Download Ch 16 Notes PPT

Evolution of
Populations
Chapter 16
Darwin and DNA
In the 1930s, Mendel’s work was
connected to Darwin’s work and
scientists understood that genes
control heritable traits
 In the 1950s, the Watson-Crick model
of DNA allowed scientists to
demonstrate the molecular nature of a
mutation and genetic variation

Where does variation come
from?


Gene Shufflingwhich is the
extensive mixing of
genes during
MEIOSIS or sexual
reproduction
Mutations- or
changes in the
structure of DNA
How do scientists study
variation and evolution?


It’s studied in populations, or a group of
individuals of the same species that interbreed
Because they interbreed they share a common
group of alleles called a gene pool
Sample Population
Homozygous Black
16%
Heterozygous Black
48%
Homozygous Brown
36%
In population on 100
mice…
40 Black Alleles (B)
60 Brown Alleles (b)
How do we know a population
is evolving?

When there are changes in the relative
frequency of alleles in the gene pool
How do the allele frequencies
change?



Natural Selection (directional, stabilizing,
disruptive, or sexual selection)
Random Change (mutations or genetic drift)
Migration or gene flow
Natural Selection on
Populations
DIRECTIONAL SELECTION:
Individuals at one end of curve have higher
levels of fitness than the individuals at the
other end of the curve
Ex: Peppered Moths
Natural Selection on
Populations
STABILIZING SELECTION:
Individuals in the center of the curve
have higher levels of fitness than the
individuals at either end
Example: Birth weight in humans or
marine creatures on ocean floor
Natural Selection on
Populations
DISRUPTIVE SELECTION:
Individuals on the ends of the
curve have higher fitness levels
than individuals in the center of
the curve
Example: Beak size in Finches
Natural Selection on
Populations
Mutations
The frequency of a mutation in the
population will depend on the effects
of natural selection- will it make the
individual more likely to survive and
reproduce?
 If the mutation increases fitness, it will
spread throughout the population!!

Genetic Drift




The random change in the gene pool
of a population
All populations are subject to genetic
drift, but small populations are
impacted more
Bottleneck Effect: Occurs when a
natural disaster greatly reduces the
size of the population and the gene
pool (population loses variation)
Founder Effect: Occurs when a few
individuals from a population colonize
an isolated island or new habitat
(population loses genetic variation)
Hardy-Weinberg Principle
Allele frequencies in a population will
remain constant unless one or more
factors causes them to change
 A population will NOT evolve if…

1. Random Mating
2. Large Populations
3. No Immigration or Emigration
4. No Mutations
5. No Natural Selection
Evolution of a Species
Biological Species Concept: a species
is a population that will interbreed and
produce fertile offspring
 When a population evolves enough
genetic change (either through natural
selection or genetic drift) that it no
longer interbreeds with the original
population, it is considered a new
species

Reproductive Isolation
When a reproductive barrier keeps species
from interbreeding. Barriers include:
 Temporal (timing)- different mating seasons
 Behavioral- different courtship or mating
behaviors
 Geographic- populations divided by barriers
like glaciers, valleys, rivers, etc.