Evolution in the Small Scale

Discussed components of evolution

Natural selection

Common descent

Time

Populations

Variation

Put all together

Evolution at Its Smallest Scale

A species is a group of organisms able to successfully interbreed in nature

A species generally consists of several smaller groups called populations

All the members of a species that live in a defined geographic region at a given time

Populations, not individuals, are the units that evolve

Evolution at Its Smallest Scale

Evolution can be defined as a change in the characteristics of a population over time

Populations are often isolated to some degree from other populations

The environments inhabited by each population is different

Each population faces the natural selection pressures of its own environment

Different populations can evolve differently

Evolution at Its Smallest Scale

e.g., Two different frog populations in two different environments may both evolve coloration patterns making them less visible to predators

A lighter coloration may benefit one population in their particular environment, while a darker coloration pattern may benefit a second population in their (different) environment

Evolution at Its Smallest Scale

Natural selection acts upon the phenotypes of individuals

Individuals with one phenotype may be more fit than individuals with a different phenotype

These phenotypes are influenced by genes

More specifically, different alleles of genes

Evolution as a Change in the Frequency of Alleles

An individual’s phenotype may make it more likely to survive longer

Surviving individuals may reproduce

Individuals that do not survive certainly cannot reproduce

Evolution as a Change in the Frequency of Alleles

Some individuals are more successful at breeding

Their alleles are passed on to the next generation in relatively greater numbers

Alternative alleles possessed by less successful frogs are passed on to the next generation in reduced numbers

Evolution as a Change in the Frequency of Alleles

The evolution of a population involves a difference in reproductive output between individuals

This is often the result of a difference in survival between individuals

Surviving individuals may reproduce

Individuals that do not survive certainly cannot reproduce

Evolution as a Change in the Frequency of Alleles

“Microevolution”is a change in allele frequencies in a population over a relatively short period of time

This is evolution within a population

“Macroevolution”is evolutionary change that results in the formation of new species

Larger changes resulting in the same way as microevolution

Five Agents of Microevolution

There are five “agents”of microevolution that can alter allele frequencies in populations

Mutation

Gene flow

Genetic drift

Sexual selection

Natural selection

Five Agents of Microevolution

A “mutation”is any permanent alteration in an organism’s DNA

Mutations can be small

“Point mutations”: change in single base pair

Mutations can be large: addition or deletion of whole or parts of chromosomes

Some mutations are heritable

Five Agents of Microevolution

Mutation rates are very low

Perhaps just one base pair per billion

Very few of these mutations are beneficial

Most are neutral, some are harmful

Beneficial mutations are very important

Mutations are the source of new genetic variation

New proteins are produced, which can modify the form or capabilities of the organism

Five Agents of Microevolution

The evolution of eyes, wings, and other structures depends upon many mutations

Many mutations, followed by rounds of genetic shuffling and natural selection are required

This process may take millions of years

Five Agents of Microevolution

Allele frequencies can change as a result of migration

Arrival of members from a different population

“Immigration”

Loss of individuals to a different population

“Emigration”

Five Agents of Microevolution

This movement may involve individuals, or may involve gametes

e.g., Movement of pollen

Five Agents of Microevolution

Allele frequencies can change simply as a matter of chance

Random fluctuations in allele frequencies are termed “genetic drift”

Alleles can increase or decrease in frequency

Alleles that are removed can only be replaced through new mutations or through migration

Five Agents of Microevolution

These fluctuations have the greatest impact on smaller populations

The bottleneck effect

The founder effect

Five Agents of Microevolution

The “bottleneck effect”is a change in a population’s allele frequencies due to chance following a sharp reduction in population size

Five Agents of Microevolution

e.g., Northern elephant seals were hunted very heavily in the 1890s

Only 50 individuals remained

Protective measures have increased these numbers somewhat

Five Agents of Microevolution

All members of the current population of northern elephant seals have descended from this few individuals that survived the bottleneck

Many alleles were removed from the population during the bottleneck

All of the members of today’s population are genetically very similar

Five Agents of Microevolution

Pull a dozen M&Ms from a two pound bag

Will all of the colors be represented?

Will the colors be represented in the same proportions as in the original bag?

If these dozen M&Ms “reproduce”to fill a new two pound bag, what proportions might you expect?

How do your answers differ if you pull 120 M&Ms from the bag?

Five Agents of Microevolution

The “founder effect”occurs when a small population migrates to a new area to start a new population

The allele frequencies in this small population will not precisely mirror those of the larger population

The effects of genetic drift can be profound in such a small population

Five Agents of Microevolution

“Sexual selection”involves differential reproductive success based on success in obtaining mating partners

A form of natural selection

This mating is based on phenotype

Five Agents of Microevolution

Birds of paradise on Papua-New Guinea

Five Agents of Microevolution

Differential mating success among members of one sex is generally based on choices made by members of the opposite sex

Females generally do the choosing

Can also be based upon the combative abilities of males

Five Agents of Microevolution

“Natural selection”is a process in which the differential adaptation of organisms to their environment selects those traits that will be passed on with greater frequency from one generation to the next

Natural selection is the means of adaptive evolution

Through natural selection, populations become better adapted to their current environment

Five Agents of Microevolution

Through natural selection

Traits of individuals more successful at reproducing will become more widespread in a population

The alleles that bring about these traits will increase in frequency from one generation to the next

Five Agents of Microevolution

“Adaptation”is a modification in the structure or behavior of organisms over generations in response to environmental change

A particular environment may change

e.g., Streams drying up, etc.

A population may migrate to a different environment

Five Agents of Microevolution

Natural selection is the only agent of microevolution that consistently works to adapt organisms to their environment

Mutation simply generates variation

Genetic drift is random

Gene flow is not related to environment

Sexual selection deals with mate choice

Natural Selection and Evolutionary Fitness

“Darwin’s finches”have been studied since the 1970s by Peter and Rosemary Grant

In 1977, a tiny GalapagosIsland suffered a severe drought

Daphne Major

This drought had a major impact on the island’s two finch species

1,300 finches fewer than 300 finches

85% of the Geospiza fortis population died

Natural Selection and Evolutionary Fitness

The G. fortis population that survived the drought had a larger average beak size than the pre-drought population

6% larger beak size

Their offspring had a larger average beak size

A slightly larger beak enables a finch to get into large, tough seeds

Natural Selection and Evolutionary Fitness

In 1984 and 1985, there was excessive rain on this island

There was an abundance of small seeds

Finches with smaller beaks survived this event in disproportionate numbers

The average beak size in the following generation decreased slightly

Natural Selection and Evolutionary Fitness

Where is the “fittest”bird in all of this?

There isn’t any

Evolution was not marching toward the production of some generally superior bird

Different traits were favored under different environmental conditions

17.5 Three Modes ofNatural Selection

Many traits are polygenic

e.g., Human height

Display continuous variation

When natural selection acts upon these traits, it can proceed in any of three ways

Stabilizing selection

Directional selection

Disruptive selection