Biologist now know that natural selection is not the only mechanism of evolution

Inheritable variation may come from any of the following: 1. Genetic drift 2. Gene flow 3. Mutation 4. Natural selection

Chapter 15 Evolution 15.3 Shaping Evolutionary Theory 1. Genetic Drift: when the same genes are combined in different ways to produce different results Due to chance with independent assortment

In smaller populations, the effects of genetic drift become more pronounced, and the chance of losing an allele becomes greater.

Chapter 15 Evolution Extreme Examples of Genetic Drift Founder Effect Occurs when a small sample of a population settles in a location separated from the rest of the population Alleles that were uncommon in the original population might be common in the new population.

Ex. Blue People of the Appalachian Mountains

Chapter 15 Evolution Extreme Examples of Genetic Drift 2. Bottleneck Occurs when a population declines to a very low number and then rebounds

The rebound group has traits that are most similar to the smallest group

Chapter 15 Evolution 15.3 Shaping Evolutionary Theory Gene Flow: Increases genetic variation as individuals move in and out of a population

3. Mutations: Random changes in the genetic code

Chapter 15 Evolution 15.3 Shaping Evolutionary Theory 4. Natural Selection Acts to select the individuals that are best adapted for survival and reproduction

Chapter 15 Evolution 15.3 Three Types of Natural Selection Stabilizing selection eliminates extreme expressions of a trait when the average expression leads to higher fitness.

Ex. Babies who have too low birth weights are less likely to survive

Chapter 15 Evolution 15.3 Shaping Evolutionary Theory Directional selection when an extreme trait makes organisms more fit it becomes more common.

Ex. Body color of the peppered moth found in England Originally, most were white, now most are brown

Why? https://www.youtube.com/watch?v=etsjB-6u-6w

Chapter 15 Evolution 15.3 Shaping Evolutionary Theory Disruptive selection a process that splits a population into two groups.

Tends to remove individuals with average traits but retains the extreme traits to better suit them for their environment Ex. Same snake species found in two areas

Speciation Speciation: The development of new species through evolution A population must diverge and then be reproductively isolated

Two Types of Speciation: 1. Allopatric speciation 2. Sympatric speciation

Allopatric Speciation Chapter 15 Evolution 15.3 Shaping Evolutionary Theory Allopatric Speciation A physical barrier divides one population into two or more populations. Abert squirrel Kaibab squirrel

A species evolves into a new species without a physical barrier. Chapter 15 Evolution 15.3 Shaping Evolutionary Theory Sympatric Speciation A species evolves into a new species without a physical barrier. The ancestor species and the new species live side by side during the speciation process but cannot reproduce.

https://www.youtube.com/watch?v=8yvEDqrc3XE

Patterns of Evolution Speciation takes a very long time (compared to a human life) but there is evidence that it occurs: Divergent Evolution (Adaptive Radiation) Coevolution Convergent Evolution

Divergent Evolution (Adaptive Radiation): when two different species share the same ancestral origins but have evolved differently 

15.3 Shaping Evolutionary Theory Chapter 15 Evolution 15.3 Shaping Evolutionary Theory Divergent Evolution (Adaptive Radiation) Can occur in a relatively short time when one species gives rise to many different species Follows large-scale extinction events

Chapter 15 Evolution 15.3 Shaping Evolutionary Theory Coevolution The relationship between two species might be so close that the evolution of one species affects the evolution of the other species. Mutualism

Chapter 15 Evolution 15.3 Shaping Evolutionary Theory Convergent Evolution Unrelated species evolve similar traits even though they live in different parts of the world.

Chapter 15 Evolution 15.3 Shaping Evolutionary Theory Rate of Speciation Evolution proceeds in small, gradual steps according to a theory called gradualism. Punctuated equilibrium explains rapid spurts of genetic change causing species to diverge quickly.

15.3 Shaping Evolutionary Theory Chapter 15 Evolution 15.3 Shaping Evolutionary Theory