Transcript How Organisms Evolve - Scituate Science Department

How Organisms Evolve
Preface
• Natural selection is a mindless, mechanical
process
• some individuals reproduce more
successfully than others and that is it
• therefore, their traits are more prevalent in
the next generation
• Darwin understood this but he had NO
knowledge of the mechanics of heredity
• today, we do
How Are Populations, Genes, and
Evolution Related?
• Understand that the changes observed as an
individual grows and develops are not
evolutionary changes
• Evolutionary changes are those that occur
from generation to generation; the changes
that cause descendants to be different from
their ancestors
• Furthermore, these changes cannot be
detected by looking at individuals
• You must look at many individuals across
many generations
• Evolution is, clearly then, a property of
populations
• But, populations are comprised of
individuals
• And, it is the actions and fates of them that
determine what traits will get passed on
• Therefore, inheritance provides the link
between the lives of individuals and the
evolution of populations
A Quick Review of the Principles
of Genetics (unit 2 starting on page 141)
• 1. Genes, influenced by the environment,
determine the traits of each individual
– there are usually more than one
allele(nucleotide sequence) for any given trait.
Different alleles generate slightly different
forms of the same enzyme which leads to
slightly different phenotypes such as, blue eyes,
green eyes, and brown eyes
• 2. The gene pool is the sum of all the
genes in a population
• In other words, the gene pool consists of
ALL the alleles of ALL the genes in ALL
the individuals of that population
Example
• In a population of 100 pea plants(sorry), the
gene for flower color would have 200
alleles (peas are diploid so there are 2 color
alleles per plant)
• All those alleles can produce only 2
phenotypes: purple or white
• Analysis of the population shows that some
individuals have alleles for white flowers
some for purple and some have both
continued
• If we add up the color alleles of all the
plants in the population, we could determine
the relative proportions of the different
alleles. This is called the allele frequency.
• So, if the gene pool for flower color consists
of 140 alleles for purple and 60 for white
the allele frequencies would be…
• correct! 0.7 or 70% for purple and 0.3 or
30% for white
• 3. Evolution is a change of gene frequencies
within a population
• many people define evolution as a change in
outward appearances or behaviors of members of
a population
• But changes in phenotypes are outward
expressions of changes in the gene pool
• So, evolution is nothing more or less than a
change in the genetic makeup of populations over
generations.
Hardy-Weinberg Equations
The Hardy-Weinberg Principle
• States that both allele and genotype
frequencies in a population remain constant
• that is to say, they are in equilibrium from
generation to generation unless disturbing
influences are introduced
Outside of the lab...
• …these disturbing influences are always in
effect
• Therefore, Hardy-Weinberg equilibrium is
impossible in nature
In the simplest case...
• …of a single locus with two alleles
• the dominant allele is denoted by T and
recessive by t (we’ll use “t’s” because of
taste)
• Their frequencies are denoted by p an q
• frequency (T) = p and frequency (t) = q
• p + q =1
If the population is in
equilibrium...
• then we will have frequency (TT) = p2 for
the TT homozygotes in the populations
• frequency (tt) = q2 for the tt homozygotes
• frequency(Tt) = 2pq for the heterozygotes
Let’s Look At It This Way
• Alleles for any given individual are chosen
randomly and independent of each other
• Consider two alleles A and a, with
frequencies p and q respectively
• The different ways to form new genotypes
can be derived using Punnet Squares
A(p)
a(q)
A (p)
2
AA(p )
Aa(pq)
a(q)
Aa(pq)
2
aa(q )
• so the formula is p2 + 2pq +q2 = 1
The Equilibrium Population
• Lets look at it from a different angle
• What if there was a population that did not
evolve?
• What would the characteristics of this
population have to be?
