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Chapter 24: The Origin of Species
Things to Know:
All those bold-faced words again.
If you look at pg 464 and the summary on page 482, you see the chapter can be
broken up into 3 essay questions. You should:
Be able to define a species and explain why a species can only reproduce
with its “own kind.”
b) Explain how a species can arise (modes of speciation)
c) Be able to define macroevolution and explain how large-scale changes have
occurred over time.
New Species and Mutation
Considering the processes of heredity and evolution, how do you think new
species arise with respect to mutation? Which is more important, the
accumulation of small mutations or a few mutations with large phenotypic
Major mutations are deleterious. Such a big change in a system
(organism) that is in balance with its environment, usually is such a shock
or great disturbance to the genetic systems within which they are arise
and therefore cause death.
• So genotypic changes caused by major mutations are not likely to be
valuable aspects of evolution
• Major changes in to a phenotype in “one jump” are practically lethal.
Small mutations can accumulate over time because the small mutations
barely show distinguishable changes in structure and function of the
organism in which they occur.
• The organism then does not find it difficult to exist in the same
So these small mutations over many generations will accumulate and have
significant evolutionary value. Fossil evidence and field studies show this.
Evolutionary changes appear to arise as a result of many small changes that
have accumulated over time and ultimately producing a change in a
particular population’s gene pool.
The Orchid
• The Orchid:
Its flower resembles a female wasp in shape and color.
Male wasps, thinking it is a female, are attracted to the flower and
are stimulated to copulate with it and in so doing get coated with
sticky pollen grains from the orchid.
Pollen is carried to next orchid.
Any mutation that gave the orchid more of a similarity to a
female wasp was selected for by its increased chances of being
Hence an advantage over those orchids not looking like female
wasps. The increased reproduction of the female wasp looking
orchids caused the propagation of the gene causing this
The Guppies
Darwin never imagined that anyone could witness natural selection.
Darwin assume evolution was too slow and gentle.
You may also be under this same impression, albeit incorrect.
UC Riverside’s David Reznick studied guppies in the streams and pools in the
forests of Trinidad.
Hypothesis to be tested: life history of animals can evolve if mutations that alter
it bring the animals more reproductive success.
Basic Characteristics of the Environment and the Guppies
At the lower elevations, the guppies were exposed to predatory fishes
At the higher elevations,s the guppies escaped such onslaught because
the fish couldn’t move upstream past waterfalls and rocks.
At the lower elevations with the predator exposure, successful guppies
should have short life spans, reaching sexual maturity quickly and have
as many offspring as possible.
The Guppies
Cost of having reach sexual maturity quickly is the guppy shortens its
lifespan and it can’t look after its babies too long (supply them with much
of her energy)
The Experiment
Reznick took the terrorized, downstream guppies and moved them
upstream where there was fewer predators.
Observed. In the 11th year. . .
The Guppies
The Results
He noticed the guppies were in less of a rush.
They took 10% longer to mature than their ancestors
They were 10% heavier by the time they were fully grown.
They were laying smaller broods of eggs but each new hatched guppy
was bigger.
The Concept
Evolution occurred within 11 years which is a flash in the pan (it’s a fish
The Finches of Peter and Rosemary Grant:
1973: went to the Galapagos to study the effects of natural selection on
the birds.
There is a standard weather pattern.
• First five months of the year: hot and rainy
• Then cool, dry period.
1977: no wet season caused by La Niňa and therefore a drought.
On one of the islands, Daphne Island, drought was lethal.
• of 1200 medium ground finches ( Geospiza fortis) living on the island,
more than 1000 died.
• G. fortis lives mainly on seeds that it cracks with its beak.
The Finches of Peter and Rosemary Grant
• Small G. fortis cracks the small seeds, but larger birds have larger
beaks and can crack the larger seeds.
• With the drought, small finches ran out of small seeds and began
dying off while the larger birds managed to survive on the larger seeds,
especially seeds seeds from caltrop which had spiked shells to protect
• These 1977 survivors mated in 1978.
The Finches of Peter and Rosemary Grant
• 1978 offspring: the beaks were on average 4% larger than those of the
previous generation.
• The surviving big-beaked finches passed on their beak trait to their
offspring and this altered the profile of the entire population (remember
that the small beaked birds had died off).
• 1983: heavy rains, abundant seeds which favored finches with smaller beaks
• by 1985, the average size of the beak had decreased 2.5%
• From 1976 – 1993, Grants found no trend in beak size. If, given the climatic
conditions, the beak helps it survive its first year, then lots of offspring. So
sometimes big is better and sometimes smaller is better.
The Finches of Peter and Rosemary Grant
Conclusion and Comments
Climatic fluctuations can make natural selection drive a population in one
direction for a while or in circles.
If a group of finches settles on an island where other finches are already
located, evolution could favor the genes in the newcomers that allow the
newcomers to find their own “niche”, that allows them to eat other kinds
of food because competition will be less.
With enough time a new kind of finch could emerge.
Lake Victoria and the cichlids (fish):
Sympatric Speciation
Reported to be the “biggest explosion of speciation occurring on Earth.”
