibrium
– Genetic equilibrium: refers to a population in which the frequency of alleles remains the same over time
– A population that is in genetic equilibrium is NOT evolving→ because allele frequencies remain the same, phenotypes remain the same, too.
– The Hardy-Weinberg Principle states that allele frequencies in a stable population will remain constant, unless one or more factors cause those frequencies to change.
– In order to maintain a stable population (genetic equilibrium) there are 5 factors that must be met.
– 5 conditions required to maintain genetic equilibrium:
- random mating: all members of a pop. must have equal opportunity to produce offspring.. In natural populations mating is rarely completely random.
- large populations: genetic drift has less of an effect on large populations
- no movement into or out of the population: the population’s gene pools must be kept together and kept separate from other populations
- no mutations: if genes mutate, new alleles may be introduced into the population, and the allele frequencies will change
- no natural selection: no phenotype can have a selective advantage over another
*These 5 conditions are rarely all met in nature, thus, most populations are continually evolving.