microevolution

Darwin’s Finches

Microevolution – A change of allele frequencies in a population over a short time. The basis for all large-scale or macroevolution.

To fully appreciate evolution, we need to look not just at the outcomes but also at the processes that created those outcomes.

Vocabulary review Population

Genotype

Phenotype

Allele

Gene pool

The two frog

populations

now have

different

environmental

pressures.

Exactly how do

populations evolve?

There are many

different alleles

in a population

but a frog can

only have two.

These two frogs

have different

alleles for

coloration.

What causes Evolution?

Five Agents of Change that can alter

allele frequencies

Mutation

Gene flow

Genetic drift

Non-random mating

Natural selection

Mutation

Any permanent alterations in the makeup of DNA.

They must be heritable

Most do nothing, a few are harmful, rarely are they beneficial.

These mutations are not working to further survival and reproduction.

These mutations are not likely to account for a change in allele frequency.

These good mutations bring new genetic information into the genetic pool.

Gene Flow

A change in allele frequency that can occur

when mating with members from another

population occurs.

The movement of genes from one

population to another.

Migration – the movement of individuals

from one population into the territory of

another.

Plants migrate, too.

The ranges of the Bullock's and Baltimore orioles. The

species overlap in a hybrid zone, where gene flow occurs.

Gene

flow

Migration

Genetic Drift

The chance alteration of allelesin a

population, with such alterations having

greatest impact on small populations.

Bottleneck effect

Founder’s effect

A small sample of alleles is likely to yield a

gene pool that is different from the

distribution found in the larger population.

If there is a drastic change (disease, migration or natural catastrophe) in the survival of a large population, there is little change in the frequency of that population's alleles. In a smaller population, drastic change can result in loss of an entire allele.

Bottleneck effect – The change in allele frequency

in a population due to chance following a sharp

reduction in the population size.

Founder effect – When a small portion of a

population migrates to another area, starting a new

population.

Bottleneck effect

Cheetahs survived a drastic bottleneck

in the nineteenth century.

Bottleneck effect

Founder effectThis Native American

woman, like all Native Americans, is

a descendant of people who crossed the Bering Strait into Alaska over 10,000

years ago. No Native Americans

have type B blood, apparently by chance, none of the emigrants had this

blood type.

Nonrandom Mating Mating in which a given member of a population is

not equally likely to mate with any other given member.

Sexual selection is a form of nonrandom mating. This is mating based on phenotype, based on the choices made by the female of the species.

The male will increase the proportion of his alleles in the next generation.

Three modes of

Natural Selection

When natural selection operates on

characters that are polygenic and

continuously variable, it can proceed in one

of three ways.

Does natural selection favor what is average

or what is extreme?

Stabilizing Selection Intermediate forms are favored

over extreme forms.

Human baby birth weight.

Infant deaths are higher at the

extremes of birth weight,

children most likely to survive

have an average birth weight.

Natural selection is working to

make average birth weights

more common

Directional Selection When natural selection

moves a character towards

one of its extremes.

Smoothness of brachiopod

shells took 10 million years to

achieve.

Disruptive Selection When natural selection moves a

character to both its extremes.

Occurs much less frequently in

nature.

Results in two separate species

Finches in West Africa

When food is scarce,

large-billed birds specialize

in cracking a large seed.

Small-billed birds feed on

soft varieties of seed.

A bird with an intermediate

sized bill would get less

food and not survive.