• The equilibrium population is a hypothetical
population that models this
The Characteristics of This
Hypothetical Population
• 1. There must be NO mutation
• 2. There must be NO gene flow between
populations (that is, there must be no net
migration of alleles into the population or
out of the population)
• 3. The population must be very large
• 4. ALL mating must be random, with NO
tendency for certain genotypes to mate with
specific other genotypes
• 5. There must be NO natural selection; that
is, ALL genotypes must be equally adaptive
and reproduce equally
• Under these conditions, allele frequencies
within a population will remain the same
indefinitely
• If one or more of these conditions are
violated, allele frequencies will change and
EVOLUTION WILL OCCUR.
What Causes Evolution?
• Based on the Hardy-Weinburg equilibrium,
we can state that evolution occurs due to
five major causes
–
–
–
–
–
mutation
gene flow
small population size
non-random mating
natural selection
Mutations Are the Ultimate
Source of Genetic Variability
• Mutations are inevitable
• Cells have a pretty efficient system for
protecting the integrity of their genes
• Some changes in the nucleotide sequence
slip past the check and repair systems
• When these changes occur in the cells that
produce gametes, the changes can enter the
gene pool and be passed on to the next
generation
Unfortunately, we
don’t get
superpowers
from this.
I don’t have this issue.
I do have this one though.
Mutations are rare
• They occur once in every 100,000-1,000,000
genes per individual in each generation
• Therefore mutation alone is not a big factor in
evolution, but...
• ...mutations are the source of new alleles, new
heritable variations on which other evolutionary
factors can act
• as such, they are the foundation of evolutionary
change
Without mutations, there would
be no evolution and no diversity
among life-forms.
MUTANTS
RULE!!!
Mutations Are NOT Goaldirected
• They do not arise as a result of or in
anticipation of environmental necessities
• They simply happen
• This may result in some change in structure
and/or function of the organism
• Whether they are helpful, harmful or neutral
depends on the environment over which the
organism has little control
• The mutation provides potential
Other forces..
• Such as migration and natural selection that
act on the potential may favor the spread of
the mutation through the population or
eliminate it.
Gene Flow Between Populations
Changes Allele Frequencies
• Gene flow: When individuals move from
one population to another, and interbreed at
the new location, alleles are transferred
from one gene pool to another.
• This alters the distributions of alleles among
populations
Example: Baboons
• Baboons live in social groups called troops
• Within each troop the females mate with a
handful of dominant males
• Young males usually leave the troop and
join another troop and perhaps become
dominant
• With this, the males carry genes from one
troop to another (gene flow)
By the way, baboons are big and strong and scary.
See what I mean?
Where is your
lip? What is
going on
here?!
What the hell is
this?
Gene Flow Can Have Very
Significant Effects
• Gene flow spreads advantageous alleles
throughout the species
• Gene flow helps maintain all the organisms
over a large area as one species
Genetic Drift
• Random events eliminate disproportionate
numbers of individuals
• This can change the allele frequency in the
population
Random Mating
• Most organisms DO NOT do this
• Only certain members of the population will
mate
• Therefore the next generation will only be
offspring of this select group, whose allele
frequencies may differ from the population
as a whole
Population bottleneck
• A form of genetic drift in which a
population becomes extremely small
• This leads to differences in allelic
frequencies and a loss of genetic variability
The Founder Effect
• A type of genetic drift in which an isolated
population founded by a small group of
individuals may develop allele frequencies
that differ from the parent population as a
result of chance inclusion of
disproportionate numbers of certain alleles
in the founders
Neanderthal Skull: Check out that
nose....FOUNDER EFFECT!!!!!!!
Survival and Reproduction
Influenced by Phenotype
• Phenotype depends, at least, partly on
genotype
• Natural selection tends to favor the
reproduction of certain alleles at the
expense of others
Extinction
Natural Selection Can Cause This
Too
• It is estimated that 99% of all the species
that ever existed are extinct
• Extinction is almost always caused by
environmental change
• the 3 major enviromental causes of
extinction are
– competition among species
– introduction of new predators or parasites
– habitat destruction