Origin of Species

Key Questions:

How do existing species give rise to new species?

How do species diversify?

What does the “family tree” of species look like?

Are there any challenges to the idea of evolution?

1. How do existing species give rise to new species?

• When populations in an existing species CAN NO LONGER give birth to fertile hybrid offspring under natural conditions

• Why does this happen? REPRODUCTIVE ISOLATING MECHANISMS make it impossible for populations to produce viable offspring

8 Categories of Reproductive Isolating Mechanisms:

• ECOLOGICAL isolation = Different populations (of the same species) adapt to different environmental niches (ex. Hominid speciation)

• SEASONAL isolation = Different populations mate at different times of the year

• SEXUAL isolation = Different populations have different courtship behaviors

• MECHANICAL isolation = Different populations have incompatible organs of reproduction

Categories of Reproductive Isolating Mechanisms (cont.):

DIFFERENT POLLINATOR isolation = In flowering plants, different populations attract different insects, birds, or bats to facilitate pollination

GAMETE isolation = Different populations have different cells of reproduction; no fertilization

HYBRID INVIABILITY = Different populations can mate and become fertile, but the hybrid zygotes do not survive

HYBRID STERILITY = Different populations produce living hybrids, but they are sterile (ex. Most mules)

When does SPECIATION occur?

• When ANY of these reproductive isolating mechanisms evolve!

• HOW do these reproductive isolating mechanisms evolve?

• Through EVOLUTIONARY PROCESSES of: Mutation, Natural Selection, Genetic Drift, Gene Flow

2. How do species diversify?

ADAPTIVE RADIATION = Spreading out of related species into new niches

When does Adaptive Radiation occur? (3 instances)

1. When an environment supports no similar, competing species

Ex. 3mya: A small group of finches migrated from South or Central America. They radiated into different environmental niches and evolved into 14 different species!

When does Adaptive Radiation occur (cont.)?

2. When EXTENSIVE EXTINCTION wipes out competing species in a set of environments

Ex. 65 mya: Mammals survive Cretaceous/Tertiary (K/T) extinction of dinosaurs

When does Adaptive Radiation occur (cont.)?

3. When a new group of a related species is adaptively GENERALIZED, it can disperse successfully into different niches, displacing species already there

Ex. 40 mya: Monkeys more generalized than Prosimians b/c larger brains, diurnal, arboreal, mixed diet, so…radiated to the New World, displacing most prosimians in Old World

Interpretations of Speciation2 Theories:

1. DARWINIAN GRADUALISM: Evolution occurs in slow changes in species over time, so “family tree” of species has few, gracefully diverging branches.

2. PUNCTUATED EQUILIBRIUM: Species tend to remain stable, experiencing “oscillating selection.” Evolution occurs in spurts of relatively rapid change.

What does the “family tree” of species look like?

A BUSH WITH MANY TWIGS!Twigs = evolution’s experiments, potential

new species

Natural Selection = “Editor” (not author) of Evolution, maintains adaptation of a species to its environment

Ex. Grants’ finches beaks changed back and forth as environmental conditions changed

Evidence for Evolution

1. FOSSIL RECORD: Fossils and the order in which they appear in layers of sedimentary rock (strongest evidence)

2. BIOGEOGRAPHY: Geographical distribution of species

3. Presence of TRANSITIONAL fossils

4. TAXONOMY: Classification of life forms

Evidence for Evolution

5. HOMOLOGOUS STRUCTURES: Structures that are similar because of common ancestry (comparative anatomy)

6. COMPARATIVE EMBRYOLOGY: Study of structures that appear during embryonic development

7. MOLECULAR BIOLOGY: DNA and proteins (amino acids)

Evidence for Scientific Creationism

• FAITH in a literal translation of the Book of Genesis in the Bible

• What do you think about Creationism?

Brief Evolutionary Timetable

15 bya Universe forms (“Big Bang”)12 bya Galaxies form5 bya Solar System forms4.5 bya EARTH forms3.8 bya LIFE on Earth (single-celled

organisms)543 mya CAMBRIAN EXPLOSION!

(all major body plans of complex multicellular organisms evolve!)

The Burgess Shale Site about 550 mya

•An "avalanche" of fine mud sliding down from the submerged reef top carried off any animals living in the shallow reef waters above

• The hard parts of all these animals caught in the mudslide were preserved as fossils, like the process at any other Cambrian site

• However, here the fine mud also penetrated and filled all available spaces within the animals, thus preserving the shapes and locations of all the soft parts. This is a rare event and has made these fossils extremely valuable

Cambrian Explosion

Amiskwia

•Three definite body segments: a head with two prominent tentacles, an unsegmented trunk with stubby side fins, and a flattened tail

•Fins and tail suggest this was an active swimmer (also suggested by its rare appearance in the Burgess Shale formation)

Cambrian Explosion (cont.)

Anomalocaris

•Fearsome-looking beast is the largest known Burgess Shale animal. Some related specimens found in China reach a length of six feet!

•Giant limbs in front, which resemble shrimp tails, were used to capture and hold its prey

•Mouth on the undersurface of the head had a squared ring of sharp teeth that could close in like nippers to crack the exoskeleton of arthropods or other prey

Cambrian Explosion (cont.)

Aysheaia

•Unusual assembly of spines and grasping arms at the head end. Its mouth lies in the center of that ring of six finger-like projections

•Thought to have been a parasite living on sponges since it is commonly found in association with their remains

•Presumably, the spiny parts at its head were designed for grasping and feeding on its prey

Cambrian Explosion (cont.)

Hallucigenia

Hallucigenia

•Even today, scientists can't be sure which end is the head!

•When originally discovered at the Burgess Shale site in Canada, the Hallucigenia fossils were squashed flat within the shale layer (like every other Burgess Shale fossil) with two sets of "spines" appearing to stick out in one direction and one set of "tentacles" in the other

Cambrian Explosion (cont.)

Opabinia

Opabinia is thought to have lived in the soft sediment on the seabed, although it presumably could have swum after prey using its side lobes. On the bottom, the proboscis could have plunged into sand burrows after worms. Sizes ranged up to 3 inches, plus that unique, amazing 1 inch proboscis!

Cambrian Explosion (cont.)

•Earliest known representative of the phylum to which we ourselves belong

•Averaging about 1 1/2 inches in length, Pikaia swam above the seafloor using its body and an expanded tail fin

•Note the characteristic muscle blocks lying along the centrally important feature, the notochord

Pikaia

Brief Evolutionary Timetable (cont.)

425 mya Fish evolve, Plants & Animals colonize land

400 mya Insects evolve350 mya Reptiles evolve250 mya MASS EXTINCTION (volcanic

eruptions in Siberia?, 95% marine & land species extinct!)

256 mya Mammal-like Reptiles evolve235 mya Dinosaurs evolve

Brief Evolutionary Timetable (cont.)

220 mya True Mammals evolve150 mya Small Dinosaurs w/feathers

evolve, ancestors of Birds100 mya Flowering Plants evolve65 mya CRETACEOUS/TERTIARY (K/T)

EXTINCTION (6 mile asteriod crashes through Earth in the MX Yucatan, dinosaurs extinct!)

55 mya PRIMATES evolve

Common Chimpanzee