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Darwin’s Theory: Homologous, Analogous, Vestigial Features
What Darwin Observed
Darwin sent specimens collected on his voyage back to England to be examined by experts
Fossils collected were confirmed to be large versions of the present day sloth and armadillo
The Galapagos Island’s bird specimens were in fact many different species of birds
Darwin originally thought they were just variations of the same species
Evidence now suggested that a single ancestral species gave rise to a number of similar but distinct species
Homologous Features
Similar in Origin
Different in Function
Recent common ancestor
May appear in embryonic development and disappear before birth.
Analogous Features
Different in Origin
Similar in Function
Lacks recent common ancestor
Vestigial Features
Structures that serve of no use to the organisms
Examples:
Dew claws in dogs and toes elevated off the ground in pigs, cattle and deer
The appendix in humans
Vestigial genes are found in DNA, which serve no function but consists of similar sequences
Artificial Selection
Artificial Selection – When offspring with desired traits are selected as ‘breeding stock’ for succeeding
generations
Humans have improved domesticated plant and animal species for thousands of years
By selecting offspring with desirable traits as breeding stock for succeeding generations
E.g. breeds of dogs, types of corn
Darwin’s Assumptions
If humans could change the behaviour and appearance of domesticated species, the environment could have
similar effects on wild species
If Lyell was right about the age of the Earth there could be time for small changes in species to accumulate
into large changes over many thousands of generations
The Struggle for Survival
Darwin was convinced that he had evidence that life had evolved
Artificial selection was a model of how evolution might operate in nature
But how does nature chose individuals with desirable traits for reproduction – like breeders do with dogs
Used Malthus’ postulate to come up with answer
In nature both plants and animals produce far more offspring than are able to survive
Survival of the Fittest
Far more offspring are born than can survive and reproduce – intense competition among individuals of the
same species to survive
Darwin: “Favourable variations would tend to be preserved, and unfavourable ones to be destroyed. The
results of this would be the formation of a new species."
Origin of Species
In 1860 Darwin published “On the Origin of Species by Means of Natural Selection"
Darwin: “…can we doubt (remembering that many more individuals are born than can possibly survive) that
individuals having any advantage, however slight, over others, would have the best chance of surviving and
procreating their kind? On the other hand, we may feel sure that any variation in the least degree injurious
would be rigidly destroyed. This preservation of favourable variations and the rejection of injurious variations I
call Natural Selection."
Natural Selection
Supportive Evidence
Many scientist were swayed by Darwin’s theory
But even with all of the evidence there were still opponents
For entirely new species to develop a great length of time was required
Not everyone believed the Earth was that old (dating not available at the time)
Fossil record at the time was limited
No transitional fossil forms from ancient to modern had been found
Basilosaurus recently found – ancient relative of whale