Evolution
What accounts for diversity of life?
1. Evolution—change over time; a process by which modern organisms have descended from ancient
organisms.
2. Components of genetic code common to all organisms
a. DNA bases (ATGC)
Converting glucose to ATP for energy
Theories of Evolution
1. Scientific Theory – a well supported testable explanation of natural phenomena that is subject to
change with new research and as new technologies are available
Ex: gravity, plate tectonics
2. Lamarck—Theory of Acquired Characteristics
a. By selective use or disuse of organs, organisms acquired or lost certain traits during their lifetime;
these traits could then be passed on to their offspring.
Ex: Giraffe could not reach the leaves so it stretched its neck so that all of its offspring had long necks, too.
b. Problem: Did not know how traits were inherited—GENETICS (meiosis!!!)
3. Darwin – Evolution by Natural Selection
a. Over time natural selection results in changes in the inherited traits of a population
Darwin’s Influences
1. Made observations in Galapagos Island as a naturalist (traveled on a boat called HMS Beagle)
a. Observations made:
i. Similar species well suited to the environment they inhabited (lived in).
ii. Traits of similar species varied noticeably among different islands of the Galapagos
b. Example: Noted similarities and differences in tortoises
and finches on Galapagos Islands in South America.
c. Hypothesized that animals adapted to local
conditions on islands after their arrival.
2. Hutton and Lyell (studied geology)
a. Helped scientists recognize that Earth is many millions of years old; older than anyone believed.
3. Malthus (studied human populations)
a. If the human population continued to grow unchecked, sooner or later there would be insufficient
living space and food for everyone creating a struggle to survive
4. Farmers and Artificial Selection—Nature provided the variation among organisms and humans
selected for the variations they found useful
Example: Farmer breeds largest hogs for most meat production.
Darwin’s Theory of Natural Selection
1. Struggle for Existence—more offspring are produced than can survive; therefore, members of each
species compete regularly to obtain food, living space, and other necessities of life.
2. Survival of the Fittest (also called Natural Selection)—individuals with an adaptation (genetic
difference) that makes them more “fit” for a certain environment will survive and reproduce; those
not suited will die or produce few offspring
3. Descent with Modification—each living species has descended, with changes, from a common
ancestor (phylogeny).
4. Darwin’s conclusions are published in a book called “On the Origin of Species”.
Evidence of Evolution:
(6 different types)
1. Fossil record—examples of many species that have lived and then became extinct
2. Geographical Distribution of living species( Biogeography)—when looking at similar environments on
different continents, different animals had similar anatomies and behavior because of adaptation.
3. Homologous body structures—structures that have different mature forms but develop from the
same embryonic tissues; provide strong evidence that all four-limbed vertebrates have descended, with
modifications, from common ancestors
.
4.Anatomical similarities like Vestigial organs—organs with functions that are useless in current
organisms, but functional in distant ancestors.
a. Ex. Existing leg bones in pythons, hip bones in whales.
5. Similarities in embryology—early stages, or embryos, of many animals with backbones are very
similar
6. Similarities in genes / amino acids- having similar genes is the best way of determining similarity
between organisms
Evolution of Populations – Change over time occurs in a population not in just one individual
1. Populations—all individuals of a species that live together in one area
2. Gene pool—consists of all genes, including all the different alleles that are present in a population
Sources of genetic variation
(Where do the differences come from ?)
* genetic variation is important for natural selection to occur. WHY?
1. Mutations—change in a sequence of DNA
2. Gene shuffling—occurs during the independent assortment of chromosomes and crossing over of
genes during meiosis
3. Genetic drift – a random change in allele frequency. Certain individuals may leave more
descendants than others, and over time this can cause an allele to become common in the
population. Not caused by natural selection, but by chance.
How new species form (Speciation)
1. Species determined by organisms that reproduce naturally only with each other
2. Speciation occurs when a population has been isolated for an extended period of time. They become
reproductively isolated from each other.
a. Behavioral Isolation—two populations are capable of interbreeding but have differences in
courtship rituals or other reproductive strategies that involve beha
b. Geographic Isolation—two populations are separated by geographic barriers such as rivers,
mountains, or bodies of water
c. Temporal Isolation—two or more species reproduce at different times
Phylogeny
1.Phylogeny is the evolutionary history of a species
2. Cladistics: a biological system of classification based on phylogeny
3.As groups of organisms diverge and evolve from a common ancestor, they keep some of the same traits
Ex. dogs and wolves both have 4 legs
4.Cladogram: branching diagram that show the derived traits of a group of organisms (like a pedigree
that shows evolution)
5.The closer two organisms are in a cladogram, the more probable that they are closely related by
evolution.
How to read a Cladogram ?
1.The beginning of the “tree” represents a common ancestor.
2.When a line splits, a new species (speciation) occurs.
3. Traits listed on the bottom of some cladograms are shared by every organism to the right of the trait.
Ex. on the cladogram from previous page… every organism to the right of “lungs” has lungs
Misconceptions about Humans:
It is important to remember that:
Humans did not evolve from chimpanzees. Humans and chimpanzees are evolutionary “cousins” and
share a recent common ancestor that was neither chimpanzee nor human.