Transcript Mechanisms of Evolution

Mechanisms of Evolution
Genetic Variation
• Genes – DNA sequences that code for specific
polypeptidesat specific loci (locations on the
chromosome)
• Most eukaryotic organisms are diploid (have
two sets of chromosomes, one set from each
parent), but some, like wheat, are polyploid
(have more than two sets of chromosomes)
• All chromosomes (except sex chromosomes)
appear as homologous pairs, where each pair
possesses the same set of genes
• Allele – particular forms of a gene; most
genes have two alleles or more (B for
dominant alleles, b for recessive)
• Homozygous individuals have two copies
of the same allele for a given gene
(represented as BB or bb)
• Heterozygous individuals have two
different alleles for a given gene
(represented as Bb)
• Differences in genotypes and
environmental influences account for
differences among phenotypes of
individuals
• Phenotype – the physical, observable
traits of an organism (i.e.: hair colour, eye
colour)
• Each individual within a species has a
different genotype (genetic identity of an
individualthe set of all alleles the
individual possesses)
Random Change
•
Allele frequencies change all the time in
natural populations due to:
Genetic Drift (random loss of an allele in small
populations)
Migration or Gene Flow (immigration and
emigration of a population)
Mutation
Non-Random Mating
Selection (adaptive evolutionary changes)
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Discussed in its own section (later)
Genetic Drift
• causes the frequencies of alleles to be altered
in small populations
• can have random loss of alleles due to:
- Few individuals
- Individuals failing to reproduce
• tends to enhance differences between
populations
Founder Effect
• few individuals create a new, isolated
population so alleles that they bring have
extreme importance
• the allele frequencies in the founder population
do not necessarily represent those found in the
source population
– if alleles were rare in the source population, they
now make up a significant fraction of the new
population’s alleles.
• the genetic features expressed today
(depending on maintained isolation factor) are
characteristic of the founders
• Amish community in Pennsylvania– 30 people
founded it; one couple had a child heterozygous
for Ellis-van Creveld syndrome (dwarfism,
shortened limbs, extra fingers and toes. Die
within a few months of birth)
– Now there is a high incidence of this rare
genetic disorder due to the fact that there has
not been a lot of genetic variation added.
This trait is present in this population at a
higher than normal rate (7% as opposed to
0.1%)
Bottleneck Effect
• dramatic, often temporary, reduction in
population size, usually resulting in significant
genetic drift
• small founder population becomes the sole
source of alleles for a given species
• cheetahs in Africa underwent crisis 10 000
years ago which depleted their numbers; within
the last century they have been almost hunted
to extinction
• the decrease in numbers has lowered the
genetic variability and has allowed for an
increase in fatal recessive disorders, which may
cause the cheetah to become extinct
Migration/Gene Flow
• movement of individuals from one population to
another
• if new animal moves to a region and its genetic
makeup allows it to survive (adapt) better than
individuals already present, then genes will be
passed on (if mating is possible)
• migration of gametes is difficult to observe but it
does happen
– male gametes on flowers can be transported large
distances
• unlike genetic drift, gene flow tends to
reduce differences between populations
Mutation
• the ultimate source of genetic variation; what
makes evolution possible
• the ONLY source of additional genetic material
and new alleles
• most mutations occur in somatic cells, but these
cannot be inherited and so do not play a part in
evolution
• only mutations in gametes have the potential to
be passed on
• Neutral, harmful or beneficial
– All depends on effects on fitness (reproductive
success)
Non-Random Mating
• individuals with certain genotypes often mate
with one another at a higher than normal rate
Inbreeding
• causes frequencies of particular genotypes to
differ greatly from HW principle
• does not change the frequency of the alleles,
just the proportion of individuals that are
homozygous.
