Causes Of Micro-evolution The Evolutionary Process Requires 2 Steps:

1. Production of variations 2. Sorting of variations

Why so much genetic variation among individuals? Five Basic Reasons: Gene mutation Crossing over during meiosis Independent assortment during meiosis Abnormal changes in chromosome number Fertilization between genetically different gametes(sexual reproduction)

Small Populations Mutations Gene Flow Non Random Mating Factors that Affect the Amount of Variation in Populations

Genetic Drift

Founder Effect = a case of genetic drift when a population is founded by a small number of individuals. Old order Amish New organisms established on islands

The old Amish settled in Lancaster, Pennsylvania in by just two couples. Because of the FOUNDER EFFECT, their gene pool contains a high concentration of dwarfism and polydactylism.

The Bottleneck Effect is another type of genetic drift. It occurs when a large population is drastically reduced in size. Natural disasters such as earthquakes, volcanic eruptions, fires, and floods kill organisms non-selectively. The small surviving population is unlikely to represent the genetic makeup of the original population.

Bottleneck Example #1 The new populations may be better adapted to the environment or perhaps less well adapted than the original population of flies.

Bottleneck Example # 2 Elephant Seals: Hunters reduced the elephant seal population to about 20 in the 1890’s, But it is now 30,000 animals. Researchers have found that no variation exists in 24 loci examined from the present population of elephant seals. (This suggests many alleles were lost during the bottleneck)

Cheetahs: The lack of genetic variation in South African cheetahs may have also resulted from genetic drift. Bottleneck Example # 3

IN ALL OF THESE CASES OF GENETIC DRIFT, THE AMOUNT OF GENETIC VARIATION IN THE POPULATION MAY BE SEVERELY LIMITED.

Mutations Mutation is important to evolution since it is the original source of genetic variation, which is the raw material for NATURAL SELECTION. It occurs randomly and may be harmful or beneficial depending upon the environment where the mutation is expressed.

More on Mutations Mutation has little quantitative effect on large populations in a single generation, since mutation at any given time is very rare. If some new allele produced by mutation increases, it is because individuals carrying this allele are producing a larger number of offspring due to NATURAL SELECTION.

Gene Flow Gene flow is the physical flow of alleles into and out of populations. Few populations of a species are isolated from other populations of the same species. Gene flow tends to decrease genetic variation between populations of a species.

More on Gene Flow Extensive gene flow can eventually group neighboring populations into a single population. Example: if an all red wildflower population was to begin receiving wind blown pollen from an all white flower population in a neighboring field.

Non Random Mating Individuals of a population usually mate with close neighbors rather than with more distant members of a population. –This violates the Hardy-Weinberg criteria that an individual must choose its mate at random from the population. –Since neighboring individuals of a large population tend to be closely related, inbreeding is promoted.

More on Non Random Mating Self-fertilization, which is common in plants, is the most extreme example of inbreeding.

Natural Selection The Hardy-Weinberg equilibrium conditions that all individuals in a population have equal ability to produce viable, fertile offspring is probably NEVER met. Variation among individuals exists and dome individuals leave more offspring than others.

More on Natural Selection THUS, alleles are passed on to the next generation in disproportionate numbers relative to their frequency in the present generation.

Examples of Natural Selection: Industrial melanism DDT and insects Sickle Cell Anemia Bacteria and antibiotics Skin Color

THE END