Download Causes of Microevolution - Effingham County Schools

Causes of Microevolution
•
Microevolution – a generation to generation
change in a population’s frequencies of
alleles
• Two main causes of microevolution are
genetic drift and natural selection
1. Genetic drift – change in population’s
allele frequencies due to chance
An Example of Genetic Drift
• Bottleneck Effect
– Disasters such as earthquakes, floods,
droughts, and fires may reduce the size
of a population dramatically
– The small surviving population may not
be representative of the
original population’s gene
pool
– Example – cheetahs
An Example of Genetic Drift
• Bottleneck Effect
An Example of Genetic Drift
• The Founder Effect
– Occurs when a few individuals from a larger
population colonize an isolation island, lake, or
some other habitat
– The smaller the sample size, the less the genetic
makeup of the colonist will represent the gene
pool of the larger population they left
– Example – Population of Amish in Lancaster
County, PA
• Original pop. size of 30; current pop. size is
12,000. Frequency of extra fingers and toes is
1 in 14 rather than 1 in 1,000 in the US pop.
Causes of Microevolution
2. Natural Selection – alleles passes on from one
generation to the next due to some variants
leaving more offspring than others
• Gene Flow – Genetic exchange
due to the migration of fertile
individuals or gametes between
populations (reduces differences
between populations)
• Mutations – Causes a change in
DNA. Can only lead to evolution
and natural selection if the
mutation is in the gametes
Genetic Variation
• Polymorphism – When 2 or more forms of a
discrete character are represented in a
population (applies only to discrete
characteristics – not those on a continuum)
Variation Between Populations
• Geographic Variation – differences in gene
pools between populations or subgroups of
populations; can occur in different populations
or within a population
– Cline – a graded
change in some
trait along a
geographic axis
Variation Between Populations
What Generates Genetic Variation?
• Mutations - Only mutations that occur in cell
lines that produce gametes can be passed
along to offspring (it is rare that this occurs)
– Only those mutations that allow an
organism to be better suited to its
environment will exist in a pop.
– Mutations are more likely to be present in
high numbers when the environment is
changing and mutations that were once
selected against are now favorable under
the new conditions
What Generates Genetic Variation?
• Sexual Recombination
– Sexual reproduction recombines old
alleles into fresh assortments every
generation
– During meiosis, crossing over and the
law of segregation provide for an 1 in 8
million chance of identical sperm and a 1
in 8 million chance of identical egg cells
Evolutionary Fitness
• Darwinian Fitness – the contribution an
individual makes to the gene pool of the next
generation relative to the contributions of other
individuals
• Relative Fitness – the contribution of a
genotype to the next generation compared to
the contributions of alternative genotypes for
the same locus
• Survival alone does not guarantee reproductive
success
Modes of Selection
• Directional Selection – shifts the frequency
curve for variations in some phenotypic character
in one direction or the other by favoring what are
initially relatively rare individuals that deviate
from the average for that character
• Diversifying Selection – occurs when
environmental conditions are varied in a way that
favors individuals on both extremes of a
phenotypic range over intermediate phenotypes
• Stabilizing Selection – acts against extreme
phenotypes and favors the more common
intermediate variants
Modes of Selection
Natural Selection Favors Sex
• Sexually reproducing individuals generate
genetic variation that natural selection
works on
• Sexual dimorphism – differences in male
and female animal species that are not
directly associated with reproduction, but
increase an animals chance of reproducing
Natural Selection Favors Sex
Four Reasons Why Natural Selection
Cannot Produce Perfection
1. Evolution is limited by historical constraints –
evolution does not scrap ancestral anatomy
and build each new complex structure from
scratch. Ex. Back problems in humans
2. Adaptations are often compromises – seals
could walk on rocks easier with legs, but
could not swim as fast
Four Reasons Why Natural Selection
Cannot Produce Perfection
3. Not all evolution is adaptive – not all alleles
fixed by genetic drift in the gene pool of the
small founding population are better suited to
the environment than alleles that are lost.
4. Selection can only edit existing variations –
Natural selection favors only the fittest
variations from the phenotypes that are
available, which may not be the ideal traits.
New alleles do not arise on demand.