• Homozygous recessive individuals are more
likely to be present more likely to have
genetic problems
Selection
• The most powerful of the 5 principle agents of
genetic change
• Stabilizing selection – selection against
individuals with traits that deviate from the
population average
• occurs when the most common phenotypes
within a population are most favoured by the
environment
• Stabilizing selection is the most common form of
selection
– once a species becomes adapted to its environment,
selective pressures maintain their traits
• An example is hummingbirds
• Hummingbirds will succeed in drawing nectar
from flowers if their bill and tongue length are
well adapted for the size of flowers they feed on
• Longer bill = more nutrients and energy to
grow, as well there’s more weight to carry
around (greater expenditure of energy)
• Shorter bill = reduces a bird’s ability to reach
food
– Ideal bill length also increase the success of flower
pollination, enhancing the reproductive success of
the flowers as well as the hummingbirds
• Environment will select against mutations that
produce birds with a bill length that differs from
the best-adapted length:
• Directional selection – selection that
favours an increase or decrease in the value
of a trait from the current population average
• Occurs when the environment favours
individuals with more extreme variations of a
trait
• When an organism migrates to a new
environment or aspects of its habitat change,
it will encounter new forces of natural
selection
• if the hummingbirds move to a new habitat with
longer flowers, individuals with bills that were
adapted to medium-length flowers will no longer
have the ideal phenotype
• birds that inherited longer bills will be more
successful
• Disruptive selection – favours individuals at
opposite extremes of a trait over individuals
with intermediate variations
• Environmental conditions may favour more than
one phenotype
– the hummingbird population may be able to feed
from two different flower species, each with
different-sized flowers
– each flower species is a good source of nectar, but
does not favour hummingbirds with medium-sized
bills
– birds with longer and shorter bills will be more
successful
• Sexual selection – favours the selection of
any trait that influences the mating success of
the individual
• Results in sexual dimorphism (striking
differences in male’s and female’s physical
appearance)
• Most common forms of sexual selection
result from female mate choice and male-vs.male competition
• Runaway Selection  bizarre extremes of sexual
dimorphisms which allow for beneficial mating
opportunities but are otherwise detrimental
• Fiddler crab (p. 560)
– Some males of certain bird species have
extraordinarily long tails, while the females’ tails are
quite short…Wouldn’t these really long tails be a
disadvantage for the males? Why do they have
them?
• Because the females like it! Females
preferentially mate with males with longer tails
– This has been proven through experiments where
they artificially lengthen the tails of male birds
– Birds that were previously unattractive (short-tailed)
have dramatically increased mating success (and,
one would assume, sex appeal) when their tails are
lengthened
The Formation of a New Species
• Species - members of groups or populations
that interbreed or have the ability to interbreed
with each other
• isolated populations become different from
each other in overall characteristics  start to
occupy different niches
• If this trend continues they may become so
different that they are said to be distinct species
– Allopatric speciation (geographically based)
• If species from different populations ever
migrate back to each other hybridization may or
may not occur depending on two isolating
mechanisms:
• Prezygotic Isolating Mechanisms  prevent
formation of zygotes and includes geographical,
ecological, temporal, behavioral and
mechanical isolation.
• Postzygotic Isolating Mechanisms  prevent
proper functioning of zygotes and therefore
development to maturity and therein ability to
reproduce.
Prezygotic Isolating Mechanisms
Geographical Isolation
• species simply do not exist together in the
same place (tigers and lions)
Ecological Isolation
• species in same area, but in different habitats
and thus will not hybridize with each other
• species formed may die due to decreased
fitness of living in area, which it is not suited for
Temporal Isolation
• differences in timing of breeding periods
• Lactuca graminifolia and L. canadensis (wild
cabbage) grow side by side and hybrids can be
created in laboratory situations but L.
graminifolia flowers in early spring and L.
canadensis flowers in the summer
Behavioral Isolation
• mating rituals are different therefore individuals
will not interbreed
Mechanical Isolation
• structural differences that can evolve that may
prevent mating (general size and mating
organs)
Postzygotic Isolating
Mechanisms
• Development is a complex process
• even though gametes might fuse, creating a
zygote, there are still factors that can hinder the
development of the organism into a normal,
functional, fertile individual
• Sibling Species – closely related species that
are often distinguished by behavior or other
non-evident characteristics. (e.g. leopard frogs
– all look the same but will not interbreed, even
in laboratory situation)
• Sympatric speciation  mutation to tetraploid vs. diploid
• If partially differentiated populations have
contact the differences between them may
become negligible over the course of time
• If, however, the offspring that are produced are
sterile/not as well adapted to the existing
habitats, the hybrids will be at a disadvantage
• Hybridization then will be of no advantage and
will not be passed on