ECOL335 Evolutionary Biology Lecture Notes PDF

ECOL335 Evolutionary Biology Spring 2025 January 21 Lecture 2 Evolution is descent with modification Our learning objectives today Compare and contrast Darwin’s view of evolution with Lamarck’s earlier theory. Understand the evidence that Darwin gathered for evolution being descent with mo...

ECOL335 Evolutionary Biology Spring 2025 January 21 Lecture 2 Evolution is descent with modification Our learning objectives today Compare and contrast Darwin’s view of evolution with Lamarck’s earlier theory. Understand the evidence that Darwin gathered for evolution being descent with modification. Relate Darwin’s observations with evolution conceptualized as a branching tree of life. Be comfortable at reading and interpreting evolutionary trees.  Thinking of evolution as a tree In his masterpiece On the Origin of Species (1859), Darwin wrote 502 pages and included only one figure: the first evolutionary tree (schematic). Thinking of evolution as a tree… Darwin’s view of evolution as tree- like was stylized by German biologist Ernst Haeckel, who coined the term phylogeny and drew the first Tree of Life. Darwin’s tree-like view of evolution replaced Lamarck’s hypothesis of ladder-like evolution. In Darwin’s theory, species are descended one from another to form a branching tree of life. In Lamarck’s view, with time species transform into more complex ones, and the simplest ones keep reappearing. FIGURE 2.12. Darwin’s theory (left) versus Lamarck’s theory (right). In Lamarck’s view, evolution was transformational. In Darwin’s view, evolution is variational. FIGURE 2.11. Different processes of change. Inspired by artificial selection, Darwin hypothesized natural selection to be the ‘sieve’ of populations’ variational change. FIGURE 1.3. The process of artificial selection. From a five-year voyage on the Beagle, documenting natural history in and around South America, Australia, South Africa, FIGURE 2.4. The voyage of the Beagle. From a five-year voyage on the Beagle, documenting natural history in and around South America, Australia, South Africa, Darwin formed his theory on the basis of three main groups of observations… First, in Australia and South America mainland, he found extant species (such as the armadillo) looking strikingly similar to fossils (such as the Glyptodont),… but with striking differences too (such as a dramatic reduction in size).  Darwin knew about recent discoveries and theories in geology, championed by British geologist Charles Lyell, that revealed that the Earth was very old – millions of years at least. Darwin inferred that the Glyptodont was very old, and that the armadillo might be a related, ‘descendant’ species which had plenty of time to evolve a smaller size, gradually across many generations. Time Second, during his voyage, Darwin was struck by the extraordinary diversity of plants and animals. For example, orchids show incredible variation among species in size, forms, colors…. But just like the striking similarity and difference of the Glyptodont and armadillo, Darwin noted that species of the same group, such as Orchids, share remarkably similar structures. To reconcile such similarities in structure with the diversity of forms, Darwin proposed that all these species sharing the same structures inherited these structures from a common ancestor. From that one species (the ancestor shared by that group), multiple species diverged, and as they did, they evolved all their differences in form and function. Homology: similarity in structure due to inheritance from common ancestor « What could be more curious than that the hand of a man, formed for grasping, that of a mole for digging, the leg of the horse, the paddle of the porpoise, and the wing of the bat, should all be constructed on the same pattern, and should include the same bones, in the same relative positions? » (Darwin 1859, p. 454) And finally, Darwin was able to hypothesize natural selection as a mechanism for gradual evolution of differences (evolutionary divergence) from a common ancestor. One of the decisive observations came from the Galapagos Islands. 15 species of finches look very similar, and yet differ greatly in their beak morphology, apparently in relation with different diet. Different islands have different species or groups of 2-3 species. To explain the diverse morphology of finches across the Galapagos archipelago, Darwin proposed the following 3-step scenario. 1) Individuals from an ancestral species accidentally traveled from South America mainland to the Galapagos islands. To explain the diverse morphology of finches across the Galapagos archipelago, Darwin proposed the following 3-step scenario. 2) The newcomer species successfully settled on one island. Individuals started competing for the food (seeds) that they found there. To explain the diverse morphology of finches across the Galapagos archipelago, Darwin proposed the following 3-step scenario. 3) As they competed in their original island, they also colonized the other islands and competed for food there too. From generation to generation, limited food availability resulted in different varieties of these birds overcompeting others and dominating the population in their island: this was natural selection. Natural selection drives the process of adaptation. As a result of natural selection, the population optimally matches the aspect of the environment from which competition arises, for example the type of available food. This match, as well as the evolutionary process leading to it, is called adaptation. Let’s take a closer look at Darwin’s schematic evolutionary tree… In geometry, what we call a tree is a graph with nodes and branches. In a tree that is oriented, one node may be the root and the end of outer branches are the tips. 1. What do big letters A, B, C etc. represent? What is the direction of time? What do number I, II, III… X represent? 2. What happens at each node? 3. What happens at each tip (except those at level X)? 4. Compare what happens from nodes A or I to what happens from the other big letters. 5. When did the lineages of species a10 and f10 start diverging? What about a10 and m10? w10 and u8? a10 and w10? a10 and u8? The hypothesized evolutionary tree of Hominins reconstructed from fossils. There is no such thing Evolution is a tree, not as « progressive » a ladder! evolution! If we focus on the order of branching events and ignore exact timing, we can align all tips: cladogram. Chronogram Evolutionary trees may be drawn in many different styles… FIGURE 4.6 How does time flow in each of these cladograms? Interior nodes represent common ancestors. FIGURE 4.7  Species A & B are more closely related than species C & D if the common ancestor of A & B is more recent than the common ancestor of C & D. Examples: Species 1 and 2 are more closely related than species 4 and 6. 11 10 Species 1 and 2 are as closely related as species 5 and 6. 9 Species 3 and 6 are more closely related than species 2 and 3. 8 Species 4 and 6 are more closely 7 related than species 3 and 4. Species 3 and 6 are as closely related as species 3 and 4. Rotating around any node leaves a phylogeny unchanged. FIGURE 4.9 Monophyletic group (clade): tetrapods as an example FIGURE 4.13 Paraphyletic group: fish as an example FIGURE 4.13 Polyphyletic group: raptors as an example FIGURE 4.12 Rooted trees from unrooted trees FIGURE 4.3 FIGURE 4.14 Unrooted Tree of Life. Unrooted tree of proteobacteria. Rooted trees from an unrooted tree FIGURE 4.15 Polytomies represent uncertainty about phylogenetic relationships. FIGURE 4.10 After today’s lecture, you should be able to define the terms… 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