Download Evolution Course Notes Theory of Evolution

Evolution Course Notes
Theory of Evolution-all organisms have arisen from a pre-existing form
Therefore, evolution suggests that all organisms have a common ancestor.
Evolution = change
1. micro-evolution -change within a species, organisms adapt to survive in their environment.
2. macro-evolution -change from one species to another forming new species.
Speciation -the process of producing a new species from existing species.
Evolution is "driven" by natural selection. That is, natural selection theory suggests a method
by which species can adapt and undergo speciation.
Useful Terms:
adaptation -the adjustment a population makes to its environment over a period of time
positive mutation -a mutation that allows an individual to be better adapted to its
environment.
gene pool -the sum of all the genes (with their respective alleles) contained in a population at
a given time.
Natural Selection -is the process that results in the survival of those organisms best suited
for their environment.
Darwin and Wallace: theory of evolution through natural selection:
1. Variation - there must be genetic variation in individuals of a population.
2. Overproduction of Offspring - many of each type of phenotype must exist.
3. Struggle for Existence - some organisms will die, the environment selects those organisms.
selective pressure = anything that acts as a selector on existing phenotypes such
as predators, weather, competition etc.
4. Differential Survival and Reproduction - those organisms that are best adapted to their
environment will survive and those organisms that survive may reproduce, therefore
contributing their genes to the gene pool.
Darwin's survival of the fittest = survival of the best adapted.
Therefore, the environment helps determine the traits of individuals within a population.
Giraffes: An Example of Natural Selection
Giraffes have always fascinated biologists as outstanding
examples of adaptation (micro-evolution). Their
extremely long necks and long legs, with the front legs
longer than the hind ones, allow them to reaching the
leaves and tender twigs of the trees on which they feed that they have no serious competitors for
that food source. The question was, what made them grow that way?
Darwin correctly recognized that variation occurs naturally in all
species and that some factor in the environment could select individuals
with certain variations to reproduce and preventing, or at least reducing,
the reproduction of others. In the case of the giraffes, the selective factor
(or selective pressure) would have been the trees, which provided food
only for giraffes that could reach high enough. It seems likely that whenever ancestors of
giraffes first found themselves in their present environment, they probably had short necks
too, and as long as some leaves and twigs were close enough to the ground, a short neck was no
disadvantage.
With the passage of thousands of years, in an environment which selects positive mutations to
create a survival advantage (like being able to reach food higher in the trees) populations
adapt to their environment.
When the giraffe population reached the carrying capacity of its environment and a food shortage
developed - as it will, sooner or later-giraffes with slightly longer than average necks had a
survival advantage. If these individuals were not the only ones to survive, they at least
survived in larger numbers. Therefore, in natural selection, some individuals die and those
with a survival advantage live to reproduce. These survival traits are the ones that we see in
the population and most individuals in the population have these traits. They are adapted to
their environment.
Natural Selection We Can See
The peppered moth is an example of natural selection that occurs over a shorter period of time.
Birds selected black moths because they were easier to spot. The peppered moth was taken less
often and so survived to reproduce more often. The population of this species of moth consists of
mostly the peppered variety. Predators are the main selective pressure.
In the vicinity of Manchester England, a rather large moth (Biston betularia) was well known.
It was called the peppered moth because it resembled a white moth on which pepper had been
sprinkled. It was nocturnal in its habits and spent the daylight hours resting on the trunks of
trees, where it blended in almost perfectly with the lichens covering the tree trunks, since the
lichens had the same "peppered" coloration. Only occasionally was a rare black member of this
species spotted.
In the 2nd half of the 19th Century, more and more black moths began to show up. That is the
population began to change from mostly peppered moths to black moths. What could have caused
this change in colour of this species?
Birds selected peppered moths leaving black moths. Black moths survived and reproduced more
frequently passing on this trait to their offspring.
Coloration of this species of moth is due to a single pair of genes, with the peppered allele being
dominant to the black allele.
Explanation:
Changes in the moth colour corresponded exactly with the progress of the Industrial Revolution.
In that coal burning region of England, the amount of soot put into the air by factories was so
great that it covered the tree trunks, killing the lichens and turning the tree trunks black.
Under these conditions (environmental change) the black moths were better camouflaged
(better adapted) Eventually 98% of the moths of this species in industrial areas were black. In
unpolluted forests, the moths retained their light coloration.
