Evolution Notes

EVOLUTION

  • Quote by Charles Darwin: "It is not the strongest of the species that survive, nor the most intelligent, but the one most responsive to change."
  • Examples of horse evolution: Eohippus, Oligohippus, Merychippus, Pliohippus, Modern horse.

I. EVOLUTION

  • Darwin & Other scientists

THE DIVERSITY OF LIFE

  • Biological Diversity: Variety of living things.
  • Evolution: Change in allele frequency over time.
  • Process by which modern organisms have descended from ancient organisms.
  • Scientific Theory: A well-supported testable explanation of natural events; supported by evidence & can be tested with new evidence.
    • Ex: Theory of Gravity, plate tectonics – NOT A GUESS

IDEAS THAT SHAPED DARWIN’S THINKING

  • Several scientists influenced Darwin:
    • Hutton and Lyell: Stated Earth is many millions of years old (older than anyone believed). Geological processes change the Earth in the past and continue to shape it in the present.
    • Lamarck: Recognized that living things change. He proposed:
      1. Tendency toward perfection.
      2. Use and Disuse.
      3. Inheritance of acquired traits.
    • Cuvier: Found deeper and older fossils to be dissimilar; new species appear and old species disappear (leaves questions – how?)
    • Malthus: Stated that if the human population continued to grow unchecked, there would be insufficient food and space for everyone.
    • Wallace: Changes in populations over time are due to environmental pressures.

CHARLES DARWIN

  • Contributed more to our understanding of evolution than anyone.
  • Over time, natural selection results in changes in inherited traits of a population.
  • Voyage of the Beagle, 1831: Sailed around the world stopping at the Galapagos Islands.
  • He made observations & collected evidence (species, fossils, etc.).
  • He noticed many plants & animals were well suited to their environment.

DARWIN’S CASE

  • He was reluctant to publish his ideas because they were so radical.
  • He was disturbed by his own findings which challenged fundamental scientific beliefs.
  • When he heard a scientist named Wallace had the same idea as his, Darwin published his work: On the Origin of Species – 1859.
    • The book provided evidence that evolution has been taking place.
    • He stated that species vary from one another, in nature and on farm.
    • Artificial selection: "Selective breeding" – Humans select the traits they find useful.
    • Natural selection: A mechanism for evolution.

II. 5 Parts of Darwin’s Theory of Natural Selection

  • These are the concepts that are on the BACK of your “Evidences of Evolution” Chart
  • Genetic Variation
  • Overproduction of Offspring
  • Struggle for Existence
  • Survival of the Fittest
  • Descent with Modification

5 PARTS OF DARWIN’S THEORY OF NATURAL SELECTION

  1. Genetic Variation: Organisms differ; some variation is heritable (can be passed down).
  2. Overproduction of Offspring: Organisms produce more offspring than can survive.
  3. Struggle for Existence: Members of each species compete to obtain food, living space, etc.
    • Predators that are faster = more food.
    • Prey that are better camouflaged = better protection, etc.
  4. Survival of the Fittest:
    • Survival of the fittest can be used interchangeably with the term “natural selection” which explains that those who are most fit for their environment will survive and reproduce.
  5. Descent with Modification:
    • All species come from common ancestors.
    • They look different from ancestors because over long periods of time, natural selection produces organisms that have different structures, niches, or occupy different habitats- meaning that, they “descended with modification”.
    • It links all organisms on Earth into a single tree of life. This principle is known as “common ancestry”.

Important Terms

  • Fitness: The ability to survive & reproduce in a specific environment.
    • Fitness is the result of adaptations (physical and/or behavioral).
    • Think of this like a grade – the higher your fitness in a given environment, the more likely you are to be able to survive in that given environment.
  • Adaptation: An inherited characteristic that increases the organism’s chance for survival.
    • Only the fittest organisms survive, and get to pass on their traits; thus, the population will change over time.
  • Common Ancestry:
    • All species (extant and extinct) were derived from common ancestor.
    • A single tree of life links all living things.

