chapter 15: evolution of populations biology 1. variation inherited differences between individuals...
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CHAPTER 15: EVOLUTION OF POPULATIONS
Biology 1
Variation
Inherited differences between individuals of a population
Can be Physical characteristic Biochemical characteristic Behavioral characteristic
If there is no variation for a trait, it is said to be fixed
Gene Pool
All of genes found within a population Relative frequency of alleles- proportion
of gene pool that the allele makes up
Sources of Variation
Mutation Creates new variation by changing parts of
the genetic code Gene Shuffling
Creates new variation by the reshuffling of genes during sexual reproduction Chromosome segregation Crossing-over
Single Gene Traits
• Traits are coded for by a single gene• If trait has simple Mendelian
(dominant/recessive) inheritance, there are 2 phenotypes possible.
• If trait has incomplete dominance or codominance, there are 3 phenotypes possible.
• If trait has multiple alleles, # of phenotypes depends on # of alleles▫ For example: ABO blood type have 3
alleles with 4 phenotypes possible
Natural Selection on Single Trait Genes
Occurs if the phenotypes are not equal in their fitness
Relative frequencies within the gene pool change as some phenotypes are selected for (or some are selected against)
This is evolution (a change in allele frequencies within a population over time)
Natural Selection Refresher
Polygenic Traits
Trait is coded for by more than one gene Various phenotypes possible Phenotypes form a bell curve
Natural Selection of Polygenic Traits
Directional Selection One phenotype extreme is selected for (or
one against) Bell curve is shifted to the left or right
Natural Selection of Polygenic Traits (continued)
Stabilizing Selection Both phenotype extremes are selected
against (average phenotype is selected for) Bell curve narrows
Natural Selection of Polygenic Traits (continued)
Disruptive Selection Average phenotype is selected against
(extremes are selected for) Bell curve splits into two peaks
Genetic Drift
Change in allelic frequencies due to random effects
Effects are seen more in smaller populations
Genetic Drift: Bottleneck Effect Event randomly removes large numbers
of individuals from a population Many variations can be lost
Genetic Drift: Founder Effect Small part of the population removes
itself (or is removed) from the larger population
Genetic Equilibrium
Allelic Frequencies remain the same No evolution
Random mating Large population No immigration or emigration No mutation No natural selection
Speciation
Species - group of individuals that can breed together and produce a fertile offspring
Speciation is the process of forming new species from existing species
To occur: Populations of one species must be isolated
from each other long enough to accumulate enough changes to become two species
Types of Isolation
Geographical Isolation Populations are
separated by a geographical barrier and cannot mate and share genes
Types of Isolation
Behavioral Isolation Populations are separated by behavioral
differences and don’t mate with each other to share genes
Temporal Isolation Populations reproduce at different times so
they cannot mate together and share genes
This leads to…
Reproductive Isolation Cannot mate and produce a fertile offspring Occurs because individuals cannot
Mate togetheror Create a zygoteor Create a viable offspringor Create a fertile offspring
EVOLUTIONARY TRENDS
Adaptive Radiation from COMMON DESCENT Species diversifies into many new species
EVOLUTIONARY TRENDS
Convergent Evolution-Analogous Structure Species evolve to a similar form from
different ancestors Both adapt to similar environment Examples
EVOLUTIONARY TRENDS
Coevolution Species evolve together because of a close
ecological relationship Coevolution Examples
EVOLUTIONARY TRENDS
Punctuated Equilibrium Evolution shows long stable period and
then rapid bursts of change