Download presentation
Presentation is loading. Please wait.
1
Evolutionary Change without Selection
2
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
3
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
4
Change without Selection
Change without selection occurs through genetic drift, the bottleneck effect, and the founder effect. Allele: a variation of a gene
5
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
6
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
7
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
8
Genetic Drift and Population
A particular trait is more likely to become very common or to disappear in a small population Chart pg 331
9
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
10
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
11
Genetic Bottlenecks Bottleneck: A dramatic reduction in population size, usually resulting in significant genetic drift, due to catastrophe A population of that suddenly drops to 50 will experience a significant genetic bottleneck
12
How Does it Happen? A population reduction can occur in any number of ways: Overhunting Habitat loss Natural disaster
13
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
14
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
15
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
16
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
17
Elephant Seals Population reduced to as few as 20 in the 1890s
Today over 125,000 exist, but all are genetically very similar
18
The Founder Effect The Founder Effect: occurs when a small number of individuals separate from their original population and establish a new population
19
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
20
Frequency Differences
Perhaps a rare variant of a gene occurs once in every 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
21
Reduced Frequencies The founder effect leads to a population with a new, smaller, gene pool We can expect less variation within the new population
22
Founder Effect
23
Founder Effect
24
Founder Effect
25
Founder Effect
26
Founder Effect
27
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
28
The Amish Community A closed-off religious community that shuns the modern world Isolation has created a smaller gene pool within the Amish community
29
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
30
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?
31
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
32
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
33
Predicting Change By considering these factors, we can predict which populations are most likely to undergo the most evolutionary change
34
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
35
Homework Questions Page 335 # 9, 10, 13
Similar presentations
© 2025 SlidePlayer.com. Inc.
All rights reserved.