Occurring in the other Great Lakes of East Africa
Huge lake: 27,000 sq. miles of East Africa; very flat floor
500 species of these small, brightly colored fish
Each species has a special feature making it unique
• some scrape algae off rocks with their teeth
• some crush shellfish
• some pluck the eyes of other species of cichlids
• some species have males that build underwater sand castles for females
to inspect (isn’t that ssspecial)
• some carry their young in their mouth
Lake Victoria and the cichlids (fish)
The Discovery
1995: Geologist discovered that the Lake has dried and filled up many
times in its history.
The ancestors of the current cichlids must have gone up into nearby
streams to avoid the emptying of the lake.
The present cichlids have similar genes and show a single lineage of
Lake Victoria and the cichlids (fish)
Evolutionary Changes: Their Mouths
Cichlids have an extra set of jaws at the back of their mouths that can be
used to break up food. This leaves the front jaws free to evolve intonew
kinds of grabbing tools.
Their teeth have turned into pegs, spikes and spatulas
Therefore their bodies have been reshaped into different forms.
Lake Victoria and the cichlids (fish)
Evolutionary Changes: Sex Lives
Some have complicated dancing “jigs.” (Didn’t I see some of you doing
some of these moves at the last dance?
some build bowers (attractive dwellings) out of sand and gravel
The choices the females make as to a mate are influenced by their genes.
• if a female prefers a particular pattern or amount of red in a mate that
trait is passed on.
• if she prefers a particular jig in the mating dance, that is also passed on
to the offspring.
• these preferences may spread throughout the females until they become
indifferent to any other male. Then these tastes isolate a population of
fish and turn it into a new species.
Lake Victoria and the cichlids (fish)
When Lake Victoria was going through changes (“going through these
changes” – Disturbed [no I’m not, yes you are, no I’m not. . ] the cichlids had to be
very flexible in their adaptations so they could survive in a variety of
streams and stream characteristics.
But once the lake formed and became stable, they could adapt to special
habitats such as rocky shores or deep waters with sandy bottoms.
Specialized ways of living had time to appear without “punishing” the
fish for their individuality!!!
Genetic differences are now being studied between the species.
In the 50’s and 60’s the Nile perch was introduced into the lake as a
source of food for people living around the lake. Yep, that’s right, the
Nile perch are eating the cichlids. The Nile perch grow up to 2 meters
long (and that is without illegal nutritional supplements!!)
Lake Victoria and the cichlids (fish)
Also, massive soil erosion from farming and logging is muddying up the
waters and so the cichlids that rely on coloration as a way too identify a
mate end up mating with other closely related species.
So the reproductive isolation that formed hundreds of new species is
Over half of the cichlid species in Lake Victoria have been eliminated in
30 years.
Sympatric Speciation: the formation of new species in geographically
overlapping populations.
Figure 24.16 Mate choice in two species of Lake Victoria cichlids
Figure 24.13 Sympatric speciation by autopolyploidy in plants
One mechanism for allopolyploid speciation in plants: Sympatric Speciation
Two different species interbreed and combine their chromosomes
Wheat is an allopolyloid
Figure 24.6 Two modes of speciation
Figure 24.7 Allopatric speciation of squirrels in the Grand Canyon
The Squirrels of the Grand Canyon:
Allopatric Speciation
Allopatric Speciation: Geography separates a species and this
separation allows for the organisms to evolve independently.
• No gene flow or little gene flow between the populations
• Could occur by a river separating land that was once joined; or by
earthquakes separating geographical areas so organisms are separated.
• So reproductive isolation occurs by water, mountains, deserts or
other kinds of terrain unsuitable for a species. These barriers reduce
or prevent the gene flow and permit the isolated population to evolve
independently of the other populations of the parental species.
The Squirrels of the Grand Canyon:
Allopatric Speciation
What Happens In This Isolated Population
Genetic processes occur:
• mutations
• certain genes may be lost due to “sampling” by nature during the
separation of the parental population.
• recombination produces a new diversity within the population.
• maybe immigration of different genes from other populations.
• Most Importantly, the isolated population is living in a new biotic
and physical environment than the parental population and is exposed
to different selection pressures acting on a new gene pool.
• If this process occurs long enough the isolated population may
become sufficiently different genetically to qualify as a different
The Squirrels of the Grand Canyon:
Allopatric Speciation
What Happens In This Isolated Population
• New isolating mechanisms occur that will prevent its interbreeding
with the parental population when the barrier disappears.
What Happens to these potential new species that are formed all the time?
Most reunite with the parental population before having reached the
species level or they become extinct.
Only a small fraction completes the speciation process.
Figure 24.8 Has speciation occurred during geographic isolation?
Figure 24.11 A model for adaptive radiation on island chains
Adaptive Radiation
What is Adaptive Radiation?
It is the successful establishment of an organism in numerous niches and
adaptive zones.
It is very common in higher organisms
• reptiles evolved into crocodilians, turtles, lizards, snakes
• mammals produced mice, monkeys, bats and whales
• birds evolved into niches of hawks, storks, songbirds, hummingbirds
(aren’t they cute) and penguins.
• each of these made its own suite of niches in nature without any major
change in its ancestor’s structural type.
