microevolution

• 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.

Terminology review

• Population

• Genotype

• Phenotype

• Allele

• Gene pool – all the alleles that exist in a population

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– Base pair, deletion, translocation, etc.– 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 allele frequencies in 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 bottleneckin the nineteenth century.

Bottleneck effect

Founder effect

This 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.

Natural Selection

• 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.

• Traits of those who are more successful in 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.

Artificial selection

• Artificial selection: Humans choose traits they prefer in animals or crops and breed to achieve those “desirable” individuals.

• Natural selection is the only one of the five agents of microevolution that consistently works to adapt organisms to their environment.– Genetic drift is random– Mutation has a negative effect, or no effect– Gene flow doesn’t necessarily bring in genes

that are better suited to the environment– Non random mating doesn’t have anything to

do with matching individuals to environment

Fitness

• The success of an organism in passing on its genes to offspring, relative to the other members of its population.

• The allele frequencies will increase.

• Also known as “survival of the fittest”

1. Large cactus finch (Geospiza conirostris) 2. Large ground finch (Geospiza magnirostris) 3. Medium ground finch (Geospiza fortis)4. Cactus finch (Geospiza scandens)5. Sharp-beaked ground finch (Geospiza difficilis) 6. Small ground finch (Geospiza fuliginosa)

7. Woodpecker finch (Cactospiza pallida) 8. Vegetarian tree finch (Platyspiza crassirostris) 9. Medium tree finch (Camarhynchus pauper)10. Large tree finch (Camarhynchus psittacula) 11. Small tree finch (Camarhynchus parvulus) 12. Warbler finch (Certhidia olivacea) 13. Mangrove finch (Cactospiza heliobates)

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.

• 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.