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Exploring non-selective mechanisms that change the genetic makeup of populations.
Imagine a giant foot accidentally steps on a colony of rare green beetles, leaving only the brown ones alive. Was it because the brown ones were 'better,' or was it just bad luck? In the game of evolution, sometimes survival isn't about being the fittest—it's about being in the right place at the right time.
While natural selection is driven by traits that provide an advantage, genetic drift is a change in the gene pool due to random chance. This process has a much larger impact on small populations. Think of it like flipping a coin: if you flip it 1,000 times, you'll likely get close to 50% heads. But if you only flip it 4 times, you might get 100% heads just by luck. In biology, if a population is small, an allele (a version of a gene) can disappear entirely just because the individuals carrying it didn't happen to reproduce, regardless of how 'good' that gene was. We represent allele frequencies as and , where . Genetic drift causes these values to fluctuate randomly over time.
Quick Check
Does genetic drift have a larger impact on a population of 50 organisms or 5,000 organisms?
Answer
A population of 50 organisms, because random events can cause much larger percentage shifts in small groups.
There are two main ways genetic drift happens. The bottleneck effect occurs when a disaster (like a fire or overhunting) kills a large portion of the population. The few survivors may not have the same genetic diversity as the original group. The founder effect happens when a few individuals leave a large population to start a new colony elsewhere. Because the 'founding' group is so small, the new population will only have the traits of those few individuals. Both effects lead to a loss of genetic diversity, making the population more vulnerable to diseases or environmental changes.
1. In the 1890s, hunters reduced the Northern Elephant Seal population to just 20 individuals. 2. This created a severe bottleneck. 3. Today, the population has grown to over 100,000, but they are all genetically very similar. 4. Because of this low diversity, a single new virus could potentially wipe out the entire species.
Quick Check
What is the primary difference between the bottleneck effect and the founder effect?
Answer
The bottleneck effect is caused by a mass die-off, while the founder effect is caused by a small group colonizing a new area.
Gene flow is the movement of alleles into or out of a population due to the migration of individuals. When a brown bird from the forest flies over a mountain to join a population of blue birds in the valley, it brings its 'brown' alleles with it. This process increases the genetic variation within the receiving population. Over time, heavy gene flow between two different populations makes them more genetically similar to each other, effectively acting as the opposite of isolation.
1. Imagine two fields of wildflowers separated by a highway. 2. Field A has mostly white flowers; Field B has mostly purple flowers. 3. A strong wind carries pollen (which contains plant sperm/DNA) from Field B to Field A. 4. The next generation in Field A now has a higher frequency of purple alleles. 5. This is gene flow without the entire plant moving!
It is vital to distinguish between these mechanisms. Natural Selection is non-random; it 'picks' the best-adapted individuals. Genetic Drift is random; it 'picks' the lucky individuals. While selection usually leads to adaptation, drift can actually cause harmful alleles to become more common or helpful alleles to be lost. In the real world, both forces act on populations simultaneously. Scientists use the Hardy-Weinberg equilibrium equation, , to calculate if a population is evolving or if these random forces are shifting the balance.
1. A storm blows 5 lizards (3 green, 2 brown) to a new volcanic island (Founder Effect). 2. On the mainland, green is a disadvantage, but on this island, there are no predators. 3. By chance, the 2 brown lizards fail to find mates and die without offspring (Genetic Drift). 4. Even though 'brown' wasn't a bad trait, the island population becomes 100% green. 5. This shows how drift can override selection in small, isolated groups.
Which of the following is an example of the founder effect?
How does gene flow typically affect two separate populations?
Genetic drift is a non-random process that always leads to better-adapted organisms.
Review Tomorrow
In 24 hours, try to explain the difference between a 'bottleneck' and 'gene flow' to a friend or family member without looking at your notes.
Practice Activity
Draw a diagram of a 'bottleneck' using different colored dots to represent alleles. Show how the 'before' and 'after' populations differ in color variety.