Evolution of Populations Chapter 16 Genes and Variation

Evolution of Populations Chapter 16

Genes and Variation Relative (allelic) frequency - the percentage of a particular allele in a gene pool.

Selection Natural Selection- In nature, unequal ability to survive and reproduce. . . Survival of the fittest. Natural Selection ACTS ON PHENOTYPE but influences genotype (thus, allelic frequency) Artificial Selection- Mankind selects for desired traits. Also known as “selective breeding”

Over time, the zebra herd becomes faster as the slower zebra (and their genes) are removed from the population… survival of the fittest Darwin’s Theory = Evolution by means of natural selection

Artificial Selection

ADAPT OR DIE! Camouflage- organisms blend-in with surrounding environment Mimicry- species copy another to insure their own survival Nat. Geo

Genes and Variation Gene pool - all the genes that exist within a population

Gene flow – movement of alleles into or out of a population Immigration – new alleles move IN Emigration – alleles move OUT

Genetic drift - change in allelic frequencies by chance Ex: sudden extinction of a dominant species; small populations most affected

Genetic equilibrium - when alleles stay the same from generation to generation 1. 2. 3. 4. 5. The Hardy Weinberg Principle: Allele frequencies will remain constant under five conditions Random Mating Large Population No movement (immigration or emigration) No Mutations No Natural Selection: equal change of survival

Hardy-Weinberg Equation (p + q)2 = 1, which is the same as p 2 + 2 pq + q 2 = 1 p = frequency of “A” allele and q = frequency of “a” allele p 2 = expected freq. of homozygotes for one allele 2 pq = expected freq. of heterozygotes q 2 = expected freq. of homozygotes for the other allele

Natural Selection effects Genetic Change in Populations Natural Selection has three affects on phenotype distribution 1. Directional Selection 2. Stabilizing Selection 3. Disruptive Selection

Directional Selection 1. Directional Selection- Individuals on one end of a curve are “better fitted” than the middle or other end Peccaries naturally choose to consume those cactus plants with the fewest spines As a result, at flowering time there are more cacti with higher spine numbers; thus, there are more of their alleles going into pollen, eggs, and seeds for the next generation.

Stabilizing Selection 2. Stabilizing Selection. Individuals near center of a curve are “better fitted” (have highest fitness) than both ends Peccaries are consuming the low-spine number plants, and the insects are killing the high-spine-number plants. As these gene combinations are removed from the cactus gene pool, there is less and less variety possible in subsequent generations.

Disruptive Selection 3. Disruptive Selection. Individuals at upper and lower ends are “better fitted” than the ones in the middle Years of collecting have left their toll on the roadside cacti. In this environment, it is maladaptive to be good looking and have a reasonable number of spines. Low spine-number plants are not picked because they don't "look right", and high spine-number varieties are left alone because they are too hard to pick. Gradually, the gene pool changes in favor of the two extreme spine number types.

Patterns of Evolution 1. 2. 3. 4. Extinction Divergent Evolution (adaptive radiation) Convergent Evolution Coevolution

Extinction Why do species go extinct?

Extinction Natural selection, climate changes, and catastrophic events have caused 99 percent of all species that have ever lived to become extinct. Mass extinctions – caused by continents moving, sea level changing, volcano eruptions, large meteors

Predict what each ecosystem will look like after the event. Catastrophic Event

Question When a mass extinction happens, what do you think will happen next?

Divergent Evolution (adaptive radiation) Divergent evolution – natural selection causes 1 species to evolve into many species with many different adaptations (homologous structures) n n n After mass extinctions, many environments will be open for inhabitation Species will migrate to that area and new environmental pressures will cause the population to change over time This is also known as Adaptive Radiation

Adaptive Radiation in honeycreepers

Convergent Evolution – when unrelated organisms come to resemble one another (analagous structures)

Coevolution When 2 species evolve in response to one another

Evidence of Evolution Homologous structures - similar structures found in related organisms that are adapted for different purposes. Ex: human arm and bat wing or whale flipper ---DIVERGENT EVOLUTION--the process of two or more related species becoming more and more dissimilar.

Homologous structures Divergent evolution

Analogous structures - structures found in unrelated organisms that have a similar function but may be structurally different Ex: bird wing and insect wing ---CONVERGENT EVOLUTION--independent evolution of similar features in species of different lineages

Analogous structures Convergent evolution

Process of Speciation speciation - evolution of a new species 4 Main Isolating Mechanisms A. Reproductive B. Behavioral C. Geographic D. Temporal

A. Reproductive Isolation: Two populations cannot interbred and produce fertile offspring B. Behavioral Isolation: Two populations capable of breeding but cannot because of courtship rituals

C. Geographic Isolation: Two populations are separated by geographic barriers Ex: Rivers, Oceans, Mountains D. Temporal Isolation: Two or more populations reproduce at different times

Genes and Variation Sources of genetic variation: 1. 2. Mutations- change in DNA sequence Gene Shuffling- random assortment of genes during gamete production (SEXUAL reproduction) Gene Expression Variation n Single-gene trait- controlled by one gene n n Ex: Widow’s Peak Polygenic trait- controlled by many genes n Skin color, eye color

Original Source of Genetic Variation? MUTATION! the evolution of sexual reproduction increased genetic diversity within a population and therefore accelerated the rate of evolution