MECHANISMS OF EVOLUTION

After studying Malthus, Darwin realized that if more individuals are produced than can survive, members of a population must compete to obtain food, living space, and other limited necessitiesDescribed as the “struggle for existence”

TARGET #3- I CAN EXPLAIN HOW VARIATION CONTRIBUTES TO AN

ORGANISM’S SURVIVAL

Variation plays a vital role in the struggle for existence Individuals have natural

variations among their heritable traits

Darwin hypothesized that some of these variations are better suited to life in their environment than others Ex: a lion with longer

claws and higher endurance is a better hunter than a lion without those traits

Adaptation: any heritable characteristic that increases an organism’s ability to survive and reproduce in its environment.Includes

Body parts Claws Wings

Coloration Bright colors Camouflage Mimicry

Physiological functions Photosynthesis Brain capacity

TA

RG

ET

#4

- I CA

N

DE

SC

RIB

E A

N

AD

AP

TA

TIO

N

Darwin recognized that there must be a connection between the way an organism “makes a living” and the environment.Differences in adaptations affect an organism’s fitness Fitness: how well an organism can survive and

reproduce in relation to others of the same species within it’s environment Individuals with adaptations that are well suited to their

environment can survive and reproduce more offspring Individuals with characteristics that are not well suited to their

environment either die without reproducing or leave few offspring The difference in rates of survival and reproduction is

called “survival of the fittest”

TARGET #5- I CAN EXPLAIN WHY ORGANISMS WITH GREATER FITNESS GENERALLY LEAVE MORE

OFFSPRING THAN ORGANISMS THAT ARE LESS FIT

Natural SelectionThe process by which organisms with variations most suited to their local environment survive and leave more offspring

The environment influences fitness

Natural selection occurs in any situation in which more individuals are born than can survive, there is a natural heritable variation, and there is variable fitness among individualsDoes not make the organism “better”Does not move in a fixed directionA process that enables species to survive and reproduce in a local environment If local conditions change, some traits

may become adaptive If conditions change faster than a

species can adapt, the species may become extinct

TA

RG

ET

#6

- I CA

N S

UM

MA

RIZ

E H

OW

N

AT

UR

AL S

ELE

CT

ION

ALLO

WS

O

RG

AN

ISM

S W

HIC

H A

RE

MO

RE

A

DA

PT

ED

SU

RV

IVE

IN T

HE

IR

EN

VIR

ON

ME

NT

Adaptations and variations among individuals in populations of species does not begin in the environmentStep 1: a mutation occurs within the DNA of an organism

Step 2: that mutation leads to the production of a protein that either does not function, over functions, or has a different function than originally intended

Step 3: the mutation is passed down to an offspring where it is expressed in the offspring’s phenotype

TARGET #7- I CAN EXPLAIN HOW A POPULATION OF ORGANISMS IN A SPECIES

UNDERGOES NATURAL SELECTION

Step 4: the mutated phenotype is naturally selected by the environment as being more advantageous, and that individual has more offspring than the non-mutated individuals

Step 5: the mutation is passed down, generation to generation, until the population of organisms in that species retain that mutation

**Step 6: if enough mutations collect within a population of individuals in a given species, then a new species is formed

Within the species black bear (Ursus americanus), there is a subspecies (Ursus americanus kermodei) There are about 400 individuals in this subspecies About 10% of this subspecies is homozygous recessive for

a mutation that turns their coats whitish Caused by a mutation in a pigment gene, known as MC1R

(Melanocortin receptor) MC1R is involved with the synthesis of the pigment

eumelanin A mutation in this gene in humans results in red hair)

Evolutionary Advantage Better at catching fish during the day than their dark bear

counterparts Are considered sacred by the local people, the Tsimshian, so

they are hunted less

EXAMPLE- THE SPIRIT BEAR

Natural selection depends on the ability of organisms to reproduce, or to leave descendantsLineage can be traced back over many generations

Over many generations, adaptation could successfully lead to the evolution of new speciesDarwin proposed that living species are descended with modification from common ancestorsKnown as descent with modification

TA

RG

ET

#8

- I CA

N E

XP

LA

IN W

HA

T

DA

RW

IN’S

ME

CH

AN

ISM

FO

R

EV

OLU

TIO

N S

UG

GE

ST

S A

BO

UT

LIV

ING

AN

D E

XT

INC

T S

PE

CIE

S

Means that life as been on earth for a long time

Basis for the explanation of the diversity of life

According to the principle of common descent, all species, living and extinct, are descended from ancient common ancestors