Evolution for Beginners

What is evolution?

A basic definition of evolution…“…evolution can be precisely

defined as any change in the frequency of alleles within a gene pool from one generation to the next." - Helena Curtis and N. Sue Barnes, Biology, 5th ed. 1989 Worth Publishers, p.974

So what does the definition mean?

• Evolution is a change in the number of times specific genes that code for specific characteristics occur within an interbreeding population

• Individuals don’t evolve, populations do• There is no implied “improvement” in

evolution

• Evolution: changes through time1. Species accumulate difference2. Descendants differ from their ancestors3. New species arise from existing ones

Genetic Variation and Evolution

A brief history of evolutionCharles Darwin was born on February 12, 1809 in Shrewsbury, England. From 1831 to 1836 Darwin served as naturalist aboard the H.M.S. Beagle on a British science expedition around the world. He observed much variation in related or similar species of plants and animals that were geographically isolated from each other.These observations were the basis for his ideas.

Darwin presumed that populations of individuals changed over time, and, in 1844, he developed the concept of the driving force for evolution. It wasn’t until many years later that he published his idea.

“I have called this principle, by whicheach slight variation, if useful, is preserved, by the term Natural Selection.”—Charles Darwin from "The Origin of Species“, 1859

A brief history of evolution

Natural selection: proposed by Darwin as the mechanism of evolution

• individuals have specific inherited characteristics • they produce more surviving offspring• the population includes more individuals with

these specific characteristics• the population evolves and is better adapted to its

present environment

Natural selection: mechanism of evolutionary change

Natural Selection

Darwin knew nothing of genes, but what he did have were two observations and a little inference that provided the motive force for evolution.

• Darwin: Evolution is descent with modification

Natural Selection

Observation 1: Organisms generally have more offspring than can survive to adulthood.

Observation 2: Offspring are not identical. There is variation in their appearance, size, and other characteristics.

Natural Selection

Inference: Those organisms that are better adapted to their environment have a greater likelihood of surviving to adulthood and passing these characteristics on to their offspring.

Survival of the “fittest.”

Darwin’s theory for how long necks evolved in giraffes

Survival of the “fittest.”

Evolution of species

• Based on 3 mechanisms– 1. Sources of variation

– 2. Method of selection for those characteristics that would be

passed on

– 3. A mechanism for retaining changes

1. Sources of variation

• Genetic diversity thru mutations that are not lethal– Physical or behavioral traits

• Sexual reproduction between genetically different individuals

1. Sources of variation

1. Sources of variation

Gene Flow• Many species are made up of local populations whose members tend to

breed within the group. • Each local population can develop a gene pool distinct from that of

other local populations. • Members of one population may breed with occasional immigrants

from an adjacent population of the same species. -introduce new genes or alter existing gene frequencies in the

residents. This is called hybridization. If the hybrids later breed with one of the parental types, new genes are

passed into the gene pool of that parent population. This process, is called introgression. It is simply gene flow between

species rather than within them.

• In either case, gene flow increases the variability of the gene pool.

Purebred naturally evolved region specific wild species can be threatened with extinction in a big way through the process of genetic pollution i.e. uncontrolled hybridization, introgression and genetic swamping which leads to homogenization or replacement of local genotypes as a result of either a numerical and/or fitness advantage of introduced plant or animal.

2. Method of selection

• Reproductive fitness is the method of selection.– Competition, escaping from and eluding

predators– Finding a good mate

Selection• Artificial selection: a breeder selects for

desired characteristics

Selection• Natural selection: environmental conditions

determine which individuals in a population produce the most offspring

• 3 conditions for natural selection to occur– Variation must exist among individuals in a population– Variation among individuals must result in differences

in the number of offspring surviving – Variation must be genetically inherited

3. Maintenance of Variation• Frequency-dependent selection: depends

on how frequently or infrequently a phenotype occurs in a population– Negative frequency-dependent selection: rare

phenotypes are favored by selection– Positive frequency-dependent selection:

common phenotypes are favored; variation is eliminated from the population

• Strength of selection changes through time

• Fitness of a phenotype does not depend on its frequency

• Environmental changes lead to oscillation in selection

Maintenance of Variation

Fitness and Its Measurement• Fitness: A phenotype with greater

fitness usually increases in frequency–Most fit is given a value of 1

• Fitness is a combination of:–Survival: how long does an organism live–Mating success: how often it mates–Number of offspring per mating that

survive

The Limits of Selection• Genes have multiple effects

– Pleiotropy: sets limits on how much a phenotype can be altered

• Evolution requires genetic variation– Thoroughbred horse speed

Selection for increased speed in racehorses is no longer effective

Directional selection: acts to eliminate one extreme from an array of phenotypes

Maintenance of Variation

Hardy–Weinberg principle states that the genotype frequencies in a population remain constant or are in equilibrium from generation to generation unless specific disturbing influences are introduced. Those disturbing influences include

1. non-random mating, 2. new mutations, selection, 3. random genetic drift 4. and gene flow.

Genetic equilibrium is a basic principle of population genetics.

Five agents of evolutionary change

Genetic Drift• Interbreeding is limited to the members of local populations. • If the population is small, Hardy-Weinberg may be violated. • Chance alone may eliminate certain members out of proportion to

their numbers in the population. In such cases, the frequency of an allele may begin to drift toward higher or lower values.

• Ultimately, the allele may represent 100% of the gene pool or, just as likely, disappear from it.

• Drift produces evolutionary change, but there is no guarantee that the new population will be more fit than the original one.

• Evolution by drift is aimless, not adaptive. Change in the allele frequencies (or gene frequencies) of a population from one generation to the next due to the phenomena of probability in which purely chance events determine which alleles (variants of a gene) within a reproductive population will be carried forward while others disappear.