How Organisms Evolve

BIO 101

Life Science

Dr. D. L. Daley

Selection of Color in Guppies

Guppies are native to northeastern South America

In some streams waterfalls have separated populations of the same species of guppy

In the pools below the waterfalls predation by the pike cichlid is a big risk& survival rates are low

However in pools above the water falls the only predator is the killifish, which rarely preys on guppies

Selection of Color in Guppies

In high predation area the guppies are drab colored & reproduce at a younger age - adults are small

Male guppies above the waterfalls display bright colors to court females &larger males are more successful at holding territories and mating with females

These differences suggest that they represent natural selection

Selection of Color in Guppies: Laboratory Experiment

2000guppies divided between 10 pools

Six months later pike cichlids were added to 4 pools, killifish to 4 pools the remaining two were “no-predation” controls

14months later (10 guppy generations) - the guppies with killifish were indistinguishable from the controls - large and brightly colored males

Those with the pike cichlids were smaller and drab

Selection of Color in Guppies: Field Experiment

Located two pools below falls with pike cichlids and guppies - upper pools only had killifish

Guppies were now transplanted to the upper pools - monitored the upper pools at several year intervals

Despite the fact the guppies originated from high predation pools the transplanted populations quickly evolved traits of low predation guppies

They were larger, more brightly colored and matured later

Control populations in the lower pools remained drab, matured early and were smaller in size

How Evolution Works

Microevolution- changes in fitness can result in different adaptations of the organisms for survival & reproduction

These occur without leading to new species

The selection of color example in guppies

Macroevolution- accumulation of adaptations can lead to the evolution of new species &higher taxa

Agents of Microevolution

Mutation- permanent change in an organism’s DNA

Generally no effect or harmful

Only rarely beneficial or adaptive

Gene Flow - movement of alleles between populations

Via migration of individuals

Gene Flow: Plant Migration Example

Agents of Microevolution

Genetic drift

Random fluctuation in allelic frequency from one generation to the next, including loss of alleles from a population

In small populations the death of an individual with a particular allele would have a great effect compared with the same loss in a large population

Many real populations are relatively small and thus genetic drift can contribute to allelic changes (evolution) in the population

Genetic Drift

Agents of Microevolution

Nonrandom mating

Occurs when one member of a population is not equally likely to mate with any other member.

Includes sexual selection - where members of a population choose mates based on the traits males exhibit

Agents of Microevolution

Natural selection

Some individuals are more successful than others in surviving and hence reproducing, owning to traits that give them a better “fit” with their environment

Thus the alleles of those who reproduce more will increase in frequency in a population.

Modes of Selection

Directional selection - discriminates against individuals at one extreme of the variation in a phenotypic character

Example- small individuals might be disadvantaged compared individuals that are normal or larger-than-normal

Thus the mean value of the character is moved in the direction of the most fit phenotypes

Modes of Selection

Stabilizing selection - discriminates against individuals the have extreme variation in the phenotypic character in either direction

Example- Favors individuals with little variation and thus it tends to reduce variation in the population

Modes of Selection

Disruptive selection - reverse of stabilizing selection - Individuals at both extremes of the range of variation

Thus individuals near the middle are disadvantaged

Thus it increases variation but doesn’t change the mean

Modes of Selection

Population Bottleneck - Example of Genetic Drift

Macroevolution

What is a species

Are groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups.

Speciation

Anew species is a group of organisms that can no longer interbreed with the population from which it originated.

Sympatricspeciation - population accumulates enough genetic changes that make it unable to reproduce with others in the population even if they live in the same area

Speciation of Fish in a Large Lake

Example of sympatric speciation

Different individuals of a species become specialized for different subsets of the environment of a lake

Eventually these different populations become unable to reproduce with each other

Sculpins in Lake Baikal in Africa is an example

Speciation

Allopatric speciation - subpopulation becomes physically separated from its parent population - then evolves differently such that interbreeding with the parent population is no possible

Founder effect - individuals on the fringe of a very large population do not mix well with majority of population (have a different gene frequency) - descendents of this subpopulation can give rise to new genetically different population

i.e. New species

Vicariance- physical separation - e.g. mountain range rises up

Allopatric Speciation in a Salamander

Another Example of Allopatric Speciation