Bio 1B Lec 1: Evidence for Evolution and Common Ancestry

Observation vs Inference

Observation: orcas are close together (CANT say that they are social as an observation, that is an INFERENCE)

Inference: orcas are social animals!

Can we observe evolutionary events directly?

NO! We rarely can do that, most of the time we are relying on historical events’ traces they leave behind

What is a scientific theory?

An explanation of a broad range of explanations

You should be able to test predictions to add to this explanation, and the theory can be revised as you have new evidence

What is the strongest hypothesis of the past (in predicting ordered fossils, nested anatomy, shared molecular machinery, and tree-like relationships)?

Consilience: convergence of independent lines of evidence

Is agreement or disagreement between evidence types more useful?

Disagreements tells us where to test assumptions

Agreements could be difficult to explain, since they could have multiple reasons for agreement

What are the four evidence streams we can use to look at the past?

Fossils: when did forms appear? compare based on time period

Homology: what structures reveal shared ancestry? how are they similar? show common ancestry

Molecular Data: what does DNA and cellular machinery preserve? look for portions of similar DNA

Phylogenies: what relationship hypothesis explains the data?T

Is the historical record random or ordered?

Ordered

If evolution is true, what should fossils show? (predictions of descent with modification)

Older rocks should generally contain older forms

Some fossils should combine traits seen in different living groups

Fossil sequences should be compatible with other kinds of evidence (DNA, anatomy, etc.)

What are strata/stratum?

They are sedimentary rock layers. Younger stratum with more recent fossils on top, older stratum with older fossils on the bottom

How to use stratum in observing evolution?

Observe similarities between strata (time periods) to see if traits are preserved

What Molecular method can you use to narrow time period of fossils?

Radiation

T or F: Fossils are a movie

FALSE, fossils are sparse and preserved under only specific conditions (like if ur a floppy jellyfish u aren’t gna turn into a rock and ur gna be forgotten forever :() Fossils are also in biased set of frames (only certain conditions turn into strata), but the frames are in order

T or F: transitional fossils are half organisms

FALSE, transitional fossils between oldest and newest are still organisms, they just existed midway with some traits of both

What makes fossil evidence powerful?

they can SUPPORT evolution (NOT PROVE IDIOT) since they occur in geologic sequence (strata are in order): temporal order

They have trait mosaics (transitional forms combine ancestral and derived features)

Independent fit: fossil ages and form agree with phylogenetic predictions

Do fossils prove evolution?

NO, they SUPPORT evolution with EVIDENCE, no PROVING can be done

Homology vs. Analogy

Homology: similarity due to shared ancestry (tetrapod forelimbs)

Analogy: similarity due to similar function (bat wing vs. bird wing)

T or F: structures can be either homologous or analogous

NO, they can be homologous on one level and analogous at another

What does descent with modification predict?

Inherited structures are modified over time

What are vestigial structures/rudimentary organs?

They are historical clues!

They are reduced/modified traits relative to the ancestral condition

Ex: whale pelvic bones, snake hindlimb remnants, blind cavefish eyes

They make sense when u think of them as inherited features that were modified over time

Can use to look for similarities in ancestors

How is looking at development useful for evolution?

Closely related organisms can share developmental features since developmental genes and pathways are inherited and modified (the manuals for how x and y bodies work are similar)

Similar developmental stages can be evidence for common ancestry

If all life shares ancestry, what should molecules show?

Similarities on the cellular level: a nearly universal genetic code

Shared macromolecules (DNA, RNA, proteins, polysaccharides), conserved cellular processes (replication, transcription, translation, metabolism)

DNA sequence similarities form nested patterns of relatedness (aka help form trees)

How is genetic code a deep homology

DNA/RNA codons —> Amino Acids [this is shared among life]

Most organisms use basically the same code to translate nucleic acid sequence into protein sequence

You can’t really say all this shared code is a coincidence

Small exceptions in this code exist, but the exceptions can tell us stuff about evolution as well

How can we use DNA sequences in determining evolution?

A few differences between two sets of DNA may suggest a close relationship, but these DNA changes could be due to convergent changes, reversals, rate differences, and small sample sizes

Due to the inability of DNA sequences to conclude anything, that is why we need phylogenies as hypotheses we use to evaluate many characterss

What are trees?

Hypotheses about common ancestry

How to read a phylogenic tree

Tips: taxa

Nodes: common ancestor

Branches: lineages

Root: the start of the tree

How is relatedness determined in trees?

The Most Recent Common Ancestor (MRCA) determines relatedness

The further you go from the root, the closer you get to the present, so MRCA of two further down the branches may be more similar than a taxa close to the root

Do scale bars matter in phylogenetic trees?

If there is no scale bar/it is a cladogram, it doesn’t matter

Can you rotate taxa around nodes?

Yes, left-to-right order is usually arbitrary (UNLESS A TIME SCALE IS SHOWN)

Synapomorphy

syn: shared

apo: new/derived

Morphy: trait

Synapomorphies help identify clades

How do phylogenies integrate evidence?

Trees use anatomical, developmental, fossil, and molecular data to form a hypotheses

A good tree should explain many characters with few made-up assumptions

Can phylogenetic trees change?

Yes, as you have new evidence, the tree should adapt and change

How do scientists pick a hypothesis?

Hypothesis should explain more evidence with fewer unsupported assumptions

Does common ancestry show natural selection?

No, while common ancestry can be connected with natural selection, there are other methods of evolution that can also relate to common ancestry

LO1: Explain how fossils, homologies, phylogenetic relationships, and molecular evidence support the theory of evolution

Fossils: They occur in ordered sequences and preserve transitional trait mosaics

Homologies: inherited structures are modified for different functions

Molecular Evidence: shared genetic machinery and sequence similarity

Phylogenies: evidence-based hypotheses about relationships and trait evolution

LO2: Distinguish observation from inference

Observation is solely based on what is observable

Inference: predictions based on observations

LO3: Use evidence to compare alternative explanations for biological similarity

Biological similarity can be either homologous or analogous