12/3/02
Outline:
Microevolution,
speciation and the fossil record
Species
definitions
Evolution
Variation
Natural selection
types of selection
Speciation
Reproductive isolation in time
Phyletic speciation and pseudoextinction
Reproductive isolation in space
Branching speciation in general: example-
allopatric speciation
Patterns
in the fossil record
Phyletic gradualism
Punctuated equilibrium
When
somebody uses the term microevolution, they are usually referring to evolution
at the level of populations and species; the details of evolution, as opposed
to the big picture of evolution (macroevolution)-evolutionary patterns, trends,
mass extinctions, bursts of evolution.
Some important
terms concepts and terms
1. species - biol. sp. definition: groups of
populations whose individuals interbreed to produce fertile offspring.
- morphological species definition: groups
of populations whose individuals are morphologically similar to each other (so
similar that they probably interbred)
2. evolution - changes in the relative
frequency of heritable characteristics that take place from generation to
generation.
3. speciation – the origin of a new species
(can have evolution w/or speciation, but no speciation w/o evolution)
The basic
ingredients of evolution: variation
& natural selection,
1. Variation. Individuals within
a species vary (skin, hair, eye color, for example).
environmental variation -(some variation is environmental - hair length, scars, body weight)
ontogenetic
variation(age-related) size, sexually mature vs. immature, age- related features.
heritable variation- For evolution to work, that variation needs
to be heritable. Eye color, skin color,
hair color,
New
heritable variation arises from mutation, the transformation of genetic
material due to mistakes in copying or damage from environmental causes. The sources of new genetic variation are
varied and whole courses are devoted to the subject. The one important thing I do want to mention is that the
direction of variation is random.
Mutations do not arise because of some "need" of the organism.
2. Natural selection.
Some of those individuals in the species may, because they possess a
heritable characteristic, be more likely to survive to reproductive age, or may
be more likely to produce offspring
Hypothetical
example: nose size in humans
OVERHEAD on pigs
To the extent
that nose size is a heritable characteristic, we have changed the relative
frequency of a characteristic from generation to generation. The population has evolved.
NATURAL SELECTION the differential survival and or
reproductive success of individuals having a particular heritable trait. Survival of the fittest: survival of the
sexiest
A few more words
on natural selection: not just the
death of individuals - that does indeed prevent the individual from passing on
their genetic makeup to the next generation.
Differential reproductive success can be achieved via: (elicit one or two of these)
-death
prior to mating (or a shortened sex life)
-greater
success in mating (higher % of fertilization)
-producing
more or healthier offspring
-longer
sex life
-greater
frequency in mating
Note that the
reasons for these greater successes may be because of greater efficiency in
food gathering, competition, resistance to disease, cold, what have you.
Types of
selection
1. Directional : shift in mean value. Persistent selection against (or for) one
extreme.
2. Stabilizing: No change in mean value, but a decrease in variation or at least
a stable range of variation thru time.
Extremes selected against (or mean selected for).
3. Disruptive.
Mean selected against, extremes selected for. May be difficult to maintain in the face of interbreeding.
Let's now
consider speciation - how to make a species.
To the two
ingredients of evolution: variation and natural selection, we need to add a
third –
the
interruption of interbreeding.
Here's one
possibility: Phyletic speciation - the evolution of one species into
another thru time. After a number of generations,
the first and last of the populations are morphologically distinct enough to
earn the title of different species.
That is, individuals in the descendant population probably couldn't
interbreed with individuals in the ancestral population.
-This is
sometimes called phyletic speciation or phyletic evolution.
-It's not clear
where to draw the line from one species to the next.
-There is no
gain or loss in the number of species present.
Note pseudoextinction
******************************
Here's a second
possibility: branching speciation (cladogenesis- the formation of a new
clade (a species in this case) in addition to the existing one. Branching.
Here,
interruption of interbreeding most often takes place through geographic
isolation (called allopatric speciation) though there are other ways to
isolate populations.
OVERHEAD
Two populations
undergo evolution in somewhat different habitats, and thus experience different
regimes of natural selection. Upon
removal of the barrier, the populations are different enough from one another
that individuals are not able to interbreed.
New species are
added and diversity increases. The
ancestor may persist along with the descendant.
Speciation in
the fossil record:
When we look at
the actual record of speciation in the rock record, that is, when we trace
actual species lineages up through stratigraphic columns, two basic patterns
have been detected:
1. Phyletic gradualism.
Evolutionary change is continuous and relatively slow. Continuous change with gradual divergence
example of Ordovician trilobites on handout.
2. Punctuated equilibrium.
Evolutionary change concentrated in relatively short periods of
time. Stasis then quick change.
Peter
Williamson's Neogene, fresh water gastropods. from Kenya. Made measurements on the shell of 13 species
(not just size). Intervals of
morphological change of 5000 to 50,000 years (short relative to total duration. Most of the time nothing is happening.
Current issue is
over how frequent one of these patterns is relative to the other. Which is the more common pattern of
microevolution?