EVOLUTION OF EVOLUTION OF POPULATIONSPOPULATIONS

What is evolution?What is evolution?

•The change in the The change in the genetic make-up of a genetic make-up of a species over timespecies over time

POPULATION GENETICSPOPULATION GENETICS Study of evolution from a genetic Study of evolution from a genetic

point of viewpoint of view What is a population?What is a population?

• A collection of individuals of the same A collection of individuals of the same species that routinely interbreed ( living species that routinely interbreed ( living in the same location)in the same location)

A species is a naturally breeding A species is a naturally breeding group of organisms that produce group of organisms that produce fertile offspring.fertile offspring.

A population is the smallest in which A population is the smallest in which evolution occursevolution occurs

Individuals within a population vary. Individuals within a population vary. Biologist study the variation . Biologist study the variation .

A bell curve represents the distribution of variants in A bell curve represents the distribution of variants in a population.a population.

What causes variation?What causes variation?

Recombination in Recombination in the diploidthe diploid• Crossing-overCrossing-over• Independent Independent

assortment of assortment of allelesalleles

MutationMutation

Random fusion of Random fusion of gametesgametes

fertilizationfertilization

Gene pool – total genetic information available in a Gene pool – total genetic information available in a population. population.

Allele frequency tends to remain the same from Allele frequency tends to remain the same from generation to generation unless acted upon by an outside generation to generation unless acted upon by an outside force. force.

What is Hardy-Weinberg What is Hardy-Weinberg Equilibrium?Equilibrium?

Assumes Assumes NONO evolution evolution occurs.occurs.

All 5 conditions must All 5 conditions must be meet.be meet.

Can never happen!Can never happen! It is a model or a It is a model or a

yardstick to measure yardstick to measure how much a how much a population or species population or species has evolved.has evolved.

Hardy-Weinberg Equilibrium Hardy-Weinberg Equilibrium ConditionsConditions

No net mutationNo net mutation

No migrationNo migration

Large population sizeLarge population size

Mating is randomMating is random

Natural selection does not occurNatural selection does not occur

Disruption of Genetic EquilibriumDisruption of Genetic Equilibrium

MutationMutation MigrationMigration Genetic DriftGenetic Drift Non- Random MatingNon- Random Mating Natural SelectionNatural Selection

MIGRATIONMIGRATION

ImmigrationImmigration EmigrationEmigration Gene flow – moving Gene flow – moving

genes from genes from population to population to anotheranother

Genetic DriftGenetic Drift

Change in the Change in the allele frequency as allele frequency as a result of random a result of random events or chanceevents or chance• Usually occurs in Usually occurs in

small populationssmall populations• After a natural After a natural

disastersdisasters FloodFlood Forest fireForest fire

In the smallest population allele frequency reaches 0 after the 45th generation = no variation

Non-random MatingNon-random Mating

Random Mating – Random Mating – mating without mating without regard to genetic regard to genetic make-upmake-up

Sometimes mating Sometimes mating selection is often selection is often influenced by influenced by geographic geographic proximityproximity

Many animals do Many animals do not mate randomlynot mate randomly

NATURAL SELECTIONNATURAL SELECTION1 All species have genetic variation.All species have genetic variation.

2.2. The environment presents many The environment presents many challengeschallenges

3 Organisms tend to produce more offspring Organisms tend to produce more offspring than the environment can support - than the environment can support - competition ( struggle for survival)competition ( struggle for survival)

4 Some individuals are better suited to cope Some individuals are better suited to cope with the challenges ( survival of fittest)with the challenges ( survival of fittest)

5 Characteristics best suited to environment Characteristics best suited to environment tend to increase in atend to increase in a population over population over timetime

(5)(5) Over long periods of time, and given a steady input of new Over long periods of time, and given a steady input of new variation into a population, these processes lead to the variation into a population, these processes lead to the emergence of new speciesemergence of new species

Essence of Darwin’s ideasEssence of Darwin’s ideas(1)(1) VariationVariation exists in natural populations exists in natural populations

(2)(2) Many Many more offspringmore offspring are born each season than are born each season than can possibly survive to maturitycan possibly survive to maturity

(3)(3) As a result, there is a As a result, there is a struggle for existencestruggle for existence

- - competitioncompetition

(4)(4) Characteristics beneficialCharacteristics beneficial in the struggle in the struggle for existence will tend to become more common for existence will tend to become more common in the population, changing the average in the population, changing the average characteristics of the populationcharacteristics of the population

- - adaptationsadaptations

LaMarckian vs. Darwinian viewLaMarckian vs. Darwinian view LaMarckLaMarck

• in reaching higher in reaching higher vegetation giraffes vegetation giraffes stretch their necks & stretch their necks & transmits the transmits the acquiredacquired longer neck to offspringlonger neck to offspring

DarwinDarwin• giraffes born with longer giraffes born with longer

necks survive better & necks survive better & leave more offspring leave more offspring who who inheritinherit their long their long necksnecks

STABALIZING SELECTIONSTABALIZING SELECTION

Individuals with Individuals with the average the average form are of a form are of a trait have the trait have the highest fitnesshighest fitness

Example:1. Birth weight in offspring2. Seed size

DIRECTIONAL SELECTIONDIRECTIONAL SELECTION The frequency of The frequency of

one allele tends to one allele tends to move in one move in one direction ( more of direction ( more of one of the one of the extremes forms of extremes forms of the traitthe trait

Example – tongue length in anteaters

Disruptive SelectionDisruptive Selection Individuals with

either extreme have an advantage over individuals with the average form of the trait.

Example: Limpet shell coloration

Sexual SelectionSexual Selection

SPECIES FORMATIONSPECIES FORMATION

Begins with geographic isolationBegins with geographic isolation

Results in reproductive isolationResults in reproductive isolation

Speciation occursSpeciation occurs

MECHANISM FOR MECHANISM FOR REPRODUCTIVE ISOLATIONREPRODUCTIVE ISOLATION

Geographical Geographical isolatiisolationon

Ecological isolationEcological isolation

Temporal isolationTemporal isolation

Mechanical isolationMechanical isolation

Reproductive failureReproductive failure

How does How does evolution/speciation occur?evolution/speciation occur?

GRADUALISMGRADUALISM• gradual process that goes on all the gradual process that goes on all the

timetime PUNCTUATED EQUILIBRIUMPUNCTUATED EQUILIBRIUM

• periods of rapid change are separated periods of rapid change are separated by periods of little or no changeby periods of little or no change

MICROEVOLUTION MICROEVOLUTION

LEADS TO LEADS TO

MACROEVOLUTIONMACROEVOLUTION