Chapter 13 Section 3 and more Objectives Describe

Chapter 13 Section 3 and more!

Objectives ▪ Describe an allele. ▪ Identify the five principles of natural selection. ▪ Describe how changes in allele frequencies are due to chance events, ▪ Summarize speciation.

What is an allele? ▪ It is a version or “flavor” of a gene that one can receive from their parents. Dad Mom

Natural Selection Principles 1. Organisms produce more offspring than their environments can support. Not all individuals in a population survive.

Natural Selection Principles 2. Offspring vary in physical appearance. Some offspring are better able to survive in their environment than others.

Natural Selection Principles 3. Much of this variation in physical appearance is caused by differences in the alleles inherited from the individual’s parents.

Natural Selection Principles 4. The inheritance of alleles determines how likely an organism is to survive and reproduce.

Natural Selection Principles 5. The ability of some organisms with certain alleles, to survive and produce more offspring will lead to a change in allele frequency in the population. Over time, more individuals will have the “helpful” alleles.

Other Mechanisms of Evolution ▪ As in the peppered moth’s population, natural selection can change the frequencies of alleles in a population. ▪ However, there are other ways in which allele frequencies may change.

Other Mechanisms of Evolution ▪ Alternate means of allele frequency changing: – Genetic drift: changes in allele frequencies that are due to chance events. ▪ Key to Genetic Drift: changes allele frequency unrelated to an allele’s impact on survival and reproduction.

Other Mechanisms of Evolution ▪ Alternate means of allele frequency changing: – Migration: the movement of individuals into or out of a population. ▪ These individuals bring or take their alleles with them.

Two Types of Migration 1. Emigration – organisms leaving an ecosystem 2. Immigration - organisms moving into an ecosystem

Other Mechanisms of Evolution ▪ Alternate means of allele frequency changing: – Founder Effect: Decreases the genetic variation in the new population by reducing the number of different alleles. ▪ Sometimes, members of a population migrate to another ecosystem. ▪ The new ecosystem has never been colonized by this organism. ▪ These migrants are the founders of a new population

Other Mechanisms of Evolution ▪ Alternate means of allele frequency changing: – Bottleneck Effect: Occurs when an entire population is reduced to just a few individuals. ▪ The population that survives after a bottleneck disaster has less genetic variation. ▪ Even when the population increases, genetic variation will remain low!

Other Mechanisms of Evolution ▪ Alternate means of allele frequency changing: – Mutations: produce new alleles. ▪ These new alleles are caused by changes in DNA sequences.

Speciation ▪ Evolution does not occur only to change existing species. ▪ Entirely new species also evolve. ▪ Speciation is the formation of new, genetically distinct species from populations of existing species.

Speciation ▪ New species arise when a population splits or two populations of the same species are under different selective pressures (ex: predators, food sources, diseases, human interference).

Types of Speciation 1. Geographical Isolation Example: A new highway is built that divides a single population of beetles into two populations. After many generations, the beetles will develop differences in their gene pool, thus possibly resulting in two new species.

Types of Speciation 2. Reproductive Isolation: Prevents members of two populations of the same species from mating, even if they inhabit the same area. Reasons: ill-attraction, sexual structures don’t match correctly, periods of sexual maturity don’t match

Speciation As separate populations evolve along separate paths, they become increasingly different. Eventually they will become so different that these organisms will form their own subspecies, which is the first step toward speciation.