Lecture 1: Introduction to Evolution

Evolution = change in allele frequencies in a populationGreat, is class over now?

No (sigh), because the process & results are interesting & important to our understanding of biology

Questions in evolution

WHY is life on earth diverse? (we’ll get to this later)

HOW is life adapted ? (questions leading to more questions)

Adaptation Questions

• Are all traits adaptive?

• Are all adaptations perfect?

• Who/what benefits from adaptation?

Where did all these questions come from?

BD : Before Darwin

• Orthodoxy – species as fixed, designed by God• Bible = literal truth • variation = imperfection• BUT, even before Darwin this was questioned• Idea of changeable/old Universe, allowed idea of

changeable life on earth (geology & paleontology)• Enlightenment – fossils: spontaneous generation,

transmutation

Lamarck

• spp. change into new spp. over time

Transformism: Acquired Characters:

-no branching e.g. Giraffe’s neck

-no extinction

So, Darwin had somewhere to start…

Darwin (1809-1882)• Noticed: geographic variation in very similar spp.• Rejected orthodoxy – b/c did not explain

adaptation • Theory : why spp. change & why they are well

designed for their lives• Influences:

– Malthus (population principle)– Lyell (continuous change vs. catastrophists) – Social influences (materialism)

Darwin’s conclusion

• ‘Struggle for existence’ only some survive• Expect favourable variations to survive

= Natural Selection • Mechanism = most important aspect of theory• Poor Alfred Russel Wallace – same

conclusions on a much smaller budget – Co-presented the idea but Darwin is remembered

because of The Origin of Species

Darwin’s Dangerous Idea

The Origin of Species (1859)

Theses:

1. Descent with Modification from Common Ancestors (Evolution)

2. Natural Selection is main agent of Evolution

Origin in a nutshell

1. Observed overproduction of offspring

Many born – few survive to reproduce

WHY?

Limited resources – pop’ns limited in size

Therefore STRUGGLE FOR EXISTENCE

2. Individuals vary

If competition for resources then only favourable variants survive to reproduce

3. Most of the variation is inherited

(e.g. domestic species – sexual repro:diversity)

Favourable variants have more offspring & increase in frequency

= Change in the population over time

Plus lots about pigeons…

DeductionDescent with modification via natural selection

Testable???

Predictions

Fossil Record

Comparative Anatomy

Comparative Embryology

Geographic Distribution

Behaviour

Why Dangerous?

• Static dynamic view of nature• Creationism = implausible• Platonic essentialism

– variation = basic/neutral feature

• Refuted teleology & anthropocentrism• Natural Selection

– No goal– No consistent direction

• EVOLUTION PROGRESS

Evolution is a bush not a ladder

Fish

Amphibians

BirdsMammals

Humans

Reception

“Evolution” generally accepted but:

1. Viewed as progressive (towards a goal)

2. Natural selection rejected• No theory of heredity

(how characteristics passed on)• Problem of uncrossable “gaps” in evolution• Back to transmutation

The Modern Synthesis

Mendelian genetics “rediscovered” in 1920sBy 30s/40s widely accepted :1. acquired characters not inherited2. Continuous variation explained by Mendelian

genetics (Fisher)3. Theoretical works show N.S. can work with what

is available in nature, nothing else required Speciation only requires N.S. not macromutation /acquired characters

4. Species are not morphotypes – dynamic concept

Tenets of Modern Synthesis

• Populations have variation from random, not adaptively directed, mutation & recombination

• Populations evolve through changes in gene frequency by drift, gene flow & N.S.

• Change is gradual because most genetic variants have slight effects on phenotype

• Diversification (speciation) is due to gradual reproductive isolation among populations

• Overtime, changes give rise to new taxa

Genetics…more important than Darwin?

• After Darwin, many accepted the idea of evolution as change in species over time

• BUT, much argument against NS

• Genetics forced the rejection of evolution as goal-oriented & the widespread acceptance of NS

Modern Evolutionary Biology

Two principle goals:

1. Inferring history of evolution

2. Elucidating the mechanisms

Modern evolutionary theory:

Provide explanation for patterns of life in space & time & the processes by which these patterns arose