Differentiate between adaptive radiation and convergent evolution.
Adaptive Radiation is also divergent evolution. A single species can
branch into many different forms that occupy different habitats.
• This is due to the constant competition for food, space, light whatever
environmental resources are needed.
• This is very advantageous to the process of evolution in that new
organisms can gain new advantages by utilizing new food sources or
escaping predators.
• The finches on the Galapagos were all derived from a single ancestor
and now exhibit different beak structure and size, feeding habitats. Many
feed on seeds, others on cacti, others live in trees and eat insects.
• The diversity of food sources allowed for the divergence from the
ancestral species while decreasing the intra-specific competition for
Differentiate between adaptive radiation and convergent evolution.
Convergent Evolution:
• occurs frequently
• is the process by which two or more groups become adapted to a
similar environment and then develop characteristics that are more or less
• Examples:
•Wings have evolved in birds but also bats, reptiles(pterosaurs) and
insects (grasshoppers)
•Sharks (a fish), dolphin and porpoise (both mammals) have
developed similar superficial similarities because of their adaptation
to a similar environment.
Punctuated Equilibrium
This deals with the time in which changes occur to form new species.
In PE, lots of changes occur just as a group of organisms branch from
their parental population, THEN they remain relatively unchanged for a
very long time like hundreds of thousands, if not millions of years.
It is referred to as a “stasis” of a widespread population species
Figure 24.17 Two models for the tempo of speciation
With what is macroevolution concerned?
the origins, diversifications and extinctions over time. That is, how we
get horses, tigers and things; the invasion of land by plants and
animals, the derivation of wings by bats, and birds, the evolutionary
explosion of mammals 120 million years ago.
Major Points
Many beautiful examples of evolutionary transformation from one
species exist but they occur in few environments that are good at r
recording such changes.
Terrestrial areas, subject to erosion and earth movements, have
spotty records whereas lakes accumulated silt or deep-sea
deposits record a steady rain of plankton from the surface
So, yes, the fossil record is incomplete and imperfect.
So is our information about living biotic environments
The nature of the record, however, makes sense within
biological constructs such as:
Macroevolution: Major Points
1) Speciation
Hardparts resist physical and chemical destruction so they
remain over time in the form of shells, teeth or pollen
grains and provide a more complete record than flimsier
parts and their respective organisms.
Rare species will be less frequently preserved than
abundant ones; localized more frequently than widespread
Macroevolution: Major Points
Punctuated Equilibrium
A “bush not a ladder”
Punctuated Equilibrium
Again, morphological changes occur in close association with the
geologically-rapid splitting of populations into new species and
then they are morphologically static over most of their histories.
“A bush with branches, not a ladder”
There is no directional change but lots of stems and twigs.
Diversification is the key to withstand the extinction that occurs
over much of time.
Complex changes like from dinosaur to bird does not occur as a
single mutational step.
Macroevolution: Major Points
4) Fossil Record
At higher levels, the fossil record is full of “missing links.”
There is a spectacular abundance of intermediate forms between
man of the major groups of plants and animals.
Reptiles to mammals have many intermediate forms
Different features evolve at different rates and times.
Archaeopteryx, earliest known bird, has feathers and a
wishbone which are common features with modern birds
but it retained the dinosaurian tail, clawed forelimbs,
pelvis and teeth of its ancestors.
Humans evolved upright posture before a shorter face
with a larger cranial capacity.
Macroevolution: Major Points
5) Morphology and Genes
Major changes in morphology don’t always require major genetic
Modest changes in gene expression can cause striking differences
in morphology
Major control genes that establish the body axes of embryo or
Hox genes that determine the positional alignments along the
anterior-posterior axis are conserved throughout all animals
and changes in their expression can produce major
evolutionary transitions in animals.
Limb development can proceed even if jaw development has
changed producing modest or vast changes.
Apes and humans can have a difference of 50 genes but be so
different if the genetic changes are located in the expression
pattern of these genes.
Figure 24.23 Hox mutations and the origin of vertebrates
Macroevolution: Major Points
6) Pulses in geologic time
Major evolutionary events are pulses in geologic time
Cambrian explosion: appearance of most multicellular animal
designs within a 10 million year window starting about 530 million
years ago. This is 0.5% of the history of Earth to that point.
Other “pulses” of evolutionary change:
Land invasion by plants
Vertebrate diversification
Mass extinctions: this removes dominant forms and provides
an opportunity for survivors to diversify.
Macroevolution: Origin of Birds
Today’s Perspective on the Origin of Birds
Major evidence comes from the anatomy of living species along with
anatomy of fossils.
Details of the head, neck, arms, hands, pelvis, hind limb, and tail all
point to a dinosaurian ancestry for birds.
Discoveries in China reveal soft tissues including “proto-feathers”
Feathers originated before the ability to fly and that feathers were only
secondarily adapted for flight.
Feathers of birds develop first for insulation and only later in
development do flight feathers and the ability to fly emerge
The first dinosaurs to have feathers were fast-running predators who
probably used their arms to grab prey. The bones of the wrist constrain
the movements of their hands to the same pattern of movement that we
see in the flight stroke of birds.