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