Download Lecture 1: Introduction to Evolution

Lecture 1:
Introduction to Evolution
Evolution = change in allele frequencies
in a population
Great, 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
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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:
-no branching
-no extinction
Acquired Characters:
e.g. Giraffe’s neck
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…
Deduction
Descent with modification via natural selection
Testable???
Comparative
Anatomy
Fossil
Record
Predictions
Comparative
Embryology
Behaviour
Geographic
Distribution
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
Humans
Mammals
Birds
Amphibians
Fish
Reception
“Evolution” generally accepted but:
1. Viewed as progressive (towards a goal)
2. Natural selection rejected
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No theory of heredity
(how characteristics passed on)
Problem of uncrossable “gaps” in evolution
Back to transmutation
The Modern Synthesis
Mendelian genetics “rediscovered” in 1920s
By 30s/40s widely accepted :
1. acquired characters not inherited
2. 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