Speciation

Gene Pool 1. Gene Pool: The total number of genes (alleles) in an interbreeding population * the larger the gene pool, the greater the genetic diversity of the population * a small gene pool indicates little genetic diversity

Genetic Drift 1. Genetic drift —is one of the basic mechanisms of evolution. In each generation, some individuals leave behind a few more descendents (and genes) than other individuals. The genes of the next generation will be the genes of the “lucky” individuals, not necessarily the healthier or “better” individuals. It happens to ALL populations.

Bottleneck Effect 1. Bottleneck effect: severe reduction in population size due to intense selective pressure or a natural calamity which alters the allele frequency 2. Because genetic drift acts more quickly to reduce genetic variation in small populations, undergoing a bottleneck can reduce a population’s genetic variation by a lot.

Founder Effect 1. Founder effect:- when a new colony is started by a few members of the original population. 2. This small population size means that the colony may have: * reduced genetic variation from the original population * a non-random sample of the genes in the original population If the red “star” is the person who has the gene for Huntington’s, then we can see how this one gene can be traced back to the original founder.

Speciation: Often Begins with a Physical Separation From the Original Population Grand Canyon is NOT a physical barrier for all species. What kind of organisms would be able to cross the canyon? 1. Geographic isolation: a barrier that physically separates members of a species into two or more groups EX: Mountains, large body of water (lake or ocean), volcanic eruption, canyon, road A barrier may physically separate some species, but not others

Reproductive Isolation Leads to Speciation Mutations cause changes in chromosome number Humans are the only primates that have 46 chromosomes Members of an original species can no longer breed together to produce fertile offspring. – Cannot interbreed. Three species of Flycatcher. Reproductive Isolation keeps these as three separate species. Why? Females only respond to mating call of males that are like them.

Temporal Isolation Temporal isolation: mate at different times or seasons Because they mate at different times, the wood frog and the leopard frog remain separate species and hybrids are not produced

Species are Not Fixed Not all organisms have been on Earth for the same amount of time Graph below shows when different groups appeared and how numerous they were during different time periods Time

Gradualism verses Punctuated Equilibrium 1. Gradualism: slow and steady changes in a species over time 2. Punctuated Equilibrium: long periods of stasis followed by rapid change

Effects of Natural Selection: Directional Selection 1. Directional selection: members at one end of distribution curve have a higher fitness than those in the middle or at the other end of the curve. 2. Can lead one population evolving into a new species. Food becomes scarce. Notice the shift in beak size

Effects of Natural Selection: Stabilizing Selection 1. Stabilizing selection: members in the center of the distribution curve have a higher fitness than those at each end. 2. This type of selection favors average individuals. 3. Reduces variation in a population; evolution is NOT likely to happen. 4. Ex: babies that are between 6 & 8 pounds at birth have a better chance of surviving 1st year of life.

Effects of Natural Selection: Disruptive Selection Slide # 20 Effects of Natural Selection: Disruptive Selection 1. Disruptive selection: when members at BOTH ends of the distribution curve have a higher fitness than those in the middle 2. In some cases, there are no intermediate forms. 3. This can lead to the evolution of two new species. Population splits into two subgroups specializing in different seeds. Number of Birds in Population Beak Size

Cladograms and Phylogenetic Trees Show Relatedness

Remember: Individuals Do not Change to Fit the Environment