Evolution and the Fossil Record

The Study of Evolution

Two domains: the present and the past

The present: Biology

Genetics

Comparative Anatomy

The past: Paleontology or Paleobiology

Comparative Anatomy of Fossilized organisms

Natural Selection

Natural selection is one of the prime mechanisms of evolutionary change. The other mechanism is mass extinction.

Charles Darwin, 1859: The Origin of Species by Natural Selection.

Natural Selection: Key Points

3 facts lead to a conclusion:

Fact 1. Overproduction of offspring

Fact 2. Variation within populations

Fact 3. Competition for resources

Conclusion: Survival of the fittest

Sources of Variation

Random mutations of DNA

>>99.9% are harmful or neutral

adaptive mutations are rare

Sexual recombination

new variants created by mixing of genes

you have 4096 direct ancestors going back 12 generations (2¹²)

Evidence of Evolution

Homology

Analogous organs or Convergent Evolution

Vestigial organs

Adaptive radiation

Homology

The recognition of common ancestry of features. e.g. all primates have 5 fingers; apes and humans lack a tail; all tetrapods have similar limb bones.

Studied by comparative anatomy of living and fossil organisms.

Analogous Organs or Convergent Evolution

Produced by evolutionary convergence. Independent origin of similar features is called convergent evolution.

Shows natural selection in operation.

e.g., insects, birds, bats, and pterosaurs all evolved wings independently

Vestigial Organs

Organs no longer used show clear evidence of evolutionary change.

e.g., pelvic bones of whales, vestiges of former legs

e.g., human body hair, a vestige of former fur

Adaptive Radiation

Natural selection can fill a variety of niches starting with a single species.

e.g., Darwin’s finches in the Galapagos Islands, 14 species evolved from one ancestral species

e.g., all birds evolved from Archaeopteryx

The Origin of New Species: Speciation

Natural selection by itself will not increase the number of species on the planet.

Species definition: Interbreeding populations reproductively isolated.

New species must be reproductively isolated from their ancestral species.

Speciation, cont’d

Allopatric speciation: produced by geographic isolation of populations.

Natural selection causes an isolated population to adapt to its local environment.

Given enough time, and no outside interbreeding, a new species will evolve.

Rates of Evolution

Phyletic gradualism - continuous and gradual change over time

Punctuated Equilibrium - long periods of stasis punctuated by rapid change, probably associated with a bottleneck in population size.

Rates of Evolution

Large populations evolve very slowly or almost not at all. Advantageous mutations pass very slowly through a population.

Small populations can evolve very rapidly. Advantageous mutations can be passed very quickly through the population.

Evolution in Action

Passing through a bottleneck:

Pesticide resistant insects

Antibiotic resistant bacteria

Role of the Fossil Record

Provides the only actual record of evolutionary change over geologic time.

Provides data on the timing of evolutionary origins.

Shows mass extinctions to be a major cause of evolutionary change.

Rates of evolution.

Evolutionary Origins

Oldest life: 3.5 BY

Oldest nucleated cells: 2.1 BY

First animals: 600 MY

First tetrapods: 375 MY

First dinosaurs: 220 MY

First mammals: 220 MY

First hominids: 4 MY

Mass Extinctions

Survival of the luckiest?

Over geologic time, mass extinctions may subvert the notion of survival of the fittest.

Is evolutionary history predictable?

Rates of Evolution

Biologists cannot measure evolutionary rates. They have only the present. Darwin assumed gradual rates.

Paleontologists can measure evolutionary rates. They have found punctuated equilibria to be the major pattern in fossil species rather than gradual change.