Note that the population changed colour, as individuals cannot change their colour.
Continuous Variation
Most traits in a population. e.g. height and body weight vary in a continuous way from one
extreme to the other.
A plot of the distribution of the trait in a population often produces a bell-shaped curve like this
one that shows the distribution of heights among a group of male secondary-school seniors.
Such a distribution could arise from
a. environmental factors - perhaps the continuous height variation in the boys is simply a
result of variation in their diet as they grew up.
b. genetic factors - tall parents tend to have tall children
or - most likely - both.
Types of Natural Selection
You might expect that natural selection would continually "improve" or make a species more fit
for its environment by weeding out unfit traits. For example, we might expect that giraffe neck
size would continue to increase generation after generation if the environment was selecting this
trait (ie. if food supply was limited and unable to sustain the entire population).
The pressures of natural selection can affect the distribution of phenotypes in a population in
several ways.
Stabilizing Selection
Natural selection often works to weed out individuals at both extremes of a range of phenotypes
resulting in the reproductive success of those near the mean. In such cases, the result is to
maintain the status quo. It is the continual elimination of extreme phenotypes, continually
selecting a particular trait. Therefore, this trait is said to be stable because it is maintained
generation after generation.
It is not always easy to see why both extremes should be handicapped; perhaps sexual selection
or liability to predation is at work. In any case, stabilizing selection is common. In humans, for
example, the incidence of infant mortality is higher for very heavy as well as for very light
babies.
Explaining Stabilizing Selection
Observation shows that their is very little variation in giraffe neck size, most have about the
same size of neck. In stabilizing selection, short necked giraffes are eliminated from the
population because this trait is a disadvantage. It is possible to suggest that having too long a
neck may also be a disadvantage, perhaps hindering the giraffes survival because it does not
have the developed musculature to hold the head up. Therefore, extremely long and short necks
are eliminated leaving a stable trait (very little neck size variation) that is found in most
individuals.
Directional selection - selects for the most extreme phenotype and so is likely to result in
the gradual replacement of one trait by another in the population. (giraffe's necks continue to
get longer)
A population may find itself in circumstances where individuals occupying one extreme in the
range of phenotypes are favored over the others.
Example: Darwin's finches in the Galapagos Islands.
When rainfall, and thus food, are plentiful, the birds tend to
1.
have a varied diet, e.g., eat seeds of a range of sizes
2.
show considerable variation in body and beak size (large beaks are better for large
seeds but can handle small seeds as well as small beaks)
From 1976 through 1977, a severe drought struck the islands, with virtually no rainfall for
over a year. This caused a large decline in the production of the seeds that are the dietary
mainstay of Geospiza fortis, the medium ground finch. The population declined from 1400 to
200 on the island of Daphne Major, a tiny (10-acre) member of the Galapagos Islands.
One of the plants to make it through the drought produces seeds in large, tough fruits that are
virtually impossible for birds with a beak smaller than 10.5 mm to eat.
Sampling the birds that died as well as those that survived showed that
1.
the larger birds were favored over the smaller ones
2.
those with larger beaks were favored over those with smaller ones.
Beak length
Beak depth
Dead birds
10.68
9.42
Survivors
11.07
9.96
Here, then was natural selection at work.
But did it produce evolution?
The answer turned out to be yes. As the population of G. fortis recovered after the rains
returned, the average body size and beak depth of their offspring was greater than before (an
increase of 4-5% for beak depth). The bell-shaped curve had been shifted to the right.
Disruptive Selection
In some circumstances, individuals at both extremes of a range of phenotypes are favored over
those in the middle. This is called disruptive selection.
An example:
The residues ("tailings") of mines often contain such high concentrations of toxic metals (e.g.,
copper, lead) that most plants are unable to grow on them. However, some hardy species (e.g.
certain grasses) are able to spread from the surrounding uncontaminated soil onto such waste
heaps. These plants develop resistance to the toxic metals while their ability to grow on
uncontaminated soil decreases. Because grasses are wind pollinated, breeding between the
resistant and nonresistant populations goes on. But evidently, disruptive selection is at work.
Higher death rates of both
1.
2.
less resistant plants growing on contaminated soil and
more resistant plants growing on uncontaminated soil
leads to increasing divergence of the populations into two sub populations with the extreme
manifestations of this trait.
