Phylogenies & History of Life- Chapter 25

Phylogeny

The evolutionary history of a group of organisms

Phylogenetic tree

shows ancestor-descendant relationships among populations or species

What is a branch?

represents a population through time

Node

the point where two branches diverge

what does a node represent

the point in time when an ancestral species split into two or more descendant species

Tip

the endpoint of a branch which represents a group that is living today or ended in extinctions

What is the cladistic approach to inferring trees

focuses on synapomorphies, the shared derived characters of the species under study

What do synapomorphies allow biologists to recognize

monophtyletic groups (clades or lineages)

Ancestral trait

characteristic that existed in an ancestor

derived trait

trait that is a modified form of the ancestral trait, found in a descendant ex. fur

Are ancestral and derived traits relative

yes

Homology

Occurs when traits are similar due to shared ancestry ex. four limbs of tetrapods

Homoplasy

Occurs when traits are similar for reasons other than common ancestry ex. ichthyosaurs and dolphins are similar, but the similarities are not due to common ancestry

Convergent evolution

when different species develop (through natural selection) similar traits to adapt to similar environments

What is a common cause of homolapsy

convergent evolution

What happens when similar traits found in distantly related lineages are similar due to common ancestry

the similar traits should be found in many intervening lineages on the tree of life

Evidence for homology

Hox genes (insects and vertebrates) - genes are derived from the same ancestral sequences

Whale evolution: A case History

pylogenetic tree places whales as an outgroup of artiodactyls
gene sequence found that whales & hippos share genes
these genes are derived traits (synapomorphies)

fossil

the physical trace left by an organism that lived in the past

Fossil record

total collection of fossils that have been found throughout the world

How do fossils form?

when all or part of an organism is buried in ash, sand, mud

Four main types of fossils

Intact fossils, compression fossils, cast fossils, permineralized fossils

Intact fossils

forms when decomposition does not occur (pollen)

compression fossils

forms when sediments accumulate on top of the material and compress it into a thin film (leaf)

Cast fossil

form when the remains decompose after burial and dissolved minerals create a cast in the remaining hole (bark)

permineralized fossils

form when the remains rot extremely slowly and dissolved minerals infiltrate the interior of the cells and harden into stone (trunk)

When did the Earth start to form and when did life begin

4.6, 3.4 billion years ago

How do researchers divide Earth history?

eons, eras, and periods

what allows researchers to assign absolute dates?

radiometric dating

Changes in the oceans and continents

Earths crust is broken into enormous plates that are in constant motion, driven by heat rising from the planets core, shifted the continents over time

Mass extinction

Rapid extinction of a large number of lineages scattered throughout the tree of life (60% of species present are wiped out within 1 million years)

Background extinction

the gradual, natural process by which species die out over time

When do background extinctions typically occur

when normal environmental change, emerging diseases, or competition reduces certain populations to zero

When do mass extinctions usually occur

when extraordinary, sudden, and temporary changes in the environment

Difference between background extinction & mass extinctions

background extinctions are thought to result primarily from natural selection while mass extinctions function like genetic drift

Impact hypothesis

for the extinction of dinosaurs proposes that an asteroid struck earth 65 mya, resulting in the extinction of an estimated 60-80% of the multicellular species alive

Conclusive evidence that killed the dinosaurs

iridium, shocked quartz, and microtektities found in rock layers dated to 65 mya, as well as a huge crater