III. EVIDENCE TO SUPPORT EVOLUTION

  • These are the same evidences as those in the articles we read in class.
  1. FOSSIL RECORD
    • Comparing fossils from older & younger rock layers documents the fact that evolution has occurred; it is a detailed record of evolution.
    • The age of a fossil is determined by comparing to others. The idea being that older fossils would be found deeper.
    • Relative Dating
  2. GEOGRAPHIC DISTRIBUTION OF LIVING SPECIES
    • Also known as biogeography
    • A given species will evolve differently in different environments along with different species evolve similarly in the same type of environment.
  3. ANATOMICAL STRUCTURES
    • A. Homologous structures of living organisms: Similarly structured/built body structures that don’t necessarily have the same functions
    • B. Analogous structures pertain to the various structures in different species having the same function but have evolved separately, thus do not share a recent common ancestor.
    • C. Vestigial Structures: Similar structures with reduced size and lesser function or no function
  4. SIMILARITIES IN EMBRYOLOGY
    • Similarities in early development: early stages, or embryos, of many animals are very similar- suggesting that organisms share common ancestry
  5. DNA/MOLECULAR EVIDENCE
    • Darwin didn’t have the benefit of DNA but we use it now.
    • Comparing macromolecules like DNA and protein allows us to compare extinct and extant species to trace their ancestry and determine species’ relatedness to others

IV. POPULATIONS

  • Genetic Variation
  • Why is it important in this unit?
  • Genetic variation is studied in populations.
    • Population: A group of individuals of the same species that interbreed; share a common group of genes called a gene pool.
    • Gene pool: All genes that are present in a population.
    • Relative gene (allele) Frequency: The # of times the allele occurs in a gene pool, compared with the # of times other alleles for the same gene occur; it’s often a %.
    • In genetic terms, evolution is any change in the relative frequency of alleles in a population.
  • What we already know:
    • Traits are controlled by genes.
    • Many genes have at least 2 forms or alleles.
    • Individuals of all species are heterozygous for many genes.

Genetic Variation: 2 main sources

  1. Mutations: Random change in DNA; phenotype is not always affected.
  2. Gene Shuffling/recombination: Occurs during gamete production; results in many different combinations due to independent assortment and crossing over.
    • *** Note- the relative frequency of alleles in a populations does not change during recombination
  • Monogenic traits: One gene controls the trait
  • Polygenic Traits: Controlled by two or more genes. Produce many different phenotypes that can be expressed in a bell-shaped curve.
  • How does it affect the population as a whole?
  • Natural Selection on single- gene traits can lead to changes in allele frequencies and thus to evolution.
    • *** Populations, not individual organisms, evolve over time!
  • How the Population evolves
  • It can affect the distribution of phenotypes in 3 ways:
    • Directional Selection: Individuals at one end of the curve have higher fitness than individuals in the middle or at the other end. The range of phenotypes shifts.
    • Stabilizing Selection: Higher fitness - in the middle of the curve. Average form of trait is "preferred". Curve narrows around the middle.
    • Disruptive Selection: When individuals at the upper and lower ends of the curve have higher fitness than the individuals near the middle. The curve has a low point in the middle.
  • How the Population evolves: Natural Selection on Polygenic traits is more complex
  • [DISRUPTIVE SELECTION IMAGE]
  • [STABILIZING SELECTION IMAGE]
  • [DIRECTIONAL SELECTION IMAGE]

OTHER MECHANISMS OF EVOLUTION

  • Gene flow is the transfer of alleles of genes from one population to another.
  • Migration into or out of a population may be responsible for a marked change in allele frequencies
  • Causes genetic variation.
  • Alleles of migrant organism become part of population’s gene pool if organism reproduces
  • What other ways can the population change?

