Natural SelectionAnd other mechanisms for

Evolution

Darwin’s Observations and InferencesGenetic Variation•Individuals in a species vary from one to another. These variations can be passed from parent to offspring. Over Production of Offspring•Individuals in populations tend to produce more offspring than can survive.Struggle for existence•Over population creates competition.•Environmental resources are limited.•Those that are more efficient will have an advantage for survival.

Differential survival and reproduction

Darwin’s Observations and InferencesInference 1: Production of more individuals than can be supported by the environment leads to a struggle for existence among individuals, with only a fraction of offspring surviving in each generation.

Inference 2: Survival in the struggle for existence is not random• depends in part on the

heritable characteristics • Those who inherit

characteristics most fit for their environment are likely to leave more offspring

Darwin’s Observations and Inferences

Inference 3:•The unequal ability of individuals to survive and reproduce leads to a gradual change in a population, with favorable characteristics accumulating over generations (natural selection).

This is Evolution by Natural Selection

Darwin’s Ideas Updated

• A major problem in Darwin’s theory was the lack of a mechanism to explain natural selection– How could favorable variations be

transmitted to later generations

• With the rediscovery of Mendel’s work and its vast extension in the first half of the 20th century, the missing link in evolutionary theory was forged.

• Darwinian theory supported by genetics is known as the modern synthesis.

Darwin in his early years.

Genetics of Evolutionary Theory

Darwin did not understand how traits were passed on to each generation. Today we define

fitness, adaptation, species, and process of evolutionary change in genetic terms.

Terms to Know

• FITNESS– Ability of an individual to survive and

reproduce• ADAPTATION

– Fitness is the result of adaptations– Adaptation is any individual characteristic that

increases an organism’s chance of survival

What is the purpose of the adaptation in this animal?

Support for Darwin’s Theory

1. Inherited variation exists within the genes of every population or species

– Law of segregation

– Law of independent assortment

Support for Darwin’s Theory

2. In a particular environment, some individuals of a population or species are better suited to survive and have more offspring

Support for Darwin’s Theory

3. Overtime, the traits that make certain individuals of a population able to survive and reproduce tend to spread in that populationDivergence –

Accumulation of differences between groups

Support for Darwin’s Theory

4. There is overwhelming evidence that living species evolve from organisms that are extinct.

Kaibab squirrel of north rim of Grand Canyon

Albert squirrel of south rim of Grand Canyon

Mechanisms for Evolution• Geographic isolation leads to reproductive

isolation– Two populations are separated by geographic

barrier•Example; river, mountain, ocean

– Two gene pools formed

Mechanisms for Evolution• Temporal isolation leads to reproductive

isolation– Two or more species reproduce at different times– Example: brown trout and rainbow trout live in

same stream but cannot interbreed because they breed in different months of the year

Brown Trout

Rainbow Trout

Patterns for Natural Selection• Divergent evolution – pattern in evolution in

which species that were once similar to an ancestral species diverge and become increasingly distinct.

• divergent evolution eventually results in a new species

Polar Bear Brown Bear

Patterns for Natural Selection• Adaptive radiation – when an ancestral species

evolves into an array of species to fit the number of diverse habitats.– Shown when new species evolve rather quickly– Happens when a colony moves to a new environment

that lacks any competing organisms– adaptive radiation is a form of divergent evolution.

•Examples: Hawaiian honeycreepers and finches of Galapagos.

Patterns for Natural Selection

A) organ pipe cactus (N. & S. America)

B) Euphorpia (African deserts)

Convergent EvolutionCan lead to development of analogous structures

PATTERNS OF EVOLUTION • Parallel

Evolution– Two related

species making similar evolutionary changes after their divergence.

– Marsupial and Placental mammals

Patterns for Natural Selection• Convergent Evolution – when two

species look alike but do not have a common ancestor

• Similar in appearance• Similar in function• Different in development, origin, &

anatomy

Opossum

Aye-Aye

DOLPHIN

SHARK

Patterns for Natural SelectionCoevolution – when two species evolve in response to changes in one another

•Flowers & Insects

•Flowers & Birds

•Flowers & Mammals

Evidence for Evolution – Structural Adaptation

• STRUCTURAL ADAPTATIONS– Mimicry– Camouflage

http://science.howstuffworks.com/animal-camouflage2.htm

Natural Selection in Action

Camouflage

Figure 22.11

(a) A flower mantidin Malaysia

(b) A stick mantidin Africa

When the environment changes over time

–Natural selection may result in adaptation to these new conditions

Natural Selection in Action Counter Shading

Natural Selection in Action

Eye spots

Natural Selection in Action

Disruptive Coloration

Natural Selection in Action

Mimicry Monarch or Viceroy Butterfly

Summary• Evolution is change in

species over time• Heritable variations

exist within a population

• These variations can result in differential reproductive success

• Over generations this can result in changes in the genetic composition of the population.