Theme 1: Topics 5-6

PHYLOGENY AND EVOLUTION

To-do:

• read

• notes/flashcards

• understand

• memorize

• quizzes

Important Concepts:

• describe the natural processes that produce new species.

• describe how the four mechanisms of evolution affect speciation.

• describe how selective pressures can shape a population over time.

Topic 5: Evolution by Natural Selection (Ch. 1 11-16, Ch. 22 498-506)

1. The Historical events that influenced Darwin’s thinking before the publication of The Origin of Species

• Darwin was influenced by early thinkers eg. Lamarck (evolution did not start with Darwin)

• 1831-1836 → HMS Beagle 5 year voyage around the world where he observed various species and their adaptations to different environments (including the Galapagos finches), which provided him with critical insights into the mechanisms of natural selection and the variation in traits that could be favoured or diminished depending on environmental factors.

• 1884 → Charles wrote an essay proposing natural selection as the mechanism of descent with modification, but he did not make it public at that time

• 1858 → Darwin received a manuscript from Alfred Russel Wallace who has developed a theory of natural selection similar to Darwin’s, which served as confirmation for his theory. They both presented their papers together to the Linnean Society but only Darwin’s extensive notes over the years made his version more convincing. Darwin took action and quickly finished On the Origin of Species by Means of Natural Selection, and published it in 1859.

2. The Theory of Evolution (“descent with modification”) by natural selection

• Species show evidence of “descent with modification” from common ancestors, so all organisms are related through descent from an ancestor that lived in the path.

• The concept of “descent with modification” explains the duality of unity and diversity in living organisms → Species share common traits because they are descended from common ancestors (unity), Differences between species arise when species accumulate heritable changes or adaptations after diverging from their common ancestor (diversity).

• Natural selection is the mechanism for “descent with modification”

3. The principles of natural selection with biological examples

• A prime example that drove Darwin to create his theory was the evidence of descent with modification of Galapagos finches

• He observed that finch species on different Galapagos island were similar but had distinct differences, specifically that variations in beak shape

• Darwin hypothesized that the beak of an ancestral species had adapted over time to form descendant species adapted to different food sources (they are a monophyletic group)

• His argument was based on observations of human propelled artificial selection

• He proposed natural selection as the mechanism for evolution, and that it gives rise to adaptation and speciation

4. Selective agents influence natural selection by creating selection pressures

• Selective agents are environmental factors acting on populations that affect the survival and/or reproduction of individuals in the populations → Biotic (predators, disease, competitors), Abiotic (space, light, temperature, water)

• When a selective agent consistently causes differences in survival and/or reproduction in a population, it is a selection pressure

• There is no natural selection without selection pressure

• Natural selection is context dependent → consistent selection pressure leads to a directional change in the population, inconsistent selection pressure results in a change in the direction of natural selection,

5. Variation in populations is essential for natural selection

• Individuals in a population vary in their inherited traits, this variation (genetic diversity) provides the raw material that natural selection acts upon.

• Genetic variation in populations arises randomly, but natural selection is not a random process.

6. How does natural selection work according to the observation of Charles Darwin?

   1. Individuals in a population vary in their inherited traits, this variation (genetic diversity) provides the raw material that natural selection acts upon

   2. Populations produce more offspring than the environment can support (hence why many don’t survive due to resource competition)

   3. Individuals with inherited traits that provide an advantage in their environment are more likely yo survive and reproduce, leaving more offspring (said inherited traits must be expressed in phenotype and be heritable in genotype)

   4. The unequal ability of individuals to survive and reproduce will lead to the accumulation of advantageous traits in the population

Topic 6: Evidence for Evolution (Ch. 22 506-514)

Four types of evidence for evolution

1. Direct observations of evolutionary change

• Galápagos medium ground finch → Natural selection has caused measurable changes in beak size among medium ground finch (Geospiza fortis) populations in response to varying environmental pressures such as drought and competition for food

• Natural selection of soapberry bug beak size (Campbell Biology Fig 22.13)

• Artificial selection

• Drug resistance in viruses and bacteria.

2. Homologies

• Homologies are similarities that arise from a shared evolutionary ancestor and include:

  a) morphological homologies (e.g. tetrapod forelimb bone structure)

  b) homologous embryonic structures

  c) vestigial structures

  d) molecular homologies

3. The fossil record

• Provides evidence of the extinction of species

• Provides evidence of the origin and diversification of new groups (chronological sequence), and changes within groups over time (transitional fossils).

4. Biogeography (the study of how species are distributed across the globe)

• Shows how geographic isolation and environmental factors shape species diversity.

• Shows how isolated populations undergo adaptive radiation forming endemic species.

• Continental patterns of species distribution reflect evolutionary histories.