OTHER MECHANISMS OF EVOLUTION

  • Genetic Drift- another source of evolutionary change
    1. Allele frequencies change.
    2. Occurs in small populations.
    3. Individuals with a particular allele leave more descendants than other individuals.
    4. Alleles become more or less common simply by chance, not selection
  • Examples:
    • Founder Effect: Allele frequencies change as a result of the migration of a small subgroup of a population – the cause of the change in frequency (gene pool) is CHANCE.
    • Bottleneck Effect: A sharp reduction in the size of a population due to environmental events or human activities. This changes the gene pool for future generations (by CHANCE, not survival of the fittest)

V. POPULATIONS NOT EVOLVING

HARDY-WEINBERG EQUILIBRIUM

  • A situation in which allele frequencies of a population remain constant and describe populations that are NOT evolving.
  • 5 conditions required to maintain genetic equilibrium:
    • Random mating.
    • Population must be very large.
    • There can be no movement into or out of the population (migration).
    • No mutations.
    • No natural selection
  • When all 5 are met, evolution will NOT occur.

The Hardy-Weinberg Principle

  • p2+2pq+q2=1p^2 + 2pq + q^2 = 1
  • frequency of homozygous dominant genotype
  • frequency of heterozygous genotype
  • frequency of homozygous recessive genotype

VI. SPECIATION

PROCESS OF SPECIATION

  • Speciation: Formation of new species

  • For 1 species to evolve into 2 new species:

    • Gene pools of 2 populations must become separated.
    • (When members of 2 populations cannot interbreed and produce fertile offspring- reproductive isolation has occurred.)

  • Reproductive Isolation

    • Geographical: Populations are separated by geographic barriers. Ex. Mountains, rivers.
    • Behavioral: Different courtship rituals. Ex. Songs.
    • Temporal: Populations reproduce at different times. Ex. Pollination.
    • Eastern meadowlark will not respond to Western meadowlark mating songs.

  • [DIAGRAM] results from which include produced by produced by produced by which result in which result in Reproductive Isolation Isolating mechanisms Behavioral isolation Temporal isolation Geographic isolation Behavioral differences Different mating times Physical separation Independently evolving populations Formation of new species

  • When speciation occurs, it is represented as branching on a phylogeny—a single ancestral lineage gives rise to two or more daughter lineages. It’s like reading a family tree!

    • Root: represents the ancestral lineage
    • Tips of the branches: represent the descendants of that ancestor.
    • As you move from the root to the tips, you are moving forward in time.

  • Phylogenetic Tree

  • Grouping organisms based on their shared derived characteristics

  • To determine how closely related two organisms on a cladogram are, TRACE from the first one to the second one. The more nodes (intersections) you pass, the farther apart the organisms are in terms of evolutionary relationship

  • CLADOGRAM

  • Humans did not evolve from chimpanzees. Humans and chimpanzees are evolutionary “cousins” and share a recent common ancestor that was neither chimpanzee nor human.

  • The phylogeny of living species most closely related to us looks like this:

  • Misconceptions about Humans

PATTERNS OF EVOLUTION

  • Convergent Evolution
  • Divergent Evolution (Adaptive Radiation)
  • Extinction
  • Coevolution
  • Punctuated Equilibrium & Gradualism
  • When we step back and look at populations evolving, these are some trends we may see.

EXTINCTION

  • The disappearance of a species.
  • Over 90% of all species that ever existed are believed to be extinct.
  • Extinction is occurring faster today than one hundred years ago.
  • A large portion of modern extinction is due to human activity.
  • COEVOLUTION
  • Process by which two species evolve in response to changes in each other.
  • Can be Cooperative (symbiotic relationships) or oppositional (camouflaging)

CONVERGENT EVOLUTION

  • Process by which unrelated organisms come to resemble each other as each species adapts to similar environments.
  • DIVERGENT EVOLUTION
  • Also called adaptive radiation;
  • Process by which 2+ species become more and more dissimilar
  • Ex: red fox and kit fox
  • The red fox lives in forest where red color blends with trees; kit fox lives in deserts where its light brown color blends in with sandy environment.
  • The similarity in structure indicates a common ancestor, but as they adapted to different environments the appearance of 2 species diverged

GRADUALISM AND PUNCTUATED EQUILIBRIUM

  • Gradualism
    • Idea that evolution is a slow even series of changes over time.
  • Punctuated Equilibrium
    • Idea that long stable periods are interrupted by brief periods of rapid evolution.