Natural Selection, Speciation, and Natural Selection, Speciation, and EvolutionEvolution

Textbook Reference

Chapter 14 & 15

Lamarck’s Early Theory on Lamarck’s Early Theory on EvolutionEvolution

• 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

Charles DarwinCharles Darwin

• Observations aboard Beagle

• Led to his theory of evolution

- change in species over time

- modern organisms descend from ancient

- change in allele frequency in gene pool

• Darwin’s mechanism: NATURAL SELECTIONNATURAL SELECTION

• Wrote On the Origin of Species

Influences that Shaped Darwin’s Theory

• Charles Lyell, geologistCharles Lyell, geologist

- Earth very old, changed over time

• Farmers, artificial selectionFarmers, artificial selection

- choose organisms with desireable traits to produce offspring

• Thomas Malthus, economistThomas Malthus, economist

- Malthusian Doctrine

Evidence for EvolutionEvidence for Evolution

• Contrasting theories over mechanism for evolution, typically not evolution itself

• Evidence of Change:1. Fossil Record2. Embryonic Development3. Anatomical Structures4. Biochemical Similarities

• All pieces support Darwin’s idea of descent from a common ancestordescent from a common ancestor

Evolution by Natural Selection

• Darwin’s observations:1 - wild species show variationsvariations2 - high birthrates & shortage of necessities, causes competitioncompetition3 - individuals whose characteristics are well-suited to their environment survive and reproduce, survival of the fittestsurvival of the fittest4- larger portion of each new generation will have favorable variation5 – over long periods of time, small changes accumulate, population has adapted and natural selectionnatural selection took placeThe environment selects which characteristics are advantageous

Video Clip

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 frequencyRelative frequency of alleles for color changed in the gene pool gene pool for the populationpopulation

• H.B.D. Kettlewell tested the theory

How do GenesGenes Fit In?• Darwin did not know about genes and

genetics• Genes:

1. Responsible for variations2. Genetic variations arise by

mutations 3. Not controlled; no goal in mind

• Phenotypic/Genotypic variation1. Raw material for natural

selection2. One genotype proves to be

advantageous example, sickle cell carrier

Modern Evolutionary TheoryModern Evolutionary Theory

• Today we define evolutionary fitnessevolutionary fitness as the success an organism has in passing on its genes to the next generation

• An adaptationadaptation 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?

Speciation• SpeciesSpecies- individuals that can breed and produce

fertile offspring.(share common gene pool)

• NicheNiche- 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 isolationreproductive isolation• Each group will become fit to the environment they are in• Overtime, two populations cannot breed, new species• Example, Darwin’s finches underwent adaptive radiation

Adaptive Radiation

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

DivergentDivergent• One species gives rise

to many species• Also known as

adaptive radiationadaptive radiation• Many species with

common ancestor• Many homologous homologous

structuresstructures

Convergent vs. Divergent Evolution

ConvergentConvergent• Similar looking

species that do not have a common ancestor

• Similar behavior and appearance

• Many analogous analogous structuresstructures

Video Clip

Evolutionary Theory Continues to Evolve

• Gene pools can change in absence of natural selection by:1. Genetic DriftGenetic Drift- allele becomes common by chance2. Genetic drift implies not all characteristics contribute to fitness

• Gene pools can remain unchanged for long periods of time ex. Horseshoe crab, living fossilliving fossil

• Gradual evolutionary change, theory of gradualismgradualism• Punctuated equilibrium,Punctuated equilibrium, long stable periods with

spurts of change