Evolution of Populations How can populations evolve to form new species?

Populations, not individuals, evolve Can an organism respond to natural selection by acquiring or losing characteristics?.....

Populations, not individuals, evolve An organism cannot develop a new phenotype by natural selection in response to its environment

Genes and Variation – genotype and phenotype in Evolution Remember, organisms inherit 2 alleles for a particular trait The combination of those alleles determines its Genotype The genotype and the environmental conditions produces the organisms Phenotype Natural Selection acts on the Phenotype Some individuals have a phenotype that is better suited to their environment

Evolution occurs as a population’s genes and their frequencies change over time

Populations and Gene Pools Many individuals make up a population All of the genes of the individuals make up the populations genes All of the alleles that make up these genes, (of every single individual) in the population is the GENE POOL Allelic frequency -is the percentage of any specific allele in the gene pool.

Genetic Equilibrium If the genes in the population do not change, the population is said to be in Genetic Equilibrium. Evolution is not occurring Something must disrupt the populations gene pool in order for evolution to occur

Sources of Genetic Variation Mutations- changes to the DNA a single mutation could have a large effect, but usually evolutionary change is based on the accumulation of many mutations Mutations are random Can be beneficial, harmful, or neutral Mutations do not “try” to make an organism better Depending where they occur they may not affect evolution If in a somatic cell they cannot be passed onto the offspring Genetic Recombination in Sexual Reproduction Crossing over and independent assortment Lateral Gene Transfer organisms that reproduce asexually, like bacteria can pass genes from one organism to the next. Gene Flow Movement of genes from one population to another

Natural Selection acting on Variations Natural Selection acts on phenotypes by producing one of three types of selection: Directional selection Stabilizing selection Disruptive selection

Natural Selection on Variations Directional Selection- When natural selection favors one of the extreme variations of a trait. Example: the change in color of a moth population as the trees they lived in turned a darker color from pollution. Population changed from paler color to darker color

Natural Selection on Variations Stabilizing Selection – Natural Selection that favors average individuals in a population. Example: Population of spiders. Large spiders are easily seen and eaten. Small spiders can’t find food. Average spiders survive

Natural Selection on Variations Disruptive Selection- Individuals with either extreme of a trait’s variation are selected for. Example: Sea Snail shells range from white, to tan, to dark brown. Snails live on rocks. White snails living on light rocks are harder to see. Same for dark snails living on darker rocks. The tan are easily seen on light and dark and are eaten.

Other Ways Allele Frequencies May Change Genetic Drift –change in allele frequencies due to chance. Has a great affect on SMALL populations because the gene pool is reduced

Example: a bird population nests on the side of a cliff Example: a bird population nests on the side of a cliff. A landslide may eliminate a portion of the population. If, just by chance, more of the birds in the landslide area have a certain allele, that allele’s frequency will decrease.

Other sources of Genetic Variation Migration -movement of individuals into or out of a population If the population is small, the effect of even a small number of migrating individuals can be great. Especially if a rare allele is removed from the population

Other sources of Genetic Variation Founder Effect -results from the formation of a new population from just a few individuals who leave a larger population The new populations frequencies will reflect those of the founding population rather than the original population.

Other sources of Genetic Variation Population Bottleneck Occurs when an entire population is reduced to just a few individuals, often as a result of some form of disaster, such as disease

The Process of Speciation: How does one species become two? Species – a population or group of populations whose members can interbreed and produce fertile offspring In order to get new species populations must become Reproductively Isolated from one another Reproductive Isolation – occurs when formerly interbreeding organisms can no longer mate and produce fertile offspring

How does one species become two? Isolating Mechanisms Reproductive Isolation-can develop in a variety of ways including Geographic Isolation Behavioral Isolation Temporal Isolation These are examples of Prezygotic mechanisms - before egg and sperm come together

Isolating Mechanisms- Geographic Isolation A physical barrier divides a population Called -Allopatric speciation Populations are separated by geographic barriers such as rivers, mountains, or bodies of water.

Isolating Mechanisms Not a physical boundary Called – Sympatric speciation Behavioral Isolation Populations that develop differences in courtship rituals or other behaviors. Temporal Isolation When two or more species reproduce at different times.

Postzygotic Mechanism After sperm and egg come together Example: Infertile offspring Huge fetus – die during child birth Weak offspring that die quickly These examples do not allow reproduction to continue and lead to isolation.