Homology: A punt to clear up some confusion:

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Homology A punt to clear up some
confusion To 19th century biologists (before
Darwin) Similarity in form, structure and
location not necessarily related to function.
Evidenced also by similarities in development
(e.g. bird wings and mammal forelimbs both
develop from embryonic limb buds) and usually by
similarities in innervation (e.g, primitive ear
and articular of upper jaw by innervated by
the same cranial nerve). To post-Darwin
biologists Similarity in form, etc., not
necessarily related to function, stemming from
descent from a common ancestor.
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Sometimes names carry legacies of obsolete,
long - discarded ideas. Ancient scholars
thought that a particular large African mammal
was a cross between two other species. This
idea survives in the species name we use for
this animal today
camelopardalis Camel camel Pardalis
leopard (or panther) Can you figure out which
animal has this name?
3
Sometimes names carry legacies of obsolete, long
- discarded ideas. Ancient scholars thought
that a particular large African mammal was a
cross between two other species. This idea
survives in the species name we use for this
animal today
camelopardalis Camel camel Pardalis
leopard (or panther) Can you figure out which
animal has this name?
Giraffa camelopardalis
Ex Africa semper aliquid novii
-Pliny
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• Goals for this lecture
• Review of Darwins observations, particularly on
  the
• Galapagos, during a voyage of the HMS
  Beagle.
• 2. Darwins mechanism for evolution, the
  struggle for
• existence.
• 3. Natural Selection as the term is now used.
• Vocabulary/concepts
• Homology
• Transitional forms
• Endemic forms or species
• Adaptive radiation
• Cultivar
• Descent with modification
• Fitness

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Vestigial structures
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Transitional forms
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Large tortoises were very common on the
Galapagos. What critical observation did Darwin
make about them?
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The tortoises on different islands often had some
different characteristic features which allowed
them to be identified even on when mixed together
on shipboard. That is, they were endemic to
different islands. An endemic is an organism
found only in a limited geographic area, usually
with one or more readily recognizable features.
The word can apply to populations, species,
genera,etc. For example Polar bears are
endemic to northern polar regions.
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Darwin recognized that the Galapagos tortoises
were closely related AND that distinguishable
endemic forms could be found on many of the
islands. The nearest population of tortoises
was on the Seychelles, thousands of miles
away.
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On one island, Albermarle, there are 5 different
types of tortoise, each endemic to a specific
volcanoe.
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Darwins finches are also endemic to the
Galapagos (their nearest relative is in South
America). Darwin recognized that these birds
were fundamentally similar but differed in bill
shape and size, and in their diets. This excited
him, for he knew that in most local bird faunas
seed eaters, insect eaters, etc. would not be
closely related.
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This finch even gets its food like a
woodpecker, and uses a tool as well.
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Later, Darwin reasoned that it was most probable
that only one finch initially reached the
islands, but that it somehow diversified into
all the specialized types that he
observed. Many scholars think that this marked
the birth of descent with modification.
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What Darwin actually observed in his finches was
an example of an adaptive radiation. We use
this term when the descendents of one(or a
few) ancestral forms occupy many different
ecological niches (or ways of making a living)
and acquire different adaptations that fit those
niches.
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The marsupial mammal fauna of Australia is
another example of an adaptive radiation. Note
that,ecologically speaking, there are
bears,wolves, moles, foxes, etc. However, all
of these are marsupials, more closely related
to each other than to any placental mammal.
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The origins of domesticated strains, stocks, or
species by artificial selection was of great
interest to Darwin and central to the
development of his ideas of descent with
modification.
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1. Individuals within species are
variable. 2. Some of this variation is passed
on to offspring. That is, it is
heritable.
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3. In every generation, more offspring are
produced than can survive.
This implies some kind of struggle for
existence or competition among
individuals.