Darwin's Evolutionary Theory

• Lecture 2 covers evolutionary biology, specifically, evolution as descent with modification

Learning Objectives

• Compare and contrast Darwin's view of evolution with Lamarck's earlier theory
• Understand the evidence that Darwin gathered for evolution as descent with modification
• Relate Darwin's observations to evolution as a branching tree of life
• Be comfortable reading and interpreting evolutionary trees

Thinking of Evolution as a Tree

• In On the Origin of Species (1859), Darwin included the first evolutionary tree.
• German biologist Ernst Haeckel stylized Darwin's view of evolution as tree-like.
• Haeckel coined the term phylogeny, and drew the first Tree of Life.

Darwin vs Lamarck

• Darwin's tree-like view replaced Lamarck's hypothesis of ladder-like evolution.
• Darwin's theory states species are descended one from another, forming a branching tree of life.
• Lamarck's view entails species transform into more complex ones over time, and the simplest ones keep reappearing.
• Lamarck's view: evolution was transformational
• Darwin's view: evolution is variational
• Darwin hypothesized natural selection is the sieve of variational change in populations.

Darwin's Voyage on the Beagle

• Darwin documented natural history in South America, Australia, and South Africa during a five-year voyage on the Beagle.
• He based his theory on three main groups of observations
• Darwin found extant species (like the armadillo) in Australia and South America that looked strikingly similar to fossils (like the Glyptodont).
• These similar species still had different sizes.
• British geologist Charles Lyell revealed the Earth was very old, millions of years at least
• Darwin inferred that the Glyptodont was very old and that the armadillo may be a related, descendant species that evolved a smaller size gradually across many generations

Darwin's Diversity Observations

• Darwin was struck by plant and animal diversity during his voyage.
• Orchids show incredible species variation in size, forms, and colors.
• Darwin reconciled structure similarities with diversity - species sharing the same structures inherited these structures from a common ancestor.
• Multiple species diverged from one species (the ancestor shared by that group) and evolved differences in form and function as they diverged.
• Homology is similarity in structure due to inheritance from a common ancestor.

Natural Selection

• Natural selection states that species adapted to their environment will survive and reproduce
• The Galapagos Islands showed many species of finches, approximately 15.
• The finches looked similar, but differed in beak morphology, based on diet
• Different islands had different species or groups of 2-3 species

Darwin's 3-Step Theory of Evolution

• To explain finch morphology across the Galapagos, Darwin proposed a 3-step scenario:
• Ancestral species traveled from South America to the Galapagos islands
• Newcomer species settled on one island, competing for food
• As they competed, they colonized other islands and competed for food there, too
• Limited food availability led to birds overcompeting others - natural selection
• Natural selection drives adaptation - the population optimally matches the aspect of the environment that causes competition.

Darwin's Schematic Evolutionary Tree

• A tree is a graph with nodes and branches
• In an oriented tree, a node may be the root, and the ends of outer branches are the tips

Evolutionary Trees

• Big letters A, B, C, etc. represent species, and the direction of time is upwards
• Numbers I, II, III... X represent time
• At each node species diverge
• At each tip is where a species reaches present day
• The lineages of species a10 and f10 start diverging at the first node, then divergence continues throughout the generations, splitting where necessary

Evolutionary Radiation

• A chart displaying the evolutionary ancestry of Hominins reconstructed from fossils
• Evolution is a tree, not a ladder
• Cladogram - If ignoring exact timing but focusing on the order of branching, we can align all tips
• Evolutionary trees come in many different styles
• Interior nodes commonly represent ancestors.

Species Relation

• Species A and B are more closely related than C and D if the common ancestor of A and B is more recent than of C and D
• Phylogeny is unchanged, even if rotating around any node
• Rotating a node does not change which species are more closely related
• Monophyletic group - a group that includes the common ancestor and all descendants
• Tetrapod vertebrates are an example of monophyletic
• Paraphyletic group - consisting of the common ancestor, but not all descendants are members
• Fish are an example of paraphyletic
• Polyphyletic group - derived from more than one common evolutionary ancestor and does not include all the descendants
• Raptors are an example of polyphyletic
• Rooted tree - A single node represents a common ancestor
• Unrooted tree - Trees that do not show a common ancestor
• Polytomies - Describes an internal node of a cladogram that has more than two immediate descendants (a branch point from which more than two taxa diverge)

Key terminology

• Variational versus transformational change
• Artificial and natural selection, adaptation
• Common ancestor, homology
• Evolutionary divergence, evolutionary stasis, 'living fossil'
• Phylogenic tree, phylogeny, chronogram, cladogram
• Branches, root, nodes, tips
• Monophyletic group (clade), paraphyletic, polyphyletic
• Species relatedness
• Unrooted and rooted phylogenies
• Polytomy

Explore Darwin's evolutionary journey, contrasting his views with Lamarck's and examining the influence of geology. Understand natural selection, phylogenetic trees, and the significance of fossil observations in shaping Darwin's theory.