196.217!Evolution: Common

Descent with Modification!Lecture 4!

!

Futuyma, Chapter 3 (pg 45-59) http://evolution.berkeley.edu/evolibrary/search/topics.php?

topic_id=13

http://www.blackwellpublishing.com/ridley/tutorials/The_evidence_for_evolution1.asp

Gayle Ferguson

g.c.ferguson@massey.ac.nz

ext.43205/room 14.07 (OR)

Darwin’s theory of evolution by natural

selection

1. Species undergo (genetic) change over time (i.e. evolution happens)

2. It takes many generations to produce substantial evolutionary change (gradualism) 3. New forms arise by the process of lineage splitting (speciation) 4. Conversely, all species derive from a common ancestor 5. The process of evolution is driven by natural selection*

5 central tenets

* we now know that natural selection is only one of several processes by which evolution occurs

“By the theory of natural selection all living species have been connected with the parent-species of each genus, by differences not greater than we see between the varieties of the same species at the present day; and these parent-species, now generally extinct, have in their turn been similarly connected with more ancient species; and so on backwards, always converging to the common ancestor of each great class. So that the number of intermediate and transitional links, between all living and extinct species, must have been inconceivably great. But assuredly, if this theory be true, such have lived upon this earth.”

Charles Darwin, Origin of Species (1859)

pp.281-282

Evolution through common descent with modification

The Logic of and Evidence for Evolution

(Common descent and modification)

1. We will construct a case for evolution based on logic from what we know about biology

2. We will look at the evidence for the predictions that evolution makes (from fossils, development, and molecular data)

EVOLUTION

VARIATION

HEREDITY = +

HEREDITY

Family Tree

HEREDITY

VARIATION

Changing Variation

"Now, here, you see, it takes all the running you can do, to keep in the same place."

Through the Looking Glass, Lewis Carroll

•  There is variation within populations •  (Some of) this variation is heritable

(genetic inheritance) •  There is always change in this variation

- populations are variable, but what that variation is fluctuates

Speciation •  Species are continuously

changing, however: •  Sometimes one species can

split into two species •  This is called speciation •  When this happens, both

species will continue to change, and therefore will become more-and-more different from each other through time

•  Thus we can think of a species tree like a family tree: a birth is equivalent to a speciation event

Darwin’s Finches

•  Gradually, through time, more species arise and species die off (extinction)

•  Species change their form through variation, selection, drift

•  Greater and greater diversity, but all come from a single origin

Evolutionary Biology

•  Prediction: more closely-related species will be more similar to each other

•  We will be able to find similar things between related organisms (called homology)

•  Finding homologous features is a key part of studying evolution

Evidence for this picture

Evolutionary Biology

•  What is evolutionary biology trying to understand? –  What is the history of life on earth –  What are the relationships between

organisms –  What has changed between different

organisms –  How did these changes occur –  Why did these changes occur

Evolutionary Biology

•  What is evolutionary biology trying to understand? –  What is the history of life on earth –  What are the relationships between

organisms –  What has changed between different

organisms –  How did these changes occur –  Why did these changes occur

1.The Hierarchical Organisation of Life

2. Homology •  When two things are inherited from a

common ancestor •  Therefore homology refers to things that

come from descent, rather than by independent acquisition

•  If we identify homologous features, we can use them to learn about the evolutionary process

•  How do we tell if some feature is homologous or not?

•  Do other related organisms have that feature?

•  Are they anatomically or structurally related?

Assessing homology

Convergent evolution (no homology)

Sometimes similarity is not a consequence of common descent - convergent evolution (homoplasy)

Tree of Life

•  The more close together on the tree, the more closely-related the species are

•  The tips of the tree represent living species; ancestral species are inside the tree

•  Nodes are the places where speciation events occurred

3. Embryological Similarities Can you match embryo

and organism?

cat dolphin human

4. Vestigial Characters

Lanugo – fine hair covering the body of a premature infant. The lanugo is shed between 6 and 9 months in utero

vestigial pelvis and femur of the whale

Whale forelimb is homologous to the forelimb of other mammals, demonstrating common ancestry

5. Convergence (homoplasy) Recurrence of Form

The convergent eyes of vertebrates and cephalopods.

6. Bad Design

7. Transitional Forms

Warning: When a fossil is called "transitional" between two types of animal, that means it shows some of the traits of both, but it does not mean it links those animals by direct descent. In short, transitional fossils are best thought of as being close relatives of the species which actually link two groups. They may have lived at the same time as those actual links, or they may not have.

So many intermediate forms have been discovered between fish and amphibians, between amphibians and reptiles, between reptiles and mammals, and along the primate lines of descent that it is often difficult to identify categorically when the transition occurs from one to another particular species. National Academy of Sciences, 1999

Archaeopteryx – dinosaur with feathers and a beak

7. Transitional Forms Rodhocetus

Fossils were important in determining what whales were related to, and they were used to rebuke the idea that whales were closely related to an extinct group of hoofed mammals called mesonychians. Whales’ closest relatives are even-toed ungulates (artiodactyls, such as pigs, hippos, camels, deer and cows)

Some patterns of evolutionary change inferred from systematics

•  Most features of organisms have been modified from pre-existing features --- homologous characters generally have similar genetic and developmental underpinnings

•  Homoplasy is common --- superficially similar features are formed by different developmental pathways --- often (but not always) adaptations by different lineages to similar environmental conditions

•  Rates of character evolution differ --- evolution of different characters at different rates within a lineage is called mosaic evolution --- every species is a mosaic of plesiomorphic (ancestral or “primitive”) and apomorphic (derived or “advanced” characteristics)

Some patterns of evolutionary change inferred from systematics

•  Evolution is often gradual --- (gradations in nature are not uncommon)

•  Change in form is often correlated with change in function --- e.g. stings of female wasps and bees

•  Similarity among species changes throughout ontogeny (development) --- are often more similar as embryos than as adults