Biol 121- 2010.03.08- Evolution- Evidence for Evolution (Ch. 24).docx
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Biol 121 225
Freeman 484-490 Mar 8, 10
Evolution 2- Ch. 24, Evidence for Evolution
Main points of Darwin’s theory?
1. Evolution – “descent with modification”
-all organisms have a common ancestor – Linnanean hierarchy, “tree of life”
-two species in a family are more closely related than those in different families in the
same class
-the environment changes traits in a population over time/over generations (allele
frequency changes in a population)
2. Natural selection – the mechanism of evolution
Premises:
-genetic variation exists
-performance affects reproductive success higher fitness
-competition differential survival
-natural selection occurs through interaction between environment and existing
genetic variation in a population
-it causes a change in the population resulting in increase in those better suited for
survival
-variable population because of adaptation of population to environment
variation + selection = adaptation
Evidence for evolution
1) Evidence for change through
time
a) Fossil record
i) Extinctions
-the fossil record
-extinct species can be noticed in fossils containing shells, bones, leaves, bird tracks,
dung, etc
-this was used as evidence by Darwin who then said that species were not static,
immutable entities
-his reasoning: since species have gone extinct, array of species living on Earth
changes over time
ii) Transitional forms
E.g.whales
-when a species evolves over generations, it may enter many transitional forms
-transitional form = fossil species with traits that are intermediate b/t those of older
and young spp.
-was known as the “law of succession” when there were resemblances b/t fossils
found in rocks underlying certain regions and the living spp. found in the same
geographic area
-Evolution of whales: Whales evolved from terrestrial mammals to the aquatic
mammals of today
-transitional forms had shorter legs with time, as well as a flatter body
-eventually, legs disappeared and became small fins
b) Changes in living organisms
i) Vestigial trait
-a vestigial trait is a reduced or incompletely developed structure that has no function
or reduced function, but is clearly similar to functioning organs or structures in closely
related species
-for example, humans have tailbones (coccyx) but we don’t have tails
-also, appendix in human is reduced version of cecum – digestive organ in other
species
ii) Artificial selection
-spindly flowering stalk (Brassica aleracea)
-if you select individuals with largest and most compact flowering stalks and breed
them, and do so for a few generations, you will get broccoli
-basically, you can breed/cross individuals that have a specific trait of interest