Evolutionary Change without Selection

Natural Selection The way nature selects for some traits over others Selection is NOT random BUT evolutionary change doesn’t always happen this way Picture of a bird

Change without Selection Changes within a gene pool can occur without the influence of successful traits These changes tend to reduce genetic diversity within a population Spix’s Macaw

Change without Selection Change without selection occurs through genetic drift, the bottleneck effect, and the founder effect. Allele: a variation of a gene

Genetic Drift Genetic drift: The random shifting of the genetic make-up between generations This shifting is entirely due to chance So some generations may see more of a particular feature, and others may see less of that feature

Genetic Drift and Population Population size has a major effect on genetic drift Large populations tend to produce a stabilizing effect on alleles Other ½ of pg 331 chart

Genetic Drift and Large Populations So in large populations, we expect to see a lot of genetic variability One trait may be more common in a generation, and less common in another generation Overall though, genetic variability remains fairly constant

Genetic Drift and Population A particular trait is more likely to become very common or to disappear in a small population Chart pg 331

Genetic Drift and Small Populations So small populations tend to have less genetic variation Certain features become more common within that population, and certain features are lost

Variety and Large Populations So in order to maintain a high level of genetic variability, a large population is best Features remain in the population at a fairly constant level

Genetic Bottlenecks Bottleneck: A dramatic reduction in population size, usually resulting in significant genetic drift, due to catastrophe A population of 10000 that suddenly drops to 50 will experience a significant genetic bottleneck

How Does it Happen? A population reduction can occur in any number of ways: Overhunting Habitat loss Natural disaster

How Does it Happen? The surviving population is likely to have a smaller gene pool than the initial population These particular traits will be passed along In this way, certain traits become favoured, while certain alleles disappear

As the Population Recovers If the population is allowed to recover, that population will be limited to those few traits left as a result of the bottleneck

Results of a Bottleneck Overall, a bottleneck reduces the size of the gene pool, and genetic variety This is generally harmful to the remaining population Increased vulnerability to disease Low reproductive success Increased infant mortality

Cheetahs The cheetah species suffered a severe bottleneck about 10,000 years ago As few as 7 cheetahs survived, and all cheetahs today are descendents of these

Elephant Seals Population reduced to as few as 20 in the 1890s Today over 125,000 exist, but all are genetically very similar

The Founder Effect The Founder Effect: occurs when a small number of individuals separate from their original population and establish a new population

The Founder Effect The founding population would have a unique mix of alleles different than the larger original population By chance, some alleles that are less common in the larger population will be more common in the founding population

Frequency Differences Perhaps a rare variant of a gene occurs once in every 10000 people in the large population But in the founding population, that trait occurs in 1 of the 20 founders Original Population = 1/10000 (0.1%) Founding Population = 1/20 (5%) This represents an 50x increase in the frequency for that variant

Reduced Frequencies The founder effect leads to a population with a new, smaller, gene pool We can expect less variation within the new population

Founder Effect

Founder Effect

Founder Effect

Founder Effect

Founder Effect

The Galapagos Tortoise The entire population is the result of as few as one pregnant female tortoise from South America Her genes, and those of her offspring, formed the entire gene pool for the new Galapagos Tortoise population

The Amish Community A closed-off religious community that shuns the modern world Isolation has created a smaller gene pool within the Amish community

Bottlenecks vs. Founder Effect Effect on Gene Pool Smaller Results in Genetic Drift? Yes How does it Happen? Catastrophe Some individuals leave the population to start their own Examples Cheetahs, Elephant Seals Galapagos Tortoise, Mennonites

A Mathematical Model for Evolution We can think of evolution as a change in the genetic makeup of a population We can also think about evolution from a math perspective: The frequency of each allele in the population Can we predict the relationship between allele frequency and the chances of those frequencies remaining constant?

The Hardy-Weinberg Principle The Hardy-Weinberg Principle: states that in large populations where only random chance is at work, alleles should stay constant between generations

The Hardy-Weinberg Principle Other factors will result in evolution, as these will lead to changes in variant frequencies Natural selection Small population size Mutation Immigration or Emigration Transfer of new alleles from a different species

Predicting Change By considering these factors, we can predict which populations are most likely to undergo the most evolutionary change

Applying the Principle to Different Species Some species will undergo evolutionary change faster than others A species with High genetic diversity Fast reproduction will respond to natural selection more quickly This is why bacteria develop resistance to antibiotics so quickly

Homework Questions Page 335 # 9, 10, 13