Sources
of genetic variation
Natural selection works only if there is variation
in the population, and this variation has a genetic (heritable) basis.
Phenotypic variation results from the inherited
genotype plus the influence of the environment. But only the genetic component
can influence adaptation through natural selection.
Polymorphism refers to characters that
have two or more traits in a given population. The ABO blood group, for
example, is a polymorphic character because there are four traits. Polygenic traits (height, eye colour, strength)
are responsible for much of the variation in a population.
Geographic
variation
arises because two different locales are likely to have at least a few
different environmental factors. A cline refers to the gradual changes in
some trait over a geographical range. The
yarrow plant shows distinct height variation associated with altitude, and some
of this variation is genetic. See
http://www.cord.edu/faculty/landa/courses/b315f99/sessions/variation/yarrow.jpg
Sources:
1. Mutations
produce new alleles. But only those mutations that occur in cell lines destined
to produce gametes can be passed on to the next generation. Mutations can include point mutations,
insertions/deletions, translocations, duplications, non-disjunction, breakage,
fusion, rearrangements, and faulty repair mechanisms. Mutations are rarely
beneficial. Mutations are very important sources of variation in organisms that
reproduce asexually and have very short generation times (ex: bacteria)
2. Sexual recombination
is the more important source of variation in plants and animals. Nearly all genetic variation in the population is the result of new
combinations of alleles. And the gametes from the same individual will vary
infinitely due to segregation and crossing over. Each generation represents new
combinations of old alleles.
Preserving genetic variation
Natural selection tends to reduce variation, but
there are several ways to counter-balance the reduction.
1. Diploidy The diploid nature of eukaryotes allows recessive
alleles to be retained in the heterozygous population. Recessive alleles are exposed to natural
selection only when they are present in two copies in the homozygote. If 1% of
a population is homozygous recessive, then 99% of the allele is protected in homozygotes. The rarer the allele, the
greater its protection.
2. Heterozygote
advantage preserves variation by favouring heterozygosity.
Another term for this is hybrid vigour. In
humans, those carriers of the sickle-cell trait have an advantage over either
homozygote. “Heinz 57” dogs are usually much healthier and live longer than
purebred dogs.
3. Neutral
variations remain in the population because there is no impact on
reproductive success. This is in no way a clear-cut distinction; what may be
neutral in one environment, may be damaging or beneficial in another.
4. Sexual
selection may be referred to as sexual dimorphism where the two sexes are
clearly distinguishable by appearance, behaviour, or
other measurable or observable traits. Since, again, much of these differences
are heritable, this variation will be preserved. Sexual selection almost always
involves females choosing males, not the other way around.
5. Frequency-dependent
selection occurs when one variation (morph) is selected against if it becomes
too common in the population. This is often seen in animals where protective colouring is varied.
Modes of natural selection
There are three ways in which natural selection
can affect the frequency of heritable traits.
1. Stabilizing
selection selects the intermediate variations at the expense of the extremes.
Best example of this is the narrow range of birth weights in humans.
2. Directional
selection will favour one extreme variation over the
other. This is commonly seen when the environment undergoes a drastic change
(ice age, e.g.) or the population migrates to a new habitat.
3.
Diversifying (disruptive, in your textbook) selection favours
either extreme at the expense of the intermediate variants. Assortative
mating may bring about conditions of diversifying selection (it certainly
reduces the frequency of heterozygotes).
The basis of each mode of natural selection is
exactly the same: differential reproductive success.
Speciation
There are two patterns of speciation
Anagesis refers to the accumulation of changes over time
leading to the transformation of one species to another (commonly seen in the
fossil record – also referred to as phyletic
evolution) and
Cladogenesis which is the emergence of two
or more species from a parent population (also referred to as branching evolution).
Cladogenesis is responsible for the diversity of species
we see today.
How does branching evolution occur? When two populations
accumulate so many changes in their gene pools that they are no longer able to interbreed,
they must be classified as two distinct species according to our definition of species
(biological concept). This can be described as reproductive isolation. Mechanisms
that bring about reproductive isolation can be classified as pre-zygotic or post-zygotic.
Pre-zygotic Reproductive Isolating Mechanisms (RIMs)
Pre-zygotic RIMs prevent
mating or fertilization. No zygote, no offspring.
A. Ecological, or habitat
isolation. If two populations are isolated by geography, they cannot meet,
and they cannot mate. Two species of garter snakes are isolated because one
lives mainly in the water and the other lives mainly in terrestrial environments.
Mountains and oceans are excellent geographic barriers. Parasites are isolated
from each other if they live in different hosts.
B. Temporal, or seasonal
isolation. Populations may mate or flower at different seasons or different
times of day. Three tropical orchid species of the genus Dendrobium flower just for a
single day, the flowers opening at dawn and withering by nightfall. Flowering inthese three species all occurs in response to certain
meteorological stimuli, such as a sudden storm on a hot day, but the lapse
between the stimulus and flowering is
eight days in one species, nine
in another, and ten or eleven in the third. Fertilization among the three
different species becomes impossible because at the time when the flowers of
one species open, those of the other species have already withered or are not
yet mature.
C. Behavioural isolation. This is often seen
when courtship rituals differ between populations. Courtship can play a significant
part in species recognition. If either of the two sexes decides that the
sequence of events in the mating process is incorrect, then the entire process will
be interrupted. Courtship and mating rituals have been extensively analyzed in
some mammals, birds, and fishes, and in a number of insect species.
It can be remarkably strong even among closely
related species. The 200+ species of Drosophila
on the
D. Mechanical isolation. Copulation is often impossible between different
animal and plant species because of incompatible shape and size of the
genitalia. Differences in body size could also come under this category.
E. Gametic isolation occurs
when fertilization cannot take place between species. In some animals with
internal fertilization, sperm may fail to survive in the sperm receptacles of
females of other species. In plants, pollen grains of one species typically
fail to germinate on the stigma of another species so that the pollen never
reaches the ovary and
fertilization cannot occur. In many aquatic animals
the ova and sperm are shed into the water. In such animals, gametes of
different species generally fail to attract one another.
Postzygotic RIMs
Post-zygotic RIMs are
more wasteful than pre-zygotic RIMs because a zygote is established but will not result in the
propagation of the species.
1. Hybrid inviability – the
zygote fails to develop properly or dies at birth
2. Hybrid
sterility - the zygote develops and matures into adulthood but cannot
reproduce. The most famous of all animal hybrids, the mule, (a cross between
horses and donkeys,) is sterile. Hybrid breakdown
3. Hybrid
breakdown – The adults may reproduce but their offspring are weak or infertile . Hybrids between the cotton species Gossypium barbadense and
G. tomentosum
appear vigorous and fertile, but their offspring die as seeds or early in
development, or they develop into sparse, weak plants unable to reproduce to
create a third generation.
If none of these conditions apply,
then two populations may potentially interbreed in nature, and, by definition,
belong to the same species.