The evolutionary significance of disruptive selection lies in the possibility that the gene pool
may become split into two distinct gene pools. This may be a way in which new species are
formed.
The formation of one or more species from a single precursor species is called speciation.
Macro-evolution: change from one species to another forming a new species. Also known as
Speciation
What Is a Species?
One of the best definitions is that of the evolutionary biologist Ernst Mayr:
A species is an actually or potentially interbreeding population that does not interbreed with
other such populations when there is opportunity to do so.
Note: sometimes breeding may take place (as it can between a horse and a donkey) but if so, the
offspring are not so fertile and/or well adapted as the parents (the mule produced is sterile).
We regard individuals to be of the same species only if they interbreed freely (or are at least
capable of interbreeding) and produce fertile offspring .
Allopatric Speciation: the Role of Isolation in Speciation
The formation of two or more species often requires geographical isolation of sub populations of
the species. Only then can natural selection or perhaps genetic drift produce distinctive gene
pools.
It is no accident that the various races (or "subspecies") of animals almost never occupy the
same territory. Their distribution is allopatric ("other country").
Leopard Frogs - are found over the entire eastern half of North America, from southern
Canada to northern Mexico. All leopard frogs were thought to belong to the same species, Rana
pipiens. However, experiments where northern frogs were mated with southern frogs and vice
versa resulted in eggs that died. It seemed that frogs at the northern and southern parts of the
frogs range were different species by our definition that requires individuals of a species to be
able to successfully reproduce. The gene pool refers to the total set of genes in the entire
population of a species at a given point in time. It is believed that all leopard frogs shared a
common gene pool at one point in time, we will call that species the ancestral species. As the
population of this species grew and spread over eastern North America, there eventually came a
time when sheer distance proved to be a barrier to free gene flow (reproduction) between
northern and southern frogs. Such situations that separate species preventing reproduction are
called isolating mechanisms. With the passage of time, natural selection would exert its
influence and that would be a different influence in different places. Temperature is one factor
that would be different from south to north and eggs from northern frogs are able to survive and
hatch in water too cold for southern frog eggs. Therefore, the northern and southern frogs
adapted through natural selection to different environments, the environment selecting traits
(mutations) that would provide a survival advantage. Over time the ancestral gene pool of both
northern and southern frogs changed as different genes were eliminated and added. With
increasing differences between southern and northern populations, successful interbreeding
was no longer possible and two species of leopard frogs had evolved from one ancestral species.
Darwin’s Finches - when members of a population undergo evolutionary divergence, new
species can develop from a common ancestor. For this to occur, members of the population must
be reproductively isolated from each other and each group must be exposed to different selective
pressures. Therefore, different groups of the population adapt to different selective pressures,
their gene pools diverge (become different) and new species may be produced, each unable to
reproduce successfully with the other. A good example of this can be seen in the Galapagos
finches first described by Darwin.
Fourteen species of finches are found on the Galapagos islands, all have similar plumage, nest
building behaviors and calls, but the different species can be easily distinguished by the size and
shape of their beaks. Five of these species are shown here. Among the animals Darwin studied
were 13 species of finches found nowhere else on earth.
Adaptive radiation suggests that an ancestral mainland finch populated the islands very soon
after they were formed. With no other species of bird to compete with, finches soon occupied all
island niches. Since ecology varied from island to island, finch populations evolved slightly
different adaptive patterns. In order to reduce competition for food, finches adapted different
feeding habits and evolved different beak structures to match their food source.
Darwin's finches are an elegant example of 2 important aspects of the evolutionary process:
geographic isolation (isolating mechanism) and adaptive radiation. Over an extended period
of time, geographic isolation allows natural selection to produce new species. The evolution of
distinctive structural adaptations and behavior patterns in a group of closely related descendant
species is called adaptive radiation and is a means by which organisms respond to competition
for resources. Natural selection favors those individuals whose characteristics are most suited
for survival and reproduction for a particular way of life.
Adaptive Radiation
The process described above can occur over and over. In the case of Darwin's finches, it must
have been repeated a number of times forming new species that gradually divided the available
habitats between them. From the first arrival have come a variety of ground-feeding and treefeeding finches as well as the warbler like finch and the tool-using woodpecker like finch.
The formation of a number of diverse species from a single ancestral one is called an adaptive
radiation.