Download Evolution: How Change Occurs

Evolution: How Change Occurs
Charles Darwin
• Proposed formal theory of evolution
- change in species over time
- modern organisms descend from ancient
- change in allele frequency in gene pool
• Darwin’s mechanism: NATURAL SELECTION
• Wrote On the Origin of Species
• Change occurs, controversy is often over how
Lamarck’s Early Theory on
Evolution
• Based on 3 ideas
1. Desire (will) to change
2. Use and Disuse
3. Passing on acquired traits (weight lifting)
• Brought idea of evolution to forefront
• Knew nothing of genes or genetics
Influences that Shaped Darwin’s
Theory
• Charles Lyell, geologist
- Earth very old, changed over time
• Farmers, artificial selection
- choose organisms with desireable traits to
produce offspring
• Thomas Malthus, economist
- Malthusian Doctrine
Evolution by Natural Selection
• Darwin’s observations:
1 - wild species show variations
2 - high birthrates & shortage of necessities, causes
competition
3 - individuals whose characteristics are well-suited to their
environment survive and reproduce, survival of the fittest
4- larger portion of each new generation will have favorable
variation
5 – over long periods of time, small changes accumulate,
population has adapted and natural selection took place
The environment selects which characteristics are
advantageous
Peppered Moths: Natural
Selection in Action
• Two varieties, population mainly light
colored prior to the Industrial
Revolution
• Soot from burning coal coated trunks
• Dark moths now “fit” the best, better
suited to survive, birds can’t spot the
moths
• Relative frequency of alleles for color
changed in the gene pool for the
population
• H.B.D. Kettlewell tested the theory
How do Genes Fit In?
• Darwin did not know about genes and
genetics
• Genes:
1. Responsible for variations
2. Genetic variations arise by
mutations
3. Not controlled; no goal in mind
• Phenotypic/Genotypic variation
1. Raw material for natural
selection
2. One genotype proves to be
advantageous example, sickle cell
carrier
Modern Evolutionary Theory
• Today we define evolutionary fitness as the
success an organism has in passing on its
genes to the next generation
• An adaptation is any genetically controlled
trait that increases an organism’s fitness
• Think about the weight lifter- big muscles
won’t be inherited but gene for the potential
to develop large muscles could be
How do new species evolve?
• Species- individuals that can breed and produce
fertile offspring.(share common gene pool)
• Niche- habitat and role/job of organism, no 2 can occupy
same niche, choices: adapt, move, or go extinct.
• Speciation occurs when populations are separated by
some barrier
• Natural selection can work differently in each group
• Groups cannot interbreed develop reproductive isolation
• Each group will become fit to the environment they are in
• Overtime, two populations cannot breed, new species
• Example, Darwin’s finches
Darwin’s Finches
on the Galapagos
Islands
Islands isolated populations
of finches; natural selection
occurred independently in
each group
Relative frequency for certain
traits changed in isolated
populations
Convergent vs. Divergent
Evolution
•
•
•
•
Divergent
One species gives rise
to many species
Also known as
adaptive radiation
Many species with
common ancestor
Many homologous
structures
Convergent vs. Divergent
Evolution
Convergent
• Similar looking
species that do not
have a common
ancestor
• Similar behavior and
appearance
• Many analogous
structures
Evolutionary Theory Continues
to Evolve
• Gene pools can change in absence of natural
selection by:
1. Genetic Drift- allele becomes common by chance
2. Genetic drift implies not all characteristics
contribute to fitness
• Gene pools can remain unchanged for long periods
of time ex. Horseshoe crab, living fossil
• Gradual evolutionary change, theory of gradualism
• Punctuated equilibrium, long stable periods with
spurts of change
Evidence for Evolution
• Contrasting theories over
mechanism for evolution, typically
not evolution itself
• Evidence of Change:
1. Fossil Record
2. Embryonic Development
3. Anatomical Structures
4. Biochemical Similarities
• All pieces support Darwin’s idea of
descent from a common ancestor