Nothing in biology makes sense except in the light of evolution. – Theodosius Dobzhansky

Darwin’s theory of Evolution]

Charles Darwin in later years

What is Evolution?

The kind we’re talking about is sometimes called organic evolution to distinguish it from non-biological changes over time.

Working definition: Evolution is the progressive change in populations over time.NOT SPECIES, NOT INDIVIDUALS

Evolution Holds a Unique Place in Biology

Other disciplines ask how?

Evolutionary biology asks why?

Evolution’s Core Principles

Natural selection.

Evolution’s Core Principles

Common descent with modification.

Evolutionary Time Scales

Macroevolution: Long time scale events that create and eliminate species.

Evolutionary Time Scales

Microevolution: Short time scale events (generation-to-generation) that change the genotypes and phenotypes of populations.

We’ll begin our more intensive look at evolution with microevolution.

Darwin’s Voyage of Discovery

A reconstruction of the HMS Beagle sailing off Patagonia.

The Voyage of the Beagle

Darwin’s Ideas Did Not Develop in a Vacuum

Contributor’s to Darwin’s thinking included:

Charles Lyell –(geologist) uniformatarianism. The Earth is older than 6,000 years

1797-1875

Georges Cuvier – species extinction. Fossils

1769-1832

Darwin’s Ideas Did Not Develop in a Vacuum

Contributor’s to Darwin’s thinking included:

Thomas Malthus – struggle for existence.Carrying capacity: J curve/S curve

1766-1834

Jean Baptiste de Lamarck – evolution by acquired characteristics. First evolutionTheory, Giraffes get taller due to stretchingIn parents

1744-1829

Alfred Russel Wallace Independently Drew the Same Conclusions as Darwin

Papers from Wallace and Darwin were jointly presented (with little impact) to the Linnaean Society in 1858.

Haeckel: ontogeny recapitulates phylogeny

• Thomas Hunt Morgan: Darwin’s bulldog

Darwin’s Observations and InferencesOrganized by Ernst Mayr

Observation 1: Left unchecked, the number of organisms of each species will increase exponentially, generation to generation.

Observation 2: In nature, populations tend to remain stable in size.

Inference 1: Production of more individuals than can be supported by the environment leads to a struggle for existence among individuals, with only a fraction of offspring surviving in each generation.

Observation 3: Environmental resources are limited.

Darwin’s Observations and Inferences

Observation 4: Individuals of a population vary extensively in their characteristics with no two individuals being exactly alike.

Observation 5: Much of this variation between individuals is heritable.

Inference 2: Survival in the struggle for existence is not random, but depends in part on the heritable characteristics of individuals. Individuals who inherit characteristics most fit for their environment are likely to leave more offspring than less fit individuals.

Darwin’s Observations and Inferences

Inference 3: The unequal ability of individuals to survive and reproduce leads to a gradual change in a population, with favorable characteristics accumulating over generations (natural selection).

Taken together, these three inferences are a statement of Darwin’s Theory of Evolution.

Darwin’s Observations and Inferences

The Weak Link of Genetics and the Modern Synthesis

A major problem in Darwin’s theory was the lack of a mechanism to explain natural selection. (No mitosis, meiosis, replication, chromosomes, laws of inheritance)

How could favorable variations be transmitted to later generations?

With the rediscovery of Mendel’s work and its vast extension in the first half of the 20th century, the missing link in evolutionary theory was forged.

Darwinian theory supported by genetics is known as the modern synthesis.

Darwin in his early years.

Discomfort With Evolution

An early disparaging view of evolutionary theory and its creator.

The upheaval surrounding evolution began with publication of On the Origin of Species and continues nearly 150 years later.

1925

Evidence for Evolution – The Fossil Record

Evidence for Evolution - Comparative MorphologyHomologous structures

Why use the same skeletal plan for these very different appendages?

Evidence for Evolution - Comparative Embryology

Why do embryos of different animals pass through a similar developmental stage?

Recent discoveries of the conservation of molecular mechanisms of development are even more compelling.

Evidence of Evolution –Conservation and Diversification at the Molecular LevelWhy should different organism possess related genes?

Why does the degree of relationship of genes match their degree of relationship established by other methods?

Evidence for Evolution – Evolution Observed

Evolution of pesticide resistance in response to selection.

Special Examples• Kin Selection: relative fitness includes fitness of close

relatives• Artificial selection: farming/animal breeding• Sexual selection: based on mating• Convergent evolution: different ancestor common lifestyle

(analogous structures result)• Divergent evolution: common ancestor different lifestyle

(homologous structures result)• Parallel evolution: evolving in the same pattern• Coevolution: two evolving each other

Speciation– Steps in the process:

» Subdivision of the population» Evolution of each subpopulation in independent

directions» Change in population that reproductively isolates the

population from the other.» Return to same setting and no longer can members of

the two populations mate.

Speciation

• Allopatric: Geographical separation leads to initial subdivision of the population.

• Rivers, volcanoes, earthquakes, roads, train lines, etc..

Sympatric speciation

• Sympatric: Isolation of populations occurs while organisms have physical contact with each other.

More sympatric

• Apple maggot flies

Reproductive Isolation

• Prezygotic vs. Postzygotic• Needed for speciation: Organisms can look

somewhat different and still be one species if this does not occur.

• Organisms can look very much alike, but be two species if this occurs.

• The idea is that eventually diversity will build up in the populations independently and thus would eventually look different.

Many Intrinsic Reproductive Isolating Mechanisms Drive Speciation

(different habits within an overlapping range)

Many Intrinsic Reproductive Isolating Mechanisms Drive Speciation

Speciation Dynamics - Gradualism or Punctuated Equilibrium?

Punctuated equilibrium appears to be a more accurate view of speciation dynamics. : long period lacking speciation where variation builds up followed by some dramatic change in the environment that leads to a struggle for survival and lots of speciation and microevolution.

Evolution possibilities• Convergent evolution: leads to analogous

structures. (same function/different ancestry)

Divergent evolution

• Divergent evolution: leads to homologous structures. (same ancestry/different function)